Autonomic Nervous System: Sympathetic Division

• The autonomic nervous system (ANS) is part of the peripheral nervous system (PNS) and comprises visceral motor neurons.
• This system supplies impulses to smooth muscles, cardiac muscles, and glands.
• The ANS is divided into two main divisions: sympathetic and parasympathetic.
• An autonomic nerve pathway includes two motor neurons that synapse in a ganglion outside the central nervous system (CNS).
• The 1st neuron, called the preganglionic neuron, extends from the CNS to the ganglion.
• The 2nd neuron, called the postganglionic neuron, extends from the ganglion to the visceral effector.
• Ganglia contain cell bodies of postganglionic neurons.

Sympathetic Nervous System (Thoracolumbar Division)

• Preganglionic sympathetic fibers are short due to the proximity of sympathetic ganglia to the vertebral column and are myelinated.

• Postganglionic sympathetic fibers are long because of the distance from the ganglion to the target effector and are unmyelinated.

• Axons from central sympathetic neurons project to the adrenal medulla (the interior portion of the adrenal gland), but do not terminate in a ganglion.

• Cells in the adrenal medulla contacted by preganglionic fibers that are called chromaffin cells.

• The adrenal medulla releases signaling molecules into the bloodstream, instead of using axons.

• In the sympathetic division, preganglionic neurons have cell bodies in the lateral horns of the gray matter of the 12 thoracic segments and the first 2 or 3 lumbar segments of the spinal cord.

• The sympathetic ganglia are either sympathetic trunk ganglia (paravertebral) or prevertebral (preaortic) ganglia, based on their location in the body

• Paired sympathetic trunk ganglia arranged anterior and lateral to the vertebral column extend from the base of the skull (neck), chest, and abdomen to the coccyx.

• The right and left coccygeal ganglia are fused and usually lie at the midline.

• There are about 23 to 26 pairs of sympathetic trunk ganglia: 3 cervical ganglia (superior, middle and inferior cervical/stellate ganglion), 11 or 12 thoracic ganglia, 4 or 5 lumbar ganglia, 4 or 5 sacral ganglia and 1 coccygeal ganglion.

• The sympathetic division of the ANS exhibits wide divergence; a single preganglionic sympathetic neuron can have 10-20 targets.

• An axon leaving a central neuron in the lateral horn of the thoracolumbar spinal cord passes through the white ramus communicans and enters the sympathetic chain, branching toward various targets:

  • Synapses on a neuron in the adjacent chain ganglion.
  • Extends up or down to a different level of the chain ganglia.
  • Passes through the chain ganglia and projects through one of the splanchnic nerves to a collateral ganglion.
  • Projects through the splanchnic nerves to the adrenal medulla.

• One preganglionic neuron, due to its branching ability, can influence different regions of the sympathetic system very broadly, acting on widely distributed organs and affecting almost the entire body simultaneously.

• Postganglionic neurons leaving the superior cervical ganglion are distributed to blood vessels in the head and innervate sweat glands, smooth muscle of the eye, lacrimal glands, nasal mucosa, salivary glands and the heart.

• Postganglionic neurons leaving the middle and inferior cervical ganglia innervate the heart and blood vessels of the neck and shoulder.

• Postganglionic neurons of the thoracic portion of the sympathetic trunk innervate thoracic blood vessels, heart, lungs, and bronchial tree, vessels, sweat glands in the skin, and arrector pili muscles of hair follicles.

• Postganglionic axons of the lumbar portion carry sympathetic output to the smooth muscle and glands of the body wall and limbs, primarily the smooth muscle of blood vessels.

• Collateral ganglia, also known as prevertebral ganglia or preaortic ganglia, have neurons.

• These receive input from the splanchnic nerve and innervate organs of the abdominal and pelvic region.

• The celiac ganglion, aortico-renal ganglion, superior mesenteric ganglion, and inferior mesenteric ganglion are found in this group.

• Prevertebral ganglia (PVG) neurons provide sympathetic input to the gastrointestinal tract, spleen, liver, pancreas, and urogenital organs (e.g., kidney, bladder, ureters, ovaries, uterus, vas deferens).

Parasympathetic Nervous System (Craniosacral Division)

• This division is also known as the craniosacral division.
• Cell bodies of preganglionic neurons of the parasympathetic division are in the brain stem (cranial nerves III, VII, IX, and X) and lateral gray horns of sacral segments 2nd through 4th of spinal cord.
• The 4 cranial nerves are oculomotor (III), facial (VII), glossopharyngeal (IX), and vagus (X).
• There are five components of cranial outflow including the four pairs of ganglia and associated plexuses with the vagus (X) nerve.
• Vagal axons, which carry about 80% of total craniosacral outflow, extend to many terminal ganglia in the thorax and abdomen to supply the heart, lungs, and liver
• Parasympathetic ganglia are sites of synapses between parasympathetic preganglionic and postganglionic neurons as well as contain postganglionic cell bodies.
• These ganglia, located near the innervated organs, innervate structures in the head.
• Parasympathetic ganglia are located close to or nearly within the wall of a visceral organ and therefore are usually named terminal ganglia.
• Most terminal ganglia do not have individual names, only those in the head such as ciliary, pterygopalatine, submandibular, and otic ganglia.
• Ciliary ganglia, receive axons of the 3rd cranial nerve (oculomotor nerve) and supply smooth muscle fibers in the eyeballs with postganglionic axons.
• Pterygopalatine ganglia receive preganglionic axons from the facial nerve and contain cell bodies of postganglionic axons that innervate the nasal mucosa, palate, pharynx, and lacrimal glands.
• Submandibular ganglia receive preganglionic axons from the facial nerve and send postganglionic axons to the submandibular and sublingual salivary glands.
• Otic ganglia receive preganglionic axons from the glossopharyngeal (IX) nerves and send postganglionic axons to the parotid salivary glands.
• The sacral parasympathetic outflow consists of preganglionic axons from the anterior roots of the 2nd through 4th sacral nerves (S2-S4), forming the pelvic splanchnic nerves.
• These nerves synapse with parasympathetic postganglionic neurons located in terminal ganglia in the walls of innervated viscera.
• From ganglia, postganglionic axons innervate smooth muscle and glands in the walls of the colon, ureters, urinary bladder, and reproductive organs.