Download Visceral motor Nerves

No. 34
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The Visceral Nervous System
Section 3 The Visceral Nervous
System
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Introduction:
The visceral nervous system is a part of
the whole nervous system. According to
the distribution it can be divided into the
central and peripheral part. The visceral
nerves are mainly distributed in the
viscera, cardiovascular system and
secretary glands. As the somatic nerve,
the visceral nerve also contains two
groups of fibers, i.e. sensory (afferent)
fibers and motor (efferent) fibers.
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The visceral motor nerves are also termed
the autonomic nervous system or the
vegetative nervous system because
they regulate the common involuntary
metabolic activities of the animal and
vegetable. They manipulate the
movements of the smooth muscle, cardiac
muscle and the secretion of glands.
Ⅰ. The Visceral motor Nerves
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There are some obvious differences in both
functions and structures between the visceral and
somatic motor system.
The main differences in structure are briefly
described as follows:
1) Difference in effectors
The somatic motor nerve innervates the skeletal
muscles, the visceral motor nerve innervates the
smooth muscle, cardiac muscle and glands.
The responses of the somatomotor fibers are
controlled voluntarily by consciousness, whereas
the visceral motor fibers are to a large extent
involuntary and unconscious.
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2) Difference in the fibrous components
The somatic motor nerve contains only
one kind of fiber, but visceral motor nerve
is divided into the sympathetic and
parasympathetic nerves by which most of
the visceral organs are innervated.
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3) Difference in number of neurons.
A single neuron is required to carry an impulse
from the lower nervous centers through the
somatic motor nerves to a skeletal muscle.
Whereas two neurons are required to transmit an
impulse from the lower nervous centers through
the visceral motor nerves to the active effector
organ in the viscera.
The first neurons, whose cell bodies are located
in the brain stem and spinal cord, are termed
preganglionic neurons which send out the
preganglionic fibers.
The second neurons, situated in the peripheral
nerve ganglia, are called postganglionic neurons
which give rise to postganglionic fibers.
One preganglionic neuron usually synapses with
one or more postganglionic neurons.
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4) Differenct in onstruction of fiber
The somatomotor fibers are all thick
myelinated fibers, whereas the
preganglionic fibers of visceral motor
nerve are thin myelinated fibers and the
postganglionic fibers are unmyelinated.
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5) Difference in distributed mood of
postanglionic fibers
The somatomotor nerves are distributed
peripherally in the form of nerve trunk,
however, the postganglionic fibers of the
nerves form the nerve plexuses around
the viscera or blood vessels.
The visceral motor nerves are divided into
the sympathetic and parasympathetic
divisions on the basis of their features in
function, shape and neurobiochemistry.
Ⅰ) The Sympathetic System
1. Lower level center
 The centers of the sympathetic system at
the lower level (preganglionic neurons)
are situated in the intermediolateral nuclei
of lateral gray horns of T1 (or C8)～L2 (or
L3) segments of the spinal cord. So the
preganglionic nerve fibers arise from these
nuclei. It is for this reason that the
sympathetic division is also termed the
thoracolumbar part of visceral motor
nerve.
2. Peripheral part
 The peripheral part of the sympathetic
nerve includes sympathetic trunks,
sympathetic ganglia, the branches from
the ganglia, and many sympathetic
plexuses.
(1) Sympathetic ganglia
 They are divided into paravertebral
ganglia and prevertebral ganglia.
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1) The paravertebral ganglia (ganglia of
sympathetic trunk)
They are arranged symmetrically on either side of
the spinal column and are connected by
interganglionic branches to form two
sympathetic trunks that extend from the base
of the skull to the coccyx and are divided into
cervical, thoracic, lumbar, sacral and coccygeal
portions. The cervical portion contains the
superior, middle and inferior cervical ganglia.
