No. 34No. 34

The Visceral Nervous SystemThe Visceral Nervous System

Section 3 The Visceral Nervous Section 3 The Visceral Nervous SystemSystem

IntroductionIntroduction:: The The visceralvisceral nervous systemnervous system is a part of is a part of

the whole nervous system. According to the whole nervous system. According to the distribution it can be divided into the the distribution it can be divided into the central and peripheral part. The visceral central and peripheral part. The visceral nerves are mainly distributed in the nerves are mainly distributed in the viscera, cardiovascular system and viscera, cardiovascular system and secretary glands. As the somatic nerve, secretary glands. As the somatic nerve, the visceral nerve also contains two the visceral nerve also contains two groups of fibers, i.e. sensory (afferent) groups of fibers, i.e. sensory (afferent) fibers and motor (efferent) fibers.fibers and motor (efferent) fibers.

The visceral motor nerves are also termed The visceral motor nerves are also termed the the autonomic nervous systemautonomic nervous system or the or the vegetative nervous systemvegetative nervous system because because they regulate the common involuntary they regulate the common involuntary metabolic activities of the animal and metabolic activities of the animal and vegetable. They manipulate the vegetable. They manipulate the movements of the smooth muscle, cardiac movements of the smooth muscle, cardiac muscle and the secretion of glands.muscle and the secretion of glands.

ⅠⅠ. The . The Visceral motorVisceral motor NervesNerves There are some obvious differences in both functions There are some obvious differences in both functions

and structures between the visceral and somatic motand structures between the visceral and somatic motor system.or system.

The main differences in structure are briefly described The main differences in structure are briefly described as follows:as follows:

1) Difference in effectors1) Difference in effectors The somatic motor nerve innervates the skeletal muscThe somatic motor nerve innervates the skeletal musc

les, the visceral motor nerve innervates the smooth mles, the visceral motor nerve innervates the smooth muscle, cardiac muscle and glands.uscle, cardiac muscle and glands.

The responses of the somatomotor fibers are controllThe responses of the somatomotor fibers are controlled voluntarily by consciousness, whereas the visceral ed voluntarily by consciousness, whereas the visceral motor fibers are to a large extent involuntary and uncmotor fibers are to a large extent involuntary and unconscious.onscious.

2) Difference in the fibrous components2) Difference in the fibrous components The somatic motor nerve contains only The somatic motor nerve contains only

one kind of fiber, but visceral motor nerve one kind of fiber, but visceral motor nerve is divided into the sympathetic and is divided into the sympathetic and parasympathetic nerves by which most of parasympathetic nerves by which most of the visceral organs are innervated.the visceral organs are innervated.

3) Difference in number of neurons.3) Difference in number of neurons. A single neuron is required to carry an impulse from thA single neuron is required to carry an impulse from th

e lower nervous centers through the somatic motor ne lower nervous centers through the somatic motor nerves to a skeletal muscle.erves to a skeletal muscle.

Whereas two neurons are required to transmit an impWhereas two neurons are required to transmit an impulse from the lower nervous centers through the visceulse from the lower nervous centers through the visceral motor nerves to the active effector organ in the visral motor nerves to the active effector organ in the viscera.cera.

The first neurons, whose cell bodies are located in the The first neurons, whose cell bodies are located in the brain stem and spinal cord, are termed preganglionic brain stem and spinal cord, are termed preganglionic neurons which send out the preganglionic fibers.neurons which send out the preganglionic fibers.

The second neurons, situated in the peripheral nerve The second neurons, situated in the peripheral nerve ganglia, are called postganglionic neurons which give ganglia, are called postganglionic neurons which give rise to postganglionic fibers.rise to postganglionic fibers.

One preganglionic neuron usually synapses with one One preganglionic neuron usually synapses with one or more postganglionic neurons.or more postganglionic neurons.

4) Differenct in onstruction of fiber4) Differenct in onstruction of fiber The somatomotor fibers are all thick myelinatThe somatomotor fibers are all thick myelinat

ed fibers, whereas the preganglionic fibers of ved fibers, whereas the preganglionic fibers of visceral motor nerve are thin myelinated fibers isceral motor nerve are thin myelinated fibers and the postganglionic fibers are unmyelinateand the postganglionic fibers are unmyelinated.d.

5) Difference in distributed mood of postanglio5) Difference in distributed mood of postanglionic fibersnic fibers

The somatomotor nerves are distributed peripThe somatomotor nerves are distributed peripherally in the form of nerve trunk, however, thherally in the form of nerve trunk, however, the postganglionic fibers of the nerves form the e postganglionic fibers of the nerves form the nerve plexuses around the viscera or blood vesnerve plexuses around the viscera or blood vessels.sels.

