Spinal cord

The spinal cord is a delicate, tube-like collection of nerves that begins at the base of the brain and travels through the vertebrae to the lower part of the middle back. It is about 18 inches (45 centimeters) long and .5 inches (1.27 centimeters) thick. It consists of nerves that send signals from the brain to the body and vice versa. These nerves are also responsible for reflexes such as a knee jerk.

The vertebrae in the spinal column protect the spinal cord in the same way that the skull protects the brain. The brain and the spinal cord make up the central nervous system (CNS).

Background

The spinal cord consists of a narrow bundle of nerve cells extending from the brain to the first lumbar vertebrae, which are between the rib cage and the pelvis. The spinal cord is like an information superhighway—it receives messages from the brain and sends them to the different parts of the body. For example, the brain may send a message via the spinal cord telling the arms and legs to move. The body also sends messages to the brain through the spinal cord. Suppose a person touches a pan on a stove that is very hot. Their skin will send a message to their brain through their spinal cord. (This particular type of message is a reflex.) The brain will quickly send a message to their hand, making it pull back quickly. Thankfully, messages sent to and from the brain travel extremely fast.

The spinal cord is protected by the spinal column, which is also called the vertebral column or backbone. The vertebrae in the spinal column are stacked one on top of the other. They have a hollow arch, called a spinal canal, which the spinal cord runs through.

Adults have thirty-three vertebrae in their spinal column. These vertebrae are separated by cartilage discs, which give the spinal column flexibility. Health care professionals refer to the vertebrae using a letter and a number. The letter refers to the anatomical region of the body. For example, the C2 vertebra is the second in the cervical region.

The vertebrae are grouped into these anatomical regions:

Cervical: The seven cervical vertebrae are in the neck and allow the head and neck to move. These vertebrae are smaller than those in the other regions.

Thoracic: The twelve thoracic vertebrae are in the chest region and attached to the ribs. The spinal canal is smaller in this region. This puts the spinal cord at a greater risk of injury if the vertebrae are damaged.

Lumbar: The lumbar vertebrae bear the weight of the body. These vertebrae are the largest and widest. Their size allows them to absorb the stress put on the lower back when heavy objects are lifted.

Sacrum: The five sacrum vertebrae connect the spine to the hips. These vertebrae are fused together.

Coccyx: The four vertebrae in this section are also fused together. They form the tailbone.

Overview

Like the spinal column, the spinal cord is divided into anatomical regions. It also has a cervical region, a thoracic region, a lumbar region, a sacrum region, and a coccyx region.

Spinal nerves make up the spinal cord. Thirty-one pairs of spinal nerves are in the spinal column; the number of nerve pairs in each anatomical region varies:

Cervical–eight nerve pairs

Thoracic–twelve nerve pairs

Lumbar–five nerve pairs

Sacrum–five nerve pairs

Coccyx–one nerve pair

Each nerve in the spinal cord is named based on the lower vertebrae it runs between. For example, the C2 nerve runs between the C1 and C2 cervical vertebrae.

Spinal nerves can be either sensory or motor nerves. Sensory nerves transmit sensations such as touch, pain, and temperature. Sensory nerves also send information to the spinal cord about the body's position. Motor nerves pass information received by the brain to the muscles to direct movement.

The spinal cord consists of gray and white matter. The gray matter is shaped like a butterfly. In the center of the gray matter is a canal filled with cerebrospinal fluid.

The four parts of what looks like a butterfly's wings are projections called horns. The two dorsal horns are near the back of the spinal cord, and the two ventral horns are near the front.

The different horns have different functions. For example, the ventral horns transmit information from the brain to the muscles while the dorsal horns transmit sensory information.

The white matter surrounds the gray matter. It consists of bundles of nerve fibers called axons that look like they are in columns. Each column is called a tract and transmits a different type of information.

The spinal cord is covered in three layers of tissue called meninges. The spinal cord and the meninges are in the spinal canal. The innermost membrane, called the pia matter, is attached to the spinal cord. The middle membrane is called the arachnoid matter. The outermost membrane, the dura matter, is the strongest of the three.

At the end of the spinal cord is a bundle of nerves called the cauda equina—this is Latin for horse's tail, which it resembles. The cauda equina carries signals to and from the legs and the pelvic organs, which include the rectum, bladder, and internal genital organs.

Injuries

A spinal cord injury disrupts the signals sent to and from the brain. Spinal cord injuries are usually serious and involve paralysis. Unlike other parts of the body, the spinal cord cannot heal and repair itself. Most spinal cord injuries do not cut through the spinal cord but instead damage vertebrae that cut into the spinal cord. Nearly half of the more than twelve thousand spinal cord injuries that occur in the United States each year are from motor vehicle accidents.

Spinal cord injuries can be either incomplete or complete. In an incomplete injury, the spinal cord is only partially severed and the individual may have some function below the injury. In a complete injury, the spinal cord is completely severed and the individual has no function below the injury.

Generally, the higher the spinal cord injury, the more serious it is. An injury to the vertebrae in the neck can cause quadriplegia, paralysis in both the arms and legs, while an injury to the lower back usually causes paralysis in only the legs.

Bibliography

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