脊髄（せきずい）

The spinal cord contains gray matter in the center, appearing butterfly-shaped in cross-section, surrounded by white matter. The gray matter includes dorsal horns that relay sensory information and ventral horns containing motor neurons. A central canal runs through the middle, containing cerebrospinal fluid that nourishes nervous tissue. Three protective membranes, the spinal meninges, surround the entire cord.

The spinal cord communicates afferent sensory information from the periphery to the brain and efferent motor information from the brain to the periphery. Dorsal nerve roots convey sensation to the spinal cord, while ventral nerve roots target skeletal muscle to control movement. This bidirectional communication allows the central nervous system to receive sensory input and send motor commands throughout the body.

The spinal cord connects at the brain stem and runs through the foramen magnum at the base of the skull. It extends downward through the vertebral column and terminates at the lumbar level between L1 and L2 vertebrae. Below this termination point, a bundle of nerves called the cauda equina continues downward through the vertebral canal.

Dermatomes are areas of skin supplied by a peripheral nerve from a single dorsal root ganglion. Each spinal cord segment innervates a specific body region represented as a dermatome on dermatome maps. Clinicians use these maps to identify the origin of pain or skin conditions and to localize spinal cord lesions or peripheral nerve root damage, though adjacent dermatomes overlap considerably.

The spinal cord is divided into four regions and segmentally organized with 31 pairs of spinal nerves: eight cervical, 12 thoracic, five lumbar, five sacral, and one coccygeal. Each region corresponds to vertebral levels. For example, cervical regions C2 to C7 innervate the neck, shoulders, arms, and hands. Paired spinal nerves exit through vertebral foramina at each level.

White matter surrounds the gray matter and contains large numbers of myelinated axon fibers arranged into longitudinal bundles called dorsal, lateral, and ventral columns. These columns form ascending and descending tracts that carry sensory and motor information between the brain and peripheral nervous system, enabling communication throughout the central nervous system.

Ventral nerve roots contain motor axons that target skeletal muscles to control movement and motor function. Dorsal nerve roots contain sensory axons that innervate the skin to convey sensation and sensory information. This functional separation allows the spinal cord to simultaneously process motor commands and sensory input from different body regions.