Sympathetic division

The preganglionic neurons of the sympathetic system arise from the thoracic and lumbar regions of the spinal cord (segments T1 through L2; see Figure 9.1). Most of these preganglionic axons are short and synapse with postgan-glionic neurons within ganglia found in sympathetic ganglion chains. Running parallel immediately along either side of the spinal cord, each of these chains consists of 22 ganglia. The preganglionic neuron may exit the spinal cord and synapse with a postganglionic neuron in a ganglion at the same spinal cord level from which it arises. This neuron may also travel more rostrally

Table 9.2 Distinguishing Features of Sympathetic and Parasympathetic Systems

Sympathetic system

Parasympathetic system

Originates in thoracic and lumbar regions of the spinal cord (T j-L2)

Ganglia located in paravertebral sympathetic ganglion chain or collateral ganglia Short cholinergic preganglionic fibers; long adrenergic postganglionic fibers Ratio of preganglionic fibers to postganglionic fibers is 1:20 Divergence coordinates activity of neurons at multiple levels of spinal cord

Activity often involves mass discharge of entire system Predominates during emergency "fight-or-flight" reactions and

Originates in brainstem (cranial nerves III, VII, IX, and X) and sacral region of spinal cord (S2-S4) Terminal ganglia located near or embedded within target tissue

Long cholinergic preganglionic fibers; short cholinergic postganglionic fibers

Ratio of preganglionic fibers to postganglionic fibers is 1:3 Limited divergence

Activity normally to discrete organs

Predominates during quiet resting conditions or caudally (upward or downward) in the ganglion chain to synapse with postganglionic neurons in ganglia at other levels. In fact, a single pregangli-onic neuron may synapse with several postganglionic neurons in many different ganglia. Overall, the ratio of preganglionic fibers to postganglionic fibers is about 1:20. The long postganglionic neurons originating in the ganglion chain then travel outward and terminate on the effector tissues. This divergence of the preganglionic neuron results in coordinated sympathetic stimulation to tissues throughout the body. The concurrent stimulation of many organs and tissues in the body is referred to as mass sympathetic discharge.

Other preganglionic neurons exit the spinal cord and pass through the ganglion chain without synapsing with a postganglionic neuron. Instead, the axons of these neurons travel more peripherally and synapse with post-ganglionic neurons in one of the sympathetic collateral ganglia (see Figure 9.1). These ganglia are located about halfway between the CNS and effector tissue.

Finally, the preganglionic neuron may travel to the adrenal medulla and synapse directly with this glandular tissue. The cells of the adrenal medulla have the same embryonic origin as neural tissue and, in fact, function as modified postganglionic neurons. Instead of the release of neurotransmitter directly at the synapse with an effector tissue, the secretory products of the adrenal medulla are picked up by the blood and travel throughout the body to all of the effector tissues of the sympathetic system.

An important feature of this system, which is quite distinct from the para-sympathetic system, is that the postganglionic neurons of the sympathetic

SYMPATHETIC PARASYMPATHETIC

SYMPATHETIC PARASYMPATHETIC

a: Greater splanchnic nerve b: Celiac ganglion c: Lesser splanchnic nerve d: Superior mesenteric ganglion e: Lumbar splanchnic nerve f: Inferior mesenteric ganglion g: Sympthetic ganglion chain h: Ciliary ganglion i: Pterygopalatine ganglion j: Submandibular ganglion k: Otic ganglion a: Greater splanchnic nerve b: Celiac ganglion c: Lesser splanchnic nerve d: Superior mesenteric ganglion

-- postganglionic nerve e: Lumbar splanchnic nerve f: Inferior mesenteric ganglion g: Sympthetic ganglion chain h: Ciliary ganglion i: Pterygopalatine ganglion j: Submandibular ganglion k: Otic ganglion

Figure 9.1 The autonomic nervous system and its effector organs. The efferent pathways of this system consist of two neurons that transmit impulses from the CNS to the effector tissue, preganglionic neuron (solid line), and postganglionic neuron (dashed line). As illustrated, most tissues receive nervous input from both divisions of the ANS: the sympathetic and the parasympathetic.

system travel within each of the 31 pairs of spinal nerves (see Chapter 7). Interestingly, 8% of the fibers that constitute a spinal nerve are sympathetic fibers. This allows for distribution of sympathetic nerve fibers to the effectors of the skin, including blood vessels and sweat glands. In fact, because most innervated blood vessels in the entire body, primarily arterioles and veins, receive only sympathetic nerve fibers, vascular smooth muscle tone and sweating are regulated by the sympathetic system only. In addition, the sympathetic system innervates structures of the head (eye, salivary glands, mucus membranes of the nasal cavity), thoracic viscera (heart, lungs) and viscera of the abdominal and pelvic cavities (e.g., stomach, intestines, pancreas, spleen, adrenal medulla, urinary bladder; see Figure 9.1).

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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