Anatomy & Physiology

Anatomy & Physiology Study Guide

14.3 Gray and White Matter Gray matter is the region of integration and command initiation, and white matter carries information from place to place. Large numbers of myelinated and unmyelinated axons are contained within the superficial white matter. The gray matter forms a butterfly shape in the spinal cord and is dominated by the cell bodies of neurons, neuroglia, and unmyelinated axons. Gray matter surrounds the narrow central canal . The projections of gray matter toward the outer surface of the spinal cord are called horns . The posterior gray horns contain somatic and visceral sensory nuclei, whereas the anterior gray horns contain somatic motor nuclei. On each side of the spinal cord, in medial to lateral sequence, are somatic motor nuclei that control muscles that position the pectoral girdle, muscles that move the arm, muscles that move the forearm and hand, and muscles that move the hand and fingers. Since the spinal cord is highly organized, the muscles that will be affected by damage to a specific area of gray matter can be predicted. The white matter on each side of the spinal cord can be separated into three areas called columns or funiculi . Each column contains tracts whose axons share structural and functional characteristics. A tract , or fasciculus, is a bundle of axons in the CNS that are relatively uniform in myelination, diameter, and conduction speed. The same type of information (motor or sensory) is relayed in the same direction along all the axons within a tract. 14.4 Anatomy of Spinal Nerves Plexuses are formed from spinal nerves and are named according to their level of emergence from the vertebral canal. Every segment of the spinal cord is connected to a pair of spinal nerves. Surrounding each spinal nerve is a series of connective tissue layers continuous with those of the associated peripheral nerves. These layers are comparable to those associated with skeletal muscles. A dense network of collagen fibers forms the outermost layer called the epineurium. The fibers of the perineurium , the middle layer, extend inward from the epineurium. These connective tissue partitions separate the nerve into sequences of compartments that include bundles of axons, or fascicles. Gentle connective tissue fibers of the endoneurium , the innermost layer, extend from the perineurium and enclose individual axons. As they extend into the periphery, the spinal nerves branch and interconnect, forming the peripheral nerves that innervate body tissues and organs. White a d Gray Rami The white rami are the myelinated preganglionic axons, and this branch has a light color. Postganglionic fibers that run directly from the ganglion to their respective effector organs innervate smooth muscles, glands, and organs in the thoracic cavity. These axons form a series of sympathetic nerves. Postganglionic fibers innervating glands and smooth muscles in the body wall or limbs return from the ganglion to rejoin the spinal nerve. These fibers, which are unmyelinated and have a darker color, form the gray ramus. The gray ramus is typically proximal to the white ramus; together, they are referred to as the rami communicantes, or “communicating branches.”

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