The visceral motor nerves are divided into the The visceral motor nerves are divided into the sympathetic and parasympathetic divisions osympathetic and parasympathetic divisions on the basis of their features in function, shape n the basis of their features in function, shape and neurobiochemistry.and neurobiochemistry.

Ⅰ Ⅰ) The ) The Sympathetic SystemSympathetic System 1. Lower level center1. Lower level center The centers of the sympathetic system at the lThe centers of the sympathetic system at the l

ower level (preganglionic neurons) are situateower level (preganglionic neurons) are situated in the intermediolateral nuclei of lateral gray d in the intermediolateral nuclei of lateral gray horns of T1 (or C8)horns of T1 (or C8) ～～ L2 (or L3) segments of thL2 (or L3) segments of the spinal cord. So the preganglionic nerve fibere spinal cord. So the preganglionic nerve fibers arise from these nuclei. It is for this reason ths arise from these nuclei. It is for this reason that the sympathetic division is also termed the at the sympathetic division is also termed the thoracolumbar part of visceral motor nervethoracolumbar part of visceral motor nerve..

2. Peripheral part2. Peripheral part The peripheral part of the sympathetic nerve iThe peripheral part of the sympathetic nerve i

ncludes sympathetic trunks, sympathetic ganncludes sympathetic trunks, sympathetic ganglia, the branches from the ganglia, and many glia, the branches from the ganglia, and many sympathetic plexuses.sympathetic plexuses.

(1) (1) Sympathetic gangliaSympathetic ganglia They are divided into paravertebral ganglia anThey are divided into paravertebral ganglia an

d prevertebral ganglia.d prevertebral ganglia.

1) The 1) The paravertebral gangliaparavertebral ganglia ( (ganglia of sympathetic ganglia of sympathetic trunktrunk))

They are arranged symmetrically on either side of the They are arranged symmetrically on either side of the spinal column and are connected by spinal column and are connected by interganglionic interganglionic branchesbranches to form two to form two sympathetic trunkssympathetic trunks that exten that extend from the base of the skull to the coccyx and are dividd from the base of the skull to the coccyx and are divided into cervical, thoracic, lumbar, sacral and coccygeaed into cervical, thoracic, lumbar, sacral and coccygeal portions. The cervical portion contains the l portions. The cervical portion contains the superiorsuperior, , middlemiddle and and inferior cervical gangliainferior cervical ganglia. There are 11 or . There are 11 or 12 12 thoracicthoracic, 3 or 4, 3 or 4 lumbarlumbar and 4 or 5 and 4 or 5 sacral gangliasacral ganglia o on each trunk. In the sacral portion, the two trunks gran each trunk. In the sacral portion, the two trunks gradually approach each other and fuse at the coccyx in tdually approach each other and fuse at the coccyx in the unpaired ganglion impar (coccygeal ganglion). So he unpaired ganglion impar (coccygeal ganglion). So there are 19-24 in number on each side.there are 19-24 in number on each side.

2) The 2) The prevertebral gangliaprevertebral ganglia They are situated in front of the vertebral coluThey are situated in front of the vertebral colu

mn, and are irregular ganglionic masses surromn, and are irregular ganglionic masses surrounding the visceral branches of the abdominal unding the visceral branches of the abdominal aorta.aorta.

They include the They include the celiac gangliaceliac ganglia, the , the aorticoreaorticorenal ganglianal ganglia, the , the superiorsuperior and and inferior mesentinferior mesenteric gangliaeric ganglia..

(2) The (2) The communicating branchescommunicating branches They link the sympathetic ganglia with the corresponThey link the sympathetic ganglia with the correspon

ding spinal nerves and are divided into white and gray ding spinal nerves and are divided into white and gray communicating branches.communicating branches.

1) The 1) The white communicating brancheswhite communicating branches (sympathetic (sympathetic preganglionic fibers)preganglionic fibers)

They are the myelinated preganglionic fibers sent by tThey are the myelinated preganglionic fibers sent by the neurons in the intermediolateral nuclei of the laterhe neurons in the intermediolateral nuclei of the lateral horns in all thoracic and the upper two or three lumal horns in all thoracic and the upper two or three lumbar segments (T1 to L3) via the anterior roots of the spbar segments (T1 to L3) via the anterior roots of the spinal nerve to communicate with the paravertebral ganinal nerve to communicate with the paravertebral ganglia.glia.

