The spinal cord is continuous with the brain. It descends from the medulla oblongata through the foramen magnum into the vertebral canal and extends to the second lumbar vertebra. Beyond this point, only the roots of the inferior spinal nerves occupy the vertebral canal.
The spinal cord is cylindrical in shape. It has two small grooves that extend throughout its length: the wider anterior median fissure and the narrower posterior median sulcus. These grooves divide the spinal cord into left and right portions. Thirty-one pairs of spinal nerves branch from the spinal cord. The spinal cord is divided into four segments-cervical, thoracic, lumbar, and sacral– based upon where the spinal nerves exit the vertebral column.
The cross-sectional structure of the spinal cord is shown in figures 8.12 and 8.17 . Gray matter, shaped like the outstretched wings of a butterfly, is centrally located and is surrounded by white matter. The central canal extends the length of the spinal cord and contains CSF.
The pointed projections of the gray matter, as seen in cross section, are called horns. The anterior horns contain the cell bodies of somatic motor neurons whose axons enter spinal nerves and carry nerve impulses to skeletal muscles. The posterior horns contain interneurons that receive nerve impulses from sensory axons in the spinal nerves and carry them to sites within the CNS. Lateral horns, found only in the thoracic and lumbar segments of the spinal cord, contain the cell bodies of autonomic motor neurons whose axons follow ANS pathways as they carry nerve impulses to cardiac and smooth muscle, glands, and adipose tissue. Interneurons form most of the gray matter in the CNS.
The horns of the gray matter divide the white matter into three regions: the anterior, posterior, and lateral funiculi (singular, funiculus). These funiculi contain nerve tracts, which are bundles of myelinated and unmyelinated axons of interneurons that extend superiorly and inferiorly within the spinal cord.
The spinal cord has two basic functions. It transmits nerve impulses to and from the brain, and it serves as a reflex center for spinal reflexes. Nerve impulses are transmitted to and from the brain by axons composing the nerve tracts. Ascending (sensory) tracts carry sensory nerve impulses to the brain; descending (motor) tracts carry motor nerve impulses from the brain.