Chapter ---15

 

 

 

AUTONOMIC NERVOUS SYSTEM

 

 

 

(Autonomous: Gr. Autos= self + nomos, law)

Autonomic nervous system (ANS) is the part of nervous system concerned with the innervation of involuntary structures such as cardiac muscle, smooth muscles and exocrine glands.

ANS is distributed throughout the central and peripheral nervous systems.

Autonomic nervous system (ANS) is concerned with maintaining a constant internal environment (homeostasis).

It does so by making fine adjustments in certain bodily functions.

It does so by controlling smooth muscle, the secretion of glands and modulation of cardiac rhythm.

 

The term ‘autonomous’ is deceptive. Autonomic nervous system (ANS) responds quickly to changes in bodily activities.  Its functions are organized and regulated in central nervous system (CNS). The concept of autonomy is mainly functional.

The activities of ANS normally do not impinge upon the conscious of the individual

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Autonomic nervous system has

►   Afferent neuron

►   Connector neuron

►   Efferent neuron

 

Afferent impulses originate in visceral receptors of cardiac muscle, smooth muscles and exocrine glands

Visceral receptors are

• Chemo-receptors [Gr. chemeia  chemistry +  L. recipere to receive, accept]

A receptor adapted for excitation by chemical substances e.g., olfactory and gustatory receptors, or a sense organ, as the carotid body or the aortic bodies, which is sensitive to chemical changes in blood, especially reduced oxygen content, and reflexly increases both respiration and blood pressure  

• Baro-receptors [Gr. baros weight  +  L. recipere to receive, accept]

A sensory nerve ending that is stimulated by changes in pressure, as those in the walls of blood vessels  

• Osmo-receptors [Gr. osmos impulse  +  L. recipere to receive, accept]

A specialized sensory nerve ending that is stimulated by changes in osmotic pressure of the surrounding medium

 

These impulses travel through afferent neurons to central nervous system

In CNS these impulses are integrated through connector neurons

Finally these impulses leave CNS via efferent neurons to visceral effector organs

 

Anatomically, ANS is formed by a collection of nerve cells located in CNS through cranial and spinal nerves, and ganglia located in the paths of these nerves.

 

Efferent pathways of ANS are made up of 2 neurons. The 1^st neuron is located in CNS. Its axon synapses with a 2^nd multipolar neuron, located in a ganglion of the peripheral nervous system.

The axons of the 1^st neuron are called preganglionic fibers

The axons of the 2^nd neuron to muscle or gland are called postganglionic fibers

 

One preganglionic neuron, through its axon, may synapse with the dendrites of many postganglionic neurons

This makes the control, exerted by ANS, very rapid

 

The chemical secreted at all preganglionic endings and parasympathetic postganglionic endings is acetylcholine, which is released from nerve endings by nerve impulses.

 

The only organ that receives preganglionic fibres is the adrenal medulla and it behaves like a ganglion, whose cells have changed into secretory cells rather ganglion cells

 

 

 

 

 

Autonomic nervous system is composed of

1. Sympathetic
2. Parasympathetic

 

These differ in morphology and function

Sympathetic nervous system is concerned with emergency situation (“fight or flight” reaction)

Parasympathetic nervous system is concerned with conservation of energy

 

Sympathetic preganglionic fibers pass out through thoracic and lumbar spinal nerves and this is known as ‘thoracolumbar outflow’. The cell bodies of these fibers lie in lateral gray horns of the spinal cord from thoracic1 to lumbar2

 

Cross section of spinal cord at the level of fifth thoracic segment

The H-shaped grey matter shows three horns. Look at the lateral horn. This is the site for the cell bodies of sympathetic connector neurons.

 

The cell bodies of postganglionic sympathetic neurons are mostly located in the ganglia of the sympathetic trunk or ganglia in more peripheral plexuses. They almost always lie closer to the spinal cord than to the areas innervated. This, however, is not true for those that innervate viscera of the pelvis

 

Parasympathetic preganglionic efferent fibers pass out via certain cranial & sacral nerves; this is known as ‘craniosacral’ outflow. The cell bodies of these fibers lie in the motor nuclei of third, seventh, ninth and tenth cranial nerves and in the gray matter of sacral 2 to 4 segments of spinal cord

The cell bodies of postganglionic parasympathetic lie close to the structures supplied or often distributed in the walls of the innervated viscus

 

 

 

 

 

 

 

 

 

 

 

 

 

The motor part of somatic nervous system is concerned with the innervation of skeletal muscles.

The cell bodies, in somatic nervous system, are in the motor nuclei of cranial nerves and in the anterior horn cells of spinal cord

The nerve fibers which leave the central nervous system run uninterruptedly to the muscle fibres. There is single neuron

 

The great difference between autonomic nervous system (ANS) and the somatic system is that the pathway from nerve cells in central nervous system (CNS) to the target organ is interrupted by synapses in a ganglion.

There are two neurons in ANS with preganglionic and postganglionic fibers

 

The cell bodies of preganglionic fibers are always in central nervous system (CNS)

In sympathetic system, they are in lateral horn of spinal cord of all thoracic and the upper two lumbar segments (sometimes third lumbar segment).