A preganglionic fiber of a white communicating brancA preganglionic fiber of a white communicating branch, after entering the sympathetic trunk, may terminath, after entering the sympathetic trunk, may terminate in three ways:e in three ways:

① ①Some fibers terminate in the corresponding paravertSome fibers terminate in the corresponding paravertebral ganglia and synapse with their neurons.ebral ganglia and synapse with their neurons.

② ②Some fibers pass up or down in the sympathetic trunSome fibers pass up or down in the sympathetic trunk for a considerable distance before terminating in the k for a considerable distance before terminating in the superior and inferior paravertebral ganglia.superior and inferior paravertebral ganglia.

③ ③The others pass through the paravertebral ganglia aThe others pass through the paravertebral ganglia and terminate in the prevertebral ganglia via the nd terminate in the prevertebral ganglia via the splansplanchnic nerveschnic nerves..

2) The 2) The gray communicating branchesgray communicating branches (sympat (sympathetic postganglionic fibers)hetic postganglionic fibers)

They are the unmyelinated postganglionic fibeThey are the unmyelinated postganglionic fibers, emitted by neurons of the paravertebral gars, emitted by neurons of the paravertebral ganglia and are situated between the sympathetinglia and are situated between the sympathetic trunk and the 31 pairs of spinal nerve.c trunk and the 31 pairs of spinal nerve.

They also course in three ways:They also course in three ways: ① ①Accompany with the corresponding spinal neAccompany with the corresponding spinal ne

rve to the peripheral blood vessels, sweat glanrve to the peripheral blood vessels, sweat gland and arrectores pilorum;d and arrectores pilorum;

② ②Surround the artery as a layer of nerve plexus Surround the artery as a layer of nerve plexus to innervate the organs accompanying the corto innervate the organs accompanying the corresponding arteries.responding arteries.

③ ③Terminate directly in certain organs from the Terminate directly in certain organs from the paravertebral ganglia.paravertebral ganglia.

3. The general distributions of the sympathetic nerves3. The general distributions of the sympathetic nerves 1) The 1) The cervical portioncervical portion The cervical sympathetic trunk consists of three ganglThe cervical sympathetic trunk consists of three gangl

ia, superior, middle and inferior.ia, superior, middle and inferior. The The superior cervical ganglionsuperior cervical ganglion: largest one, situating : largest one, situating

in front of the transverse processes of the C2 and C3.in front of the transverse processes of the C2 and C3. The The middle cervical ganglionmiddle cervical ganglion: the smallest of the thr: the smallest of the thr

ee cervical ganglia, at the level of the transverse proceee cervical ganglia, at the level of the transverse process of the C6.ss of the C6.

The The inferior cervical ganglioninferior cervical ganglion: at the level of the C7, : at the level of the C7, may be fused with the first thoracic ganglion to form tmay be fused with the first thoracic ganglion to form the he cervicothoracic ganglioncervicothoracic ganglion ( (stellate ganglionstellate ganglion).).

The postganglionic fibers arising from the three cervicThe postganglionic fibers arising from the three cervical ganglia are distributed in the following ways.al ganglia are distributed in the following ways.

①①They communicate with the eight cervical nerves thrThey communicate with the eight cervical nerves through the gray communicating branches to the blood vough the gray communicating branches to the blood vessels, sweat glands and arrectores pilorum of the heaessels, sweat glands and arrectores pilorum of the head, neck and upper limbs, are distributed in the followid, neck and upper limbs, are distributed in the following ways:ng ways:

② ②The fibers surround the internal and external carotid The fibers surround the internal and external carotid arteries to form the internal carotid plexus, external carteries to form the internal carotid plexus, external carotid plexus, to innervate the glands (lacrimal gland, arotid plexus, to innervate the glands (lacrimal gland, salivary glands, mucous glands in the oral and nasal csalivary glands, mucous glands in the oral and nasal cavities, thyroid gland), arrectores pilorum, blood vessavities, thyroid gland), arrectores pilorum, blood vessels, dilator pupillae of the head and neck.els, dilator pupillae of the head and neck.

The internal carotid plexus sends a branch to the iris tThe internal carotid plexus sends a branch to the iris to control the activity of the dilator pupillae.o control the activity of the dilator pupillae.

③ ③The fibers, with the cardiac branch of the vagus nervThe fibers, with the cardiac branch of the vagus nerve, comprise the cardiac plexus at the base of the heart.e, comprise the cardiac plexus at the base of the heart.

2) The 2) The thoracic portionthoracic portion The The thoracic sympathetic trunkthoracic sympathetic trunk is situated in is situated in

front of the heads of the ribs and contains a sefront of the heads of the ribs and contains a series of ganglia, 10ries of ganglia, 10 ～～ 12 in number on each sid12 in number on each side.e.

