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Type B Nerve fibres cause exactly what kind of autonomic sensation to the preganglionic neurons?

Type B Nerve fibres cause exactly what kind of autonomic sensation to the preganglionic neurons?


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Ganong's Review of Medical Physiology (25th ed) presents the Erlanger and Gasser classification of mammalian Nerve Fibres as such:

Type B Fibres are concerned with Preganglionic autonomic sensations, but there is no mention of this being sympathetic or parasympathetic. Does it mean that it is both? The table does specify it as sympathetic for the Type C Fibres, though. (I have tried searching this on google, but I can't seem to find anything specific.)


Vagus nerve

The vagus nerve, historically cited as the pneumogastric nerve, is the tenth cranial nerve or CN X, and interfaces with the parasympathetic control of the heart, lungs, and digestive tract. The vagus nerves are normally referred to in the singular. It is the longest nerve of the autonomic nervous system in the human body and comprises sensory and motor fibers. The sensory fibers originate from neurons of the nodose ganglion, whereas the motor fibers come from neurons of the dorsal motor nucleus of the vagus and the nucleus ambiguus. [1]


Peripheral Nervous System Function

The primary function of the peripheral nervous system is to connect the brain and spinal cord to the rest of the body and the external environment. This is accomplished through nerves that carry information from sensory receptors in the eyes, ears, skin, nose, and tongue, as well as stretch receptors and nociceptors in muscles, glands and other internal organs. When the CNS integrates these varied signals and formulates a response, motor nerves of the PNS innervate effector organs and mediate the contraction or relaxation of skeletal, smooth or cardiac muscle.

Thus, the PNS regulates internal homeostasis through the autonomic nervous system, modulating respiration, heart rate, blood pressure, digestion reproduction, and immune responses. It can increase or decrease the strength of muscle contractility across the body, whether it is sphincters in the digestive and excretory systems, cardiac muscles in the heart, or skeletal muscles for movement. It is necessary for all voluntary action, balance, and maintenance of posture.

Sensory Nervous System

The functional classification of the PNS divides it into three categories. The first is the sensory nervous system, carrying signals from the viscera, sense organs, muscles, bones and joints towards the CNS. Nerve fibers that carry this information are part of the afferent division. Sensory receptors can transduce a physical stimulus such as pressure, sound waves, electromagnetic radiation, or chemical composition, into an electrochemical signal.

This signal, when it reaches a certain threshold, is transmitted as an action potential along an afferent neuron, and relayed to the CNS, where the signal is perceived and interpreted. Thus the sensory nervous system consisting of the receptor and neural pathway deliver information about the intensity, location, type, and duration of a stimulus to the CNS.

Somatic Nervous System

The second functional division of the PNS is the somatic nervous system. It controls the voluntary muscular movement of skeletal muscles in the limbs, back, shoulders, neck, and face. It also mediates reflex actions, where an afferent nerve fiber is nearly directly connected to a motor nerve fiber, to quickly generate a response to a stimulus. These include protective responses, like the movement of the body away from acute injurious stimuli like extremes in temperature, as well as those like the patellar ‘knee-jerk’ response when the patellar ligament is struck.

Autonomic Nervous System

The autonomic nervous system is related to all the involuntary visceral activity of the body. It consists of the sympathetic and parasympathetic nervous systems, and their effector organs include cardiac muscle, smooth muscle, and various glands. The anatomy of the autonomic nervous system is distinct because the effector arm involves two neurons that synapse with each other at specific ganglia.

The neurons of the sympathetic nervous system have short preganglionic neurons that can excite multiple postganglionic nerve fibers. The sympathetic nervous system is said to have a thoracic and lumbar outflow. The parasympathetic nervous system, on the other hand, uses cranial and sacral nerves and their ganglia are situated close to the target organ.


Sympathetic ganglia

Sympathetic ganglia can be divided into two major groups, paravertebral and prevertebral (or preaortic), on the basis of their location within the body. Paravertebral ganglia generally are located on each side of the vertebrae and are connected to form the sympathetic chain, or trunk. There are usually 21 or 22 pairs of these ganglia—3 in the cervical region, 10 or 11 in the thoracic region, 4 in the lumbar region, and 4 in the sacral region—and a single unpaired ganglion lying in front of the coccyx, called the ganglion impar. The three cervical sympathetic ganglia are the superior cervical ganglion, the middle cervical ganglion, and the cervicothoracic ganglion (also called the stellate ganglion). The superior ganglion innervates viscera of the head, and the middle and stellate ganglia innervate viscera of the neck, thorax (i.e., the bronchi and heart), and upper limbs. The thoracic sympathetic ganglia innervate the trunk region, and the lumbar and sacral sympathetic ganglia innervate the pelvic floor and lower limbs. All the paravertebral ganglia provide sympathetic innervation to blood vessels in muscle and skin, arrector pili muscles attached to hairs, and sweat glands.

The three preaortic ganglia are the celiac, superior mesenteric, and inferior mesenteric. Lying on the anterior surface of the aorta, preaortic ganglia provide axons that are distributed with the three major gastrointestinal arteries arising from the aorta. Thus, the celiac ganglion innervates the stomach, liver, pancreas, and the duodenum, the first part of the small intestine the superior mesenteric ganglion innervates the small intestine and the inferior mesenteric ganglion innervates the descending colon, sigmoid colon, rectum, urinary bladder, and sexual organs.


Biology Question Bank – 34 MCQs on “Nervous System and Sense Organs” – Answered!

34 Questions with Answers and Explanations on “Nervous System & Sense Organs” for Biology Students.

1. Acute vision is present in

Image Source: mcinroeanatomy.files.wordpress.com

Answer and Explanation:

1. (a): Bird’s (e.g. vulture) sense of sight has much higher resolution than ours. Their eyes are much larger in proportion to the sizes of their heads than our eyes are.

2. Sensitive pigmented layer of eye is

Answer and Explanation:

2. (b): The retina consists of both pigmented layer and the sensory layer. The pigment cells reinforce the light absorbing property of choroid in reducing the scattering of light in the eye. The sensory layer consists of rods and cones required for vision.

3. Which of the following cranial nerves can regulate heart beat?

Answer and Explanation:

3. (a): X nerve i.e. the vagus nerve (mixed) that arises from the side of medulla controls the visceral sensations and movements of larynx, lungs, heart, stomach and intestines. IX nerve innervates pharynx and tongue. VIII nerve innervates internal ear and VII nerve innervates face, neck, taste buds and salivary glands.

4. One function of parasympathetic nervous system is

(a) contraction of hair muscles

(b) stimulation of sweat glands

(c) acceleration of heart beat

Answer and Explanation:

4. (d): The action of the parasympathetic nervous system is opposite to that of the sympathetic nervous system. If the sympathetic nervous system accelerates an action, the parasympathetic nervous system slows it. However, neither system is exclusively exitatory or inhibitory. The parasympathetic fibres constrict the pupil, decrease the rate and force of heart beat, dilate many blood vessels, lower the arterial blood pressure, quicken the peristaltic movements, and contract the urinary bladder.

5. Third ventricle of brain is also known as

Answer and Explanation:

5. (d): The ventricles consist of four hollow, fluid filled spaces inside the brain. The third ventricle is also known as diacoel. The third ventricle consists of a narrow channel between the hemispheres through the area of the thalamus. It is connected by the cerebral aqueduct or aqueduct of Sylvius or iter in the midbrain portion of the brainstem to the fourth ventricle in the pons and medulla. Metacoel is the IV ventricle, rhinocoel is the I ventricle and paracoel are the II ventricles.

6. Vagus nerve is

Answer and Explanation:

6. (a): Vagus nerve is X nerve. It arises from the side of medulla oblongata. It innervates the larynx, trachea, oesophagus, stomach, lungs, heart and intestines. It is a mixed nerve. It controls the visceral sensations and visceral movements, i.e., heartbeat, respiratory movements, peristalsis, sound production, etc. Glossopharyngeal is the IX nerve. Facial is the VII nerve and trigeminal is the V nerve.

7. Afferent nerve fibres carry impulses from

Answer and Explanation:

7. (b): Afferent nerve fibres carry impulses from the receptors to the central nervous system. Efferent nerve fibres conduct nerve impulses from the central nervous system to the effector organs such as muscles and glands.

Answer and Explanation:

8. (d): At the junction of the sclera and the cornea, the vascular coat sharply bends into the cavity of the eyeball to form a thin, coloured partition. This partition is called iris.

9. Function of iris is to

(a) move lens forward and backward

(c) bring about movements of eye lids

(d) alter the size of pupil.

Answer and Explanation:

9. (d): At the junction of the sclera and the cornea, the vascular coat sharply bends into the cavity of the eyeball to form a thin, coloured partition.

This partition is called iris. It is perforated at the middle by an aperture called pupil. The iris contains two sets of smooth muscles: sphincters and dilators. These muscles regulate the amount of light entering the eyeball by varying the size of the pupil.

The sphincter muscles are arranged in rings. Their contraction makes the pupil smaller in bright light so that less light enters the eye. The dilator muscles are arranged in a radial manner. Their contraction widens the pupil in dim light to let in more light. Iris, by regulating the size of the pupil, allows light to pass only through the centre of the lens, which is optically the most effective part.

10. Retina is most sensitive at

Answer and Explanation:

10. (d): A small area of the optical part of the retina lying exactly opposite to the centre of the cornea is called the macula lutea, or yellow spot which has a yellow pigment (xanthophyll). The macula lutea has at its middle a shallow depression, the fovea centralis. The fovea has cone cells only, and is the place of most distinct vision. Away from the fovea, the rod and cone cells occur in equal numbers, and at the periphery of the retina, the rods are more numerous than the cones.

