A thumb-thick cable that outsources reflexes from the brain
The spinal cord is a long thin tube of nerve tissue running from the medulla through the vertebral canal to the lumbar region. In people it averages about forty-five centimeters in men and forty-three in women—shorter than the spine that shelters it—and carries signals both ways while hosting reflex circuits.
The cord continues straight down from the brainstem, leaving the skull through the foramen magnum and entering the canal of the neck vertebrae. It stops well short of the bottom of the spine, tapering into the conus medullaris between the first and second lumbar vertebrae, after which a bundle of nerve roots called the cauda equina, Latin for horse's tail, carries on to the coccyx. A thin strand, the filum terminale, anchors it to the tailbone. The cord swells in two places: at the neck, from roughly C4 to T1, where nerves serve the arms, and lower down, serving the legs.
It is organised into 31 segments, 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and one coccygeal, each giving off a pair of sensory roots at the back and motor roots at the front. Sensory roots end in dorsal root ganglia holding the cell bodies of their neurons, while motor fibres start from cells in the front grey matter. Because the backbone grows faster than the cord after the fetal stage, adult segments sit higher than the vertebrae that share their names; lumbar and sacral segments lie between vertebral levels T9 and L2, and their nerves travel down before exiting.
A cross-section shows a butterfly of grey matter, holding nerve cell bodies, interneurons and glia, around a central canal filled with cerebrospinal fluid. Surrounding it is white matter made almost entirely of myelinated axons grouped into tracts that carry messages up or down. Three membranes, the meninges, wrap the cord: the tough outer dura mater, with a fat-filled epidural space between it and the bone, and two finer layers within.
The cord's main job is relay: motor commands from the cortex travel down to the body, and sensory signals travel up to the brain. It also runs reflex arcs that act without waiting for the brain, and houses networks of interneurons that coordinate movement. Blood arrives through one anterior and two posterior spinal arteries running its full length, reinforced below the neck by radicular arteries entering alongside the nerve roots.
Source: Spinal cord