In parasympathetic system, they are in third, seventh, ninth and tenth cranial nerves’ nuclei and in lateral horn of spinal cord of second, third and forth sacral segments.

 

The cell bodies of postganglionic fibers are in ganglia outside CNS

In sympathetic system, the ganglia are either in sympathetic trunk or in collateral ganglia (such as coeliac ganglia).

In parasympathetic system, the ganglia are terminal ganglia. They are usually within the walls of the viscera concerned.

There are four ganglia which are some little distance from the structures innervated and they are ciliary, pterygopalatine, submandibular and otic ganglia.

 

Autonomic nervous system (ANS) has two components

1. Sympathetic nervous system

It prepares the body for an emergency. It accelerates the heart rate, causes constriction of peripheral blood vessels and raises the blood pressure. It brings about a redistribution of the blood, so that blood leaves the areas of the skin and intestine and becomes available to brain, heart, and skeletal muscle. At the same time it inhibits peristalsis of intestinal tract and closes the sphincters.

 

2. Parasympathetic nervous system

It conserves and restores energy. It slows the heart rate, increases peristalsis of intestines, increases glandular activity and opens the sphincters.

 

 

 

 

 

 

 

 

 

 

 

 

Sympathetic nervous system

Sympathetic system is the larger of the two parts of ANS and is widely distributed throughout the body.

It innervates heart and two lungs

It innervates the smooth muscles of blood vessels and viscera in abdomen and pelvis

It innervates glands of the body including all the sweat glands of skin and arrector muscles of hair follicles

 

Sympathetic system prepares the body for an emergency

• Heart rate is increased.
• It is interesting to note that arterioles of skin and intestines are constricted and those of skeletal muscle are dilated.
• Blood pressure is raised.
• There is a redistribution of blood. The blood supply to skin and gastrointestinal tract is decreased and the blood supply to brain, heart, and skeletal muscle is increased.
• Pupils dilate, smooth muscles of bronchi are inhibited, peristalsis in intestines decreases, and urinary bladder is relaxed.  
• The sphincters are closed.
• The hair is made to stand on end and sweating occurs.

 

Sympathetic nervous system consists of

1. Lateral horns in spinal cord
2. Efferent nerve fibers
3. Rami communicantes
4. Two sympathetic trunks
5. Nerve branches
6. Nerve plexuses
7. Regional ganglia
8. Afferent nerve fibers

 

The lateral gray columns (horns) of the spinal cord from the first thoracic segment to the second lumbar segment (sometimes third lumbar segment) possess the cell bodies of sympathetic connector neurons

 

Cross section of spinal cord at the level of fifth thoracic segment.

The H-shaped grey matter shows three horns. Look at the lateral horn. This is the site for the cell bodies of sympathetic connector neurons.

 

 

The myelinated axons of the cell bodies of sympathetic connector neurons leave spinal cord through anterior nerve roots and pass via the white rami communicantes to paravertebral ganglia of sympathetic trunk. These are called preganglionic fibers

 

 

Cross section of spinal cord at the level of fifth thoracic segment

On left side general arrangement of somatic system is shown

On right side general arrangement of sympathetic system is shown

Gray ramus is shown in gray color and white ramus is shown in white color. The ganglion of sympathetic trunk is shown in blue color

Note the postganglionic fiber entering into anterior or ventral ramus through gray ramus.

 

Ventral or anterior rami of spinal nerves are connected to ganglia of sympathetic trunk by two bundles of nerve fibers. The connecting links between the ganglia of sympathetic trunk and anterior rami are called rami communicantes.  There are two rami communicantes that join an anterior ramus of a spinal nerve with associated paravertebral ganglion of sympathetic trunk.

 

White rami communicantes

They are white because they contain myelinated preganglionic fibers that leave anterior rami and pass to paravertebral ganglia of sympathetic trunk. White rami also contain afferent sympathetic fibers. These fibers are also myelinated. White ramus is always the distal one.

Within the trunk, the fibers of the white rami communicantes run longitudinally. They end on the nerve cells in the ganglia throughout the length of the sympathetic trunk.

Through these nerve fibers the central nervous system controls the activity of all the nerve cells in the sympathetic trunk. Thus it can alter the secretion of sweat, the amount of blood flowing through the various tissues, and the erection of hairs (goose-flesh) throughout the body by way of the processes of the sympathetic nerve cells that are distributed through the spinal (and cranial) nerves.

 

It is important to note that the nerve fibers which connect the central nervous system to the sympathetic nervous system run only in the first thoracic to second or third lumber spinal nerves. If all these nerves or the white rami communicants arising from them were cut, the sympathetic nervous system would be separated from the control of the central nervous system. This would result in the loss of a number of responses which arise from afferent impulses discharging directly into the central nervous system over the dorsal roots, e.g. the sweating and dilatation of skin vessels on exposure to heat and the contraction of skin vessels with goose-flesh in response to cold or fear.

 

Cross section of spinal cord at the level of fifth thoracic segment. On left side general arrangement of somatic system is shown. On right side general arrangement of sympathetic system is shown. Gray ramus is shown in gray color and white ramus is shown in white color. Note the postganglionic fiber entering into dorsal ramus. Do remember that gray ramus is connected to anterior or ventral ramus.