There are 11 or 12 thoracic, 3 or 4 lumbar and
4 or 5 sacral ganglia on each trunk. In the
sacral portion, the two trunks gradually approach
each other and fuse at the coccyx in the unpaired
ganglion impar (coccygeal ganglion). So there
are 19-24 in number on each side.
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2) The prevertebral ganglia
They are situated in front of the vertebral
column, and are irregular ganglionic
masses surrounding the visceral branches
of the abdominal aorta.
They include the celiac ganglia, the
aorticorenal ganglia, the superior and
inferior mesenteric ganglia.
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(2) The communicating branches
They link the sympathetic ganglia with the
corresponding spinal nerves and are divided into
white and gray communicating branches.
1) The white communicating branches
(sympathetic preganglionic fibers)
They are the myelinated preganglionic fibers sent
by the neurons in the intermediolateral nuclei of
the lateral horns in all thoracic and the upper two
or three lumbar segments (T1 to L3) via the
anterior roots of the spinal nerve to communicate
with the paravertebral ganglia.
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A preganglionic fiber of a white communicating
branch, after entering the sympathetic trunk,
may terminate in three ways:
①Some fibers terminate in the corresponding
paravertebral ganglia and synapse with their
neurons.
②Some fibers pass up or down in the
sympathetic trunk for a considerable distance
before terminating in the superior and inferior
paravertebral ganglia.
③The others pass through the paravertebral
ganglia and terminate in the prevertebral ganglia
via the splanchnic nerves.
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2) The gray communicating branches
(sympathetic postganglionic fibers)
They are the unmyelinated postganglionic
fibers, emitted by neurons of the
paravertebral ganglia and are situated
between the sympathetic trunk and the 31
pairs of spinal nerve.
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They also course in three ways:
①Accompany with the corresponding
spinal nerve to the peripheral blood
vessels, sweat gland and arrectores
pilorum;
②Surround the artery as a layer of nerve
plexus to innervate the organs
accompanying the corresponding arteries.
③Terminate directly in certain organs
from the paravertebral ganglia.
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3. The general distributions of the sympathetic
nerves
1) The cervical portion
The cervical sympathetic trunk consists of three
ganglia, superior, middle and inferior.
The superior cervical ganglion: largest one,
situating in front of the transverse processes of
the C2 and C3.
The middle cervical ganglion: the smallest of
the three cervical ganglia, at the level of the
transverse process of the C6.
The inferior cervical ganglion: at the level of
the C7, may be fused with the first thoracic
ganglion to form the cervicothoracic ganglion
(stellate ganglion).
The postganglionic fibers arising from the three
cervical ganglia are distributed in the following
ways.
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①They communicate with the eight cervical
nerves through the gray communicating branches
to the blood vessels, sweat glands and arrectores
pilorum of the head, neck and upper limbs, are
distributed in the following ways:
②The fibers surround the internal and external
carotid arteries to form the internal carotid
plexus, external carotid plexus, to innervate the
glands (lacrimal gland, salivary glands, mucous
glands in the oral and nasal cavities, thyroid
gland), arrectores pilorum, blood vessels, dilator
pupillae of the head and neck.
The internal carotid plexus sends a branch to the
iris to control the activity of the dilator pupillae.
③The fibers, with the cardiac branch of the
vagus nerve, comprise the cardiac plexus at the
base of the heart.
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2) The thoracic portion
The thoracic sympathetic trunk is
situated in front of the heads of the ribs
and contains a series of ganglia, 10～12 in
number on each side.
The postganglionic fibers arising from the
thoracic sympathetic trunk are distributed
in the following ways.
①The postganglionic fibers communicate
with the 12 pairs of thoracic nerves
through the gray communicating branches,
are distributed in the blood vessels, sweat
glands, arrectores pilorum of the thoracic
and abdominal walls.
②The postganglionic fibers arising from
the upper 5 paires of thoracic ganglia,
with the branches of the vagus nerve,
form the pulmonary plexus whose
filaments innervate the trachea, bronchi,
lungs and other thoracic viscera.
