Spinal Cord Health Article

Definition

The spinal cord is the elongated bundle of nervous tissue that carries nerve impulses between the brain and the rest of the body. It lies in the vertebral canal of the vertebral column.

Description

The spinal cord lies within the vertebral canal, which is the hollow part of the vertebral column, or spine, that consists of 33 bones called vertebrae. The canal is formed by the stacked vertebrae which all contain a central vertebral foramen, or hole. The spinal cord extends from the lowest part of the brain, called the brainstem, through a hole located at the base of the skull, the foramen magnum, and continues down the vertebral canal to the twenty-first vertebra of the spine.

Like the brain, the spinal cord is protected by three layers of membranes, called meninges. The inner meninge that makes direct contact with the spinal cord is called the pia mater. It is separated from the second layer by a space called the subarachnoid space. This space is filled with cerebrospinal fluid (CSF), the colorless fluid that bathes the entire brain and spinal cord. The second layer is the thin and spider web-like arachnoid mater and it is separated from the outermost layer by a space called the subdural space. The outermost layer is the dura mater, a protective sheath made of tough fiber. Between the dura mater and the bone of the vertebral canal is a space, called the epidural space, which contains a small amount of fatty tissue and blood vessels. The spinal dura mater

prolongs the dura mater that lines the skull cavity and extends to the sacrum, the second to last bone of the vertebral column. It also covers each of the spinal nerves as they leave the vertebral canal. Both the arachnoid and pia mater also prolong the arachnoid and pia surrounding the brain, but unlike the arachnoid, which continuously follows the dura mater, the pia ends where the spinal cord ends. A stringy extension of the pia mater, called the filum terminale joins the end of the spinal cord to the end of the dura mater. Additionally, the pia mater contains thin projections called denticulate ligaments, that connect the spinal cord to the dura mater.

Function

The major function of the spinal cord is to carry nerve impulses between the brain and the rest of the body. Together, the brain and the spinal cord constitute the central nervous system (CNS). The other nerves of the nervous system, that is the motor and sensory nerves, constitute the peripheral nervous system (PNS).

The spinal cord consists of a core of grey nervous tissue surrounded by a thicker section of white tissue. The grey matter looks like a butterfly with outspread wings and the upper and lower sections of these wings are called the posterior and anterior horns. The tissues of the spinal cord are full of nerve cells, also called neurons. Neurons with large cell bodies that are located in the anterior horns give rise to motor nerve fibers that connect to spinal nerves which pass out of the cord to skeletal muscle. The grey matter of the spinal cord also contains other neurons that connect together to form nerve pathways and the white matter contains nerves that are wrapped in myelin sheaths and form nerve tracts. The tracts that conduct sensory impulses from the body to the brain are called ascending tracts and those that conduct motor impulses from the brain to muscles and glands are called descending tracts.

Thirty-one pairs of spinal nerves emerge from the spinal cord. They are all mixed nerves, meaning that they provide a two-way communication system for sensory and motor information exchange between the spinal cord and the rest of the body. Spinal nerves are numbered according to the vertebral column level from which they stem. There are eight pairs of cervical nerves, C1 to C8, twelve pairs of thoracic nerves, T1 to T12, five pairs of lumbar nerves, L1 to L5, five pairs of sacral nerves, S1 to S5, and one pair of coccygeal nerves.