Innervation of Blood Vessels

The chief outflow of sympathetic preganglionic fibers is through the anterior roots of spinal nerves T1 to L2. The fibers pass in white rami communicantes to adjacent sympathetic trunk ganglia, where many relay. The axons of these ganglionic cells (postganglionic fibers) may pass in nerves to nearby structures, such as midline vessels and prevertebral plexuses (cardiac, celiac, mesenteric), or they may join the lowest cervical, thoracic, and upper lumbar spinal nerves through gray rami communicantes, to be distributed with them to vessels and glands in the thoracic and abdominal cavities and limbs.

Other preganglionic fibers, however, do not relay in adjacent trunk ganglia, but ascend or descend in the sympathetic trunks to form synapses in the cervical or lower lumbar and sacral ganglia. The axons (postganglionic fibers) of the cervical ganglionic cells supply the vessels and glands in the head and neck, while others contribute to the sympathetic cervical cardiac nerves. Some of the postganglionic fibers arising in the lumbar and sacral ganglia run in lumbar and sacral splanchnic nerves to the mesenteric and hypogastric plexuses, but others pass through gray rami communicantes to the lumbar, sacral, and coccygeal spinal nerves to be distributed with them and their branches to vessels, sweat glands, and arrectores pilorum muscles in the loin, lower abdominal wall, buttocks, perineum, and lower limbs.

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