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③The greater splanchnic nerve is formed by
the preganglionic fibers from the 5th (6th) to the
9th (or 10th) thoracic ganglia, which terminate in
the celiac ganglion.
④The lesser splanchnic nerve is formed by
the preganglionic fibers from the ninth and tenth
thoracic ganglia, which end in the aorticorenal
ganglion.
The postganglionic fibers of the celiac and
aorticorenal ganglia, with the branches of the
vagus nerve, comprise the celiac plexus in front
of the beginning of the abdominal aorta and
surrounding the roots of the celiac and superior
mesenteric artery. The bundles and filaments of
the celiac plexus branches of the abdominal aorta
to the liver, spleen, kidney and the alimentary
tract as far as the left colonic flexure.
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3) The lumbar portion
The lumbar sympathetic trunk includes
4 or 5 sympathetic ganglia which are
situated ventrally to the bodies of the
lumbar vertebrae, along the medial
margin of the psoas major.
①Some of the postganglionic fibers of the lumbar
trunk, which are the gray communicating
branches, accompany the lumbar nerves to the
blood vessels, sweat glands and arrectores
pilorum of the anteromedial surface of the lower
limbs.
②The lumbar splanchnic nerves are composed
of the preganglionic fibers penetrating the lumbar
ganglia, terminate the prevertebral ganglia in the
abdominal aortic plexus and the inferior
mesenteric plexus. The postganglionic fibers
are distributed to the alimentary tracts below the
left colonic flexure, pelvic visceral organs and the
lower limbs.
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4) The pelvic portion
The pelvic sympathetic trunk lies
against the ventral surface of the sacrum,
medial to the sacral foramina. It is a direct
continuation of the lumbar portion and
contains 2 or 3 sacral sympathetic
ganglia and the ganglion impar.
The postganglionic fibers arising from the
pelvic sympathetic trunk are distributed in
the following ways.
①The postganglionic fibers communicate
with the thoracic and coccygeal nerves
through the gray communicating branches,
are distributed in the blood vessels, sweat
glands, arrectores pilorum of the lower
limbs and the perineum.
②The some small branches of the sacral
sympathetic ganglia and ganglion impar
join the pelvic plexus, and are supplied
through it to the pelvic viscera.
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The regularity of the distribution of the
preganglionic and postganglionic fibers.
①After the preganglionic fibers from the
intermediolateral nuclei of the 1st to 5th thoracic
segments of spinal cord interchange neurons, the
postgnglionic fibers are distributed in the visceral
organs of the head, neck, and thoracic cavity,
blood vessels, sweat glands, and arrectores
pilorum of the upper limbs.
②After the preganglionic fibers from the
intermediolateral nuclei of the 5th to 12th
thoracic segments of spinal cord interchange
neurons, the postgnglionic fibers are distributed
in the liver, spleen, and kidney, and the
alimentary tract upper the left colonic flexure.
③After the preganglionic fibers from the
intermediolateral nuclei of the 1st to 3rd
lumbar segments of spinal cord
interchange neurons, the postgnglionic
fibers are distributed in the alimentary
tracts below the left colonic flexure, pelvic
visceral organs and blood vessels, sweat
glands, and arrectores pilorum of the
lower limbs.
Ⅱ) The Parasympathetic System
 The lower level center of the
parasympathetic system:
 The centers of the parasympathetic
system at the lower level (preganglionic
neurons) are situated in certain cranial
parasympathetic nuclei and in the sacral
parasympathetic nuclei 2-4 segments of
sacral portion of the spinal cord. It is for
this reason that the parasympathetic
division is also termed the craniosacral
portion of visceral motor nerve.
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1. The cranial portion of the
parasympathetic system
The cranial outflow includes fibers in the
oculomotor, facial, glossopharyngeal and
vagus nerves.
