Parasympathetic nervous system
Parasympathetic system also widely distributed throughout the body but it is much smaller than sympathetic nervous system
Heart rate slows, pupils constrict, peristalsis in intestines increases, secretions of glands increase, sphincters open, and urinary bladder contracts under the influence of parasympathetic nervous system
Sympathetic system prepares the body for an emergency. The activities of parasympathetic nervous system are to conserve and restore energy.
Sympathetic fibers supply all parts of the body but the distribution of parasympathetic fibers is visceral and not to the trunk or limbs. Similarly suprarenal glands and gonads appear to have only a sympathetic supply.
Parasympathetic nervous system consists of
Cranial nerve nuclei in brainstem
Gray matter in sacral segments of spinal cord
Efferent nerve fibers
Nerve branches
Nerve plexuses
Regional ganglia
Afferent nerve fibers
The connector nerve cells of parasympathetic part of ANS are located in the brainstem and the sacral segments of the spinal cord.
Cranial nerve nuclei in brainstem
The connector nerve cells located in brainstem form nuclei in the following cranial nerves:
1. Oculomotor
Accessory parasympathetic nucleus or Edinger-Westphal nucleus
It is situated posterior to the main oculomotor nucleus
2. Facial
Superior salivatory nucleus and lacrimatory nucleus
3. Glossopharyngeal
Inferior salivatory nucleus
4. Vagus
Dorsal nucleus of vagus
The axons of these connector nerve cells are myelinated and emerge from brainstem along the cranial nerves
Gray matter in sacral segments of spinal cord
Sacral connector nerve cells are found in the lateral gray horn of second, third, and fourth sacral segments of spinal cord. In fact these cells are not numerous enough to form an obvious lateral gray horn, as do the sympathetic connector neurons in thoracolumbar region.
The myelinated axons leave the spinal cord in anterior nerve roots of the corresponding spinal nerves. They then leave the sacral nerves and form pelvic splanchnic nerves.
Efferent Nerve Fibers
The efferent parasympathetic fibers (craniosacral outflow) are called preganglionic fibers. They are myelinated fibers and synapse in peripheral ganglia. Here, again, acetylcholine is the neurotransmitter. The postganglionic parasympathetic fibers are nonmyelinated and of relatively short length
Regional ganglia
The cranial parasympathetic ganglia are four:
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 ovale.
These four ganglia are very similar in plan. Each has parasympathetic, sympathetic and sensory roots, and branches of distribution.
Parasympathetic root carries preganglionic fibers from the cells of origin in a brainstem nucleus. This is the essential functional root of the ganglion. Preganglionic fibers synapse in it.
The fibers of other roots simply pass through the ganglion without synapse
Sympathetic root contains postganglionic fibers from the superior cervical ganglion. Their preganglionic fibers are coming from the lateral grey horn of thoracic 1-3 segments of spinal cord.
Sensory root contains the peripheral processes of the cell bodies in trigeminal ganglion.
The branches of each ganglion carry postganglionic fibers to target organs and structures.
From ciliary ganglion short ciliary nerves leave to eye. They supply ciliary muscle and sphincter pupillae.
From the pterygopalatine ganglion the nerves pass through the zygomatic and lacrimal nerves to lacrimal gland; through maxillary nerve branches to mucous glands in the nose, nasopharynx and palate.
From submandibular and otic ganglia nerves leave for salivary glands.
The other fibers in the branches are sympathetic fibers to the same structures. They are mainly for their blood vessels.
The sacral parasympathetic ganglia are located close to the viscera they innervate.
Nerve plexuses
Cardiac plexus,
Pulmonary plexus
The plexuses associated with gastrointestinal tract are Myenteric plexus (Auerbach’s plexus) and Mucosal plexus (Meissner’s plexus)
Hypogastric plexuses
Efferent parasympathetic fibers
Preganglionic parasympathetic fibers leaving the brainstem pass through the cranial nerves (oculomotor, facial, glossopharyngeal and vagus).
Preganglionic fibers coming from accessory parasympathetic nucleus go to ciliary ganglion and synapse there. They pass through the inferior division of oculomotor nerve and a branch from the nerve to the inferior oblique muscle.
Postganglionic fibers go to eyeball. They pass through short ciliary nerves to constrictor pupillae and ciliary muscles.
