Sympathetic Division

Within the sympathetic division (thoracolumbar division), the preganglionic fibers originate from neurons within the lateral horn of the spinal cord. These neurons are found in all of the thoracic segments and in the upper two or three lumbar segments of the cord. Their axons exit through the ventral roots of spinal nerves along with various somatic motor fibers.

After traveling a short distance, preganglionic fibers leave the spinal nerves through branches called white

Saphenous n.

Saphenous n.

Figure 11.35

Nerves of the lumbosacral plexus. (a) Anterior view. (b) Posterior view.

Superior gluteal n

Sacral plexus

Inferior gluteal n

Sciatic n

Common peroneal n

Superior gluteal n

Sacral plexus

Inferior gluteal n

Sciatic n

Common peroneal n

Pudendal n.

Tibial n.

Figure 11.35

Nerves of the lumbosacral plexus. (a) Anterior view. (b) Posterior view.

Pudendal n.

Tibial n.

rami (sing., ramus) and enter sympathetic ganglia. Two groups of such ganglia, called paravertebral ganglia, are located in chains along the sides of the vertebral column. These ganglia, with the fibers that connect them, comprise the sympathetic trunks (fig. 11.37).

The paravertebral ganglia lie just beneath the parietal pleura in the thorax and beneath the parietal peritoneum in the abdomen (see chapter 1, p. 12). Although these ganglia are located some distance from the viscera they help control, other sympathetic ganglia are nearer to the viscera. The collateral ganglia, for example, are found within the abdomen, closely associated with certain large blood vessels (fig. 11.38).

Some of the preganglionic fibers that enter paraver-tebral ganglia synapse with neurons within these ganglia. Other fibers extend through the ganglia and pass up or down the sympathetic trunk and synapse with neurons in ganglia at higher or lower levels within the chain. Still other fibers pass through to collateral ganglia before they synapse. Typically, a preganglionic axon will synapse with several other neurons within a sympathetic ganglion.

The axons of the second neurons in sympathetic pathways, the postganglionic fibers, extend out from the sympathetic ganglia to visceral effectors. Those leaving paravertebral ganglia usually pass through branches called gray rami and return to a spinal nerve before proceeding to an effector (fig. 11.38). These branches appear gray because the postganglionic axons generally are un-myelinated, whereas the preganglionic axons in the white rami are nearly all myelinated.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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