Porth's Essentials of Pathophysiology, 4e

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Nervous System

U N I T 1 0

and decreased activity results in vasodilatation caused by decreased tone. In structures such as the sinoatrial node and atrioventricular node of the heart, which are innervated by both divisions of the ANS, one division predominates in controlling tone. In this case, the toni- cally active parasympathetic nervous system exerts a constraining or braking effect on heart rate, and when parasympathetic outflow is withdrawn, similar to releasing a brake, the heart rate increases. The increase in heart rate that occurs with vagal withdrawal can be further augmented by sympathetic stimulation. Autonomic Efferent Pathways The outflow of both divisions of the ANS follows a two- neuron efferent pathway (Fig. 34-22). The cell body of the first motor neuron, called the preganglionic neuron , lies in the brain stemor the spinal cord. The secondmotor neuron, called the postganglionic neuron , synapses with a pregangli- onic neuron in an autonomic ganglion located in the PNS. The two divisions of the ANS differ in terms of location of preganglionic cell bodies, relative length of preganglionic fibers, general function, nature of peripheral responses, and preganglionic and postganglionic neurotransmitters and their postsynaptic receptors (Table 34-3). Most visceral organs are innervated by both sympa- thetic and parasympathetic fibers. Exceptions include

structures such as blood vessels and sweat glands that have input from only sympathetic division of the ANS. The fibers of the sympathetic nervous system are distrib- uted to effectors throughout the body, and as a result, sympathetic actions tend to be more diffuse than those of the parasympathetic nervous system, in which there is a more localized distribution of fibers. The pregangli- onic fibers of the sympathetic nervous system may tra- verse a considerable distance and pass through several ganglia before synapsing with postganglionic neurons, and their terminals make contact with a large num- ber of postganglionic fibers. In some ganglia, the ratio of preganglionic to postganglionic cells may be 1:20; because of this, the effects of sympathetic stimulation are diffuse. There is considerable overlap, and one ganglion cell may be supplied by several preganglionic fibers. In contrast to the sympathetic nervous system, the para- sympathetic nervous system has its postganglionic neu- rons located very near or in the organ of innervation. Because the ratio of preganglionic to postganglionic communication often is 1:1, the effects of the parasym- pathetic nervous system are much more circumscribed. Sympathetic Nervous System The preganglionic neurons of the sympathetic nervous system are located primarily in the thoracic and upper lumbar segments (T1 to L2) of the spinal cord; thus, the

Parasympathetic

Sympathetic

Lacrimal gland

Carotid artery

III

Iris

Salivary glands

Ciliary ganglion

VII

Sphenopalatine ganglion

X IX

Submandibular and sublingual glands

Superior, middle, and inferior cervical ganglia

Submandibular ganglion

T1 T2 T3 T4 T5 T6 T7 T8 T9 L1 L2 L3 L4 L5 S1 S2 S3 S4 S5 C5 C6 C7 C8 T10 T11 T12

Parotid gland

Otic ganglion

Heart

Celiac ganglion

Lungs

Superior mesenteric ganglion

Stomach

Pancreas

Small bowel

Inferior mesenteric ganglion

Small bowel

Paravertebral chain of sympathetic ganglia

Liver

Adrenal gland

FIGURE 34-22. The autonomic nervous system with the parasympathetic division (craniosacral) indicated in red on the right and the sympathetic division (thoracolumbar) indicated in blue on the left. (Adapted from DeMyer W. Neuroanatomy, 2nd ed. Baltimore, MD: Williams &Wilkins; 1998.)

Splenic flexure of colon

Colon

Pelvic splanchnic nerve

Urinary bladder

Gonad

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