![]() The released norepinephrine binds to the pre- and postsynaptic adrenergic receptors. In general, only 1% of the total stored norepinephrine is released with each depolarization thus, there is a tremendous functional reserve. Then the vesicles merge with the cell membrane and release their contents into the synapse ( Fig.5). The neurotransmitters are stored in vesicles until the postganglionic nerve is stimulated. In the adrenal medulla, norepinephrine is methylated to epinephrine. DOPA is then converted to dopamine and, once inside the storage vesicle at the nerve terminal, is β-hydroxylated to norepinephrine. The rate-limiting step is the transformation of tyrosine to dihydroxyphenylalanine (DOPA), which is catalyzed by the enzyme tyrosine hydroxylase. Sympathetic neurotransmitters are synthesized from tyrosine in the postganglionic sympathetic nerve ending ( Fig.4). In contrast, the postganglionic fibers run a long course before innervating effector organs ( Fig.2). The sympathetic preganglionic fibers are relatively short because sympathetic ganglia are generally close to the central nervous system (CNS). The postganglionic neurons of the SNS then travel to the target organ. ![]() ![]() A sympathetic response, therefore, is not confined to the segment from which the stimulus originates, as discharge can be amplified and diffuse. Preganglionic sympathetic fibers not only synapse at the ganglion of the level of their origin in the spinal cord but can also course up and down the paired ganglia. The nerve fibers extend to paired ganglia, creating the sympathetic chains that lie immediately lateral to the vertebral column or extend to unpaired distal plexuses (e.g., the celiac and mesenteric plexuses). The cell bodies of these neurons lie in the spinal gray matter. Parasympathetic nervous system fibers arising from the sacral portion of the spinal cord innervate the large intestine, rectum, and bladder.The preganglionic fibers of the SNS originate from the thoracolumbar region (T1 to L2 or 元) of the spinal cord ( Fig.1). Additionally, salivary glands also receive parasympathetic stimulation through the glossopharyngeal (IX) nerve. The eye receives parasympathetic stimulation via the occulomotor (III) nerve, and the lacrimal and salivary glands are stimulated through fibers from the facial (VII) nerve. This includes parasympathetic stimulation to the heart, lungs, stomach, small intestine, liver, gallbladder, and pancreas. A large percentage of parasympathetic innervation to the thorax and abdomen stems from the vagus (X) nerve. ![]() Parasympathetic stimulation arising from the cranial nerves innervate viscera of the head, thorax, and abdomen. The preganglionic fibers of the parasympathetic system differ from those of the sympathetic system in that they travel uninterrupted to their effector organ before synapsing with a short postganglionic fiber. It is sometimes referred to as the craniosacral division. The parasympathetic nervous system cell bodies stem from cranial nerves III, VII, IX, and X and the sacral segment of the spinal cord Figure 14.1. Because the preganglionic fibers are myelinated, the signal speed is quick, causing a rapid release of norepinephrine and epinephrine from cells within the adrenal medulla. Therefore, preganglionic fibers do not synapse prior to reaching the adrenal gland. Medulla is unique in that the secretory cells are considered modified postganglionic neurons. Synapses with these outlying ganglia, sometimes also referred to as collateral ganglia, innervate the visceral organs below the diaphragm. Some preganglionic fibers exit the sympathetic chain and synapse with outlying ganglia such as the celiac ganglia or the superior and inferior mesenteric ganglia. After reaching the ganglia, the impulse may travel in one of three ways: (1) directly across the ganglion to synapse with cell bodies of the postganglionic fibers, (2) cephalad or caudad to synapse with a higher or lower postganglionic neuron, or (3) through the sympathetic chain without synapsing. The myelinated effector nerves leave the spinal cord and enter the ganglia. For this reason, it is sometimes referred to as the thoracolumbar division. The sympathetic nervous system preganglionic fibers originate in the thoracic (T1-T12) segments and the first three lumbar (L1-元) segments of the spinal cord ( Figure 14.1).
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