Preganglionic fibers coming from superior salivatory nucleus go to pterygopalatine ganglion and submandibular ganglion. They synapse in these ganglia.
Preganglionic fibers, going to pterygopalatine ganglion, pass through nervus intermedius. Then they pass through greater petrosal nerve and the nerve of pterygoid canal and reach the ganglion.
Postganglionic fibers go to lacrimal, nasal and palatal glands. They leave the ganglion and join maxillary nerve. Then they pass through zygomatic branch and zygomaticotemporal nerve. The lacrimal fibers pass through lacrimal nerve to go to lacrimal gland.
Preganglionic fibers, going to submandibular ganglion, pass through nervus intermedius, then via chorda tympani nerve and lingual nerve to reach submandibular gland.
Postganglionic fibers are distributed to submandibular and sublingual glands via branches of the lingual nerve.
Preganglionic fibers coming from inferior salivary nucleus go to otic ganglion and synapse there. They pass through tympanic branch of glossopharyngeal nerve to the tympanic plexus and then through lesser petrosal nerve reach otic ganglion.
Postganglionic fibers go to parotid gland via filaments of the auriculotemporal nerve.
Unlike the other three ganglia, the otic ganglion has an additional somatic motor root, from the nerve to the medial pterygoid; the fibers pass through (without synapse) to supply tensor tympani and tensor palati.
The preganglionic fibers synapse in peripheral ganglia. In certain locations the ganglion cells are placed in nerve plexuses, such as cardiac plexus, pulmonary plexus, myenteric plexus, and mucosal plexus.
The postganglionic cells for the first three of the above groups are in the four parasympathetic ganglia (ciliary, pterygopalatine, submandibular, otic). The vagal fibers synapse with postganglionic cell bodies in the walls of the viscera supplied (heart, lungs and gut).
Preganglionic parasympathetic fibers arising from the sacral segments of spinal cord leave the spinal cord in anterior nerve roots of the corresponding spinal nerves. They then leave the sacral nerves and form pelvic splanchnic nerves. The pelvic splanchnic nerves synapse in ganglia in the hypogastric plexuses.
Leaving the anterior rami of the appropriate sacral nerves near the anterior sacral foramina they pass forwards to enter into the formation of inferior hypogastric plexuses (pelvic plexus). From there they run to pelvic viscera and to the hindgut as far up as the splenic flexure. Fibers reach the viscera either by running along their blood vessels or making their own way retroperitoneally, and they synapse around postganglionic cell bodies in the walls of these viscera.
The postganglionic parasympathetic fibers are nonmyelinated and of relatively short length as compared with sympathetic postganglionic fibers.
The viscera and the eye also receive an additional efferent innervation from the parasympathetic nervous system which is not found in other parts of the body. This consists of peripheral ganglion (nerve) cells in small groups (ganglia) or scattered in or near the viscera which they innervate and not primarily associated with the arterial supply. These parasympathetic ganglion cells usually affect the structures which they innervate, e.g. glands and smooth muscle of the gut tube, by the release of acetylcholine from the terminals of their short postganglionic fibres (cholinergic nerve fibers) while the sympathetic postganglionic fibres usually act by the release of noradrenalin (adrenergic fibers) although the sympathetic innervation of sweat glands is effected by the release of acetylcholine.
Like the sympathetic ganglion cells, parasympathetic ganglion cells are controlled by the central nervous system through preganglionic nerve fibres which arise from cells in the central nervous system and emerge from it in certain cranial nerves and the second, third and fourth sacral nerves to pass to its ganglion cells. Because of this the parasympathetic nervous system is some times called the craniosacral part of the involuntary (autonomic) nervous system while the sympathetic nervous system, for the same reason, is the thoracolumbar part of the involuntary nervous system. These two parts together form the autonomic nervous system and their balanced activities control the visceral structures of the body.
Parasympathetic afferent fibers
Afferent myelinated parasympathetic fibers arise from the viscera supplied by efferent parasympathetic fibers. They travel to their cell bodies, present in the sensory ganglia of cranial nerves or in posterior root ganglia of sacrospinal nerves. The central processes from these ganglia go to central nervous system. Once the afferent fibers gain entrance to the spinal cord or brain, they are thought to travel alongside, or mixed with, the somatic afferent fibers. They take part in the formation of local reflex arcs or pass to higher centers of autonomic nervous system, such as hypothalamus.