The postganglionic fibers arising from the thorThe postganglionic fibers arising from the thoracic sympathetic trunk are distributed in the facic sympathetic trunk are distributed in the following ways.ollowing ways.

① ①The postganglionic fibers communicate with The postganglionic fibers communicate with the 12 pairs of thoracic nerves through the grathe 12 pairs of thoracic nerves through the gray communicating branches, are distributed in y communicating branches, are distributed in the blood vessels, sweat glands, arrectores pilthe blood vessels, sweat glands, arrectores pilorum of the thoracic and abdominal walls.orum of the thoracic and abdominal walls.

② ②The postganglionic fibers arising from the upThe postganglionic fibers arising from the upper 5 paires of thoracic ganglia, with the brancper 5 paires of thoracic ganglia, with the branches of the vagus nerve, form the hes of the vagus nerve, form the pulmonary plpulmonary plexusexus whose filaments innervate the trachea, b whose filaments innervate the trachea, bronchi, lungs and other thoracic viscera.ronchi, lungs and other thoracic viscera.

③③The The greater splanchnic nervegreater splanchnic nerve is formed by the preg is formed by the preganglionic fibers from the 5th (6th) to the 9th (or 10th) tanglionic fibers from the 5th (6th) to the 9th (or 10th) thoracic ganglia, which terminate in the celiac gangliohoracic ganglia, which terminate in the celiac ganglion.n.

④ ④The The lesser splanchnic nervelesser splanchnic nerve is formed by the prega is formed by the preganglionic fibers from the ninth and tenth thoracic ganglnglionic fibers from the ninth and tenth thoracic ganglia, which end in the aorticorenal ganglion.ia, which end in the aorticorenal ganglion.

The postganglionic fibers of the celiac and aorticorenThe postganglionic fibers of the celiac and aorticorenal ganglia, with the branches of the vagus nerve, compal ganglia, with the branches of the vagus nerve, comprise the celiac plexus in front of the beginning of the arise the celiac plexus in front of the beginning of the abdominal aorta and surrounding the roots of the celiabdominal aorta and surrounding the roots of the celiac and superior mesenteric artery. The bundles and filac and superior mesenteric artery. The bundles and filaments of the celiac plexus branches of the abdominal ments of the celiac plexus branches of the abdominal aorta to the liver, spleen, kidney and the alimentary traorta to the liver, spleen, kidney and the alimentary tract as far as the left colonic flexure.act as far as the left colonic flexure.

3) The 3) The lumbar portionlumbar portion TheThe lumbar sympathetic trunklumbar sympathetic trunk includes 4 or includes 4 or

5 sympathetic ganglia which are situated ventr5 sympathetic ganglia which are situated ventrally to the bodies of the lumbar vertebrae, aloally to the bodies of the lumbar vertebrae, along the medial margin of the psoas major.ng the medial margin of the psoas major.

① ①Some of the postganglionic fibers of the lumbar trunSome of the postganglionic fibers of the lumbar trunk, which are the gray communicating branches, accok, which are the gray communicating branches, accompany the lumbar nerves to the blood vessels, sweat mpany the lumbar nerves to the blood vessels, sweat glands and arrectores pilorum of the anteromedial surglands and arrectores pilorum of the anteromedial surface of the lower limbs.face of the lower limbs.

② ②The The lumbar splanchnic nerveslumbar splanchnic nerves are composed of the are composed of the preganglionic fibers penetrating the lumbar ganglia, tpreganglionic fibers penetrating the lumbar ganglia, terminate the prevertebral ganglia in the erminate the prevertebral ganglia in the abdominal aabdominal aortic plexusortic plexus and the and the inferior mesenteric plexusinferior mesenteric plexus. The . The postganglionic fibers are distributed to the alimentary postganglionic fibers are distributed to the alimentary tracts below the left colonic flexure, pelvic visceral orgtracts below the left colonic flexure, pelvic visceral organs and the lower limbs.ans and the lower limbs.

4) The 4) The pelvic portionpelvic portion The The pelvic sympathetic trunkpelvic sympathetic trunk lies against the lies against the

ventral surface of the sacrum, medial to the saventral surface of the sacrum, medial to the sacral foramina. It is a direct continuation of the cral foramina. It is a direct continuation of the lumbar portion and contains 2 or 3 lumbar portion and contains 2 or 3 sacral symsacral sympathetic gangliapathetic ganglia and the and the ganglion imparganglion impar..

The postganglionic fibers arising from the pelvThe postganglionic fibers arising from the pelvic sympathetic trunk are distributed in the follic sympathetic trunk are distributed in the following ways.owing ways.