This is why we see better in dim light by looking out of the corner of the eye. The point on the retina from where the optic nerve starts is called the blind spot, or optic disc, as it lacks the receptor cells and is insensitive to light.

11. Light rays entering the eye is controlled by

Answer and Explanation:

11. (a): Pupil is the opening which controls the amount of light entering in eye. When light intensity is high, it decrease in size and when light intensity is low it dilates to allow more light in the eye to make eye enable to see the object.

12. Ivan Pavlov performed experiments on

Answer and Explanation:

12. (b): By training, a particular response can be obtained to a stimulus other than the one which normally evokes that response. Such a reflex is known as the conditioned reflex.

The conditioned reflexes were first demonstrated in 1920’s by the Russian physiologist I.P. Pavlov. He found that the “sight and smell of food reflexly cause flow of saliva in hungry animals. He rang a bell every time he offered food to a dog. The bell did not induce salivation by itself in the beginning of the experiment. Gradually, the dog learnt to associate the bell with food. Eventually, mere ringing of bell, without presenting food, induced salivation in the dog.

Thus, ringing of bell can substitute sight of food to cause salivation. Pavlov called sound of the bell as ‘ conditioned stimulus, salivation in response to bell a conditioned response, food itself as unconditioned stimulus, and salivation in response to food an unconditioned response. A conditioned reflex is established when a new sensory clue (the bell) becomes associated with an inborn reflex (salivation).

13. By which nervous system and of what type, the blood is supplied into visceral organs?

(a) both SNS and PNS, involuntary

(b) para-sympathetic nervous system, involuntary

(c) sympathetic nervous system, involuntary

(d) sympathetic nervous system, voluntary.

Answer and Explanation:

13. (a): The blood is supplied into visceral organs by both SNS (sympathetic nervous system) and PNS (parasympathetic nervous system) involuntarily. The sympathetic fibres increase the rate and force of heart beat, constrict most blood vessels and raise the arterial blood pressure. The parasympathetic fibres decrease the rate and force of heart beat, dilate many blood vessels and lower the arterial blood pressure.

14. The vagus nerve is the cranial nerve numbering

Answer and Explanation:

14. (c): Vagus nerve is the tenth cranial nerve. It arises from the side of medulla oblongata. It innervates the larynx, trachea, oesophagus, stomach, lungs, heart and intestines. It is a mixed nerve. It controls the visceral sensations and visceral movements, i.e., heartbeat, respiratory movements, peristalsis, sound production, etc.

15. In the chemistry of vision in mammals, the photosensitive substance is called

Answer and Explanation:

15. (a): Photosensitive means sensitive to light. The rod cells of retina contain a purplish pigment called rhodopsin. They function in dim light and at night. Rhodopsin consists of a protein component, opsin, linked to a nonprotein chromophore, retinal (or retinene), a derivative of vitamin A, Light falling on the rod is absorbed by the retinal, which changes its form and separates from the opsin component.

This initiates the transmission of a nerve impulse to the brain. Retinol is the another name of vitamin A. Deficiency of this affects the eyes, causing night blindness and xerophthalmia. Melanin is a pigment that gives colour to the eyes, skin and hair in vertebrates.

16. The Nissl’s granules of nerves cell are made up of

Answer and Explanation:

16. (c): Cell body of a nerve cell contains basophilic granules called Nissl’s granules. These granules appear to be cisternae of rough endoplasmic reticulum with numerous attached and free ribosomes. They probably synthesize proteins for the cell.

17. Which of the following is regarded as a unit of nervous tissue?

Answer and Explanation:

17. (a): Neurons or nerve cells are the structural and functional unit of nervous system. These have a special structure but vary greatly in size and shape. Each neuron has a cell body which encloses cytoplasm and has a nucleus. A number of processes arise from the cell body. There is usually a single axon and a variable number of dendrites.

The medullated nerve a fibre is composed of a shining, white, fatty substance called myelin.

18. The junction between the axon of one neuron and the dendrite of the next is called

Answer and Explanation:

18. (d): Synapse is the close proximity of the axon of one neuron and the dendrite or cyton of another neuron with a gap of just about 200 A in between. A nerve impulse is transmitted across the synapse by the release from the presynaptic membrane of neurotransmitter, which diffuses across the synaptic cleft to the post synaptic membrane. This triggers the propagation of the impulse from the dendrite along the length of the post synaptic neuron.

19. Sympathetic nervous system induces

(a) secretion of digestive juices

Answer and Explanation:

19. (b): Sympathetic nervous system is a component of autonomic nervous system consisting of a pair of sympathetic trunks, preganglionic sympathetic fibres, postganglionic sympathetic fibres and collateral ganglia. It quickens rate and force of heart beat while it inhibits secretion of saliva and gastric juice.

20. Which cranial nerve has the highest number of branches?

Answer and Explanation:

20. (b): Trigeminal nerve is the largest 5th cranial nerve. It has 3 branches –

(i) Ophthalamic, a sensory branch from skin of the nose, eyelids, forehead and scalp, and from the conjunctiva and lacrimal glands.

(ii) Maxillary, also sensory branch from skin and mucous membrane of cheeks and upper lip, and from lower eyelids.

(iii) Mandibular, a mixed branch innervating the lower jaw, lower lip, pinna and tongue.

Vagus nerve is the 10th cranial nerve and innervates larynx, trachea, oesophagus, stomach, lungs, heart and intestines. Facial nerve is the 7th cranial nerve and innervates muscles of face and back, taste buds and salivary glands.

21. Depolarization of axolema during nerve conduction takes place because of

(a) equal amount of Na + and K + move out across axolema

(b) Na + move inside and K + move more outside.

Answer and Explanation:

21. (b): Depolarization of a nerve cell membrane occurs during the passage of an action potential along the axon where the nerve is transmitting an impulse. During depolarization, the activation gates of Na channels open, and the K channels remain closed. Na + rush into the axon. Entry of sodium ions leads to depolarization (reversal of polarity) of the nerve membrane, so that the nerve fibre contents become electropositive with respect to the extracellular fluid.

22. Which of the following statements is the characteristic of human cornea?

(a) secreted by conjuctiva and glandular layer

(b) it is a lacrimal gland which secrete tears

(c) blood circulation is absent in cornea

(d) in old age it becomes the cause of cataract.

Answer and Explanation:

22. (c): Cornea forms the anterior one-sixth of the fibrous coat. It is transparent, circular and fully visible from in front. It is composed of a peculiar variety of connective tissue covered externally by stratified non- keratinized squamous epithelium and internally by simple squamous epithelium. It lacks blood vessels. It is nourished by lymph from adjacent area.

23. When we migrate from dark to light, we fail to see for sometime but after a time visibility becomes normal. It is example of

Answer and Explanation:

23. (b): The rod cells of eye contain a purplish pigment called visual purple, or rhodopsin. They function in dim light and at night. Bright light splits rhodopsin into a lipoprotein scotopsin and a carotenoid pigment retinene. The splitting of rhodopsin depolarizes the rod cell. In the dark, rhodopsin is resynthesized from scotopsin and retinene.

This process is called “dark adaptation.” It makes the rods functional. It takes some time for rhodopsin to be reformed. This is why on entering a dark room at daytime or on coming out of a well lighted room at night, we feel blind for a while. When we go from darkness into bright light, we feel difficulty in seeing properly for a moment till rhodopsin is bleached and cones become functional.

Accommodation is the reflex mechanism by which the focus of the eye changes to make the images of distant and near objects sharp on the retina. Mutation is a change in the genetic material (DNA) of a cell, or the change in a characteristic of an individual, which is not caused by normal genetic processes. Photoperiodism is the response of an organism to the day length.

24. Which of the following statement is correct for node of Ranvier of nerve?

(a) neurilemma is discontinuous

(b) myelin sheath is discontinuous

(c) both neurilemma and myelin sheath are discontinuous

(d) covered by myelin sheath.

Answer and Explanation:

24. (b): At the level of node of Ranvier the myelin sheath is discontinuous but not the neurilemma lining. Actually myelin sheath is an integral part of Schwann’s cell – which forms a continuous neurilemmal covering. Each Schwann’s cell wrap-around the neurite to form concentric layers of plasma membrane. But at the level of junction between two Schwann’s cells myelin cannot be formed and thus a gap appears.

25. What used to be described as Nissl’s granules in a nerve cell are now identified as?

Answer and Explanation:

25. (c): Refer answer 16.

26. Injury to vagus nerve in humans is not likely to affect

(b) gastrointestinal movements

Answer and Explanation:

26. (a): Vagus nerve arises from the side of medulla oblongata. It innervates the larynx, trachea, oesophagus, stomach, lungs, heart and intestines. It is a mixed nerve. It controls the visceral sensations and visceral movements, i.e., heartbeat, respiratory movements, peristalsis, sound production, etc. Movement of the tongue is controlled by hypoglossal nerve as it innervates the muscles of the tongue.

27. In the resting state of the neural membrane, diffusion due to concentration gradients, if allowed, would drive

(b) K + and Na + out of the cell

Answer and Explanation:

27. (c): In the resting nerve fibre, in the external medium (tissue fluid), sodium ions (Na 4 ) predominate, whereas within the fibre (intracellular fluid) potassium ions (K) predominate. Due to different concentrations of ions on the two sides of the membrane, sodium ions tend to passively diffuse into the nerve fibre and potassium ions tend to diffuse out of the nerve fibre down their electrochemical gradients.

