Descending pathways of the autonomic nervous system

Antagonistic control

Most of the vital organs are under antagonistic control, where one control unit is excitatory, and the other is inhibitory. I.e. heart rate is increased by activation of the sympathetic nervous system, whereas it is decreased with the activation of the parasympathetic nervous system. This means that heart rate is regulated by changes in sympathetic and parasympathetic activity.
 
Sweat glands and smooth muscles of most blood vessels are not dually innervated; rather, they are only supplied with sympathetic nerves that control the intensity of their activity by increasing or decreasing neural stimulation.

Distribution of neurotransmitter receptors in target tissues defines the responses of those tissues. Most blood vessels have one type of adrenergic receptor ( alpha) that, when activated, causes the blood vessels to constrict. On the other hand, arterioles of vital organs have different types of receptors (beta) that, when activated, cause the smooth muscle of that vessel to relax. Norepinephrine activates both of these receptors, which means that the response is determined by the type of receptor, rather than the neurotransmitter.

Enteric nervous system

This system refers to the autonomic neurons scattered throughout the digestive tract (esophagus, stomach, intestines, and rectum). These neurons are grouped into organized networks called plexuses. There are two major plexuses included in the enteric nervous system:

• Meissner’s plexus: lies in the connective tissue between the longitudinal muscle and  circular muscle layers in the digestive tract – In other words, it can be found under the mucosa layer, which is why it is commonly referred to as the submucosal plexus.
• Auerbach's plexus: also referred to as the myenteric plexus, lies in the connective tissue between the circular muscle layer and the muscularis mucosae.

Each plexus consists of multipolar, bipolar, and unipolar neurons. The system also includes glial astrocytes that have a supportive role for the neurons. Astrocytes enwrap the neurons and their processes and form a barrier between the blood and the enteric nervous system.

The enteric nervous system provides projections onto the pancreas, gallbladder, and elsewhere. The unipolar and bipolar neurons appear to function as sensory neurons with multipolar cells forming many of the connections.

Vagus nerve

The parasympathetic component of the vagus nerve arises from its dorsal motor nucleus in the medulla. It descends through the carotid sheath of the neck that contains the common carotid artery, internal jugular vein, and the deep cervical lymph nodes. In this sheath, the artery is medial and the vein is lateral, whereas the vagus nerve is positioned posteriorly to these two structures.

After passing through the superior thoracic aperture, the vagus nerve branches to contribute to plexuses in the thoracic and abdominal viscera. These fibers provide preganglionic parasympathetic innervation to the pharynx, larynx, esophagus, lungs, heart, stomach, and intestines down to the transverse colon. The postganglionic fibers originate from ganglia in the mentioned organs. The bodies of these postganglionic neurons are located within the submucosal plexus of Meissner, and in the myenteric plexus of Auerbach in the enteric nervous system.

Oculomotor nerve

The parasympathetic fibers arise from the Edinger-Westphal nucleus in the midbrain and innervate the constrictors of the pupil via the parasympathetic ciliary ganglion which contains the postganglionic parasympathetic neurons.

Facial nerve

The axons originate in the superior salivatory nuclei of the pons and innervate glands in the nasal cavity and palate, together with the lacrimal, submandibular, and sublingual glands.

Glossopharyngeal nerve

The axons arise from the inferior salivatory nucleus in the medulla. The axons exit with CN IX and end in the otic ganglion, which innervates the parotid gland. The sacral portion of the preganglionic parasympathetic neurons contains the neurons lying in the intermediolateral nucleus and intermediomedial nucleus of the sacral spinal cord. These neurons send their axons through the third and fourth sacral root to form the somatic pudendal nerve and the parasympathetic pelvic splanchnic nerves (S2-S4).
 
Fibers arising from the latter contribute to the inferior hypogastric plexus and pelvic organs (urinary bladder, rectum, and genitals). Synapses with postganglionic neurons are formed in the inferior hypogastric plexus or in small ganglia of the various organ plexuses.

Cervical and upper thoracic segments

The cervical portion consists of three ganglia:

• The superior cervical ganglion lies below the base of the skull. It receives fibers from the upper thoracic segment via the interganglionic branches, whereas its postganglionic fibers form plexuses around the internal carotid artery and external carotid artery. Branches from the internal carotid plexus extend to:
  • The meninges
  • The eyes
  • The glands of the head region
  • The superior tarsal muscle
  • The ophthalmic muscles
• The middle cervical ganglion is the smallest and may be absent.
• The inferior cervical ganglion typically fuses with the first thoracic ganglion to form the stellate ganglion. Its postganglionic fibers form plexuses around the subclavian artery and around the vertebral artery.

Nerves from the cervical and upper thoracic ganglia send fibers to the heart and lungs, where they participate with the parasympathetic fibers of the vagus nerve in the formation of the cardiac plexus and the pulmonary plexus.

Lower thoracic and abdominal segments

This group of ganglia is the location where preganglionic fibers pass the sympathetic chain without synapsing. After passing through the ganglia, these nerves extend to the prevertebral ganglia of the abdominal aortic plexus.

The most important nerves belonging to these segments are the splanchnic nerves. They are named according to their length: the greater, the lesser, and the least splanchnic nerves. 

• The greater splanchnic nerve leaves the CNS through the roots of the spinal nerves from T5 to T10. The nerve courses through the thoracic ganglia without synapsing to end within the celiac ganglion.
• The lesser and the least splanchnic nerves arise from the preganglionic neurons that are located within the segments T11 and T12 of the spinal cord. Similar to the greater splanchnic nerve, they run through the paravertebral ganglion to reach the ganglia of their final synapse. For the lesser splanchnic nerve, the place of ending is the celiac ganglion, whereas for the least splanchnic nerve, it’s the superior mesenteric ganglion.

The postganglionic fibers arising from these ganglia form branches to supply the aorta, duodenum, pancreas, liver, spleen, etc.

Lumbar segment

This portion consists of small ganglia that lie along the anterior border of the psoas muscle. The preganglionic fibers originate from the segments L1 and L2 of the spinal cord. They pass through the lumbar ganglia, and end in the inferior mesenteric ganglion, where they innervate:

• The descending colon
• The rectum
• The urinary bladder
• The ureter
• The external genitalia

Innervation in the pelvis and perineum is dual, meaning that these organs have both parasympathetic and sympathetic innervations. The parasympathetic fibers supply the uterus, vagina, testes, erectile tissue (penis and clitoris), sigmoid colon, rectum, and bladder. They cause contraction and emptying of the bladder and erection of the clitoris or penis. The sympathetic fibers cause vasoconstriction and ejaculation of semen and inhibit peristalsis in the sigmoid colon and rectum.