 

Gray rami communicantes

Soon after its formation, each ventral ramus receives a slender bundle of non-myelinated nerve fibers from the corresponding ganglion of sympathetic trunk. This bundle of non-myelinated nerve fibers is given the name the gray ramus communicans. It is proximal to white ramus. They are gray colored because the nerve fibers in these rami are devoid of myelin. They bring postganglionic sympathetic fibers to ventral rami and are distributed through their branches. They also enter every branch of dorsal ramus.

 

Gray ramus contains non-myelinated efferent postganglionic fibers that leave the paravertebral ganglion and pass to anterior (ventral) or posterior (dorsal) ramus of spinal nerve. These sympathetic fibers supply smooth muscles of blood vessels, smooth muscles of the hair (arrector pilorum) and the sweat glands. Thus the spinal nerve supplies involuntary and voluntary structures.

 

The fibers in gray ramus are those that are distributed via the branches of spinal nerve to blood vessels (vasomotor fibers), sweat glands (sudomotor fibers) and arrector pili muscles (pilomotor fibers).

 

 

 

 

 

 

Every spinal nerve receives a gray ramus

All thoracic and the upper two lumbar nerves have gray rami and white rami.

All cervical, lower lumbar and all sacral nerves do not have gray rami, because there is no sympathetic outflow from these segments of spinal cord. The ganglia with which these nerves are connected by their white rami receive their preganglionic fibers from thoracolumbar lateral horn cells whose fibers after entering sympathetic trunk have gone up or down in it

 

 

The arrangement of sympathetic fibers in cervical region

Note that

There is no gray ramus

The preganglionic fibers do not synapse in the sympathetic ganglion and ascend upwards

 

Superior cervical ganglion gives off four gray rami.

Middle cervical ganglion gives off two gray rami.

Inferior cervical ganglion gives off two gray rami.

Stellate ganglion which is formed by the fusion of inferior cervical ganglion and first thoracic ganglion, gives three gray rami and one white ramus communicans.

Note that the gray rami contain only efferent postganglionic fibers but white rami contain efferent preganglionic and afferent fibers.

 

 

The arrangement of sympathetic fibers in lower lumbar and all sacral nerves

Note that

There is no gray ramus

The preganglionic fibers do not synapse in the sympathetic ganglion and descend downwards

 

There are two sympathetic trunks one on each side of the vertebral column. Each trunk consists of a vertical chain of ganglia. These ganglia are united by nerve fibers. They extend the whole length of vertebral column. Each sympathetic trunk extends alongside the vertebral column from the base of the skull to the coccyx.

 

In neck, the trunks lie anterior to the transverse processes of cervical vertebrae. There are three ganglia in cervical region

      Superior cervical

      Middle cervical and

      Inferior cervical ganglia

 

Occasionally middle cervical ganglion is absent

Inferior cervical ganglion and first thoracic ganglion often unite to form stellate ganglion. Sometimes second thoracic may also be included or even third and fourth thoracic may be

 

In thorax, sympathetic trunks lie on the sides of vertebral bodies, anterior to the heads of ribs.

There are eleven or twelve ganglia in thoracic region.

They are named according to their number; first thoracic ganglion, second thoracic ganglion, etc.

 

 

 

In abdomen, the trunks are anterolateral to the sides of the bodies of lumbar vertebrae.

There are four or five ganglia in lumbar region.

They are first lumber ganglion, second lumbar ganglion and so on.

 

In pelvis, sympathetic trunks are anterior to sacrum.

There are four or five ganglia in sacral region.

They are first sacral ganglion, second sacral ganglion and so on.

 

Superiorly the proximal ends of sympathetic trunks are separate but inferiorly the two trunks join each other at a single ganglion. This ganglion where the two trunks join is called ganglion impar.

 

Theoretically there is a ganglion for each spinal nerve, but fusion occurs, especially in the cervical region.

First four ganglia unite to form superior cervical ganglion.

Fifth and sixth ganglia join to form middle cervical ganglion.

Seventh and eighth cervical ganglia fuse and form inferior cervical ganglion.

 

Elsewhere there is usually one ganglion less than the number of nerves

There are usually 11 thoracic ganglia (but there may be twelve also)

There may be 4 lumbar ganglia (but may be 5also) and

There are 4 sacral ganglia

 

 

Diagrammatic representation of paravertebral sympathetic ganglia.  The anterior aspect of vertebral column is shown. The paravertebral ganglia are linked together and form two (right and left) sympathetic chains.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Splanchnic nerves

The preganglionic efferent sympathetic nerve fibers enter the sympathetic ganglia via white rami communicantes

Some of them synapse their

Some ascend and some descend without synapsing

Some pass through the ganglia without synapsing. They do not ascend or descend. These myelinated fibers leave the sympathetic trunk as greater splanchnic, lesser splanchnic and lowest or least splanchnic nerves.

 

 

A part of sympathetic system showing preganglionic fibers which do not synapse in sympathetic chain ganglia and pass through these ganglia without synapsing and synapse in prevertebral ganglia with postganglionic neurons

Postganglionic fibers arise here in prevertebral ganglia 

 

Greater splanchnic nerves

Right and left greater splanchnic nerves are formed from the branches from fifth to ninth thoracic ganglia on both sides. They descend obliquely on the sides of the bodies of thoracic vertebrae and pierce the crura of diaphragm and synapse with excitor cells in the ganglia of celiac plexus, renal plexus, and suprarenal medulla.