The membrane of a resting nerve fibre is, however, more permeable to potassium than to sodium. Because of this selective permeability of the membrane, potassium leaves the nerve fibre faster than sodium enters it. This makes the membrane of the resting nerve fibre polarized, extracellular fluid outside it being electropositive (positively charged) with respect to the cell contents inside it.

28. Parkinson’s disease (characterized by tremors and progressive rigidity of limbs) is caused by degeneration of brain neurons that are involved in movement control and make use of neurotransmitter

Answer and Explanation:

28. (c): Parkinsonism is caused by degenerations of neurons in Substantia Nigra tract which are essentially dopaminergic. This striatum controls muscle tones and coordinates movements. An imbalance is caused by deficiency of dopamine (an inhibitory neurotransmitter) vis a vis. Epinephrine (cholinergic which is an excitatory neurotransmitter) results in motor deficits.

Hence to restore a balance central anticholinergics are given. Parkinson’s disease is a clinical picture characterized by tremor, rigidity, slowness of movement, and postural instability. The commonest symptom is tremor, which often affects one hand, spreading first to the leg on the same side and then to the other limbs.

The patient has an expressionless face, an unmodulated voice, an increasing tendency to stoop, and a shuffling walk.

29. In a man, abducens nerve is injured. Which one of the following functions will be affected?

(a) movement of the eyeball

Answer and Explanation:

29. (a): Abducens is the sixth cranial nerve which innervates the external rectus muscle of the eye ball. It is responsible for turning the eye outwards. Movement of the tongue is controlled by the hypoglossal nerve. Neck movements are controlled by the facial nerve. Swallowing is by glossopharyngeal.

30. Which one of the following is the example of the action of the autonomous nervous system?

(c) peristalsis of the intestine

Answer and Explanation:

30. (c): Options (a), (b) and (d) are reflex actions. Autonomic nervous system is involved in peristalsis of “tine which is affected through mysentric plexus, pathetic fibres decrease peristaltic movements while sympathetic fibres increase these movements.

31. Which one of the following does not act as a neurotransmitter?

Answer and Explanation:

31. (a): Neurotransmitters are chemicals that are used lorelay, amplify and modulate electrical signals between a neuron and another cell. Substances that act as leurotransmitters can be categorized into three major groups: (1) amino acids (primarily glutamic acid, GABA, aspartic acid & glycine), (2) peptides (vasopressin, somatostatin, neurotensin, etc.), and (3) monoamines (norepinephrine, dopamine & serotonin) plus acetylcholine. Cortisone is a glucocorticoid steroid (hormone, produced by the adrenal glands and has anti-inflammatory and immune-system suppressing Properties.

32. Bowman’s glands are found in

(a) juxtamedullary nephrons

(c) external auditory canal

Answer and Explanation:

32. (b): Bowman’s gland, also called olfactory gland is any of the branched tubuloalveolar glands situated in the mucous membrane of the olfactory region of the nasal cavity that produce mucus to moisten the olfactory epithelium and dissolve odour-containing gases.

33. Bowman’s glands are located in the

(b) female reproductive system of cockroach

(c) olfactory epithelium of our nose

(d) proximal end of uriniferous tubules.

Answer and Explanation:

33. (c): Refer answer 32.

34. During the transmission of nerve impulse through a nerve fibre, the potential on the inner side of the plasma membrane has which type of electric change?

(a) first positive, then negative and continue to be negative

(b) first negative, then positive and continue to be positive

(c) first positive, then negative and again back to positive

(d) first negative, then positive and again back to negative.

Answer and Explanation:

34. (d): Nerve is a strand of tissue comprising many nerve fibres plus supporting tissue enclosed in a connective tissue sheath. The signal that travels along the length of a nerve fibre and is the means by which information is transmitted through the nervous system is called nerve impulse.

It is marked by the flow of ions across the membrane of the axon caused by changes in the permeability of the membrane, producing a reduction in potential difference that can be detected as the action potential. The strength of the impulse produced in any nerve fibre is constant.


Neuroanatomy of the Autonomic Nervous System

The ANS can be described best after certain characteristics common to both the sympathetic and parasympathetic divisions have been reviewed. The parasympathetic and sympathetic innervation of autonomic effectors (i.e., organs, vessels, glands) is organized differently than the innervation of skeletal muscle (Fig. 10-1). Although the axons of alpha and gamma motor neurons course directly to skeletal muscles, the innervation of ANS effectors requires a chain of two neurons (see Fig. 10-1), called the preganglionic and postganglionic neurons. The cell body of the preganglionic neuron always is located in the CNS either in the spinal cord or brain stem (Fig. 10-2). The axon is thinly myelinated and immediately leaves the CNS within a specific ventral root of the spinal cord or within certain cranial nerves exiting the brain stem. The cell body of the postganglionic neuron is located in an autonomic ganglion that may be found in numerous places outside the CNS (see Fig. 10-2). The preganglionic neuron synapses with the postganglionic neuron within this ganglion. The axon of the postganglionic neuron is unmyelinated and innervates the effector. Both the preganglionic and postganglionic neurons frequently travel in components of the peripheral nervous system (PNS) (i.e., spinal nerves, cranial nerves) and are intermingled with afferents and somatic motor neurons of peripheral nerves. As stated, most effectors are innervated by both sympathetic and parasympathetic fibers (Table 10-1 see also Fig. 10-2). These fibers produce antagonistic but coordinated responses in the effectors. Descending input from higher integrative centers such as the hypothalamus and areas of the brain stem reaches the cell bodies of preganglionic fibers to regulate and adjust their activity. This descending input is a part of several specific visceral reflex pathways and also is used by higher centers to institute widespread bodily changes.















































































































































Structure Sympathetic Function (Adrenergic Receptors) Parasympathetic Function
Eye
Sphincter muscle Contraction → Constricts
Dilator muscle Contraction → Dilates (α 1 )
Ciliary muscle Relaxes (slightlyfar vision) (β 2 ) Contracts (near vision)
Heart
Rate and force of atrial and ventricular contractions Increases (β 1 and some β 2 ) Decreases
Lungs
Bronchial muscle Relaxation → Dilates airway (β 2 ) Contraction → Constricts airway
Glands Stimulates secretion
Skin
Sweat glands Increases secretion (cholinergic) 1
Arrector pili muscle Contracts (α 1 )
Glands of Head
Lacrimal Vasoconstriction → Reduces secretion (α) Increases secretion
Salivary Vasoconstriction → Secretion reduced and viscid (α)amylase secretion (β 2 ) Secretion increased and watery
Blood Vessels
Arterioles:coronary, skeletal muscle, pulmonary, abdominal viscera, renal Contraction → Vasoconstriction (α 1 ) Relaxation → Vasodilation (β 2 ) Vasodilation
Skin, cerebral Contraction → Vasoconstriction (α 1 )
Systemic veins Contraction → Vasoconstriction (α 1 )
Relaxation → Vasodilation (β 2 )
Gastrointestinal Tract
Motility/tone Inhibits (α 1 ,β 2 ) Stimulates
Sphincters Constricts (α 1 ) Relaxes
Secretion Vasoconstriction → Inhibits secretion (α 1 ) Stimulates
Liver Breaks down glycogen (glycogenolysis), gluconeogenesis, decreased bile secretion (α, β 2 ) Glycogen synthesisincreases bile secretion
Gallbladder Relaxes (β 2 ) Contracts
Pancreas Inhibits secretion of digestive enzymes, glucagon, and insulin (α 2 )increases secretion of insulin and glucagon (β 2 ) Secretion of digestive enzymes, insulin, and glucagon
Spleen Capsule Contraction (α 1 )
Relaxation (β 2 )
Adipose Lipolysis (β 1 )release of fatty acids into blood (β 1 , β 3 β 3 in brown adipose tissue)
Kidney (juxtaglomerular cells) Secretion of renin (β 1 )
Urinary Bladder
Detrusor muscle Relaxes (minimal role) (β 2 ) Contracts
Sphincter (nonstriated) Contracts (α 1 ) Relaxes
Sex Organs Contracts smooth muscle of vas deferens, seminal vesicle, prostate → Ejaculation (α 1 ) Vasodilation → Erection of clitoris (females) and penis (males)
Uterus Variable (depends on hormonal status, pregnancy, and other factors) (α 1 , β 2 ) Minimal effect
Adrenal Medulla Stimulates secretion of epinephrine and norepinephrine (nicotinic ACh receptors) via preganglionic fibers
Pineal Increases melatonin synthesis and secretion (β)

From Benarroch EE et al. (1999). Medical neurosciences (4th ed.). New York: Lippincott Williams & Wilkins Bray JJ et al. (1994). Lecture notes on human physiology (3rd ed.). Cambridge, UK: Blackwell Science FitzGerald MJT & Folan-Curran J. (2002). Clinical neuroanatomy and related neuroscience (4th ed.). Philadelphia: WB Saunders Tortora GJ & Grabowski SR. (2003). Principles of anatomy and physiology (10th ed.). New York: John Wiley & Sons, Inc. Waxman SG. (2003). Clinical neuroanatomy (25th ed.). Chicago: McGraw-Hill.

