Anatomy of the spinal cord

The spinal cord is a continuation of the medulla oblongata. It starts at the foramen magnum of the occipital bone and descends in the vertebral column ending at the lower border of the first lumbar vertebra.

It is about 45 centimeters in an adult male and about 42 centimeters in an adult female.

It contains nerve fibers to and from the brain. It also has nerve cells responsible for reflex actions. A reflex action is involuntary and instantaneous and occurs at the subconscious level.

Knee jerk is an example of a reflex. With the leg dangling loosely tap the tendon below the patella with a firm object. The leg will immediately kick.

The anterior and posterior surfaces of the spinal cord have grooves which incompletely divide the spinal cord into right and left halves.

31 pairs of nerves called spinal nerves are attached to the sides of the spinal cord. Like the vertebral column which provides a secure covering for it, the spinal cord has cervical, thoracic, lumbar, sacral and coccygeal segments.

The cervical portion gives rise to 8 pairs of nerves, thoracic part 12 pairs of nerves the lumbar and sacral 5 pairs each and the coccygeol one pair making a total of 31 pairs of spinal nerves.

Each spinal nerve has a dorsal root (towards the back of the body) and a vertral root (towards the front of the body). The dorsal root has a bulb like structure called dorsal root ganglion. It contains the cell bodies of the dorsal root fibers.

The dorsal root contains sensory nerve fibers. These fibers carry impulses from various parts of the body to the brain. They are also called afferent nerves.

The ventral root contains fibers which carry impulses from the brain to various organs of the body. The spinal nerves which are formed by the union of dorsal and ventral roots are mixed nerves. A nerve is somewhat like an electric cable with insulation. Just as an electric cable is made of filaments a nerve is made of fibers.

The structural and functional unit of the nervous system is the neuron. What distinguishes a neuron from other cells is the presence of elongated processes projecting from the body of the neuron.

These processes are of two types-axons and dendrites. Dendrites carry impulses towards the cell body while axons carry impulses away from the body.

The cell bodies are situated in the center and this portion of the spinal cord is known as the gray matter. On a cross section of the spinal cord, the demarcation between gray matter and white matter (which contains nerve fibers) can be seen.

The gray matter consists of two symmetrical portions one on each side joined by a transverse portion. On a cross section the gray matter roughly resembles the letter H.

Its shape varies in the different regions of the spinal cord. A minute canal called central canal runs in the center of the horizontal portion of the gray matter.

Like the brain the spinal cord is covered by three membranes. The outermost is called the dura mater which is a tough fibrous membrane.

It extends below the spinal cord and ends as a sac at the level of lower border of second sacral vertebra. The space between the wall of the vertebral canal and the dura mater is called the epidural space.

The middle one is called the arachnoid mater. This is thin and somewhat transparent. Between this and the innermost layer is a space called the subarachnoid space.

This contains a clear fluid called the cerebrospinal fluid. This fluid is taken out and examined by a procedure called lumbar puncture whenever meningitis is suspected.

The innermost layer is called pia mater. This is closely attached to the spinal cord.

Back to human body from spinal cord