The spinal cord has a core of tissue containing nerve cells, surrounded by long tracts of nerve fibers consisting of axons. The tracts extend up and down the spinal cord, carrying signals to and from the brain. The average size of the spinal cord varies in circumference along its length from the width of a thumb to the width of one of the smaller fingers. The spinal cord extends down through the upper two thirds of the vertebral canal, from the base of the brain to the lower back, and is generally 15 to 17 inches long depending on an individual's height.
The interior of the spinal cord is made up of neurons, their support cells called glia, and blood vessels. The neurons and their dendrites (branching projections that help neurons communicate with each other) reside in an H-shaped region called "grey matter."
The H-shaped grey matter of the spinal cord contains motor neurons that control movement, smaller interneurons that handle communication within and between the segments of the spinal cord, and cells that receive sensory signals and then send information to the centers in the brain.
Surrounding the grey matter of neurons is white matter. Most axons are covered with an insulating substance called myelin, which allows electrical signals to flow freely and quickly. Myelin has a whitish appearance due to which the outer section of the spinal cord is called "white matter."
Most of these electrical signals are then relayed to the brain. The spinal cord also contains neuronal circuits that control reflexes and repetitive movements, such as walking, which can be activated by incoming sensory signals without input from the brain.
The functions of these nerves are determined by their location in the spinal cord. They control everything from body functions, such as breathing, sweating, digestion, and elimination, to gross and fine motor skills, as well as sensations in the arms and legs.
This answer is based on source information from the National Institute of Neurological Disorders and Stroke.