These nerves have been described in
previous pages and the details will be
repeated here only as far as the supply to
the visceral efferent fibers.
1) The parasympathetic preganglionic
fibers in the oculomotor nerve
 They arise from neurons in the EdingerWestphal nucleus (or accessory nucleus of
oculomotor nerve) and form synapses
with the ciliary ganglion cells after
entering the orbit.
 The postganglionic fibers proceed in the
short ciliary nerves to the eyeball,
penetrate the sclera, and reach the ciliary
muscle and sphincter pupillae.
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2) The parasympathetic preganglionic fibers in
the facial nerve
They arise from the cells in the superior
salivatory nucleus.
① Certain of the preganglionic fibers pass
through the greater petrosal nerve to synapse
with the cells in the pterygopalatine ganglion.
Some of the postganglionic fibers reach the
lacrimal gland via the maxillary, zygomatic, and
lacrimal nerves successively; others accompany
the branches of the maxillary nerve to the glands
in the mucosa of the nasal cavity and palate.
② Other preganglionic fibers accompany
the chorda tympani and join the lingual
nerve to reach the submandibular
ganglion. Then, they form synapses with
the cells in the ganglion. The
postganglionic fibers form the
secretomotor supply to the submandibular
and the sublingual glands.
3) The parasympathetic preganglionic
fibers contained in the glossopharyngeal
nerve
 They arise from the inferior salivatory join
the tympanic plexus through the tympanic
nerves. The lesser petrosal nerve arises
from this plexus and form synapses in the
otic ganglion. The postganglionic fibers
join the auriculotemporal nerve, and are
distributed to the parotid gland.
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4) The parasympathetic preganglionic
fibers contained in the vagus nerve
They arise from the cells in the dorsal
nucleus of vagus in the medulla oblongata,
and run in the vagus nerve to the ganglia
which are situated in or near the
organs innervated.
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2. The sacral portion of the parasympathetic
nerve
The cells which give rise to the sacral outflow are
in the second to fourth sacral segments of the
spinal cord, and pass out with the corresponding
sacral nerves.
They leave the sacral nerve as a pelvic
splanchnic nerve and join the pelvic plexus
whose branches synapse with the scattered
ganglia in or near the walls of the pelvic organs
and of the descending and sigmoid colon and
rectum.
The postganglionic fibers are distributed to the
organs mentioned above.
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Ⅲ) The Main Differences between the
Sympathetic and Parasympathetic
Systems
1. The different lower center
The lower centers of the sympathetic
nerve are situated in the lateral horn of
the thoracolumbar segments of the spinal
cord.
The lower centers of the parasympathetic
nerve are in the brain stem and gray
matter of the 2nd -4th of sacral segments
of the spinal cord.
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2. The different locations of the peripheral ganglia
The sympathetic ganglia are divided into
paravertebral and prevertebral.
The parasympathetic ganglia are situated in or
near the walls of the innervated organs.
So, the preganglionic fibers of the
parasympathetic nerve are longer than those of
the sympathetic nerve, vice versa, the
postganglionic fibers of the parasympathetic are
shorter than those of the sympathetic.
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3. The different ratio of the preganglionic
fibers to the postganglionic fibers
A preganglionic sympathetic fiber
synapses with many more postganglionic
neurons than the parasympathetic, so its
effect is more widespread than that of the
parasympathetic.
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4. The different distributions
The peripheral distribution of the
sympathetic nerve is much wider than that
of the parasympathetic.
It is generally believed that the blood
vessels, sweat glands, arrectores pilorum
and medullary part of the suprarenal
gland are supplied only by the
sympathetic nerve.
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5. The different actions to a visceral organ
The viscera receives a dual autonomic
supply. In most cases, the two sets of
nerves function antagonistically to one
another. However, in some cases, the
action of the two divisions may not be
antagonistic.
