20.10: The Spinal Cord

The spinal cord is the body’s major nerve tract of the central nervous system, communicating afferent sensory information from the periphery to the brain and efferent motor information from the brain to the body. The human spinal cord extends from the hole at the base of the skull, or foramen magnum, to the level of the first or second lumbar vertebra.

Cross-sectional View

The spinal cord is cylindrical and contains both white and grey matter. In the center is the central canal, which is the remnant of the lumen of the primitive neural tube and is part of the internal system of cerebrospinal fluid cavities. In cross-section, the grey matter surrounding the central canal appears butterfly-shaped.

The wings of the butterfly are divided into dorsal and ventral horns. The dorsal horn contains sensory nuclei that relay sensory information, and the ventral horn contains motor neurons that give rise to the axons that innervate skeletal muscle.

White matter surrounds the gray matter and contains large numbers of myelinated fibers. The white matter is arranged into longitudinal bundles called dorsal, lateral, and ventral columns. Three membranes surround the spinal cord: the pia adheres closely to the surface of the spinal cord, followed by the arachnoid, and the dura mater—the tough outermost sheath.

Organization

The spinal cord is divided into four different regions and is segmentally organized. There are 31 pairs of nerves exiting the cord: eight cervical, 12 thoracic, five lumbar, five sacral, and one coccygeal.

Dermatomes

Each segment of the spinal cord innervates a different region of the body and is represented on the skin as a dermatome. A dermatome is an area of the skin supplied by a peripheral nerve originating from a single dorsal root ganglion. Clinically, dermatome maps are useful in determining the level of spinal cord lesions and for localizing peripheral nerve root lesions. However, it must be taken into account that there is considerable variation and overlapping of dermatomes of adjacent spinal segments. Although there are 31 pairs of spinal nerves, there are only 30 dermatomes, as C1 generally has no spinal root, and therefore, the first dermatome corresponds to C2.

The spinal cord is a thick cylinder of nervous tissue that comprises part of the central nervous system, communicating afferent sensory information from the periphery to the brain, and efferent motor information from the brain to the periphery.

In cross-section the gray matter on the interior of the spinal cord appears butterfly-shaped. The ventral horn of the gray matter contains motor neurons. It is surrounded by white matter with groups of axons of both ascending and descending tracts. In the center is a central canal which contains cerebrospinal fluid that nourishes the nervous tissue. The cord itself is surrounded by membranes, the spinal meninges, that include dura mater.

Connected at the brain stem the spinal cord runs through the foramen magnum and the series of bones of the vertebral column until it terminates at the lumbar level of L1-2, and forms a long bundle of nerves called the cauda equina. It's subdivided into levels or regions determined by the position of the vertebrae with paired spinal nerves.

For instance, the cervical spine, regions C2 to C7, correspond to the nerves innervating the neck, shoulders, arms and hands. Within each vertebra nerve root pairs exit through the vertebral foramen into separate branches. The ventral nerve root targets muscles to control movement whereas the dorsal nerve root innervates the skin to convey sensation. Each area of the skin innervated by a specific spinal nerve is called a dermatome and portrayed by dermatome maps. These maps are designed to help clinicians identify where a particular pain or a skin condition may be originating.