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The Spine
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An Overview

The spinal cord is the most important structure between the body and the brain. The spinal cord extends from the foramen magnum, where it is continuous with the medulla to the level of the first or second lumbar vertebrae. It is a vital link between the brain and the body, and from the body to the brain.

The spine is of great clinical importance because it is a major site of traumatic injury and the focal point for many disease processes.  Although the spinal cord constitutes only about 2% of the central nervous system (CNS), its functions are vital.

The spinal cord begins at the base of the brain and extends into the pelvis. Many of the nerves of the periphery nervous system, or PNS, branch out from the spinal cord and travel to various parts of the body.  Information from the senses travels through the nerves of the PNS to the spinal cord and then to the brain for processing, and commands from the brain travel down the spinal cord and then to the appropriate part of the PNS, where nerves transport the instructions to the body part where action is needed.

To facilitate this process, the spinal cord is divided into two kinds of pathways, called "tracts."  Acending tracts carry sensory input from the body to the brain, and descending tracts carry commands from the brain.

The spinal cord is also essential for reflex function. Reflexes are the body’s way of coping with stimuli that require an immediate response.  Jerking away from something hot or sharp is a reflex action. It happens immediately because instructions come from the spine, rather than the brain, to avoid injury.

The spinal cord, like the brain, has two major layers of protection. First are the vertebrae of the spine, and underneath those are three layers of tough membrane called the "meninges."  The meninges surround both brain and spinal cord and are filled with a liquid called "cerebrospinal fluid." The fluid has several functions, and one of them is shock absorption.

The spinal cord can suffer physical damage that can hamper or even halt communication between brain and body. If the spinal cord is severed, the part of the body below the damage is cut off from the commands of the brain, which causes paralysis.

Spinal Cord and Nerves

The spinal cord is 40 to 50 cm long and 1 cm to 1.5 cm in diameter.  It extends from the skull to your lower back and travels through the middle part of each stacked vertebra, called the "central canal."

Two consecutive rows of nerve roots emerge on each side of the spinal cord. These nerve roots join distally to form 31 pairs of spinal nerves, which branch out from the spinal cord through openings in the vertebrae and carry messages between the brain and muscles.

The spinal cord is a cylindrical structure of nervous tissue composed of white and gray matter, is uniformly organized, and is divided into four regions, each of which is comprised of several segments.  The spinal nerve contains motor and sensory nerve fibers to and from all parts of the body.  Each spinal cord segment innervates a dermatome (an area of the skin supplied by nerves from a single spinal root).

The spinal cord ends around the first and second lumbar vertebrae in the lower back and continues as nerve roots. This bundle of nerve roots is called the "cauda equina."  They exit the spinal canal through openings in the vertebrae (foramen), just like other nerve roots.