Autonomic Nerve Fibers

Autonomic Nerve Fibers

Sympathetic fibers are thoracolumbar, originating in the thoracic and lumbar regions of the spinal cord (FIGURE 14 -3). They have short preganglionic fibers and long postganglionic fibers. Sympathetic ganglia lie closer to the spinal cord than parasym-pathetic ganglia. The sympathetic division innervates more organs than the parasympathetic division, includ-ing the visceral­ organs of the body cavities and the vis-ceral structures in the somatic (superficial) areas of the body. Certain glands and smooth muscle structures in the sweat glands and arrector pili muscles require auto-nomic innervation, served exclusively by sympathetic fibers. Sympathetic fibers also innervate the smooth muscle walls of the arteries and veins. Autonomic nerves innervate organs in the central body cavities. In the autonomic nervous system, there is always a syn-apse between the CNS and the effector organ.

All preganglionic fibers arise from cell bodies of preganglionic neurons in the lateral gray horns of spinal cord segments T₁ to L₂; hence, the thoracolumbar division is the alternate name for the sympathetic divi-sion. Many preganglionic sympathetic neurons exist in the spinal cord’s gray matter, which form its lateral horns. There are no lateral horns in the sacral spinal cord regions because parasympathetic preganglionic neurons are far less abundant there when compared with the sympathetic neurons in the thoracolumbar regions. The preganglionic fibers leave the spinal cord via the ventral root, passing through a white ramus communicans and entering the sympathetic trunk gan-glion to form part of the sympathetic trunk. The sym-pathetic trunks flank each side of the vertebral column and appear like strands of beads that are glistening and white. Each sympathetic trunk is also known as a sym-pathetic chain, and the sympathetic trunk ganglia are also called paravertebral or chain ganglia.

Sympathetic fibers arise only from the thoracic and lumbar spinal cord segments. They leave the spinal cord at the first thoracic vertebrae. There are usually 23­ganglia in each sympathetic trunk (3 cervical, 11 thoracic, 4 ­lumbar, 4 sacral, and 1 coccygeal). Three different things occur when a preganglionic axon reaches a trunk ganglion: The pre- and postganglionic neurons can either synapse at the same level, synapse at a higher or lower level, or synapse in a distant collateral ganglion. When they synapse in a distant collateral ganglion, the pregan-glionic fibers play a role in forming several splanchnic nerves. They synapse in collateral ganglia anterior to the vertebral column. Collateral ganglia are not paired or arranged in segments and only occur in the abdomen and pelvis. However, all sympathetic ganglia are close to the spinal cord. There are three sympathetic collateral ganglia located in the abdominal cavity.