B. Root filaments converge to form the dorsal root, which contains afferent (sensory) fibers, and the ventral root, which contains efferent (motor) fibers.
C. The dorsal and ventral roots join to form the mixed spinal nerve near the intervertebral foramen.
1. The white communicating rami contain myelinated preganglionic sympathetic nerve fibers and are present only in spinal nerves Tl — L3.
2. The gray communicating rami contain unmyelinated postganglionic sympathetic nerve fibers and are present in all spinal nerves.
IV. DERMATOMES (Figure 2-3) are strips of skin that extend from the posterior midline to the anterior midline. Dermatomes are supplied by sensory branches of the dorsal and ventral rami of a single spinal nerve. A clinical finding of sensory deficit in a dermatome helps to assess which spinal nerve, nerve root, or spinal cord segment is damaged.
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