Gr. Autos= self + nomos, law)
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.
is distributed throughout the central and peripheral nervous systems.
nervous system (ANS) is concerned with maintaining a constant internal
does so by making fine adjustments in certain bodily functions.
does so by controlling smooth muscle, the secretion of glands and modulation of
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
nervous system has
impulses originate in visceral receptors of cardiac muscle, smooth muscles and
[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
[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
[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
impulses travel through afferent neurons to central nervous system
CNS these impulses are integrated through connector neurons
these impulses leave CNS via efferent neurons to visceral effector organs
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.
pathways of ANS are made up of 2 neurons. The 1st neuron is located
in CNS. Its axon synapses with a 2nd multipolar neuron, located in a
ganglion of the peripheral nervous system.
axons of the 1st neuron are called preganglionic fibers
axons of the 2nd neuron to muscle or gland are called postganglionic
preganglionic neuron, through its axon, may synapse with the dendrites of many
makes the control, exerted by ANS, very rapid
chemical secreted at all preganglionic endings and parasympathetic
postganglionic endings is acetylcholine, which is released from nerve endings
by nerve impulses.
only organ that receives preganglionic fibres is the adrenal medulla and it
behaves like a ganglion, whose cells have changed into secretory cells rather
nervous system is composed of
differ in morphology and function
nervous system is concerned with emergency situation (“fight or flight” reaction)
nervous system is concerned with conservation of energy
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
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
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
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
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
There are four ganglia which are some
little distance from the structures innervated and they are ciliary, pterygopalatine,
submandibular and otic ganglia.
nervous system (ANS) has two components
- 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.
- Parasympathetic nervous system
It conserves and restores energy. It
slows the heart rate, increases peristalsis of intestines, increases glandular
activity and opens the sphincters.
system is the larger of the two parts of ANS and is widely distributed
throughout the body.
innervates heart and two lungs
innervates the smooth muscles of blood vessels and viscera in abdomen and
innervates glands of the body including all the sweat glands of skin and
arrector muscles of hair follicles
system prepares the body for an emergency
rate is increased.
- It is
interesting to note that arterioles of skin and intestines are constricted
and those of skeletal muscle are dilated.
pressure is raised.
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.
dilate, smooth muscles of bronchi are inhibited, peristalsis in intestines
decreases, and urinary bladder is relaxed.
sphincters are closed.
hair is made to stand on end and sweating occurs.
nervous system consists of
horns in spinal cord
- Rami communicantes
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
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
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
Note the postganglionic fiber entering into
anterior or ventral ramus through gray ramus.
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.
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.
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.
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
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
Every spinal nerve receives a gray
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
arrangement of sympathetic fibers in cervical region
is no gray ramus
preganglionic fibers do not synapse in the sympathetic ganglion and ascend
Superior cervical ganglion gives off
four gray rami.
Middle cervical ganglion gives off two
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.
arrangement of sympathetic fibers in lower lumbar and all sacral nerves
is no gray ramus
preganglionic fibers do not synapse in the sympathetic ganglion and descend downwards
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.
neck, the trunks lie anterior to the transverse processes of cervical vertebrae.
There are three ganglia in cervical region
middle cervical ganglion is absent
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
thorax, sympathetic trunks lie on the sides of vertebral bodies, anterior to
the heads of ribs.
are eleven or twelve ganglia in thoracic region.
are named according to their number; first thoracic ganglion, second thoracic
abdomen, the trunks are anterolateral to the sides of the bodies of lumbar
are four or five ganglia in lumbar region.
are first lumber ganglion, second lumbar ganglion and so on.
pelvis, sympathetic trunks are anterior to sacrum.
are four or five ganglia in sacral region.
are first sacral ganglion, second sacral ganglion and so on.
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
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.
preganglionic efferent sympathetic nerve fibers enter the sympathetic ganglia
via white rami communicantes
of them synapse their
ascend and some descend without synapsing
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
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
Sympathetic nervous system highlighting
greater splanchnic nerve
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
or Least splanchnic nerves
present they arise from right and left twelfth thoracic ganglia, pierce the
diaphragm to synapse with excitor neurons in the ganglia of renal plexus.