 

 

 

 

Sympathetic nervous system highlighting greater splanchnic nerve

 

 

 

 

 

 

Lesser splanchnic nerves

There are also right and left lesser splanchnic nerves. They are formed from the branches of tenth and eleventh thoracic ganglia (sometimes twelfth also) on both sides. They descend with greater splanchnic nerves and pierce the diaphragm to synapse the excitor cells in the ganglia of the lower part of celiac plexus.

 

 

Sympathetic nervous system

Concentrate on lesser splanchnic nerve

 

 

 

 

 

 

 

 

 

 

 

Lowest or Least splanchnic nerves

When present they arise from right and left twelfth thoracic ganglia, pierce the diaphragm to synapse with excitor neurons in the ganglia of renal plexus.

 

Nerve plexuses

Large collections of sympathetic and parasympathetic efferent nerve fibers and their associated ganglia, together with visceral afferent fibers, form autonomic nerve plexuses in the thorax, abdomen, and pelvis. Branches from these plexuses innervate the viscera.

• In the thorax there are the cardiac, pulmonary, and esophageal plexuses.
• In the abdomen there are preaortic plexuses that are associated with aorta and its branches. These autonomic plexuses are named according to the branch of aorta along which they are lying: celiac, superior mesenteric, and inferior mesenteric plexuses.
• In the pelvis there are the superior and inferior hypogastric plexuses.

 

Regional ganglia

They are present in prevertebral or preaortic position. They are in the plexuses that surround the origins of ventral branches of abdominal aorta.

They are called prevertebral ganglia or preaortic or subsidiary ganglia or collateral ganglia

There are three sets of prevertebral ganglia

• Celiac ganglia two in number present in celiac plexus that surround the origin of celiac artery.
• Superior mesenteric ganglion present in superior mesenteric plexus that surrounds the origin of superior mesenteric artery.
• Inferior mesenteric ganglion present in inferior mesenteric plexus that surrounds the origin of inferior mesenteric plexus.

 

Structure of an autonomic ganglion

Autonomic ganglia are often irregular in shape. They are the site where preganglionic nerve fibers synapse on postganglionic neurons.

Ganglia are situated along the course of efferent nerve fibers of the autonomic nervous system.

Sympathetic ganglia are part of sympathetic trunk and they are present around the roots of ventral branches of abdominal aorta.

Parasympathetic ganglia, on the other hand, are situated close to or within the walls of the viscera.

 

Majority of autonomic ganglia resemble sensory ganglia in having a similar connective tissue capsule and framework.

 

But unlike sensory ganglia, autonomic ganglia contain synapses.

An autonomic ganglion consists of a collection of multipolar neurons. There are numerous branched dendrites and an axon which forms unmyelinated postganglionic visceral efferent fibers.

In larger ganglia, each neuron cell is surrounded by a layer of satellite cells as in spinal sensory ganglia.

 

 

Nerve bundles are attached to these ganglia consist of

1. Preganglionic nerve fibers that enter the ganglion,
2. Postganglionic nerve fibers that are leaving the ganglion,
3. Afferent and efferent nerve fibers that pass through the ganglion without synapsing.

 

The preganglionic fibers are myelinated. The postganglionic fibers are unmyelinated.

Postganglionic fibers are much more numerous than preganglionic fibers

Preganglionic axons may synapse with many postganglionic neurons for wide dissemination and amplification of sympathetic activity

While an autonomic ganglion is the site where preganglionic fibers synapse on postganglionic neurons, the presence of small interneurons and collateral branches suggests that a ganglion may play a greater role than simply relaying information

 

Preganglionic and postganglionic fibers

The myelinated axons of sympathetic connector neurons (the lateral horn cells) leave spinal cord through anterior nerve roots (with the axons of anterior horn cells) to reach the spinal nerve and its anterior ramus.

Then they pass via the white rami communicantes to paravertebral ganglia of sympathetic trunk. These are preganglionic fibers.

 

After reaching a paravertebral ganglion, the preganglionic fibers have one of the five possible synaptic alternatives

 

      The commonest is for them to synapse with the cell bodies of an excitor neuron. The gap between the two (connector and excitor) neurons is bridged by acetylcholine. Acetylcholine is a neurotransmitter. The postganglionic nonmyelinated axons leave the paravertebral ganglion and pass to the thoracic spinal nerves through gray rami communicantes. These postganglionic fibers are distributed in branches of the spinal nerves to smooth muscle in the walls of blood vessel, sweat glands, and arrector pili muscles of skin.

 

      Most of the preganglionic fibers entering the upper part of sympathetic trunk from the upper thoracic segments of spinal cord travel superiorly to cervical sympathetic ganglia. They synapse in these ganglia in the cervical region. The postganglionic nerve fibers pass via gray rami communicantes to join the cervical spinal nerves. These postganglionic fibers are distributed in branches of the spinal nerves to smooth muscle in the blood vessel walls, sweat glands, and arrector pili muscles of skin like thoracic region.