It is important to realize that the afferent component of autonomic system is identical to the afferent component of somatic nerves and that it forms part of the general afferent segment of the entire nervous system.
The nerve endings in the autonomic afferent component may not be activated by such sensations as heat or touch but rather by stretch or lack of oxygen.
The routes taken by the parasympathetic nerve fibers (pre-and postganglionic) are also routes for the distribution of afferent visceral nerve fibres which have no peripheral functional connection with the parasympathetic system but discharge directly into the central nervous system through the appropriate cranial and spinal nerves.
It is important to appreciate that preganglionic fibres of the autonomic nervous system synapse with the cells of its ganglia. Hence drugs which act on synapse will affect the central nervous control over the autonomic nervous system but will have no effect on the transmission of afferent impulses through sensory ganglia where there are no synapses.
There are afferent and efferent nerve fibers in both sympathetic and parasympathetic systems.
The autonomic nervous system exerts control over the functions of many organs and tissues in the body. Along with the endocrine system, it brings about fine internal adjustments necessary for the optimal internal environment of the body.
The autonomic nervous system, like the somatic nervous system, has afferent, connector, and efferent neurons. The afferent impulses originate in visceral receptors and travel via afferent pathways to the central nervous system, where they are integrated through connector neurons at different levels and then leave via efferent pathways to visceral effector organs.
The efferent pathways of the autonomic system are made up of preganglionic and postganglionic neurons. The cell bodies of the preganglionic neurons are situated in the lateral gray column of the spinal cord and in the motor nuclei of the third, seventh, ninth, and tenth cranial nerves. The axons of these cell bodies synapse on the cell bodies of the postganglionic neurons that are collected together to form ganglia outside the central nervous system.
The control exerted by the autonomic system is widespread, since one preganglionic axon may synapse with several postganglionic neurons. Large collections of afferent and efferent nerve fibers and their associated ganglia form autonomic plexuses in the thorax, abdomen, and pelvis.
The visceral receptors include chemoreceptors, baroreceptors, and osmoreceptors. Pain receptors are present in viscera and certain types of stimuli, such as oxygen lack or stretch, may cause extreme pain.
Enteric Nervous System
Enteric nervous system is made up of ganglionated plexuses located in the walls of gastrointestinal tract. It is responsible for regulating contractions of muscle of alimentary tract, gastric acid secretion, intestinal transport of water and electrolytes, mucosal blood flow etc.
The system of neurons and their supporting cells found in the walls of GIT, including the neurons within the pancreas and gall bladder. These neurons arise from neural crest tissue that is different from that giving rise to sympathetic and parasympathetic systems.
The majority of innervations of the alimentary tract comes from 2 main interconnected ganglionated plexuses, (i) the myenteric (Auerbach’s) plexus lying between the longitudinal and circular muscle layers (ii) the submucous (Meissner’s) plexus lying between the circular muscle and muscularis mucosae.
The myenteric plexus consists of small interconnected ganglia & extends from the esophagus to the internal anal splinter.
The submucous plexus extends from the stomach to internal anal splinter.
The total no. of nerve cells in these plexuses has been estimated between 10-100 million neurons. The gall bladder also has a ganglionated plexus and ganglia are fond in the gall bladder and pancreas.
Systems of nerve bundles connect ganglia run from ganglionated plexus is to form plexuses in the muscle layers, in the mucosa and around blood vessels. The plexuses are named by their location as subserous, longitudinal muscle, mucosal, perivascular and plexus of muscularis mucosae.
The enteric nervous system retains many functions after all central connections are cut off. The motility of the gut is transiently affected. This highlights the dominant role of intrinsic plexuses which contain complete reflex pathways consisting of enteric sensory neurons which monitor intestinal wall tension and intestinal contents; interneurons that link them together; and motor neurons which change the intestinal activity.
References
Tele Anatomy Website
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J Autonomic Nervous System 8 303 1983
Furness J B and Costa M
The types of nerves in the enteric nervous system
Neuroscience 5 1-20 1980
Gershon M D
The enteric nervous system
Annual Review Neuroscience 4 227 1981
Gibbins I L Jobling P Messenger J P et al
Neuronal morphology and the synaptic organization of sympathetic ganglia
J Autonomic Nervous System 81 104 2000
