The autonomic nervous system

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Introduction

The autonomic nervous system is a part of the peripheral nervous system. This part is essential for regulation of the  involuntary physiological processes, including heart rate, blood pressure, respiration, digestion, and sexual arousal. It consists of three anatomically different divisions: sympathetic, parasympathetic and enteric.

The development of the autonomic nervous system

The development of the autonomic nervous system results from the migration of the neural crest cells like other portions of the nervous system. The migration of these neural cells takes the dorsolateral and ventromedial directions. The ventromedial direction forms the cells that will present the autonomic nervous system.                                                    

This process is found that it depends in an obvious way on the growth factors. These growth factors coordinate the migration and the growth of the axons. These growth factors also stimulate growth of others by releasing neurotransmitters.

The structure of the sympathetic nervous system

The  intermediolateral columns, or lateral horns, of the spinal cord have the cell bodies of the sympathetic neurons. The presynaptic fibers emergence from the spinal cord through anterior roots and enter the anterior rami of T1-L2 spinal nerves and onto the sympathetic trunks via white rami communicantes. 

Then the fibers may ascend or descend the sympathetic trunk to a superior or inferior paravertebral ganglion then pass to adjacent anterior spinal nerve rami via gray rami communicantes, or these fibers may arrive to prevertebral ganglia across the abdominolpelvic splanchnic nerve. 

Paravertebral ganglia present as nodules throughout the sympathetic trunk, adjacent to the spinal column, where pre- and postganglionic neurons synapse.

Generally, there are three cervical, twelve thoracic, four lumbar, and five sacral ganglia. This distribution may vary individually. Of these, only the cervical have names of superior, middle, and inferior cervical ganglia. There is the  stellate ganglion that forms by the fusion of the inferior cervical ganglion and the first thoracic ganglion. 

There are splanchnic nerves that  convey afferent and efferent fibers between the CNS and the viscera. Cardiopulmonary splanchnic nerves transfer the postsynaptic fibers for the thoracic cavity.

There are the abdominopelvic splanchnic nerves  include the greater, lesser, least, and lumbar splanchnic nerves. The presynaptic nerves finally synapse in prevertebral ganglia that are nearer to their target organ. 

Prevertebral ganglia includes the celiac, aorticorenal, and superior and inferior mesenteric ganglia. 

The celiac ganglion innervates organs derived from the foregut: distal esophagus, stomach, proximal duodenum, pancreas, liver, biliary system, adrenal glands, and the spleen. 

The superior mesenteric ganglion innervates organs derived from the midgut: distal duodenum, jejunum, ileum, cecum, appendix, ascending colon, and proximal transverse colon. 

The inferior mesenteric ganglion provides sympathetic innervation to the structures derived from the hindgut: distal transverse, descending, and sigmoid colon; rectum and upper anal canal; as well as the bladder and external genitalia.

The function of the sympathetic nervous system

The sympathetic nervous system enables the body to use energy appropriately to respond to stressful situations and emergencies, as in the “fight or flight” response. 

Activation of the sympathetic system leads to pupil dilation, piloerection, vasoconstriction of cutaneous blood vessels, sweating, release of adrenaline, bronchodilation, increased cardiac contraction and decreased digestion.

During normal conditions, blood vessels are existing in a resting state of moderate vasoconstriction.

 If sympathetic signals are sent, vasoconstriction increases and vice-versa. However, in coronary vessels, vessels of the external genitalia and the skeletal muscles sympathetic stimulation leads to vasodilation.

The structure of the parasympathetic nervous system

There are the parasympathetic fibers that emergences from  the central nervous system via cranial nerves III, VII, IX, and X, and also through the S2-4 nerve roots. There are four pairs of parasympathetic ganglia, and they are existing in the head.

The third cranial nerve, via the ciliary ganglion, innervates the iris and ciliary muscles of the eye.

The seventh cranial nerve innervates the lacrimal, nasal, palatine, and pharyngeal glands via the pterygopalatine ganglion, and also the sublingual and submandibular glands via the submandibular ganglion.

 The ninth cranial nerve innervates the parotid glands via the otic ganglion.

Each other presynaptic parasympathetic fiber synapses in a ganglion near or on the wall of the target tissue; this leads to the presynaptic fibers being obviously longer than the postsynaptic. 

The tenth cranial nerve (the vagus nerve) forms  about 75% of the PNS and supplies parasympathetic input to most of the thoracic and abdominal viscera, with the sacral parasympathetic fibers innervating the descending and rectum and the sigmoid colon.  The vagus nerve has four cell bodies in the medulla oblongata. These include the following:

  • Dorsal nucleus: supplies parasympathetic output to the viscera.
  • Nucleus ambiguus: produces motor fibers and neurons that innervate the heart.
  • Nucleus solitarius: receives afferents of taste sensation and that from the viscera.
  • Spinal trigeminal nucleus: receives information of touch, pain, and temperature of the outer ear, part of the dura and the mucosa of the larynx.

The function of the parasympathetic nervous system

The parasympathetic fibers are sent to various viscera to do different involuntary functions as:

  • Constriction of the pupil when the eyes are exposed to excessive bright light.
  • Contraction of the ciliary muscle to allow the lens to thicken for near vision.
  • Advancement of the secretion of the lacrimal glands, the secretion of the salivary glands, decreasing the rate and strength of contraction.
  • Constriction of the bronchi and the coronary vessels.
  • Advancement of bronchi mucus secretion of the lungs.
  • Sending impulses to stimulate peristalsis and secretion of digestive juices.
  • Contraction of the rectum during defecation and inhibition of the internal anal sphincter to cause defecation.
  • Advancement the building/conservation of glycogen.
  • Raising the secretion of bile.
  • Inhibition of the contraction of internal sphincter of urinary bladder and they are also responsible for contraction of the detrusor muscle of the urinary bladder wall causing urination.
  • Stimulation of engorgement (erection) of erectile tissues of the external genitalia.
  • In order to achieve the homeostasis in the activities of these organs, the sympathetic nervous system also innervates those structures in an antagonistic way.

The structure of the enteric nervous system

The enteric nervous system  consists of two ganglionated plexuses: 

The myenteric  and the submucosal. The myenteric plexus exists between the longitudinal and circular smooth muscle of the gastrointestinal tract, while the submucosal plexus presents within the submucosa.

The enteric nervous system neurons can be classified functionally into three groups: intrinsic primary afferent neurons, interneurons and motor neurons.

The intrinsic primary afferent neurons receive chemical and mechanical stimuli from ingestion, usually food and drink.

The motor neurons in the enteric nervous system coordinate peristalsis, rhythmic muscle contractions that move material along the digestive tract.

Enteric neurons are responsible for regulation the vomiting reflex, which is particularly critical in those cases in which vomiting results as a side-effect of medical treatments.

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