 

      Many of the preganglionic fibers entering the lower part of the sympathetic trunk from the lower thoracic and upper two lumbar segments of the spinal cord travel inferiorly to synapse in ganglia in the lower lumbar and sacral regions. Here again, the postganglionic nerve fibers pass via gray rami communicantes to join the lumbar, sacral, and coccygeal spinal nerves.

 

 

 

 

 

      The fourth alternative is to leave paravertebral ganglion without synapsing and to pass to a prevertebral ganglion for synapse. These myelinated fibers leave sympathetic trunk as the greater, lesser, and lowest splanchnic nerves. These splanchnic nerves are formed by preganglionic fibers.

Postganglionic fibers arise from the excitor cells in the peripheral plexuses and are distributed to the smooth muscle and glands of the viscera.

 

      A few preganglionic fibers, traveling in the greater splanchnic nerve, end directly on the cells of suprarenal medulla. These medullary cells, which may be regarded as modified sympathetic excitor neurons, are responsible for the secretion of epinephrine and norepinephrine.

 

Each sympathetic trunk ganglion has a collateral or visceral branch, usually called a splanchnic nerve in the thoracic, lumbar and sacral regions but in the cervical region called a cardiac branch because it proceeds to the cardiac plexus.

 

The visceral branches generally arise high up and descend steeply to form plexuses for the viscera. Thus cardiac branches arise from the three cervical ganglia to descend into the mediastinum to the cardiac plexus, which is supplemented by fibers from upper thoracic ganglia.

 

 

From lower thoracic ganglia three splanchnic nerves pierce the diaphragm to reach celiac plexus.

 

From upper lumbar ganglia the lumbar splanchnic nerves descend to superior hypogastric plexus and this divides to enter the left and right inferior hypogastric plexuses. The inferior hypogastric plexuses (collectively forming the pelvic plexus) are joined by visceral branches from all the sacral ganglia (sacral splanchnic nerves).

 

 

Sympathetic visceral plexuses thus formed are joined by parasympathetic nerves (vagus nerve to celiac plexus and pelvic splanchnic nerves to inferior hypogastric plexuses). The mixed visceral plexuses reach the viscera by branches that hitch-hike along the relevant arteries. These visceral branches supply not only the smooth muscle and glands of viscera but also the blood vessels of these viscera.

 

 

 

All sympathetic trunk ganglia give off vascular branches to adjacent large blood vessels.

The cervical ganglia give branches to the carotid arteries and their branches, including the internal carotid plexus along the internal carotid arteries.

The thoracic and lumbar ganglia give filaments to the various parts of the aortic plexus and its derivatives, including those along the common iliac and median sacral arteries.

The sacral ganglia give branches to lateral and median sacral arteries.

 

Note that the head and neck arteries receive direct branches from cervical trunk ganglia.

On the other hand limb vessels get their sympathetic innervation by nerve fibers that run with the adjacent peripheral nerves before passing to the vessels.

In limbs fibers do not run long distances along the vessels themselves. Thus the nerve filaments to the vessels of the tip of a finger or toe run not with the digital arteries but with the digital nerves and only leave the nerves near the actual site of innervation.

 

            From the information given above, it should be clear that branches of nerves to skin (cutaneous branches) are not entirely sensory but also contain sympathetic efferent fibers.

            Similarly, branches to muscles are not entirely efferent but also contain sensory fibers and sympathetic fibers.

            Thus the signs of nerve injury are not simply paralysis of muscle and loss of sensation, but also loss of sweating, blood-vessel control, and goose-flesh.

 

                        In addition to its gray rami communicantes to the spinal nerves and equivalent branches to the cranial nerves, the sympathetic trunk distributes postganglionic fibres through branches which pass on to the arteries of the body wall, limbs, head, and neck. Hence there is a dual route via nerves and arteries to these structures from sympathetic trunk ganglion cells.

                       

The internal organs (viscera), including the gut tube from mouth to anus, also receive postganglionic sympathetic nerve fibers. These arise from separate sympathetic ganglia closely associated with the arteries which supply these organs and pass to these organs along the arteries as periarterial plexuses of nerves fibers. These visceral or splanchnic ganglia are brought under the control of the central nervous system through preganglionic nerve fibres which emerge in the white rami communicantes, enter the sympathetic trunk, and pass through it to the splanchnic ganglia as splanchnic nerves. They appear as branches of the sympathetic trunk but merely pass through it, emerging at every level to run to the splanchnic ganglia which lie at that level in early development. Subsequently, the developing viscera and their splanchnic ganglia move caudally relative to the vertebral column and to the sympathetic trunks and the nerves to which they are attached by rami communicantes so that the splanchnic nerves come to run caudally from the sympathetic trunk in the adult.

 

 

 

 

 

 

Sympathetic nervous system

Look at the nerve supply to the viscera

 

 

Afferent sympathetic nerve fibers

These are myelinated nerve fibers. They do not have their cell bodies in sympathetic ganglia. They have their cell bodies in the posterior root ganglia of spinal nerves.

The peripheral processes travel from the viscera through some plexus or subsidiary ganglia with which the efferent fibers were involved. Then they enter sympathetic ganglia without synapsing. They pass to the spinal nerve via white rami communicantes and reach their cell bodies in the posterior root ganglion of the corresponding spinal nerve.

