Schematic diagram showing the general arrangement of sympathetic and parasympathetic neurons of the ANS. The sympathetic outflow is shown on the right; the parasympathetic, on the left. The sympathetic (thoracolumbar) outflow leaves the CNS from the thoracic and upper lumbar segments (T1-L2 or L3) of the spinal cord. These presynaptic fibers communicate with postsynaptic neurons in two locations, the paravertebral and prevertebral ganglia. Paravertebral ganglia are linked together and form two sympathetic trunks (yellow columns on each side of the spinal cord). Prevertebral ganglia are associated with the main branches of the abdominal aorta (yellow circles). Note the distribution of postsynaptic sympathetic nerve fibers to the viscera. The parasympathetic (craniosacral) outflow leaves the CNS from the gray matter of the brainstem within cranial nerves III,
VII, IX, and X and the gray matter of sacral segments (S2-S4) of the spinal cord and is distributed to the viscera. The presynaptic fibers traveling with cranial nerves III, VII, and IX communicate with postsynaptic neurons in various ganglia located in the head and neck region (yellow circles). The presynaptic fibers traveling with cranial nerve X and with pelvic splanchnic nerves have their synapses with postsynaptic neurons in the wall of visceral organs (terminal ganglia). The viscera, thus, contain both sympathetic and parasympathetic innervation. Note that a two-neuron chain carries impulses to all viscera except the adrenal medulla. (Modified from Moore KL, Dalley AF. Clinically Oriented Anatomy. Baltimore: Lippincott Williams & Wilkins, 1999:48-50.)
and the sacral segments of the spinal cord (S2 through S4) to visceral ganglia. The ganglia in or near the wall of abdominal and pelvic organs and the visceral motor ganglia of cranial nerves III, VII, IX, and X contain cell bodies of the postsynaptic effector neurons of the parasympathetic division (Figs. 11.27c and 11.28).
The sympathetic and parasympathetic divisions of the ANS often supply the same organs. In these cases, the actions of the two are usually antagonistic. An obvious example of this antagonistic action is that sympathetic stimulation increases the rate of cardiac muscle contractions, whereas parasympathetic stimulation reduces the rate.
Many functions of the SNS are similar to those of the adrenal medulla, an endocrine gland. This functional similarity is partly explained by the developmental relationships between the cells of the adrenal medulla and postsynaptic sympathetic neurons. Both are derived from the neural crest, are innervated by presynaptic sympathetic neurons, and produce closely related physiologically active agents, EPI and NE. The difference is that the sympathetic neurons deliver the agent directly to the effector, whereas the cells of the adrenal medulla deliver the agent indirectly through the bloodstream. The innervation of the adrenal medulla may constitute an exception to the rule that autonomic innervation consists of a two-neuron chain from CNS to an effector; for the adrenal medulla, it is only one neuron, unless the adrenal medullary cell is considered the functional equivalent of the second neuron, in effect, a neurosecretory neuron.
The enteric division of the ANS consists of the ganglia and postsynaptic neuronal networks of the alimentary canal
Ganglia and postsynaptic neurons of the enteric division are located in the lamina propria, muscularis mucosae, submucosa, muscularis externa, and subserosa of the alimentary canal from the esophagus to the anus (see page 476). The enteric division can function independently of presynaptic input from the vagus nerve and sacral outflow; e.g., the intestine will continue peristaltic movements even after the vagus nerve is cut. There are many millions more ganglion cells in the enteric division than could be supplied by the presynaptic neurons of the vagal and sacral outflows.
Figures 11.27 and 11.28 summarize diagrammatically the origins and distribution of the ANS. Refer to these figures as you read the descriptive sections. Note that the diagrams indicate both the paired innervation (parasympathetic and sympathetic) common to the ANS as well as the important exceptions to this general characteristic.
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