① ①The postganglionic fibers communicate with The postganglionic fibers communicate with the thoracic and coccygeal nerves through the the thoracic and coccygeal nerves through the gray communicating branches, are distributed gray communicating branches, are distributed in the blood vessels, sweat glands, arrectores in the blood vessels, sweat glands, arrectores pilorum of the lower limbs and the perineum.pilorum of the lower limbs and the perineum.

② ②The some small branches of the sacral sympThe some small branches of the sacral sympathetic ganglia and ganglion impar join the athetic ganglia and ganglion impar join the pepelvic plexuslvic plexus, and are supplied through it to the , and are supplied through it to the pelvic viscera.pelvic viscera.

The regularity of the distribution of the preganglionic The regularity of the distribution of the preganglionic and postganglionic fibers.and postganglionic fibers.

① ①After the preganglionic fibers from the intermediolatAfter the preganglionic fibers from the intermediolateral nuclei of the 1st to 5th thoracic segments of spinaeral nuclei of the 1st to 5th thoracic segments of spinal cord interchange neurons, the postgnglionic fibers arl cord interchange neurons, the postgnglionic fibers are distributed in the visceral organs of the head, neck, e distributed in the visceral organs of the head, neck, and thoracic cavity, blood vessels, sweat glands, and and thoracic cavity, blood vessels, sweat glands, and arrectores pilorum of the upper limbs.arrectores pilorum of the upper limbs.

② ②After the preganglionic fibers from the intermediolatAfter the preganglionic fibers from the intermediolateral nuclei of the 5th to 12th thoracic segments of spineral nuclei of the 5th to 12th thoracic segments of spinal cord interchange neurons, the postgnglionic fibers al cord interchange neurons, the postgnglionic fibers are distributed in the liver, spleen, and kidney, and thare distributed in the liver, spleen, and kidney, and the alimentary tract upper the left colonic flexure.e alimentary tract upper the left colonic flexure.

③③After the preganglionic fibers from the intermAfter the preganglionic fibers from the intermediolateral nuclei of the 1st to 3rd lumbar segediolateral nuclei of the 1st to 3rd lumbar segments of spinal cord interchange neurons, the ments of spinal cord interchange neurons, the postgnglionic fibers are distributed in the alimpostgnglionic fibers are distributed in the alimentary tracts below the left colonic flexure, pelentary tracts below the left colonic flexure, pelvic visceral organs and blood vessels, sweat glvic visceral organs and blood vessels, sweat glands, and arrectores pilorum of the lower limbands, and arrectores pilorum of the lower limbs.s.

Ⅱ Ⅱ) The ) The Parasympathetic SystemParasympathetic System The lower level center of the parasympathetic The lower level center of the parasympathetic

system:system: The centers of the parasympathetic system at The centers of the parasympathetic system at

the lower level (preganglionic neurons) are sitthe lower level (preganglionic neurons) are situated in certain cranial parasympathetic nucleuated in certain cranial parasympathetic nuclei and in the sacral parasympathetic nuclei 2-4 i and in the sacral parasympathetic nuclei 2-4 segments of sacral portion of the spinal cord. Isegments of sacral portion of the spinal cord. It is for this reason that the parasympathetic dit is for this reason that the parasympathetic division is also termed the vision is also termed the craniosacral portion craniosacral portion of visceral motor nerveof visceral motor nerve..

1. The 1. The cranial portion of the parasympathetic cranial portion of the parasympathetic systemsystem

The cranial outflow includes fibers in the oculoThe cranial outflow includes fibers in the oculomotor, facial, glossopharyngeal and vagus nermotor, facial, glossopharyngeal and vagus nerves.ves.

These nerves have been described in previous These nerves have been described in previous pages and the details will be repeated here onlpages and the details will be repeated here only as far as the supply to the visceral efferent fiy as far as the supply to the visceral efferent fibers.bers.

1) The parasympathetic preganglionic fibers in t1) The parasympathetic preganglionic fibers in the oculomotor nervehe oculomotor nerve

They arise from neurons in the Edinger-WestpThey arise from neurons in the Edinger-Westphal nucleus (or accessory nucleus of oculomothal nucleus (or accessory nucleus of oculomotor nerve) and form synapses with the or nerve) and form synapses with the ciliary gciliary ganglionanglion cells after entering the orbit. cells after entering the orbit.

The postganglionic fibers proceed in the short The postganglionic fibers proceed in the short ciliary nerves to the eyeball, penetrate the scleciliary nerves to the eyeball, penetrate the sclera, and reach the ciliary muscle and sphincter ra, and reach the ciliary muscle and sphincter pupillae.pupillae.