The cell bodies of the preganglionic neurons are found in four nuclei within the intermediate gray matter of the spinal cord (Fig. 10-3) (Cabot, 1990). The largest group of these cell bodies is the intermediolateral (IML) cell column that forms the lateral horn. Throughout this column are clusters of 20 to 100 neurons that are separated by distances ranging from 200 to 500 μm in the thoracic region and 100 to 300 μm in the lumbar region. The cell bodies are approximately 12 to 13 μm in diameter and histologically are similar to motor neurons (Harati, 1993). The diameters of the axons range from 2 to 5 μm, and their speed of conduction is approximately 3 to 15 m/sec. These fibers often are classified in the B group (see Chapter 9). At the T6 and T7 levels, the mean number of these cells is approximately 5000, but it has been shown that the number decreases with age at the rate of approximately 8% per decade (Harati, 1993).


(From Cabot JB. [1990]. Sympathetic preganglionic neurons: Cytoarchitecture, ultrastructure, and biophysical properties. In AD Loewy & KM Spyer [Eds.]. Central regulation of autonomic functions . New York: Oxford University Press.)

The other three nuclear groups of preganglionic neurons have been described by Cabot (1990) and are the lateral funicular area (located lateral and dorsal to the intermediolateral group), the intercalated cell group (located medial to the IML column and possibly the same cluster of neurons typically called the intermediomedial group), and the central autonomic nucleus (located lateral and dorsal to the central canal). The combination of these groups forms a ladderlike structure in longitudinal sections in which the paired IML cell columns form the sides of the ladder and the interconnected central autonomic nucleus and intercalated cell group form the rungs (Fig. 10-3). The IML cell column is the origin of the vast majority of preganglionic fibers, but the other three nuclei also give rise to some preganglionic fibers. The four nuclei are the recipients of extensive input from higher centers such as the hypothalamus and brain stem nuclei. These sources release various neurotransmitters that have been identified as monoamines, neuropeptides, and amino acids. Although the anatomic characteristics of these four nuclei have been described, the exact functions of each specific nucleus still remain unclear.

According to the general rule of organization of the ANS, two neurons are necessary for the impulse to reach the effector. One is the preganglionic neuron, just discussed. The second neuron in the pathway to an autonomic effector is the postganglionic neuron. This neuron’s axon is classified as a group C fiber (see Chapter 9). Generally it is described as unmyelinated, with a diameter ranging from 0.3 to 1.3 μm and a slow conduction speed ranging from 0.7 to 2.3 m/sec (Carpenter and Sutin, 1983). The cell body is located outside the CNS in an autonomic ganglion. Unlike a sensory ganglion of cranial nerves and a dorsal root ganglion of spinal nerves, in which no synapses occur, an autonomic ganglion is the location of the synapse between the preganglionic and postganglionic neurons. Preganglionic fibers disseminate their information by diverging and synapsing on numerous postganglionic fibers. This principle of divergence is based on studies of the superior cervical ganglion of mammals. Results of different studies show preganglionic to postganglionic ratios of 1:4 (Loewy, 1990a), 1:15 to 1:20, and 1:196 in a human superior cervical ganglion (Williams et al., 1995). (The parasympathetic division also has been found to exhibit divergence, but to a lesser degree.) This divergence may allow the effects of sympathetic stimulation to be more widespread throughout the body and to be of greater magnitude.




(From Tortora GJ & Grabowski SR. [2003]. Principles of anatomy and physiology [10th ed.]. New York: John Wiley & Sons, Inc.]

Sympathetic preganglionic fibers sending nerve impulses to effectors in the head enter the sympathetic trunk, ascend to the superior cervical ganglion, and synapse with postganglionic neurons. The postganglionic fibers course with large blood vessels to reach effectors located in the head region (Fig. 10-7, A ). Such effectors include glands, the smooth muscle of blood vessels, and the smooth muscle of the eye. Some preganglionic fibers sending impulses to smooth muscle, cardiac muscle, and glands of the thorax also ascend on entering the trunk and synapse at rostral levels, whereas others synapse with postganglionic fibers at the level of entry. These postganglionic fibers leave the chain as branches that merge with other nerve fibers, including parasympathetic vagal fibers, to form plexuses innervating the heart and lungs. Abdominal and pelvic effectors are innervated in a different manner than the effectors of the head, thorax, and cutaneous regions. Preganglionic fibers enter the sympathetic trunk via white rami communicantes but do not synapse in the chain ganglia. Instead they pass through the chain ganglia and emerge as a collection of fibers called sympathetic splanchnic (referring to the viscera) nerves. These nerves course inferiorly in an anteromedial direction, pass through the diaphragm, and end in various prevertebral ganglia. Here they synapse on postganglionic neurons that then continue to the effectors of the abdominal and pelvic cavities (Fig. 10-7, B ). The sympathetic prevertebral ganglia are enmeshed in plexuses of sympathetic and parasympathetic fibers and are located near large arteries found in the abdominal cavity. Examples are the celiac, superior mesenteric, aorticorenal, and inferior mesenteric ganglia.



The fusion of the eight cervical ganglia results in three distinct ganglia in the region of the neck (Figs. 10-9, A and 10-10 see also Fig. 10-5). These are known as the superior, middle, and cervicothoracic (stellate) ganglia. (Twenty percent of the time the T1 ganglion is separate, in which case the cervicothoracic ganglion is called the inferior cervical ganglion [Harati, 1993].) The superior ganglion (Fig. 10-11, A see also Figs. 10-9, A and 10-10) is the largest of the three and lies high in the neck adjacent to vertebrae C2 and C3, anterior to the longus capitis muscle and posterior to the cervical part of the internal carotid artery. It is also in the vicinity of the internal jugular vein and the glossopharyngeal, vagus, spinal accessory, and hypoglossal cranial nerves (Williams et al., 1995). The proximity of the ganglion to these nerves may account for the autonomic effects seen when these nerves are lesioned in this location (Cross, 1993b). The ganglion is formed by the fusion of the first four cervical ganglia, is 2.5 to 3.0 cm long, and includes more than 1 million neurons (Carpenter and Sutin, 1983 Harati, 1993 Williams et al., 1995). Postganglionic fibers leaving this ganglion course to various regions. Some ascend as perivascular plexuses on the internal and external carotid arteries. A large branch (internal carotid nerve) from the superior cervical ganglion ascends with the internal carotid artery and divides into branches that form the internal carotid plexus (see Fig. 10-9, A ) (Williams et al., 1995). This plexus, which surrounds the artery and innervates its wall, continues to travel with that artery, and within the cranial cavity the fibers innervate the autonomic effectors. Examples of these effectors are the dilator pupillae muscle of the eye, the superior tarsal muscle (Müller’s muscle) of the eyelid, and sweat glands in the lateral part of the forehead (Watson and Vijayan, 1995 Salvesen, 2001). In addition, some are sympathetic vasoconstrictor fibers and innervate cerebral branches of the internal carotid artery. Other postganglionic fibers leave the ganglion as medial, lateral, and anterior branches and course directly to effectors. The lateral branches include slender filaments that communicate with the glossopharyngeal, vagus, and hypoglossal nerves and gray rami that join the first four cervical spinal nerves. The latter travel with those spinal nerves to effectors in the areas of distribution of the nerves. The medial branches include laryngopharyngeal and cardiac (efferent) branches. The anterior branches travel with the common and external carotid arteries. Fibers continue with branches of the external carotid artery to innervate such structures as the facial sweat glands by traveling with terminal branches of the trigeminal nerve (cranial nerve [CN] V) (Williams et al., 1995).




The cervicothoracic (stellate) ganglion (see Figs. 10-9, A and 10-11, B ) is formed by the fusion of the seventh, eighth, and first thoracic ganglia (and sometimes even the second, third, and fourth thoracic ganglia). It is approximately 2.8 cm long and is located between the base of the TP of C7 and the neck of the first rib lying on or just lateral to the longus colli muscle. A small detached portion of the stellate ganglion (or middle cervical ganglion), called the vertebral ganglion, may be present on the sympathetic trunk near the origin of the vertebral artery.

Some postganglionic fibers of the stellate (cervicothoracic) ganglion travel in gray rami communicantes to enter the C7, C8, and T1 spinal nerves, whereas others form a cardiac branch. Some other fibers form branches that course on the subclavian artery and its branches. One of these is large, and because it ascends with the vertebral artery (see Figs. 10-9, A and 10-11, B ), frequently it is called the vertebral nerve (see Chapter 5 and Fig. 5-22). This nerve is joined by other branches and forms the vertebral plexus. Deep rami communicantes branch from the vertebral plexus and travel with ventral rami of the first five or six cervical spinal nerves. This plexus travels into the cranial cavity on the vertebral artery and continues on the basilar artery (and its branches) as far as the posterior cerebral artery, where it continues anteriorly to join the internal carotid artery plexus. Some consider the vertebral plexus to be the major continuation of the sympathetic system into the cranium (Williams et al., 1995).

Eleven small ganglia usually (70% of the time) are found in the thoracic sympathetic chain (Fig. 10-12 see also Fig. 10-9, B ). (Note that 80% of the time the T1 ganglion is fused with the inferior cervical ganglion, in which case the succeeding ganglion is still named the second.) Each ganglion includes 90, 000 to 100, 000 neurons (Harati, 1993). The thoracic chain lies adjacent to the heads of the ribs and posterior to the costal pleura. In this region of the chain, white rami communicantes, as well as the gray rami communicantes, are clearly evident (see Fig. 10-12, B ). The white rami lay more distal (lateral) than the gray rami, and two or more rami may be connected to one spinal nerve. A mixed ramus formed by the fusion of the gray and white rami sometimes may be present. Postganglionic fibers originating from all thoracic ganglia enter the thoracic spinal nerves and travel with them to effectors. Some postganglionic fibers from the T1 to T5 ganglia form direct branches to the thoracic aortic, cardiac, and pulmonary plexuses of the thorax. Other large branches of the T5 to T12 ganglia supply the aorta and are associated with the three splanchnic nerves involved with the sympathetic innervation of the abdominal and pelvic viscera. These splanchnic nerves consist of preganglionic fibers that synapse in prevertebral ganglia located in the abdominal cavity.