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.
are present in prevertebral or preaortic position. They are in the plexuses
that surround the origins of ventral branches of abdominal aorta.
are called prevertebral ganglia or preaortic or subsidiary ganglia or
are three sets of prevertebral ganglia
ganglia two in number present in celiac plexus that surround the origin of
mesenteric ganglion present in superior mesenteric plexus that surrounds
the origin of superior mesenteric artery.
mesenteric ganglion present in inferior mesenteric plexus that surrounds
the origin of inferior mesenteric plexus.
Structure of an autonomic ganglion
ganglia are often irregular in shape. They are the site where preganglionic
nerve fibers synapse on postganglionic neurons.
are situated along the course of efferent nerve fibers of the autonomic nervous
ganglia are part of sympathetic trunk and they are present around the roots of ventral
branches of abdominal aorta.
ganglia, on the other hand, are situated close to or within the walls of the
of autonomic ganglia resemble sensory ganglia in having a similar connective
tissue capsule and framework.
unlike sensory ganglia, autonomic ganglia contain synapses.
autonomic ganglion consists of a collection of multipolar neurons. There are
numerous branched dendrites and an axon which forms unmyelinated postganglionic
visceral efferent fibers.
larger ganglia, each neuron cell is surrounded by a layer of satellite cells as
in spinal sensory ganglia.
bundles are attached to these ganglia consist of
nerve fibers that enter the ganglion,
nerve fibers that are leaving the ganglion,
and efferent nerve fibers that pass through the ganglion without
preganglionic fibers are myelinated. The postganglionic fibers are unmyelinated.
fibers are much more numerous than preganglionic fibers
axons may synapse with many postganglionic neurons for wide dissemination and
amplification of sympathetic activity
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
Preganglionic and postganglionic fibers
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.
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.
Because there is
no sympathetic outflow from the cervical part of the cord, nor from the lower
lumbar and sacral parts, those preganglionic fibres which are destined to
synapse with the cell bodies whose fibers are going to run with cervical nerves
must ascend in the sympathetic trunk to cervical ganglia, and those for lower
lumbar and sacral nerves must descend in the trunk to lower lumbar and sacral
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
Postganglionic fibers arise from the
excitor cells in the peripheral plexuses and are distributed to the smooth
muscle and glands of the viscera.
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
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
Mediastinum is the
central part of thorax between the two lungs.
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).
Do not confuse
sacral splanchnic nerves which are sympathetic with pelvic splanchnic nerves
(S2, 3, 4), which are parasympathetic.
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
sympathetic nerve fibers
are myelinated nerve fibers. They do not have their cell bodies in
sympathetic ganglia. They
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
A part of sympathetic system shown
Look at the black colored afferent neuron
between stomach and lateral horn of spinal cord
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
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
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
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.
system also widely distributed throughout the body but it is much smaller than
sympathetic nervous system
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
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.
nervous system consists of
nerve nuclei in brainstem
matter in sacral segments of spinal cord
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
connector nerve cells located in brainstem form nuclei in the following cranial
parasympathetic nucleus or Edinger-Westphal nucleus
is situated posterior to the main oculomotor nucleus
nucleus and lacrimatory nucleus
Dorsal nucleus of vagus
axons of these connector nerve cells are myelinated and emerge from brainstem along
the cranial nerves
Gray matter in sacral segments of spinal cord
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
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 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
cranial parasympathetic ganglia are four:
- 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.
- 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.
- Submandibular ganglion. It hangs suspended from
the lingual nerve on the surface of hyoglosus muscle.
- Otic ganglion. It is a small body lying between
the tensor veli palatini and the mandibular nerve, just below the foramen
These four ganglia are very similar in
plan. Each has parasympathetic, sympathetic and sensory roots, and branches of
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
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
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
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
sacral parasympathetic ganglia are located close to the viscera they innervate.
- Cardiac plexus,
- Pulmonary plexus
- The plexuses
associated with gastrointestinal tract are Myenteric plexus (Auerbach’s plexus) and Mucosal plexus (Meissner’s
- Hypogastric plexuses
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
Preganglionic fibers coming from
superior salivatory nucleus go to pterygopalatine
ganglion and submandibular ganglion. They synapse in these
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.
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.
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.
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
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
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.
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.
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
Parasympathetic afferent fibers
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.
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.
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.
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
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
are afferent and efferent nerve fibers in both sympathetic and parasympathetic
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
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.
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.
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.
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
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
The myenteric plexus consists of small
interconnected ganglia & extends from the esophagus to the internal anal
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
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
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
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