A part of sympathetic system shown

Look at the black colored afferent neuron between stomach and lateral horn of spinal cord

 

The central axons (processes) then enter the spinal cord via the posterior nerve root (like any other afferent fibres) at approximately the same segmental level as the preganglionic cells.

Here they may form the afferent component of a local reflex arc or ascend to higher centers, such as the hypothalamus.

 

Visceral pain fibers enter the posterior horn of spinal cord, and thereafter the pain pathway is the same as that for spinal nerve pain fibers. Others concerned with reflex activities may synapse with interneurons in spinal cord or ascend to the hypothalamus and other higher centers.

 

 

 

Many afferent nerve fibres which innervate sensory nerve endings in the viscera travel with the sympathetic nerve fibres (in rami communicantes, sympathetic trunk, splanchnic nerves, and periarterial plexuses) but have no functional connection with them, merely transmitting their sensory information directly to the central nervous system through the dorsal roots of the spinal nerves. They are, therefore, part of the general sensory system and not of the sympathetic nervous system

 

Sympathectomy

For the control of excessive sweating and vasoconstriction in the limbs, parts of the sympathetic trunk with appropriate ganglia can be removed to abolish the normal sympathetic influence.

 

In cervical sympathectomy for the upper limb (so called because it is usually carried out through the neck, not because any cervical part of the trunk is removed), the second and third thoracic ganglia with their rami and the intervening part of the trunk are resected. The first thoracic ganglion is not removed, since the preganglionic fibers for the upper limb do not usually arise above T2 level and its removal would result in Horner’s syndrome.

 

For lumbar sympathectomy the third and fourth lumbar ganglia and the intervening trunk are removed; preganglionic fibers do not arise below L2. The first lumbar ganglion should be preserved otherwise ejaculation may be compromised; the exact positions of the ganglia vary, and they have no constant relation to the lumbar vertebrae.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Parasympathetic nervous system

Parasympathetic system also widely distributed throughout the body but it is much smaller than sympathetic nervous system

 

Heart rate slows, pupils constrict, peristalsis in intestines increases, secretions of glands increase, sphincters open, and urinary bladder contracts under the influence of parasympathetic nervous system

 

Sympathetic system prepares the body for an emergency. The activities of parasympathetic nervous system are to conserve and restore energy.

 

Sympathetic fibers supply all parts of the body but the distribution of parasympathetic fibers is visceral and not to the trunk or limbs. Similarly suprarenal glands and gonads appear to have only a sympathetic supply.

 

Parasympathetic nervous system consists of

1. Cranial nerve nuclei in brainstem
2. Gray matter in sacral segments of spinal cord
3. Efferent nerve fibers
4. Nerve branches
5. Nerve plexuses
6. Regional ganglia
7. Afferent nerve fibers

 

The connector nerve cells of parasympathetic part of ANS are located in the brainstem and the sacral segments of the spinal cord.

 

Cranial nerve nuclei in brainstem

The connector nerve cells located in brainstem form nuclei in the following cranial nerves:

1.    Oculomotor

Accessory parasympathetic nucleus or Edinger-Westphal nucleus

It is situated posterior to the main oculomotor nucleus

2.    Facial

Superior salivatory nucleus and lacrimatory nucleus

3.    Glossopharyngeal

Inferior salivatory nucleus

4.    Vagus

Dorsal nucleus of vagus

 

The axons of these connector nerve cells are myelinated and emerge from brainstem along the cranial nerves

 

Gray matter in sacral segments of spinal cord

Sacral connector nerve cells are found in the lateral gray horn of second, third, and fourth sacral segments of spinal cord. In fact these cells are not numerous enough to form an obvious lateral gray horn, as do the sympathetic connector neurons in thoracolumbar region.

 

The myelinated axons leave the spinal cord in anterior nerve roots of the corresponding spinal nerves. They then leave the sacral nerves and form pelvic splanchnic nerves.

 

Efferent Nerve Fibers

The efferent parasympathetic fibers (craniosacral outflow) are called preganglionic fibers. They are myelinated fibers and synapse in peripheral ganglia. Here, again, acetylcholine is the neurotransmitter. The postganglionic parasympathetic fibers are nonmyelinated and of relatively short length

 

Regional ganglia

The cranial parasympathetic ganglia are four:

1. Ciliary ganglion. It lies in the apex of orbit anterior to the medial end of superior orbital fissure. It is minute body (2mm dm) lying on the lateral side of the optic nerve between the nerve and the lateral rectus muscle.
2. Pterygopalatine ganglion. It lies immediately in front of the opening of the pterygoid canal and nerve of that canal runs straight into the back of the ganglia.
3. Submandibular ganglion. It hangs suspended from the lingual nerve on the surface of hyoglosus muscle.
4. Otic ganglion. It is a small body lying between the tensor veli palatini and the mandibular nerve, just below the foramen ovale.

 

These four ganglia are very similar in plan. Each has parasympathetic, sympathetic and sensory roots, and branches of distribution.

 

Parasympathetic root carries preganglionic fibers from the cells of origin in a brainstem nucleus. This is the essential functional root of the ganglion. Preganglionic fibers synapse in it.

 

The fibers of other roots simply pass through the ganglion without synapse

Sympathetic root contains postganglionic fibers from the superior cervical ganglion. Their preganglionic fibers are coming from the lateral grey horn of thoracic 1-3 segments of spinal cord.