2) The parasympathetic preganglionic fibers in the faci2) The parasympathetic preganglionic fibers in the facial nerveal nerve

They arise from the cells in the superior salivatory nucThey arise from the cells in the superior salivatory nucleus.leus.

① ① Certain of the preganglionic fibers pass through the Certain of the preganglionic fibers pass through the greater petrosal nerve to synapse with the cells in the greater petrosal nerve to synapse with the cells in the pterygopalatine ganglionpterygopalatine ganglion..

Some of the postganglionic fibers reach the lacrimal gSome of the postganglionic fibers reach the lacrimal gland via the maxillary, zygomatic, and lacrimal nerves land via the maxillary, zygomatic, and lacrimal nerves successively; others accompany the branches of the msuccessively; others accompany the branches of the maxillary nerve to the glands in the mucosa of the nasal axillary nerve to the glands in the mucosa of the nasal cavity and palate.cavity and palate.

② ② Other preganglionic fibers accompany the chOther preganglionic fibers accompany the chorda tympani and join the lingual nerve to reaorda tympani and join the lingual nerve to reach the ch the submandibular ganglionsubmandibular ganglion. Then, they f. Then, they form synapses with the cells in the ganglion. Thorm synapses with the cells in the ganglion. The postganglionic fibers form the secretomotor e postganglionic fibers form the secretomotor supply to the submandibular and the sublingusupply to the submandibular and the sublingual glands. al glands.

3) The parasympathetic preganglionic fibers co3) The parasympathetic preganglionic fibers contained in the glossopharyngeal nerventained in the glossopharyngeal nerve

They arise from the inferior salivatory join the They arise from the inferior salivatory join the tympanic plexus through the tympanic nerves. tympanic plexus through the tympanic nerves. The lesser petrosal nerve arises from this plexThe lesser petrosal nerve arises from this plexus and form synapses in the us and form synapses in the otic ganglionotic ganglion. Th. The postganglionic fibers join the auriculotempoe postganglionic fibers join the auriculotemporal nerve, and are distributed to the parotid glral nerve, and are distributed to the parotid gland.and.

4) The parasympathetic preganglionic fibers co4) The parasympathetic preganglionic fibers contained in the vagus nerventained in the vagus nerve

They arise from the cells in the dorsal nucleus They arise from the cells in the dorsal nucleus of vagus in the medulla oblongata, and run in tof vagus in the medulla oblongata, and run in the vagus nerve to the he vagus nerve to the ganglia which are situaganglia which are situated in or near the organs innervatedted in or near the organs innervated..

2. The 2. The sacral portion of the parasympathetic nervesacral portion of the parasympathetic nerve The cells which give rise to the sacral outflow are in thThe cells which give rise to the sacral outflow are in th

e second to fourth sacral segments of the spinal cord, e second to fourth sacral segments of the spinal cord, and pass out with the corresponding sacral nerves.and pass out with the corresponding sacral nerves.

They leave the sacral nerve as a They leave the sacral nerve as a pelvic splanchnic nerpelvic splanchnic nerveve and join the pelvic plexus whose branches synapse and join the pelvic plexus whose branches synapse with the scattered ganglia in or near the walls of the pwith the scattered ganglia in or near the walls of the pelvic organs and of the descending and sigmoid colon elvic organs and of the descending and sigmoid colon and rectum.and rectum.

The postganglionic fibers are distributed to the organThe postganglionic fibers are distributed to the organs mentioned above.s mentioned above.

Ⅲ Ⅲ) The Main Differences between the Sympath) The Main Differences between the Sympathetic and Parasympathetic Systemsetic and Parasympathetic Systems

1. The different lower center1. The different lower center The lower centers of the sympathetic nerve arThe lower centers of the sympathetic nerve ar

e situated in the lateral horn of the thoracolue situated in the lateral horn of the thoracolumbar segments of the spinal cord.mbar segments of the spinal cord.

The lower centers of the parasympathetic nervThe lower centers of the parasympathetic nerve are in the brain stem and gray matter of the e are in the brain stem and gray matter of the 2nd -4th of sacral segments of the spinal cord.2nd -4th of sacral segments of the spinal cord.

2. The different locations of the peripheral ganglia2. The different locations of the peripheral ganglia The sympathetic ganglia are divided into paravertebrThe sympathetic ganglia are divided into paravertebr

al and prevertebral.al and prevertebral. The parasympathetic ganglia are situated in or near thThe parasympathetic ganglia are situated in or near th

e walls of the innervated organs.e walls of the innervated organs. So, the preganglionic fibers of the parasympathetic neSo, the preganglionic fibers of the parasympathetic ne

rve are longer than those of the sympathetic nerve, vicrve are longer than those of the sympathetic nerve, vice versa, the postganglionic fibers of the parasympathee versa, the postganglionic fibers of the parasympathetic are shorter than those of the sympathetic.tic are shorter than those of the sympathetic.

