Review of Medical Physiology - William F. Ganong 2002

Functions of the Nervous System
Autonomic Nervous System
Structural Organization of Autonomic Efferent Pathways

The peripheral segment of the efferent division of the Autonomic Nervous system is formed by pre- and postganglionic Neurons (Figs. 13-1, 13-2). The Cell bodies of preganglionic neurons are located in the visceromotor intermediolateral gray columns (IMLC) of the Spinal Cord or in the homologous cranial nerve nuclei. Their axons, which are predominantly myelinated, are relatively slow-conducting B-fibers that form synaptic contacts with the cell bodies of postganglionic neurons located exclusively outside the CNS. Each preganglionic axon forms synapses with an average of eight to nine postganglionic neurons. This mechanism provides signal divergence within the efferent autonomic pathways. The axons of postganglionic neurons are predominantly unmyelinated C-fibers that terminate on visceral effectors. Anatomically, the efferent division of the autonomic nervous system is subdivided into sympathetic and parasympathetic parts. Both parts are interconnected by the enteric nervous system, which is often referred to as the third component of the autonomic nervous system.

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Fig. 13-1. Autonomic nervous system. Pre - preganglionic neuron; Post - postganglionic neuron; CG - communicating branch.

Fig. 13-2. Diagram of efferent autonomic pathways. Preganglionic fibers are represented by solid lines, and postganglionic fibers by dashed lines. Thick lines denote parasympathetic fibers, and thin lines denote sympathetic fibers (modified and reproduced with permission from Youmans W: Fundamentals of Human Physiology, 2nd ed. Year Book, 1962).

Sympathetic Division

The axons of sympathetic preganglionic neurons emerge from the spinal cord through the ventral roots of the Spinal Nerves, extending from the first thoracic to the third or fourth lumbar segment. From the spinal nerves, these axons pass via white rami communicantes into the paravertebral ganglia that form the Sympathetic trunk, where the majority of them terminate on the cell bodies of postganglionic neurons. Some postganglionic axons travel to visceral Organs as part of various sympathetic nerves, whereas the remaining axons leave the paravertebral ganglion via gray rami communicantes to join the spinal nerve and supply autonomic effectors in the regions distributed by that spinal nerve. Postganglionic sympathetic fibers supplying the HEAD region originate from the superior, middle, and stellate ganglia of the sympathetic trunk and course to their effectors along Blood Vessels. The axons of certain preganglionic neurons pass uninterrupted through the paravertebral ganglia to terminate on the cell bodies of postganglionic neurons located in prevertebral (collateral) ganglia, which lie closer to the visceral organs. PARTS OF THE Uterus and the male reproductive tract are innervated by a specialized system of short noradrenergic neurons whose cell bodies reside within ganglia located in or immediately adjacent to the walls of these organs. Consequently, the nerves projecting to these organs consist entirely of preganglionic axons (see Fig. 13-2).

Furthermore, as noted above, at least in rats, there are intrinsic cardiac adrenergic Cells (ICA cells) that contain adrenaline and noradrenaline, accounting for about 15% of the total catecholamine content in The Heart. Although their exact function remains to be fully elucidated, Gene knockout experiments have demonstrated that catecholamines are essential for normal cardiac development.

Parasympathetic Division

The cranial efferent fibers of the parasympathetic nervous system that innervate visceral structures of the head run within the oculomotor, facial, and glossopharyngeal nerves, whereas those supplying the thoracic and abdominal organs travel within the Vagus nerve. The sacral efferent fibers of the parasympathetic nervous system, which innervate the pelvic organs, run within the pelvic splanchnic nerves—Branches of the second, third, and fourth sacral spinal nerves. The preganglionic fibers of both divisions terminate on the cell bodies of postganglionic neurons located within or immediately adjacent to the walls of visceral organs (see Fig. 13-2).