The Spinal Cord

 

The Spinal Cord

The spinal cord is a cylindrical shaped bundle of nerve fibers that is connected to the brain at the brain stem. The spinal cord runs down the center of the protective spinal column extending from the neck to the lower back. The brain and spinal cord are the major components of the central nervous system (CNS). The CNS is the processing center for the nervous system, receiving information from and sending information to the peripheral nervous system. Peripheral nervous system cells connect various organs and structures of the body to the CNS through the cranial nerves and spinal nerves. Spinal cord nerves transmit information from body organs and external stimuli to the brain and send information from the brain to other areas of the body.

Spinal Cord Anatomy

The spinal cord is composed of nervous tissue. The interior of the spinal cord consists of neurons, nervous system support cells called glia, and blood vessels. Neurons are the basic unit of nervous tissue. They are composed of a cell body and projections that extend from the cell body that are able to conduct and transmit nerve signals. These projections are axons (carry signals away from the cell body) and dendrites (carry signals toward the cell body). The neurons and their dendrites are contained within an H-shaped region of the spinal cord called "grey matter." Surrounding the grey matter area is a region called "white matter." The white matter section of the spinal cord contains axons that are covered with an insulating substance called myelin. Myelin is whitish in appearance and allows electrical signals to flow freely and quickly. Axons carry signals along descending and ascending tracts away from and toward the brain.
Neurons are classified as either motor, sensory, or interneurons. Motor neurons carry information from the central nervous system to organs, glands, and muscles. Sensory neurons send information to the central nervous system from internal organs or from external stimuli. Interneurons relay signals between motor and sensory neurons. The descending tracts of the spinal cord consist of motor nerves that send signals from the brain to control voluntary and involuntary muscles. They also help to maintain homeostasis by assisting in the regulation of autonomic functions such as heart rate, blood pressure, and internal temperature. The ascending tracts of the spinal cord consist of sensory nerves that send signals from internal organs and external signals from the skin and extremities to the brain. Reflexes and repetitive movements are controlled by spinal cord neuronal circuits that are stimulated by sensory information without input from the brain.
The axons that link the spinal cord to the muscles and the rest of the body are bundled into 31 pairs of spinal nerves, each pair with a sensory root and a motor root that make connections within the grey matter. These nerves must pass between the protective barrier of the spinal column to connect the spinal cord to the rest of the body. The location of the nerves in the spinal cord determine their function.

Spinal Column

The spongy spinal cord is protected by the irregular shaped bones of the spinal column called vertebrae. Spinal vertebrae are components of the axial skeleton and each contain an opening that serves as a channel for the spinal cord to pass through. Between the stacked vertebrae are discs of semi-rigid cartilage, and in the narrow spaces between them are passages through which the spinal nerves exit to the rest of the body. These are places where the spinal cord is vulnerable to direct injury. The vertebrae can be organized into sections, and are named and numbered from top to bottom according to their location along the backbone:

• Cervical vertebrae (1-7) located in the neck
• Thoracic vertebrae (1-12) in the upper back (attached to the ribcage)
• Lumbar vertebrae (1-5) in the lower back
• Sacral vertebrae (1-5) in the hip area
• Coccygeal vertebrae (1-4 fused) in the tail-bone

The spinal cord is also organized into segments and named and numbered from top to bottom. Each segment marks where spinal nerves emerge from the cord to connect to specific regions of the body. Locations of spinal cord segments do not correspond exactly to vertebral locations, but they are roughly equivalent.

• Cervical spinal nerves (C1 to C8) control signals to the back of the head, the neck and shoulders, the arms and hands, and the diaphragm.
• Thoracic spinal nerves (T1 to T12) control signals to the chest muscles, some muscles of the back, and parts of the abdomen.
• Lumbar spinal nerves (L1 to L5) control signals to the lower parts of the abdomen and the back, the buttocks, some parts of the external genital organs, and parts of the leg.
• Sacral spinal nerves (S1 to S5) control signals to the thighs and lower parts of the legs, the feet, most of the external genital organs, and the area around the anus.

The single coccygeal nerve carries sensory information from the skin of the lower back.

Spinal Cord Injury

The consequences of a spinal cord injury vary depending on the size and severity of the injury. A spinal cord injury may cut off normal communication with the brain that can result in a complete or incomplete injury. A complete injury results in a total lack of sensory and motor function below the level of injury. In the case of an incomplete injury, the ability of the spinal cord to convey messages to or from the brain is not completely lost. This type of injury enables a person to maintain some motor or sensory function below the injury.