3. The different ratio of the preganglionic fibers 3. The different ratio of the preganglionic fibers to the postganglionic fibersto the postganglionic fibers

A preganglionic sympathetic fiber synapses wiA preganglionic sympathetic fiber synapses with many more postganglionic neurons than thth many more postganglionic neurons than the parasympathetic, so its effect is more widespe parasympathetic, so its effect is more widespread than that of the parasympathetic.read than that of the parasympathetic.

4. The different distributions4. The different distributions The peripheral distribution of the sympathetic The peripheral distribution of the sympathetic

nerve is much wider than that of the parasympnerve is much wider than that of the parasympathetic.athetic.

It is generally believed that the blood vessels, It is generally believed that the blood vessels, sweat glands, arrectores pilorum and medullasweat glands, arrectores pilorum and medullary part of the suprarenal gland are supplied onry part of the suprarenal gland are supplied only by the sympathetic nerve.ly by the sympathetic nerve.

5. The different actions to a visceral organ5. The different actions to a visceral organ The viscera receives a dual autonomic The viscera receives a dual autonomic

supply. In most cases, the two sets of supply. In most cases, the two sets of nerves function antagonistically to one nerves function antagonistically to one another. However, in some cases, the another. However, in some cases, the action of the two divisions may not be action of the two divisions may not be antagonistic.antagonistic.

The sympathetic division is thrown into activity in The sympathetic division is thrown into activity in preparation of the organism for “flight or fight”. In preparation of the organism for “flight or fight”. In action, it tends to produce vasoconstriction of the action, it tends to produce vasoconstriction of the skin and viscera, shifting more blood to the brain skin and viscera, shifting more blood to the brain and skeletal muscles, increasing the rate of the and skeletal muscles, increasing the rate of the heart beating and respiration, the wider opening heart beating and respiration, the wider opening of the pupil, the elevation of the blood pressure of the pupil, the elevation of the blood pressure and the dilatation of the bronchi.and the dilatation of the bronchi.

While, the actions of parasympathetic division to While, the actions of parasympathetic division to the above mentioned organs, are on the contrary.the above mentioned organs, are on the contrary.

ⅣⅣ) The Autonomic Plexuses) The Autonomic Plexuses The fibers of two divisions of the autonomic system are combined intThe fibers of two divisions of the autonomic system are combined int

o extensive plexuses in the thorax, abdomen and pelvis.o extensive plexuses in the thorax, abdomen and pelvis. 1. The cardiac plexus1. The cardiac plexus Superficial cardiac part and deep cardiac part.Superficial cardiac part and deep cardiac part. 2. pulmonary plexus2. pulmonary plexus 3. The celiac plexus3. The celiac plexus Secondary plexus, such as:Secondary plexus, such as: Hepatic plexus,Hepatic plexus, Splenic plexus,Splenic plexus, Pancreatic plexus,Pancreatic plexus, Superior and inferior mesenteric plexuses.Superior and inferior mesenteric plexuses. 4. The abdominal aortic plexus4. The abdominal aortic plexus 5. The hypogastric plexus5. The hypogastric plexus Superior hypostric plexusSuperior hypostric plexus Inferior hypogastric plexus (pelvic plexus)Inferior hypogastric plexus (pelvic plexus)

Ⅱ Ⅱ. The . The Visceral SensoryVisceral Sensory NervesNerves They conduct the impulses from interoceptors of the vThey conduct the impulses from interoceptors of the v

iscera to the central nervous system.iscera to the central nervous system. Sensory fibers, myelinated or unmyelinated, from the Sensory fibers, myelinated or unmyelinated, from the

thoracic, abdominal and pelvic viscera traverse sympthoracic, abdominal and pelvic viscera traverse sympathetic and splanchnic nerves to reach the sympathetiathetic and splanchnic nerves to reach the sympathetic trunk.c trunk.

Some of them pass uninterrupted through the trunk aSome of them pass uninterrupted through the trunk and white communicating branch to their perikarya of nd white communicating branch to their perikarya of origin in the spinal ganglia whose central processes eorigin in the spinal ganglia whose central processes enter the spinal cord and end in the dorsal horn.nter the spinal cord and end in the dorsal horn.