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The sympathetic division is thrown into activity in
preparation of the organism for “flight or fight”. In
action, it tends to produce vasoconstriction of the
skin and viscera, shifting more blood to the brain
and skeletal muscles, increasing the rate of the
heart beating and respiration, the wider opening
of the pupil, the elevation of the blood pressure
and the dilatation of the bronchi.
While, the actions of parasympathetic division to
the above mentioned organs, are on the contrary.
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Ⅳ) The Autonomic Plexuses
The fibers of two divisions of the autonomic
system are combined into extensive plexuses in
the thorax, abdomen and pelvis.
1. The cardiac plexus
Superficial cardiac part and deep cardiac part.
2. pulmonary plexus
3. The celiac plexus
Secondary plexus, such as:
Hepatic plexus,
Splenic plexus,
Pancreatic plexus,
Superior and inferior mesenteric plexuses.
4. The abdominal aortic plexus
5. The hypogastric plexus
Superior hypostric plexus
Inferior hypogastric plexus (pelvic plexus)
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Ⅱ. The Visceral Sensory Nerves
They conduct the impulses from interoceptors of
the viscera to the central nervous system.
Sensory fibers, myelinated or unmyelinated, from
the thoracic, abdominal and pelvic viscera
traverse sympathetic and splanchnic nerves to
reach the sympathetic trunk.
Some of them pass uninterrupted through the
trunk and white communicating branch to their
perikarya of origin in the spinal ganglia whose
central processes enter the spinal cord and end in
the dorsal horn.
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Others are the peripheral processes of the inferior
ganglia of glossopharyngeal and vagus nerves
whose central processes accompany these two
nerves to terminate in the solitary nucleus of the
brain stem.
The afferent visceral fibers are important in the
initiation of various visceral and viscerosomatic
reflexes mediated through the spinal cord and
brain stem.
Although the viscera are insensitive to cutting,
crushing or burning, the excessive tension and
contraction of smooth muscle and certain
pathological conditions can produce visceral pain.
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The afferent impulses also give rise to
distress, nausea, hunger and other poorly
localized visceral sensations, and are
responsible for the general feeling of
internal well being.
The visceral afferent fibers have their own
features in morphology which are different
activities are at the subconscious level.
1. The number of the visceral sensory
fibers is less than that of the
somatosensory fibers and have a high
pain threshold, which may be the reason
that most of the visceral activities are at
the subconscious level.
2. The sensory fibers from a visceral
organ pass through several segments of
spinal nerves to the central centers, and a
spinal nerve may contain the sensory
fibers from several organs.
Ⅲ. The Central Centers of
Visceral Nerves
Ⅰ) The limbic lobe of the cerebrum is
closely related to the visceral activities, in
which there are representative motor
areas of respiration, gastrointestine,
bladder, blood pressure, pupil etc.
Ⅱ) The hypothalamus is believed to have
close relationship with the visceral nerve,
through which the limbic lobe regulates
the visceral activities.
Ⅲ) The brain stem and cerebellum also
play the important roles in regulation of
the visceral activities.
Ⅳ. The Referred Pain
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Although many and perhaps most of the
physiological impulses carried by visceral afferent
fibers fail to reach the level of consciousness,
pathological conditions or excessive stimulation
may bring into action of sensory fibers which
carry pain.
The central nervous system has a poorly
developed power for localizing the source of such
pain, and by some mechanism not clearly
understood, the pain may be referred to the
region innervated by somatic afferent fibers
whose central connections are the same as those
of the visceral afferents.
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For example, when patient suffers from
angina pectoris, the visceral afferent from
the heart enter the upper thoracic nerves,
and impulses traversing them may cause
pain on the medial side of the left arm,
and in the precordial region.
The patient suffering from hepatic or
cystic diseases may feel pain in the right
shoulder.
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The study of clinical cases of referred pain
has been very useful in tracing the path of
the afferent fiber from the various viscera,
and a knowledge of those paths may be of
great assistance to the diagnostician in
locating a pathological process.