The greater splanchnic nerve (see Figs. 10-9, B and 10-12, A ) contains preganglionic fibers exiting from the T5 to T9 or T10 ganglia. As it descends obliquely on the vertebral bodies, it sends branches to the descending thoracic aorta and then pierces the diaphragm. It courses to the medulla of the adrenal gland, the celiac ganglion, and sometimes the aorticorenal ganglion. In the ganglia, the preganglionic fibers of the greater splanchnic nerve synapse on postganglionic neurons. The lesser splanchnic nerve consists of preganglionic fibers from the T9 and T10 or T10 and T11 ganglia and is present 94% of the time. It traverses the diaphragm with the greater splanchnic nerve and enters the abdominal cavity to synapse in the aorticorenal ganglion (the detached lower part of the celiac ganglion). The third splanchnic nerve is the lowest or least splanchnic nerve and is present 56% of the time. Sometimes this nerve is called the renal nerve and emerges from the T12 (or lowest) ganglion to terminate in many small ganglia located in the renal plexus (Harati, 1993 Williams et al., 1995). From these prevertebral ganglia, postganglionic fibers participate in the formation of the various perivascular plexuses as they travel to abdominal effectors.

The thoracic sympathetic trunk passes posterior to the medial arcuate ligament (or sometimes through the crura of the diaphragm) to become continuous with the lumbar sympathetic trunk found within the abdominal cavity. The trunk has been described as consisting of four interconnected lumbar ganglia (each of which contains 60, 000 to 85, 000 neurons) (Harati, 1993 Williams et al., 1995). However, other data indicate that the number of ganglia varies (Mitchell, 1953 Rocco, Palombi, and Raeke, 1995). Murata et al. (2003) studied cadaveric specimens and looked at the anatomic variations of the ganglia and associated rami. They found that the number of ganglia on one side ranged from 2 to 6 (mean, 3.9) and that the majority of ganglia were located on the L2 and L3 vertebrae. Typically no ganglia were found on the L1 and L4 vertebrae. Approximately 40% of the cadavers showed the same number of ganglia on both sides, and those were asymmetrically located. In addition, Murata et al. (2003) found 5 to 12 rami communicantes per side (mean of 7.2) and noted that more than one rami communicantes could connect to a lumbar ventral ramus and that a lumbar ventral ramus often received rami communicantes from more than one ganglion. In fact, one third of the ganglia associated with the L2 and L3 vertebrae included rami that traveled to three spinal nerves. Rami associated with the L1-5 spinal nerves were measured, and it was found that the ramus of the L4 spinal nerve was significantly similar in length to the L2 and L3 rami. Although this is contrary to what would be expected because the sympathetic chain lies closer to the intervertebral foramina (IVFs) at the L4-5 vertebral levels, it may be due to the fact that there are fewer ganglia at the L4 level and the rami have farther to travel to reach a more superiorly located ganglion. The L1 rami also were significantly longer than the other lumbar rami, most likely for the same reason. The rami of L5 were significantly shorter than those of L1-4. The presence of anatomic variations in the rami and ganglia in this region may be of importance relative to the nociceptive pathway for lower lumbar structures. Based on studies in rats that may apply to humans as well, it has been suggested that nociceptive input from low back structures is carried in two different pathways. Some nociceptive fibers from low back pain generators travel in a segmental fashion, directly within spinal nerves, and terminate in local cord segments. Other nociceptive fibers terminate centrally in a nonsegmental fashion. These course in lower lumbar spinal nerves, enter the sympathetic chain and ascend, and then exit the chain through more superiorly located rami connected to the L2 spinal nerve. These fibers terminate in the lower region of the thoracolumbar cord segments associated with the sympathetic division (Murata et al., 2003). If this pathway is present in humans it may contribute to the wide referral pattern seen in lower lumbar intervertebral discs, lower lumbar zygopophysial (Z) joints, and sacroiliac joint pain conditions (Murata et al., 2000) (see Chapters 7 and 11 for further information).

The lumbar trunk lies adjacent to the anterolateral aspect of the upper lumbar vertebrae and becomes more posterior relative to lower lumbar vertebrae (Murata et al., 2003). It also lies adjacent to the medial margin of the psoas major muscle (Fig. 10-13 see also Fig. 10-9, C ). The inferior vena cava, right ureter, and lumbar lymph nodes lie anterior to the right sympathetic trunk. The left sympathetic trunk lies posterior to the aortic lymph nodes and lateral to the aorta. These relationships are important surgically because lumbar ganglia may have to be removed (lumbar sympathectomy) to treat certain arterial diseases of the lower extremities (Moore, 1980).


The pelvic chain consists of four or five ganglia that lie on the anterior aspect of the sacrum. Each side unites to form the ganglion impar on the anterior aspect of the coccyx (Fig. 10-14 see also Fig. 10-9, C ). Postganglionic fibers leave the chain in gray rami to enter the sacral spinal nerves and coccygeal nerve. Fibers destined for blood vessels in the leg and foot course primarily with the tibial nerve to connect subsequently with (and supply) the popliteal artery and its branches in the leg and foot. Other fibers travel with the pudendal and gluteal nerves to the internal pudendal artery and gluteal arteries. In addition, some fibers from the first two sacral ganglia send postganglionic branches into the inferior hypogastric plexus (or hypogastric nerve).


The cardiac, pulmonary, celiac, and hypogastric plexuses are the major plexuses (Williams et al., 1995), although secondary plexuses may emanate from each one. The cardiac plexus, which is divided into deep and superficial parts, consists of cardiac branches from cervical and upper thoracic ganglia mixed with cardiac branches of the vagus nerve (see Fig. 10-9, B ). A continuation of the cardiac plexus forms secondary coronary and atrial plexuses. The pulmonary plexus is an extension of fibers of the cardiac plexus that course with the pulmonary arteries to the lungs. Therefore the cardiac and pulmonary plexuses consist of the same sympathetic and vagal branches.

The celiac plexus is the largest autonomic plexus (see Fig. 10-9, C ). It is located at the level of the T12 and L1 vertebrae and surrounds the celiac artery and the base of the superior mesenteric artery. This plexus is a dense fibrous network that interconnects the paired celiac ganglia. Mingling with the celiac plexus and ganglia are the greater and lesser splanchnic nerves and also branches of the vagus and phrenic nerves (Williams et al., 1995). Numerous subsidiary ganglia and fibers extend from the celiac plexus and course along abdominal blood vessels to autonomic effectors. These fibers and ganglia form (in some cases with the help of the lesser and least splanchnic nerves) plexuses that include the phrenic, hepatic, gastric, splenic, testicular, ovarian, superior mesenteric (to small and large intestines), renal, inferior mesenteric (to lower GI tract), and abdominal aortic (intermesenteric). The latter three plexuses also include lumbar splanchnic nerves. The hepatic plexus is the largest of these smaller plexuses and supplies innervation to the liver, gallbladder, bile ducts, stomach, duodenum, and pancreas. It contains afferent and efferent sympathetic branches and parasympathetic fibers. Consequently the celiac plexus and its secondary plexuses are responsible for the innervation of the abdominal viscera. Anterior to the bifurcation of the aorta at the level of the L4 to L5 vertebral bodies, the superior hypogastric plexus (Fig. 10-13, A, and see Fig. 10-9, C ) is formed by the third and fourth lumbar splanchnic nerves and fibers of the aortic plexus. This plexus sends branches to the testicular, ureteric, ovarian, and common iliac plexuses. As the superior hypogastric plexus descends into the pelvic cavity, it divides into left and right hypogastric nerves that continue caudally to form the inferior hypogastric (pelvic) plexuses (see Fig. 10-9, C ). Within the pelvis, pelvic splanchnic parasympathetic fibers (originating from the S2-4 cord segments) join each inferior hypogastric plexus. Preganglionic sympathetic fibers in the plexus originate in the T10-L2 cord segments and travel as lumbar splanchnics or postganglionic fibers of the lumbar and sacral chain ganglia. Extensions of the inferior hypogastric plexus, which include the middle rectal, prostatic, uterovaginal, and vesical plexuses, continue along the branches of the internal iliac artery to innervate autonomic effectors of the pelvis. The ANS innervation of the most clinically important effectors of the pelvis is discussed later in this chapter.

The oculomotor nerve (CN III) emerges from the ventral surface of the midbrain of the brain stem (see Chapter 9, Fig. 9-20). The origin of the autonomic efferents is in the Edinger-Westphal nucleus, which is located in the midbrain ventral to the cerebral aqueduct of Sylvius. These preganglionic fibers course within the oculomotor nerve to the ciliary ganglion, where they synapse with postganglionic neurons (Fig. 10-15). This ganglion is less than 2 mm long and contains 3000 multipolar neurons (Harati, 1993). It is located in the orbit just anterior to the superior orbital fissure. Postganglionic fibers course in the short ciliary nerves to the eye and travel between the choroid and sclera of the eye wall. Here the fibers innervate the smooth muscle of the iris (sphincter pupillae) and ciliary body (ciliary muscle). The sphincter muscle of the iris functions to constrict the pupil during the pupillary light reflex and during the accommodation-convergence reflex. Contraction of the ciliary muscle occurs during the accommodation-convergence reflex. The result of this contraction is a thickening of the lens, which improves near vision.