Sensory root contains the peripheral processes of the cell bodies in trigeminal ganglion.

 

The branches of each ganglion carry postganglionic fibers to target organs and structures.

From ciliary ganglion short ciliary nerves leave to eye. They supply ciliary muscle and sphincter pupillae.

From the pterygopalatine ganglion the nerves pass through the zygomatic and lacrimal nerves to lacrimal gland; through maxillary nerve branches to mucous glands in the nose, nasopharynx and palate.

From submandibular and otic ganglia nerves leave for salivary glands.

The other fibers in the branches are sympathetic fibers to the same structures. They are mainly for their blood vessels.

 

The sacral parasympathetic ganglia are located close to the viscera they innervate.

 

Nerve plexuses

1. Cardiac plexus,
2. Pulmonary plexus
3. The plexuses associated with gastrointestinal tract are Myenteric plexus (Auerbach’s plexus) and Mucosal plexus (Meissner’s plexus)
4. Hypogastric plexuses

 

 

Efferent parasympathetic fibers

 

 

Preganglionic parasympathetic fibers leaving the brainstem pass through the cranial nerves (oculomotor, facial, glossopharyngeal and vagus).

 

Preganglionic fibers coming from accessory parasympathetic nucleus go to ciliary ganglion and synapse there. They pass through the inferior division of oculomotor nerve and a branch from the nerve to the inferior oblique muscle.

Postganglionic fibers go to eyeball. They pass through short ciliary nerves to constrictor pupillae and ciliary muscles.

 

Preganglionic fibers coming from superior salivatory nucleus go to pterygopalatine ganglion and submandibular ganglion. They synapse in these ganglia.

Preganglionic fibers, going to pterygopalatine ganglion, pass through nervus intermedius. Then they pass through greater petrosal nerve and the nerve of pterygoid canal and reach the ganglion.

Postganglionic fibers go to lacrimal, nasal and palatal glands. They leave the ganglion and join maxillary nerve. Then they pass through zygomatic branch and zygomaticotemporal nerve. The lacrimal fibers pass through lacrimal nerve to go to lacrimal gland.

 

Preganglionic fibers, going to submandibular ganglion, pass through nervus intermedius, then via chorda tympani nerve and lingual nerve to reach submandibular gland.

Postganglionic fibers are distributed to submandibular and sublingual glands via branches of the lingual nerve.

 

Preganglionic fibers coming from inferior salivary nucleus go to otic ganglion and synapse there. They pass through tympanic branch of glossopharyngeal nerve to the tympanic plexus and then through lesser petrosal nerve reach otic ganglion.

Postganglionic fibers go to parotid gland via filaments of the auriculotemporal nerve.

 

Unlike the other three ganglia, the otic ganglion has an additional somatic motor root, from the nerve to the medial pterygoid; the fibers pass through (without synapse) to supply tensor tympani and tensor palati.

 

The preganglionic fibers synapse in peripheral ganglia. In certain locations the ganglion cells are placed in nerve plexuses, such as cardiac plexus, pulmonary plexus, myenteric plexus, and mucosal plexus.

The postganglionic cells for the first three of the above groups are in the four parasympathetic ganglia (ciliary, pterygopalatine, submandibular, otic). The vagal fibers synapse with postganglionic cell bodies in the walls of the viscera supplied (heart, lungs and gut).

 

Preganglionic parasympathetic fibers arising from the sacral segments of spinal cord leave the spinal cord in anterior nerve roots of the corresponding spinal nerves. They then leave the sacral nerves and form pelvic splanchnic nerves. The pelvic splanchnic nerves synapse in ganglia in the hypogastric plexuses.

Leaving the anterior rami of the appropriate sacral nerves near the anterior sacral foramina they pass forwards to enter into the formation of inferior hypogastric plexuses (pelvic plexus). From there they run to pelvic viscera and to the hindgut as far up as the splenic flexure. Fibers reach the viscera either by running along their blood vessels or making their own way retroperitoneally, and they synapse around postganglionic cell bodies in the walls of these viscera.

 

The postganglionic parasympathetic fibers are nonmyelinated and of relatively short length as compared with sympathetic postganglionic fibers.

 

                        The viscera and the eye also receive an additional efferent innervation from the parasympathetic nervous system which is not found in other parts of the body. This consists of peripheral ganglion (nerve) cells in small groups (ganglia) or scattered in or near the viscera which they innervate and not primarily associated with the arterial supply. These parasympathetic ganglion cells usually affect the structures which they innervate, e.g. glands and smooth muscle of the gut tube, by the release of acetylcholine from the terminals of their short postganglionic fibres (cholinergic nerve fibers) while the sympathetic postganglionic fibres usually act by the release of noradrenalin (adrenergic fibers) although the sympathetic innervation of sweat glands is effected by the release of acetylcholine.

 

Like the sympathetic ganglion cells, parasympathetic ganglion cells are controlled by the central nervous system through preganglionic nerve fibres which arise from cells in the central nervous system and emerge from it in certain cranial nerves and the second, third and fourth sacral nerves to pass to its ganglion cells. Because of this the parasympathetic nervous system is some times called the craniosacral part of the involuntary (autonomic) nervous system while the sympathetic nervous system, for the same reason, is the thoracolumbar part of the involuntary nervous system. These two parts together form the autonomic nervous system and their balanced activities control the visceral structures of the body.

