Anatomy of the Spine and Peripheral Nervous System

The spinal cord is an extension of the central nervous system (CNS), which consists of the brain and spinal cord. It begins at the bottom of the brain stem (at the area called the medulla oblongata), and it ends in the lower back as it tapers to form a cone called the conus medullaris.

Anatomically, the spinal cord runs from the top of the highest neck bone (the C1 vertebra) to approximately the level of the L1 vertebra, which is the highest bone of the lower back and which is found just below the rib cage. The spinal cord is about 18 inches (45 centimeters) in length and is more or less cylindrical in shape, but two areas of enlargement are found in the cervical (neck) and lumbar (lower back) segments. Beginning at the tip of the conus medullaris, a fibrous band called the filium terminale extends down to the pelvis.

At the bottom of the spinal cord (conus medullaris) is a collection of nerves known as the cauda equina, which is Latin for "horse's tail". Early anatomists thought the collection of nerves looked like a horse's tail.

The spinal cord is bathed in cerebrospinal fluid (CSF) and surrounded by three protective layers called the meninges (dura, arachnoid, and pia mater).

The spinal cord lies inside the spinal column, which is made up of 33 bones called vertebrae. Five vertebra are fused together to form the sacrum (part of the pelvis), and four small vertebra are fused together to form the coccyx (tailbone).

The spine itself is divided into four sections, not including the tailbone:

Between the vertebral bodies (except C1 and C2) are discs that serve as supporting structure for the spine. These discs are oval-shaped, with a tough outer layer (annulus) that surrounds a softer material called the nucleus pulposus. These discs act as shock absorbers for the spinal bones. Ligaments attached to the vertebrae also serve as supporting structures.

There are 31 pairs of spinal nerves and roots. Eight pairs of cervical nerves (called C1-C8) exit from the cervical cord at each vertebral level. One member of the pair exits on the right side, and the other exits on the left. The first cervical root exits above the C1 vertebra. The second cervical root exits between the C1-C2 segment, and the remaining roots exit just below the correspondingly numbered vertebra. The C8 nerve root exits between the C7 and T1 vertebra. There are 12 thoracic nerve pairs (T1-T12). The T1 nerve root exits between the T1 and T2 vertebrae. There are five lumbar nerve pairs (L1-L5). The L1 nerve root exits between L1 and L2. There are five sacral nerve pairs (S1-S5). The S1 nerve root exits between S1 and S2. One pair of coccygeal (Co1) nerves meets in the area of the tailbone.

Nerve impulses travel to and from the brain through the spinal cord to a specific location by way of the peripheral nervous system (PNS). The PNS is a complex system of nerves that branch off from the spinal nerve roots. These nerves travel outside of the spinal canal to the upper extremities (arms, hands and fingers), to the muscles of the trunk, to the lower extremities (legs, feet and toes), and to the organs of the body.

Any interruption of spinal cord function by disease or injury at a particular level may result in a loss of sensation and motor function below that level. Depending on the severity of the disease or injury, the loss of function may be permanent.

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