Chapter 19: The Autonomic Nervous System

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19.4:

Sympathetic Pathways: Collateral Ganglia and Adrenal Medulla

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Anatomy and Physiology

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Sympathetic Pathways: Collateral Ganglia and Adrenal Medulla

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The collateral ganglia are the synapse sites between the preganglionic and postganglionic fibers that innervate the abdominal and pelvic region organs. The ganglia include the celiac, mesenteric, aorticorenal, and renal ganglia. Preganglionic fibers enter the collateral ganglia as various splanchnic nerves and synapse with postganglionic fibers, innervating their targets. The greater splanchnic nerve extends to the celiac ganglion and regulates structures in the stomach, spleen, pancreas, and gallbladder. The lesser splanchnic nerve enters the aorticorenal and superior mesenteric ganglion, controlling the small intestine, colon, and kidney function. The least splanchnic nerve extends to the renal plexus, regulating kidney function. The lumbar and sacral splanchnic nerves travel to the inferior mesenteric ganglion and the hypogastric plexus, which supply sympathetic signals to the large intestine, urinary bladder, uterus, and genitals. Some preganglionic fibers pass through the collateral ganglia without synapsing. They enter the adrenal medulla and stimulate its chromaffin cells to release hormones such as epinephrine and norepinephrine in the bloodstream. As a result, the adrenal medulla can regulate the activity of distant organs.

19.4:

Sympathetic Pathways: Collateral Ganglia and Adrenal Medulla

The sympathetic pathways of the collateral ganglia and adrenal medulla serve unique but interconnected roles in the sympathetic response.

Collateral Ganglia

Sympathetic preganglionic axons reach the collateral ganglia along the route of splanchnic nerves. These nerves bypass the sympathetic trunk and communicate with sympathetic postganglionic neurons housed in the prevertebral ganglia. These ganglia supply the organs of the abdominopelvic cavity.

The greater splanchnic nerve, formed by the preganglionic axons from the thoracic ganglia (T5–T9 or T10), interfaces with the stomach, spleen, liver, kidneys, and small intestine. The lesser splanchnic nerve (T10–T11) reaches the aorticorenal and the superior mesenteric ganglia, affecting the small intestine and proximal colon. The lumbar splanchnic nerve, formed by axons from the lumbar ganglia (L1–L4), interfaces with the distal colon, rectum, urinary bladder, and genital organs through the inferior mesenteric ganglion.

Adrenal Medulla

The adrenal medulla pathway, on the other hand, is unique. Some sympathetic preganglionic axons traverse the sympathetic trunk, greater splanchnic nerves, and the celiac ganglion to reach the adrenal medullae without synapsing. This area houses chromaffin cells, which are modified sympathetic postganglionic neurons. But, unlike typical neurons, they lack dendrites and axons, and instead of transmitting to another organ, these cells discharge hormones into the bloodstream. Upon stimulation by sympathetic preganglionic neurons, chromaffin cells release catecholamine hormones, predominantly epinephrine (80%) and norepinephrine (20%), with dopamine in trace amounts. These hormones disperse into the bloodstream, intensifying the body's responses initiated by sympathetic postganglionic neurons.