Efferent Autonomic Pathways

The autonomic output to effector organs consists of a two-neuron pathway with one synapse in the peripheral autonomic ganglia. The central neuron is called preganglionic, and the ganglionic neuron is called (illogically) postganglionic. Sympathetic preganglionic neurons are located in the intermediolateral cell column of the gray matter of the spinal cord at all thoracic and high lumbar levels (T1-L3) (thoracolumbar outflow) ( ..F.i9,;.?..1:3 A). Parasympathetic preganglionic neurons are located both in the brain stem and at the sacral (S2-S4) level of the spinal cord (craniosacral outflow) ( ...Fig, 21-3. B). Sympathetic and parasympathetic preganglionic axons emerge from the brain stem or spinal cord as small myelinated fibers, but postganglionic sympathetic and parasympathetic axons to the peripheral effector organs are unmyelinated.

SYMPATHETIC OUTFLOW

Preganglionic sympathetic neurons are organized into different functional units that control blood flow to the skin and muscles, secretion of sweat glands, skin hair follicles, systemic blood flow, as well as the function of viscera. Preganglionic sympathetic neurons receive direct descending excitatory input from the paraventricular and lateral nuclei of the hypothalamus as well as the ventromedial and ventrolateral medulla. These pathways coordinate selective rather than massive patterns of sympathetic discharge by innervating different specific subsets of preganglionic sympathetic neurons. Selectivity is further refined by the release of different neurotransmitters.

Preganglionic sympathetic axons exit through the ventral roots and pass through the white rami of the corresponding spinal nerve toward paravertebral or prevertebral ganglia (see following paragraphs). These axons have a segmental organization, but their distribution does not follow the dermatomal pattern of somatic nerves. Segments T1-T3 innervate the head and neck, T3-T6 the upper extremities and thoracic viscera, T7-T11 the abdominal viscera, and T12-L2 the lower extremities and pelvic and perineal organs.

Sympathetic ganglia are organized into two anatomical and functionally distinct groups: paravertebral and prevertebral ganglia. At the paravertebral ganglia (located at the sides of the spinal cord), preganglionic fibers may synapse on a postganglionic neuron at the same level or branch and run rostrally or caudally to synapse on a large number of postganglionic neurons at different levels. Preganglionic fibers also pass through the paravertebral ganglia without synapsing to form the splanchnic nerves that innervate the prevertebral ganglia (located anterior to the spinal cord) or the adrenal medulla.

The paravertebral ganglia provide long unmyelinated axons to all sympathetically innervated tissues and organs except those in the abdomen, pelvis, and perineum. The superior cervical ganglion (T1-T2) provides pupilodilator

Figure 21-3 Sympathetic^; and parasympathetic^; divisions of the autonomic nervous system: efferent system(Modified from Noback C, Demarest R: The Human Nervous System. Basic Principles of Neurobiology. New York, McGraw Hill, 1981, p 228.)

and sudomotor fibers to the face. The stellate ganglion (T2-T6) innervates the upper limb through branches of the brachial plexus, and the lumbar sympathetic ganglia (T9-L1) innervate the lower limb through branches of the lumbosacral plexus. The postganglionic sympathetic fibers join the peripheral somatic nerve via the gray rami communicantes, and thus their distribution is similar to that of the corresponding somatic nerve. Sympathetic fibers in somatic nerves provide vasomotor, sudomotor, and pilomotor innervation to the extremities and trunk. The lower cervical and upper thoracic ganglia innervate the heart via the cardiac plexus and the tracheobronchial tree via the pulmonary plexus.

The prevertebral ganglia innervate the abdominal, pelvic, and perineal organs. Preganglionic fibers from T5-T12 levels are carried by the thoracic splanchnic nerves, which pierce the diaphragm and enter the abdomen to synapse on the celiac and superior mesenteric ganglia. These form the celiac plexus, which innervates all abdominal viscera except the descending colon. Sympathetic activation produces splanchnic vasoconstriction, inhibition of secretion and motility of the gut, as well as secretion of epinephrine, renin, and glucagon. Preganglionic axons from L1-L3 levels, carried by the lumbar splanchnic nerves, synapse on the inferior mesenteric ganglion. The postganglionic axons innervate the descending colon, rectum, bladder, and genitalia via the hypogastric plexus. Sympathetic nerves inhibit muscle contractility of the bladder and bowel, thus allowing storage of urine and feces. These postganglionic neurons also produce the contraction of the vas deferens that is necessary for ejaculation. y

PARASYMPATHETIC OUTFLOW

The cranial parasympathetic neurons are located in the general visceral efferent column of the midbrain, pons, and medulla. Their preganglionic axons travel in the third, seventh, ninth, and tenth cranial nerves (see Chapters 9 , 1..1 , and 13 ). Parasympathetic outputs carried via the third nerve produce pupillary constriction and accommodation of the lens. Output carried via the seventh and ninth cranial nerves stimulates lacrimal and salivary secretion. The vagus nerve (tenth nerve) provides the most widespread brain stem parasympathetic output. Vagal preganglionic neurons are located in the dorsal motor nucleus, which controls respiratory and abdominal viscera, and in the region of the nucleus ambiguus, which innervates the heart. The vagus provides inputs to parasympathetic ganglia located in or near the target organs, and its main effects are cardioinhibitory, visceromotor, and secretomotor.

The sacral preganglionic parasympathetic neurons are located in the intermediolateral cell column at the S2-S4

levels of the spinal cord. Their axons pass through the ventral roots to the pelvic splanchnic nerves (nervi erigentes), which join the inferior hypogastric (pelvic) plexus. The sacral parasympathetic system is critical for defecation, micturition, and erection. y

EFFERENT AUTONOMIC NEUROTRANSMITTERS

Acetylcholine is the neurotransmitter of the sympathetic and parasympathetic preganglionic neurons. Acetylcholine produces fast excitation of ganglion neurons via nicotinic receptors. The main postganglionic sympathetic neurotransmitter is norepinephrine, which acts through alpha- and beta-adrenergic receptors. The sympathetic innervation of the sweat glands is mediated by acetylcholine, acting via muscarinic receptors. The main postganglionic parasympathetic neurotransmitter is acetylcholine, which acts through different subtypes of muscarinic receptors. Postganglionic sympathetic terminals to the heart and smooth muscles also release adenosine triphosphate and neuropeptide Y, whereas postganglionic parasympathetic terminals may release vasoactive intestinal polypeptide or nitric oxide.

Alpha-adrenergic receptors mediate sympathetically induced pupillary dilatation (mydriasis), vasoconstriction, and contraction of the vas deferens and bladder and rectal internal sphincters. Beta receptors mediate cardiac stimulation, vasodilation, bronchodilatation, relaxation of the bladder, and endocrine-metabolic effects. Muscarinic receptors mediate pupil constriction (miosis), salivary and lacrimal secretion, cardiac inhibition, bronchoconstriction, stimulation of motility and secretion in the gastrointestinal tract, evacuation of the bladder and rectum, and erection.

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