The facial nerve (CN VII) also contains preganglionic fibers. The cell bodies of these fibers are located in the superior salivatory nucleus. This nucleus is located in the caudal part of the pons near the facial motor nucleus. The fibers emerge from the pontomedullary junction in the nervus intermedius portion of CN VII (see Chapter 9, Fig. 9-20). Some of the fibers travel in the chorda tympani nerve, which in turn joins the lingual branch of the mandibular division of the trigeminal nerve (CN V). These preganglionic fibers continue to the submandibular (sublingual) ganglion, where they synapse with postganglionic neurons (see Fig. 10-15). The postganglionic fibers are secretomotor and course to minor salivary glands, as well as to the larger submandibular and sublingual salivary glands. (It has been reported also that stimulation of the chorda tympani nerve results in vasodilation in the salivary glands [Williams et al., 1995].) In addition to the preganglionic fibers en route to the submandibular ganglion, other secretomotor preganglionic fibers from the lacrimal portion of the superior salivatory nucleus course in the greater petrosal nerve to the pterygopalatine ganglion (see Fig. 10-15). This ganglion is approximately 3 mm long and contains 56, 500 compactly arranged neurons (Harati, 1993). It is located in the pterygopalatine fossa behind and below the orbit. Postganglionic fibers exit from here and travel in the zygomatic nerve (a branch of the maxillary division of the trigeminal nerve) and terminate in the lacrimal gland. Other secretomotor branches of the pterygopalatine ganglion course to the glands and mucous membranes of the palate and nasal mucosa.

The glossopharyngeal nerve is CN IX. Preganglionic neurons that course in this nerve originate in the inferior salivatory nucleus, which is located caudal to the superior salivatory nucleus. CN IX emerges as three to five rootlets from the dorso-olivary sulcus on the lateral side of the medulla of the brain stem (see Chapter 9, Fig. 9-20). The preganglionic fibers travel in the lesser petrosal nerve to the otic ganglion, where they synapse with postganglionic neurons (Fig. 10-16, A ). The postganglionic fibers are secretomotor, and the axons of these neurons travel in the auriculotemporal nerve (a branch of the mandibular division of the trigeminal nerve) to reach the parotid gland that they innervate. Evidence shows that stimulation of the lesser petrosal nerve results in vasodilation in the parotid gland, as well as serous secretion (Williams et al., 1995).


Types of Nerve Fibres and their Functions – Local and General Anaesthesia

Action and Functions of Nerve Fibres is very important in understanding their action and the use of Local and General Anaesthetics in controlling Pain, Proprioception, Touch and Pressure. Each Nerve Fibre has a specific function and the Anaesthetic agents should be such that they act on those nerve fibres to get the desired effect.

It is important to note that the diameter of the Nerve Fibre plays an important role in transmission speed of nerve impulse. The Larger the Diameter of Nerve Fibre the Higher the Speed of Conduction. A-Fibres are the largest in Diameter and have the fastest speed of conduction at 70-120 m/sec

In Local Anaesthesia the A-Delta fibres are targeted to stop pain transmission.

The action of Various Nerve Fibres:

A Fibres: These have Motor Function in Muscle Spindles

A – Alpha Fibres: Motor Function

A – Beta Fibres: Also called as Type II fibres

A- Delta Fibres: Also called as Type III fibres

B Fibres: These are Preganglionic Autonomic Fibres

C Dorsal Root: Type IV Fibres

C Sympathetic Fibres: These are Post ganglionic sympathetic fibers

Article by Varun Pandula

I am Varun, a Dentist from Hyderabad, India trying my bit to help everyone understand Dental problems and treatments and to make Dental Education simplified for Dental Students and Dental fraternity. If you have any doubts feel free to contact me or comment in the post, thanks for visiting.


INTRODUCTION

This manuscript presents a detailed review of the autonomic nervous system (ANS). A thorough knowledge of this system is quite important as it prepares the pharmacy student for further studies in pathophysiology, pharmacology, and therapeutics. The ANS plays a crucial role in the maintenance of homeostasis. Furthermore, this system may play a role in many systemic diseases (eg, heart failure) and drugs that affect this system may improve (eg, β2-adrenergic agonists and asthma) or exacerbate (eg, α1-adrenergic agonists and hypertension) various disease symptoms and processes. Although this manuscript focuses primarily on the basic anatomy and physiology of the ANS, references to diseases and medications involving the ANS are included to illustrate the application of this system to the practice of pharmacy.

The ANS and the accompanying case studies are discussed over 5 lectures and 2 recitation sections during a 2-semester course in Human Physiology. The lectures typically include 300-325 students, although the recitation sections are much smaller with 20-30 students. The students are in the first professional year of a doctor of pharmacy program.

Also known as the visceral or involuntary nervous system, the ANS functions without conscious, voluntary control. Because it innervates cardiac muscle, smooth muscle, and various endocrine and exocrine glands, this nervous system influences the activity of most tissues and organ systems in the body. Therefore, the ANS makes a significant contribution to homeostasis. The regulation of blood pressure, gastrointestinal responses to food, contraction of the urinary bladder, focusing of the eyes, and thermoregulation are just a few of the many homeostatic functions regulated by the ANS.

At this point in the class discussion, we take a break from our traditional classroom format for a story about my next door neighbor, Joe, and my skeleton, Matilda. Interestingly, the ANS is discussed in this Human Physiology course in mid to late October (ie, around Halloween time). Joe leaves for work at 5:00 am when it is still quite dark outside. On Halloween Eve, we placed Matilda in the driver's seat of Joe's pickup truck. Halloween morning, we arose at 4:45 am , poured coffee, and waited patiently by the window located nearest to Joe's truck. Completely unsuspecting, Joe came walking down the driveway at his usual time. When he opened the truck door, the sound of 𠇊ghhhh. ” shattered the quiet of the morning. Poor Joe stood by his truck wide-eyed and clutching his chest. Upon opening our window, we cheerfully wished our friend a “Happy Halloween!” Although Joe's response to our holiday greeting cannot be published in this article, suffice it to say that the students always enjoy it immensely.

I now ask the class “What happened to Joe?” Several events occurred in his body at once. His heart began racing, his blood pressure increased, his pupils dilated, he began sweating, the hair on his arms and the back of his neck stood on end, and he felt a surge of adrenaline. These are some of the effects of sympathetic nervous activity in Joe's body. Meanwhile, as we waited for Joe's early morning arrival, the events occurring in my body were quite different. My heart rate was comparatively slower and my digestive system was processing the cream and sugar in my coffee. These are some of the effects of parasympathetic nervous activity. I tell my students that during the next several class periods they will learn in great detail about the many functions of the sympathetic and parasympathetic nervous systems, the neurotransmitters released by their neurons, the receptors to which they bind, and how it is all regulated. At this point, the students often look as afraid as Joe did that Halloween morning. I reassure them (and remind them repeatedly) that it is not necessary to memorize very much at all. I encourage them to let it make sense. The sympathetic system controls 𠇏ight-or-flight” responses. In other words, this system prepares the body for strenuous physical activity. The events that we would expect to occur within the body to allow this to happen do, in fact, occur. The parasympathetic system regulates “rest and digest” functions. In other words, this system controls basic bodily functions while one is sitting quietly reading a book.

Specific learning objectives for the discussion of the autonomic nervous system include the following:

Explain how various regions of the central nervous system regulate autonomic nervous system function

Explain how autonomic reflexes contribute to homeostasis

Describe how the neuroeffector junction in the autonomic nervous system differs from that of a neuron-to-neuron synapse

Compare and contrast the anatomical features of the sympathetic and parasympathetic systems

For each neurotransmitter in the autonomic nervous system, list the neurons that release them and the type and location of receptors that bind with them

Describe the mechanism by which neurotransmitters are removed

Distinguish between cholinergic and adrenergic receptors

Describe the overall and specific functions of the sympathetic system

Describe the overall and specific functions of the parasympathetic system and

Explain how the effects of the catecholamines differ from those of direct sympathetic stimulation.


Type B Nerve fibres cause exactly what kind of autonomic sensation to the preganglionic neurons? - Biology

The autonomic nervous system regulates organ systems through circuits that resemble the reflexes described in the somatic nervous system. The main difference between the somatic and autonomic systems is in what target tissues are effectors. Somatic responses are solely based on skeletal muscle contraction. The autonomic system, however, targets cardiac and smooth muscle, as well as glandular tissue. Whereas the basic circuit is a reflex arc , there are differences in the structure of those reflexes for the somatic and autonomic systems.

The Structure of Reflexes

One difference between a somatic reflex, such as the withdrawal reflex, and a visceral reflex, which is an autonomic reflex, is in the efferent branch. The output of a somatic reflex is the lower motor neuron in the ventral horn of the spinal cord that projects directly to a skeletal muscle to cause its contraction. The output of a visceral reflex is a two-step pathway starting with the preganglionic fiber emerging from a lateral horn neuron in the spinal cord, or a cranial nucleus neuron in the brain stem, to a ganglion—followed by the postganglionic fiber projecting to a target effector. The other part of a reflex, the afferent branch, is often the same between the two systems. Sensory neurons receiving input from the periphery—with cell bodies in the sensory ganglia, either of a cranial nerve or a dorsal root ganglion adjacent to the spinal cord—project into the CNS to initiate the reflex (Figure 13.6). The Latin root “effere” means “to carry.” Adding the prefix “ef-” suggests the meaning “to carry away,” whereas adding the prefix “af-” suggests “to carry toward or inward.”