Others are the peripheral processes of the inferior ganOthers are the peripheral processes of the inferior ganglia of glossopharyngeal and vagus nerves whose centglia of glossopharyngeal and vagus nerves whose central processes accompany these two nerves to terminaral processes accompany these two nerves to terminate in the solitary nucleus of the brain stem.te in the solitary nucleus of the brain stem.

The afferent visceral fibers are important in the initiatiThe afferent visceral fibers are important in the initiation of various visceral and viscerosomatic reflexes meon of various visceral and viscerosomatic reflexes mediated through the spinal cord and brain stem.diated through the spinal cord and brain stem.

Although the viscera are insensitive to cutting, crushinAlthough the viscera are insensitive to cutting, crushing or burning, the excessive tension and contraction of g or burning, the excessive tension and contraction of smooth muscle and certain pathological conditions casmooth muscle and certain pathological conditions can produce visceral pain.n produce visceral pain.

The afferent impulses also give rise to The afferent impulses also give rise to distress, nausea, hunger and other poorly distress, nausea, hunger and other poorly localized visceral sensations, and are localized visceral sensations, and are responsible for the general feeling of responsible for the general feeling of internal well being.internal well being.

The visceral afferent fibers have their own The visceral afferent fibers have their own features in morphology which are different features in morphology which are different activities are at the subconscious level.activities are at the subconscious level.

1. The number of the visceral sensory fibers is l1. The number of the visceral sensory fibers is less than that of the somatosensory fibers and ess than that of the somatosensory fibers and have a high pain threshold, which may be the rhave a high pain threshold, which may be the reason that most of the visceral activities are at eason that most of the visceral activities are at the subconscious level.the subconscious level.

2. The sensory fibers from a visceral organ pass 2. The sensory fibers from a visceral organ pass through several segments of spinal nerves to tthrough several segments of spinal nerves to the central centers, and a spinal nerve may conhe central centers, and a spinal nerve may contain the sensory fibers from several organs. tain the sensory fibers from several organs.

ⅢⅢ. The . The Central Centers of Central Centers of Visceral NervesVisceral Nerves

Ⅰ Ⅰ) The limbic lobe of the cerebrum is closely re) The limbic lobe of the cerebrum is closely related to the visceral activities, in which there alated to the visceral activities, in which there are representative motor areas of respiration, gre representative motor areas of respiration, gastrointestine, bladder, blood pressure, pupil astrointestine, bladder, blood pressure, pupil etc.etc.

Ⅱ Ⅱ) The hypothalamus is believed to have close ) The hypothalamus is believed to have close relationship with the visceral nerve, through wrelationship with the visceral nerve, through which the limbic lobe regulates the visceral actihich the limbic lobe regulates the visceral activities.vities.

Ⅲ Ⅲ) The brain stem and cerebellum also play th) The brain stem and cerebellum also play the important roles in regulation of the visceral ae important roles in regulation of the visceral activities.ctivities.

ⅣⅣ. The . The Referred PainReferred Pain Although many and perhaps most of the Although many and perhaps most of the

physiological impulses carried by visceral afferent physiological impulses carried by visceral afferent fibers fail to reach the level of consciousness, fibers fail to reach the level of consciousness, pathological conditions or excessive stimulation pathological conditions or excessive stimulation may bring into action of sensory fibers which may bring into action of sensory fibers which carry pain.carry pain.

The central nervous system has a poorly The central nervous system has a poorly developed power for localizing the source of such developed power for localizing the source of such pain, and by some mechanism not clearly pain, and by some mechanism not clearly understood, the pain may be referred to the understood, the pain may be referred to the region innervated by somatic afferent fibers region innervated by somatic afferent fibers whose central connections are the same as those whose central connections are the same as those of the visceral afferents.of the visceral afferents.

For example, when patient suffers from angina For example, when patient suffers from angina pectoris, the visceral afferent from the heart epectoris, the visceral afferent from the heart enter the upper thoracic nerves, and impulses tnter the upper thoracic nerves, and impulses traversing them may cause pain on the medial raversing them may cause pain on the medial side of the left arm, and in the precordial regioside of the left arm, and in the precordial region.n.

The patient suffering from hepatic or cystic disThe patient suffering from hepatic or cystic diseases may feel pain in the right shoulder.eases may feel pain in the right shoulder.

The study of clinical cases of referred pain The study of clinical cases of referred pain has been very useful in tracing the path of has been very useful in tracing the path of the afferent fiber from the various viscera, the afferent fiber from the various viscera, and a knowledge of those paths may be of and a knowledge of those paths may be of great assistance to the diagnostician in great assistance to the diagnostician in locating a pathological process.locating a pathological process.