Spinal cord: anatomy, location and function

The spinal cord (Medulla spinalis) is the elongated part of the central nervous system, which comprises the brain and spinal cord. It lies inside the vertebral canal, the passage within the vertebral column, and continues from the brain at the foramen magnum, the large opening in the skull base.

Where does the spinal cord begin and end?

At the foramen magnum, the spinal cord continues from the medulla oblongata, the lowest part of the brainstem. The brainstem is the part of the brain connecting its higher regions with the spinal cord. Within the vertebral canal, the cord lies behind the vertebral bodies, the weight-bearing anterior portions of the vertebrae.

In adults, the cord usually ends near the level of the first or second lumbar vertebra. Its tapered lower end is the conus medullaris. Below this, descending spinal nerve roots form the cauda equina, a bundle named for its resemblance to a horse's tail.

The filum terminale is a slender connective-tissue strand extending from the conus medullaris towards the coccyx, the terminal bone of the vertebral column. It helps anchor the lower end of the cord.

What coverings surround the spinal cord?

The meninges are connective-tissue membranes surrounding the brain and spinal cord. From outside inward, they are the dura mater, arachnoid mater and pia mater. The pia mater closely follows the cord's surface.

The epidural space lies between the spinal dura mater and the vertebral canal and contains fat and a network of veins. The subarachnoid space, between the arachnoid mater and pia mater, contains cerebrospinal fluid, the fluid surrounding the central nervous system.

Below the conus medullaris, this fluid-filled space continues around the cauda equina. Denticulate ligaments are lateral extensions of pia mater that attach to the dura mater and help stabilise the cord.

What are the spinal cord segments?

A spinal cord segment is the portion associated with one pair of spinal nerves. There are 31 segments, grouped into cervical, thoracic, lumbar, sacral and coccygeal regions. These names describe their associated nerves, not necessarily the vertebrae beside them.

Because the vertebral column grows longer than the cord during development, lower spinal cord segments lie above their correspondingly named vertebrae. Their roots therefore travel downwards before leaving the canal.

What are the spinal cord's external features and nerve roots?

The cervical and lumbosacral enlargements are widened regions associated with the nerve supply to the upper and lower limbs. Along the front of the cord, the anterior median fissure forms a deep midline groove. The posterior median sulcus is a shallower groove on its back surface.

Small bundles of nerve fibres, called rootlets, join to form anterior and posterior roots. Anterior roots carry motor fibres away from the cord, while posterior roots carry sensory fibres towards it.

A typical posterior root has a spinal ganglion, a collection of sensory nerve-cell bodies outside the central nervous system. The C1 posterior root and its ganglion may be absent. The anterior and posterior roots unite to form a mixed spinal nerve, containing both motor and sensory fibres.

How are grey and white matter arranged?

The grey matter of spinal cord forms an inner, roughly H-shaped region containing nerve-cell bodies, their processes and supporting cells. Its posterior horns receive and process sensory information. Its anterior horns contain motor nerve cells supplying skeletal muscles, the muscles responsible for movement of the skeleton.

Lateral horns are prominent in thoracic and upper lumbar segments and contain sympathetic nerve-cell bodies. These belong to the autonomic nervous system, which regulates involuntary activity in structures such as smooth muscle and glands.

The white matter of spinal cord surrounds the grey matter and contains mainly axons, the long processes that carry signals away from nerve-cell bodies. Many axons have myelin, an insulating sheath that assists signal conduction. Axons form tracts, bundles travelling together through the central nervous system.

The central canal of spinal cord is a narrow channel within the central grey matter. It is lined by ependymal cells, specialised cells lining the central nervous system's fluid spaces, although its lumen—the internal passage—often becomes partly or completely closed in adults.

What does the spinal cord do?

Ascending tracts carry sensory information towards the brain, including information about touch and body position. Descending tracts carry signals from the brain that influence movement and other spinal activity.

Local circuits also produce reflexes, automatic responses to incoming signals. In a spinal reflex, sensory input can activate motor output through connections within the cord, without requiring the response to be initiated by the brain. Spinal nerve cells also contribute to autonomic control of internal organs.

What supplies blood to the spinal cord?

The anterior spinal artery runs along the front of the cord, while paired posterior spinal arteries supply its posterior region. Segmental medullary arteries entering through openings between vertebrae reinforce this supply at different levels.

Surface arteries give rise to branches entering the cord. Veins drain into the internal vertebral venous plexus, an interconnected network of veins within the vertebral canal.

How can you explore the spinal cord in the 3D model?

In the 3D model next to the article, select the spinal cord and isolate it, then rotate and zoom to examine its overall shape. Switch layers on and off or fade surrounding structures to compare its position with the vertebral column. A transverse section—a cut across the body's long axis—provides an orientation for studying cross-sectional relationships.