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.
