The spinal cord is perhaps the most simply arranged part of the central nervous system (CNS). Its basic structure, indicated in a schematic drawing of the eighth cervical segment ( Fig. 2.1 ), is the same at every level—a butterfly-shaped core of gray matter surrounded by white matter. An often indistinct central canal in the middle of the butterfly is the remnant of the lumen of the embryonic neural tube.
The extensions of the gray matter posteriorly and anteriorly are termed the posterior and anterior ( dorsal and ventral ) horns , respectively. The zone where the two horns meet is the intermediate gray . At every level, the posterior horn is capped by a zone of closely packed small neurons, the substantia gelatinosa . Beyond this, there are level-to-level variations in the configuration of the spinal gray ( Fig. 2.2 ). For example, the motor neurons that innervate skeletal muscle are located in the anterior horns, so these horns expand laterally in lumbar and lower cervical segments to accommodate the many motor neurons required for the muscles of the lower and upper extremities. Other examples are pointed out in Fig. 2.2 . When studied in microscopic detail, the spinal gray matter can be partitioned into a series of 10 layers (Rexed laminae) , as indicated on the right side of Fig. 2.1 . Some of these laminae have clear functional significance. For example, lamina II corresponds to the substantia gelatinosa, which plays an important role in regulating sensations of pain and temperature.