 

Parasympathetic afferent fibers

Afferent myelinated parasympathetic fibers arise from the viscera supplied by efferent parasympathetic fibers. They travel to their cell bodies, present in the sensory ganglia of cranial nerves or in posterior root ganglia of sacrospinal nerves. The central processes from these ganglia go to central nervous system. Once the afferent fibers gain entrance to the spinal cord or brain, they are thought to travel alongside, or mixed with, the somatic afferent fibers. They take part in the formation of local reflex arcs or pass to higher centers of autonomic nervous system, such as hypothalamus.

 

It is important to realize that the afferent component of autonomic system is identical to the afferent component of somatic nerves and that it forms part of the general afferent segment of the entire nervous system.

 

The nerve endings in the autonomic afferent component may not be activated by such sensations as heat or touch but rather by stretch or lack of oxygen.  

 

The routes taken by the parasympathetic nerve fibers (pre-and postganglionic) are also routes for the distribution of afferent visceral nerve fibres which have no peripheral functional connection with the parasympathetic system but discharge directly into the central nervous system through the appropriate cranial and spinal nerves.

It is important to appreciate that preganglionic fibres of the autonomic nervous system synapse with the cells of its ganglia. Hence drugs which act on synapse will affect the central nervous control over the autonomic nervous system but will have no effect on the transmission of afferent impulses through sensory ganglia where there are no synapses.       

 

There are afferent and efferent nerve fibers in both sympathetic and parasympathetic systems.

 

The autonomic nervous system exerts control over the functions of many organs and tissues in the body. Along with the endocrine system, it brings about fine internal adjustments necessary for the optimal internal environment of the body.

 

The autonomic nervous system, like the somatic nervous system, has afferent, connector, and efferent neurons. The afferent impulses originate in visceral receptors and travel via afferent pathways to the central nervous system, where they are integrated through connector neurons at different levels and then leave via efferent pathways to visceral effector organs.

 

The efferent pathways of the autonomic system are made up of preganglionic and postganglionic neurons. The cell bodies of the preganglionic neurons are situated in the lateral gray column of the spinal cord and in the motor nuclei of the third, seventh, ninth, and tenth cranial nerves. The axons of these cell bodies synapse on the cell bodies of the postganglionic neurons that are collected together to form ganglia outside the central nervous system.

 

The control exerted by the autonomic system is widespread, since one preganglionic axon may synapse with several postganglionic neurons. Large collections of afferent and efferent nerve fibers and their associated ganglia form autonomic plexuses in the thorax, abdomen, and pelvis.

 

The visceral receptors include chemoreceptors, baroreceptors, and osmoreceptors. Pain receptors are present in viscera and certain types of stimuli, such as oxygen lack or stretch, may cause extreme pain.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Enteric Nervous System

 

Enteric nervous system is made up of ganglionated plexuses located in the walls of gastrointestinal tract. It is responsible for regulating contractions of muscle of alimentary tract, gastric acid secretion, intestinal transport of water and electrolytes, mucosal blood flow etc.

 

The system of neurons and their supporting cells found in the walls of GIT, including the neurons within the pancreas and gall bladder. These neurons arise from neural crest tissue that is different from that giving rise to sympathetic and parasympathetic systems.

 

The majority of innervations of the alimentary tract comes from 2 main interconnected  ganglionated plexuses, (i) the myenteric (Auerbach’s) plexus lying between the longitudinal and circular muscle layers (ii) the submucous (Meissner’s) plexus lying between the circular muscle and muscularis mucosae.

 

The myenteric plexus consists of small interconnected ganglia & extends from the esophagus to the internal anal splinter.

 

The submucous plexus extends from the stomach to internal anal splinter.

 

The total no. of nerve cells in these plexuses has been estimated between 10-100 million neurons. The gall bladder also has a ganglionated plexus and ganglia are fond in the gall bladder and pancreas.

 

Systems of nerve bundles connect ganglia run from ganglionated plexus is to form plexuses in the muscle layers, in the mucosa and around blood vessels. The plexuses are named by their location as subserous, longitudinal muscle, mucosal, perivascular and plexus of muscularis mucosae.

 

The enteric nervous system retains many functions after all central connections are cut off. The motility of the gut is transiently affected. This highlights the dominant role of intrinsic plexuses which contain complete reflex pathways consisting of enteric sensory neurons which monitor intestinal wall tension and intestinal contents; interneurons that link them together; and motor neurons which change the intestinal activity.

 

 

 

 

 

 

 

 

 

 

 

 

 

References

 

Bulygin I A

A consideration of the general principles of organization of sympathetic ganglia

J Autonomic Nervous System  8  303  1983

 

Furness J B and Costa M  

The types of nerves in the enteric nervous system  

Neuroscience  5  1-20  1980

 

Gershon M D

The enteric nervous system

Annual Review Neuroscience  4  227  1981

 

Gibbins I L  Jobling P  Messenger J P  et al

Neuronal morphology and the synaptic organization of sympathetic ganglia

J  Autonomic Nervous System  81  104  2000