The sympathetic preganglionic neuron cell bodies are situated in the thoracic and upper two or three lumbar segments of the spinal cord. The cell bodies lie in the lateral horn of the spinal gray matter. The (usually) short preganglionic fibers leave the spinal cord in the ventral nerve root, and join the spinal nerve. These fibers synapse with the postganglionic fibers, either in one of the sympathetic ganglia, which lie in a bilateral longitudinal, paraverte-bral chain on either side of the spinal column, or in one of the plexuses, which surround the main branches of the abdominal aorta. These plexuses are the coeliac, superior mesenteric, and inferior mesenteric ganglia, and are unpaired. These ganglia are also termed prevertebral ganglia.
An exception to this general arrangement of the sympathetic division is that of the adrenal medulla. The adrenal gland lies above the kidney, and is structurally two separate organs. The outer shell of the adrenal gland is concerned with production of the steroid hormones, while the inner core is the adrenal medulla, a modified sympathetic ganglion. Thus, pregan-glionic cholinergic fibers run to the adrenal medulla, where they synapse with postganglionic cell bodies, which are in effect hormone-secreting cells. These cells respond to the arrival of impulses down the preganglionic fibers, by secreting the catecholamine hormones epinephrine and norepinephrine into the bloodstream.
The sympathetic division innervates most of the organs innervated by the para-sympathetic division, and generally (but not always) opposes parasympathetic effects. Thus, the eye, lacrimal, and salivary glands are innervated by postganglionic fibers from the superior cervical ganglion, while fibers from the paravertebral chain innervate the heart, larynx, trachea,
Greenstein, Color Atlas of Neuroscience © 2000 Thieme and bronchi. Fibers from the coeliac ganglion innervate the oesophagus, stomach, and small intestine, and some interconnect the coeliac ganglion and the superior mesenteric ganglion, which innervates the large intestine. The inferior mesenteric ganglion innervates the kidney, bladder, and the reproductive organs. In addition, several postganglionic fibers leave the paravertebral chain of ganglia and run to the blood vessels, erector pili, which are the muscles responsible for piloerection (hair raising), and the sweat glands.
The preganglionic fibers, like those of the parasympathetic division, release the neurotransmitter, ACh, which binds to nicotinic receptors on ganglionic postsy-naptic cell bodies of postganglionic fibers. But the postganglionic fibers of the sympathetic division differ from those of the parasympathetic division since they release as their neurotransmitter the cate-cholamine norepinephrine, which binds to a or p receptors on the presynaptic nora-drenergic nerve terminal, or on the postsy-naptic membrane of the target organ. An exception to this general rule is the presence in the sympathetic division of post-ganglionic fibers, which innervate the sweat glands. These are cholinergic, and release ACh, which acts on muscarinic receptors on the membranes of the sweat glands. Stimulation of the sympathetic division of the ANS results in increased heart rate and force of contraction, raised blood pressure, and mobilization of glucose, and this division is therefore the focus of much attention in the treatment of diseases such as essential hypertension, and cardiac disorders.
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