Afferent Branch

The afferent branch of a reflex arc does differ between somatic and visceral reflexes in some instances. Many of the inputs to visceral reflexes are from special or somatic senses, but particular senses are associated with the viscera that are not part of the conscious perception of the environment through the somatic nervous system. For example, there is a specific type of mechanoreceptor, called a baroreceptor , in the walls of the aorta and carotid sinuses that senses the stretch of those organs when blood volume or pressure increases. You do not have a conscious perception of having high blood pressure, but that is an important afferent branch of the cardiovascular and, particularly, vasomotor reflexes. The sensory neuron is essentially the same as any other general sensory neuron. The baroreceptor apparatus is part of the ending of a unipolar neuron that has a cell body in a sensory ganglion. The baroreceptors from the carotid arteries have axons in the glossopharyngeal nerve, and those from the aorta have axons in the vagus nerve.

Though visceral senses are not primarily a part of conscious perception, those sensations sometimes make it to conscious awareness. If a visceral sense is strong enough, it will be perceived. The sensory homunculus—the representation of the body in the primary somatosensory cortex—only has a small region allotted for the perception of internal stimuli. If you swallow a large bolus of food, for instance, you will probably feel the lump of that food as it pushes through your esophagus, or even if your stomach is distended after a large meal. If you inhale especially cold air, you can feel it as it enters your larynx and trachea. These sensations are not the same as feeling high blood pressure or blood sugar levels.

When particularly strong visceral sensations rise to the level of conscious perception, the sensations are often felt in unexpected places. For example, strong visceral sensations of the heart will be felt as pain in the left shoulder and left arm. This irregular pattern of projection of conscious perception of visceral sensations is called referred pain. Depending on the organ system affected, the referred pain will project to different areas of the body (Figure 13.7). The location of referred pain is not random, but a definitive explanation of the mechanism has not been established. The most broadly accepted theory for this phenomenon is that the visceral sensory fibers enter into the same level of the spinal cord as the somatosensory fibers of the referred pain location. By this explanation, the visceral sensory fibers from the mediastinal region, where the heart is located, would enter the spinal cord at the same level as the spinal nerves from the shoulder and arm, so the brain misinterprets the sensations from the mediastinal region as being from the axillary and brachial regions. Projections from the medial and inferior divisions of the cervical ganglia do enter the spinal cord at the middle to lower cervical levels, which is where the somatosensory fibers enter.

Disorders of the Nervous System: Kehr’s Sign

Kehr’s sign is the presentation of pain in the left shoulder, chest, and neck regions following rupture of the spleen. The spleen is in the upper-left abdominopelvic quadrant, but the pain is more in the shoulder and neck. How can this be? The sympathetic fibers connected to the spleen are from the celiac ganglion, which would be from the mid-thoracic to lower thoracic region whereas parasympathetic fibers are found in the vagus nerve, which connects in the medulla of the brain stem. However, the neck and shoulder would connect to the spinal cord at the mid-cervical level of the spinal cord. These connections do not fit with the expected correspondence of visceral and somatosensory fibers entering at the same level of the spinal cord.

The incorrect assumption would be that the visceral sensations are coming from the spleen directly. In fact, the visceral fibers are coming from the diaphragm. The nerve connecting to the diaphragm takes a special route. The phrenic nerve is connected to the spinal cord at cervical levels 3 to 5. The motor fibers that make up this nerve are responsible for the muscle contractions that drive ventilation. These fibers have left the spinal cord to enter the phrenic nerve, meaning that spinal cord damage below the mid-cervical level is not fatal by making ventilation impossible. Therefore, the visceral fibers from the diaphragm enter the spinal cord at the same level as the somatosensory fibers from the neck and shoulder.

The diaphragm plays a role in Kehr’s sign because the spleen is just inferior to the diaphragm in the upper-left quadrant of the abdominopelvic cavity. When the spleen ruptures, blood spills into this region. The accumulating hemorrhage then puts pressure on the diaphragm. The visceral sensation is actually in the diaphragm, so the referred pain is in a region of the body that corresponds to the diaphragm, not the spleen.

Efferent Branch

The efferent branch of the visceral reflex arc begins with the projection from the central neuron along the preganglionic fiber. This fiber then makes a synapse on the ganglionic neuron that projects to the target effector.

The effector organs that are the targets of the autonomic system range from the iris and ciliary body of the eye to the urinary bladder and reproductive organs. The thoracolumbar output, through the various sympathetic ganglia, reaches all of these organs. The cranial component of the parasympathetic system projects from the eye to part of the intestines. The sacral component picks up with the majority of the large intestine and the pelvic organs of the urinary and reproductive systems.

Short and Long Reflexes

Somatic reflexes involve sensory neurons that connect sensory receptors to the CNS and motor neurons that project back out to the skeletal muscles. Visceral reflexes that involve the thoracolumbar or craniosacral systems share similar connections. However, there are reflexes that do not need to involve any CNS components. A long reflex has afferent branches that enter the spinal cord or brain and involve the efferent branches, as previously explained. A short reflex is completely peripheral and only involves the local integration of sensory input with motor output (Figure 13.8).

The difference between short and long reflexes is in the involvement of the CNS. Somatic reflexes always involve the CNS, even in a monosynaptic reflex in which the sensory neuron directly activates the motor neuron. That synapse is in the spinal cord or brain stem, so it has to involve the CNS. However, in the autonomic system there is the possibility that the CNS is not involved. Because the efferent branch of a visceral reflex involves two neurons—the central neuron and the ganglionic neuron—a “short circuit” can be possible. If a sensory neuron projects directly to the ganglionic neuron and causes it to activate the effector target, then the CNS is not involved.

A division of the nervous system that is related to the autonomic nervous system is the enteric nervous system. The word enteric refers to the digestive organs, so this represents the nervous tissue that is part of the digestive system. There are a few myenteric plexuses in which the nervous tissue in the wall of the digestive tract organs can directly influence digestive function. If stretch receptors in the stomach are activated by the filling and distension of the stomach, a short reflex will directly activate the smooth muscle fibers of the stomach wall to increase motility to digest the excessive food in the stomach. No CNS involvement is needed because the stretch receptor is directly activating a neuron in the wall of the stomach that causes the smooth muscle to contract. That neuron, connected to the smooth muscle, is a postganglionic parasympathetic neuron that can be controlled by a fiber found in the vagus nerve.

Read this article to learn about a teenager who experiences a series of spells that suggest a stroke. He undergoes endless tests and seeks input from multiple doctors. In the end, one expert, one question, and a simple blood pressure cuff answers the question. Why would the heart have to beat faster when the teenager changes his body position from lying down to sitting, and then to standing?

Balance in Competing Autonomic Reflex Arcs

The autonomic nervous system is important for homeostasis because its two divisions compete at the target effector. The balance of homeostasis is attributable to the competing inputs from the sympathetic and parasympathetic divisions (dual innervation). At the level of the target effector, the signal of which system is sending the message is strictly chemical. A signaling molecule binds to a receptor that causes changes in the target cell, which in turn causes the tissue or organ to respond to the changing conditions of the body.

Competing Neurotransmitters

The postganglionic fibers of the sympathetic and parasympathetic divisions both release neurotransmitters that bind to receptors on their targets. Postganglionic sympathetic fibers release norepinephrine, with a minor exception, whereas postganglionic parasympathetic fibers release ACh. For any given target, the difference in which division of the autonomic nervous system is exerting control is just in what chemical binds to its receptors. The target cells will have adrenergic and muscarinic receptors. If norepinephrine is released, it will bind to the adrenergic receptors present on the target cell, and if ACh is released, it will bind to the muscarinic receptors on the target cell.

In the sympathetic system, there are exceptions to this pattern of dual innervation. The postganglionic sympathetic fibers that contact the blood vessels within skeletal muscle and that contact sweat glands do not release norepinephrine, they release ACh. This does not create any problem because there is no parasympathetic input to the sweat glands. Sweat glands have muscarinic receptors and produce and secrete sweat in response to the presence of ACh.

At most of the other targets of the autonomic system, the effector response is based on which neurotransmitter is released and what receptor is present. For example, regions of the heart that establish heart rate are contacted by postganglionic fibers from both systems. If norepinephrine is released onto those cells, it binds to an adrenergic receptor that causes the cells to depolarize faster, and the heart rate increases. If ACh is released onto those cells, it binds to a muscarinic receptor that causes the cells to hyperpolarize so that they cannot reach threshold as easily, and the heart rate slows. Without this parasympathetic input, the heart would work at a rate of approximately 100 beats per minute (bpm). The sympathetic system speeds that up, as it would during exercise, to 120–140 bpm, for example. The parasympathetic system slows it down to the resting heart rate of 60–80 bpm.

Another example is in the control of pupillary size (Figure 13.9). The afferent branch responds to light hitting the retina. Photoreceptors are activated, and the signal is transferred to the retinal ganglion cells that send an action potential along the optic nerve into the diencephalon. If light levels are low, the sympathetic system sends a signal out through the upper thoracic spinal cord to the superior cervical ganglion of the sympathetic chain. The postganglionic fiber then projects to the iris, where it releases norepinephrine onto the radial fibers of the iris (a smooth muscle). When those fibers contract, the pupil dilates—increasing the amount of light hitting the retina. If light levels are too high, the parasympathetic system sends a signal out from the Eddinger–Westphal nucleus through the oculomotor nerve. This fiber synapses in the ciliary ganglion in the posterior orbit. The postganglionic fiber then projects to the iris, where it releases ACh onto the circular fibers of the iris—another smooth muscle. When those fibers contract, the pupil constricts to limit the amount of light hitting the retina.

