has numerous dorsal (posterior) and ventral (anterior) rootlets that arise from the respective surfaces
of the spinal cord. Dorsal rootlets contain neuronal processes that conduct afferent impulses to the spinal cord, whereas the ventral rootlets conduct efferent impulses from the spinal cord. Respective
rootlets from each segment unite to form dorsal and ventral roots:
■ The dorsal root contains the central processes of sensory neuronal cell bodies that are located in the dorsal root ganglion (DRG). The DRG is also called a spinal ganglion. The peripheral processes of these neurons are located in the spinal nerve, its rami, and their branches. These processes end at or form receptors.
■ The ventral root contains motor fibers. Their neuronal cell bodies are found in the gray matter of the spinal cord: ventral horn if the axons innervate skeletal muscle; lateral horn if the axons supply smooth muscle, cardiac muscle, or glands. The dorsal and ventral roots join to form a short, mixed spinal nerve. Almost immediately after its formation, the spinal nerve divides into mixed dorsal and ventral rami:
■ Dorsal rami supply intrinsic (deep) muscles of the back and neck, joints of the vertebral column, and skin on the dorsal surface of the trunk, neck, and head.
■ Ventral rami innervate all other muscles of the neck and trunk (including the diaphragm), skin of the anterior and lateral body walls, and all muscles, and skin of the limbs. In general, ventral rami are larger than dorsal rami. The 31 pairs of spinal nerves arise from the five regions of the spinal cord. Most of the spinal nerves (C2-L5) are formed in an intervertebral foramen. The C1 spinal nerve emerges between the skull and the first cervical vertebra. Sacral spinal nerves are formed in the vertebral canal and their rami exit through respective dorsal and ventral sacral foramina. The Co1 spinal nerve exits inferior to the rudimentary transverse process of the first segment of the coccyx.
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