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 G…
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.
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Q1: What are collateral ganglia and where are they located?
Collateral ganglia are synapse sites between preganglionic and postganglionic sympathetic fibers that innervate abdominal and pelvic organs. They include the celiac, mesenteric, aorticorenal, and renal ganglia. These prevertebral ganglia receive preganglionic axons via splanchnic nerves that bypass the sympathetic chain ganglia, allowing direct regulation of visceral organs.
Q2: How do splanchnic nerves distribute sympathetic signals to abdominal organs?
The greater splanchnic nerve extends from thoracic ganglia to the celiac ganglion, regulating the stomach, spleen, pancreas, and gallbladder. The lesser splanchnic nerve reaches the aorticorenal and superior mesenteric ganglia, controlling the small intestine, colon, and kidney function. The least splanchnic nerve supplies the renal plexus for kidney regulation.
Q3: What is the role of the adrenal medulla in sympathetic signaling?
The adrenal medulla contains chromaffin cells, modified postganglionic neurons that lack dendrites and axons. When stimulated by preganglionic fibers, these cells release catecholamine hormones—epinephrine and norepinephrine—directly into the bloodstream. This hormonal pathway allows the sympathetic nervous system to regulate distant organs throughout the body.
Q4: How do chromaffin cells differ from typical postganglionic neurons?
Chromaffin cells are modified sympathetic postganglionic neurons that lack dendrites and axons. Instead of transmitting signals to target organs, they release catecholamine hormones into the bloodstream upon stimulation. This unique structure allows them to function as endocrine cells, amplifying sympathetic responses systemically rather than locally.
Q5: What hormones does the adrenal medulla release and in what proportions?
The adrenal medulla releases catecholamine hormones upon sympathetic stimulation: epinephrine comprises approximately 80% of the hormonal output, norepinephrine accounts for about 20%, and dopamine appears in trace amounts. These hormones enter the bloodstream and intensify the body's sympathetic responses initiated by postganglionic neurons.
Q6: Which organs does the lumbar and sacral splanchnic nerve pathway supply?
The lumbar splanchnic nerves, formed from lumbar ganglia (L1–L4), travel to the inferior mesenteric ganglion and hypogastric plexus. This pathway supplies sympathetic signals to the large intestine, urinary bladder, uterus, and genitals, coordinating sympathetic control of lower abdominal and pelvic organs.
Q7: Why do some preganglionic fibers bypass collateral ganglia to reach the adrenal medulla?
Some preganglionic fibers pass through collateral ganglia without synapsing and continue to the adrenal medulla. This direct pathway allows rapid hormonal release into the bloodstream, enabling the sympathetic nervous system to produce widespread, systemic effects more quickly than local postganglionic signaling alone.