In this example, the autonomic system is controlling how much light hits the retina. It is a homeostatic reflex mechanism that keeps the activation of photoreceptors within certain limits. In the context of avoiding a threat like the lioness on the savannah, the sympathetic response for fight or flight will increase pupillary diameter so that more light hits the retina and more visual information is available for running away. Likewise, the parasympathetic response of rest reduces the amount of light reaching the retina, allowing the photoreceptors to cycle through bleaching and be regenerated for further visual perception this is what the homeostatic process is attempting to maintain.

Interactive Link

Watch this video to learn about the pupillary reflexes. The pupillary light reflex involves sensory input through the optic nerve and motor response through the oculomotor nerve to the ciliary ganglion, which projects to the circular fibers of the iris. As shown in this short animation, pupils will constrict to limit the amount of light falling on the retina under bright lighting conditions. What constitutes the afferent and efferent branches of the competing reflex (dilation)?

Autonomic Tone

Organ systems are balanced between the input from the sympathetic and parasympathetic divisions. When something upsets that balance, the homeostatic mechanisms strive to return it to its regular state. For each organ system, there may be more of a sympathetic or parasympathetic tendency to the resting state, which is known as the autonomic tone of the system. For example, the heart rate was described above. Because the resting heart rate is the result of the parasympathetic system slowing the heart down from its intrinsic rate of 100 bpm, the heart can be said to be in parasympathetic tone.

In a similar fashion, another aspect of the cardiovascular system is primarily under sympathetic control. Blood pressure is partially determined by the contraction of smooth muscle in the walls of blood vessels. These tissues have adrenergic receptors that respond to the release of norepinephrine from postganglionic sympathetic fibers by constricting and increasing blood pressure. The hormones released from the adrenal medulla—epinephrine and norepinephrine—will also bind to these receptors. Those hormones travel through the bloodstream where they can easily interact with the receptors in the vessel walls. The parasympathetic system has no significant input to the systemic blood vessels, so the sympathetic system determines their tone.

There are a limited number of blood vessels that respond to sympathetic input in a different fashion. Blood vessels in skeletal muscle, particularly those in the lower limbs, are more likely to dilate. It does not have an overall effect on blood pressure to alter the tone of the vessels, but rather allows for blood flow to increase for those skeletal muscles that will be active in the fight-or-flight response. The blood vessels that have a parasympathetic projection are limited to those in the erectile tissue of the reproductive organs. Acetylcholine released by these postganglionic parasympathetic fibers cause the vessels to dilate, leading to the engorgement of the erectile tissue.

Homeostatic Imbalances: Orthostatic Hypotension

Have you ever stood up quickly and felt dizzy for a moment? This is because, for one reason or another, blood is not getting to your brain so it is briefly deprived of oxygen. When you change position from sitting or lying down to standing, your cardiovascular system has to adjust for a new challenge, keeping blood pumping up into the head while gravity is pulling more and more blood down into the legs.

The reason for this is a sympathetic reflex that maintains the output of the heart in response to postural change. When a person stands up, proprioceptors indicate that the body is changing position. A signal goes to the CNS, which then sends a signal to the upper thoracic spinal cord neurons of the sympathetic division. The sympathetic system then causes the heart to beat faster and the blood vessels to constrict. Both changes will make it possible for the cardiovascular system to maintain the rate of blood delivery to the brain. Blood is being pumped superiorly through the internal branch of the carotid arteries into the brain, against the force of gravity. Gravity is not increasing while standing, but blood is more likely to flow down into the legs as they are extended for standing. This sympathetic reflex keeps the brain well oxygenated so that cognitive and other neural processes are not interrupted.

Sometimes this does not work properly. If the sympathetic system cannot increase cardiac output, then blood pressure into the brain will decrease, and a brief neurological loss can be felt. This can be brief, as a slight “wooziness” when standing up too quickly, or a loss of balance and neurological impairment for a period of time. The name for this is orthostatic hypotension, which means that blood pressure goes below the homeostatic set point when standing. It can be the result of standing up faster than the reflex can occur, which may be referred to as a benign “head rush,” or it may be the result of an underlying cause.

There are two basic reasons that orthostatic hypotension can occur. First, blood volume is too low and the sympathetic reflex is not effective. This hypovolemia may be the result of dehydration or medications that affect fluid balance, such as diuretics or vasodilators. Both of these medications are meant to lower blood pressure, which may be necessary in the case of systemic hypertension, and regulation of the medications may alleviate the problem. Sometimes increasing fluid intake or water retention through salt intake can improve the situation.

The second underlying cause of orthostatic hypotension is autonomic failure. There are several disorders that result in compromised sympathetic functions. The disorders range from diabetes to multiple system atrophy (a loss of control over many systems in the body), and addressing the underlying condition can improve the hypotension. For example, with diabetes, peripheral nerve damage can occur, which would affect the postganglionic sympathetic fibers. Getting blood glucose levels under control can improve neurological deficits associated with diabetes.


Function of the autonomic nervous system

The autonomic nervous system controls internal body processes such as the following:

Heart and breathing rates

Metabolism (thus affecting body weight)

The balance of water and electrolytes (such as sodium and calcium)

The production of body fluids (saliva, sweat, and tears)

Many organs are controlled primarily by either the sympathetic or the parasympathetic division. Sometimes the two divisions have opposite effects on the same organ. For example, the sympathetic division increases blood pressure, and the parasympathetic division decreases it. Overall, the two divisions work together to ensure that the body responds appropriately to different situations.

Autonomic Nervous System

Generally, the sympathetic division does the following:

Prepares the body for stressful or emergency situations—fight or flight

Thus, the sympathetic division increases heart rate and the force of heart contractions and widens (dilates) the airways to make breathing easier. It causes the body to release stored energy. Muscular strength is increased. This division also causes palms to sweat, pupils to dilate, and hair to stand on end. It slows body processes that are less important in emergencies, such as digestion and urination.

The sympathetic division is part of the autonomic nervous system—the part of the nervous system that works automatically (autonomously), without a person's conscious effort, to regulate internal body processes.

Generally, the sympathetic division prepares the body for stressful or emergency situations—fight or flight. For example, it increases heart rate and the force of heart contractions and widens (dilates) the airways to make breathing easier.

The parasympathetic division does the following:

Controls body process during ordinary situations.

Generally, the parasympathetic division conserves and restores. It slows the heart rate and decreases blood pressure. It stimulates the digestive tract to process food and eliminate wastes. Energy from the processed food is used to restore and build tissues.

The parasympathetic division is part of the autonomic nervous system—the part of the nervous system that works automatically (autonomously), without a person's conscious effort, to regulate internal body processes.

The parasympathetic division regulates body process during ordinary situations. Generally, it conserves and restores. For example, it slows the heart rate and decreases blood pressure. It stimulates the digestive tract to process food and eliminate wastes.

Both the sympathetic and parasympathetic divisions are involved in sexual activity, as are the parts of the nervous system that control voluntary actions and transmit sensation from the skin (somatic nervous system).

Divisions of the Autonomic Nervous System

Heart rate and force of heart contractions

Release of energy stored in the liver

The speed at which energy is used to perform body functions while a person is at rest (basal metabolic rate)

Widens the airways to make breathing easier

Decreases functions that are less important in an emergency (such as digestion and urination)

Controls the release of semen (ejaculation)

Stimulates the digestive tract to process food and eliminate wastes (in bowel movements)

Two chemical messengers (neurotransmitters) are used to communicate within the autonomic nervous system:

Nerve fibers that secrete acetylcholine are called cholinergic fibers. Fibers that secrete norepinephrine are called adrenergic fibers. Generally, acetylcholine has parasympathetic (inhibiting) effects and norepinephrine has sympathetic (stimulating) effects. However, acetylcholine has some sympathetic effects. For example, it sometimes stimulates sweating or makes the hair stand on end.


Surgical Considerations

Horner syndrome is a mild, rare condition often presenting with unilateral ptosis, miotic, but a reactive pupil, and facial anhidrosis secondary to sympathetic nerve damage in the oculosympathetic pathway.[46] This damage may have a central cause such as infarction of the lateral medulla, or peripheral such as from damage secondary to thoracic surgery or from partial/total resection of the thyroid gland.[46][47] More centralized lesions tend to correlate with a constellation of symptoms that include Horner syndrome.[46] For more information, please see the associated StatPearls articles, here.[48][49] 

Hyperhidrosis is a common disease characterized by excessive sweating, primarily of the face, palms, soles, and/or axilla. While the cause of primary hyperhidrosis is not fully understood, it has been attributed to increased cholinergic stimulation. Treatment can be either clinical or surgical.[50] Treatment on the clinical side centers on anticholinergic agents such as topical glycopyrrolate or oral oxybutynin, or less commonly, alpha-adrenergic agonists such as clonidine, calcium channel blockers, or gabapentin.[50][51] The most common and permanent surgical technique is the resection, ablation, or clipping of the thoracic sympathetic chain. While permanent, the procedure may lead to compensatory hyperhidrosis in a small number of individuals. These hyperhidrosis symptoms are the same if not more severe than prior to the procedure due to possible overcompensation by the hypothalamus. Research has demonstrated that surgical reconstruction of the sympathetic chain can reduce this compensatory response.[52]


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