Linda Dinsmore, age 13, of Camden, N.J. for her question:
WHAT MAKES UP THE CENTRAL NERVOUS SYSTEM?
A human's nervous system can be separated into three divisions: the central nervous system, the peripheral nervous system and the autonomic nervous system. The central nervous system consists of the brain and the spinal cord.
Your spinal cord is a long, thick nerve trunk that runs from the base of the brain down through the spinal column, or backbone. The cord is composed of white matter and gray matter. The white matter is nerve fibers covered with a white myelin sheath while the gray matter is made up of cell bodies and dendrites of neurons.
A cross section view of the spinal cord shows the gray matter as an H shaped area surrounded by white matter.
Thirty one pairs of spinal nerves leave the spinal cord. Each of these nerve trunks is attached to the cord in two places. The root of the nerve that leaves the cord toward the front of the body is called the ventral root or anterior root. The root that leaves toward the rear of the body is called the dorsal root or posterior root.
Sensory nerves enter the spinal cord at the dorsal root. Motor nerves leave the spinal cord at the ventral root.
If the ventral root of a nerve is cut, the part of the body to which the nerve goes cannot move, but it still has sensation. If the dorsal root of a nerve is cut, sensation disappears, but the body part can still move.
Some nerve immpulses entering the spinal cord are directed to the brain. Others are routed to nerve centers and then to the parts of the body that the nerves control. Responses from impulses that go almost directly to motor nerves are called spinal reflexes.
The peripheral nervous system consists of 12 pairs of cranial nerves and the 31 spinal nerves.
The autonomic nervous system regulates the internal organs and blood vessels.
Control of the autonomic nervous system is largely automatic and is strongly affected by an individual's emotional state. Some degree of voluntary control can be developed by a conditioning method called biofeedback.
Nerves of the autonomic nervous system are connected to the central nervous system and are normally regulated at a subconscious level. Each nerve consists of a chain of two or more neurons leading from the spinal cord, through a ganglion and to an organ.
The nerve fibers have little or no myelin sheath. Most nerves in the autonomic system are effectors or motor nerves, but each trunk also has reaceptor fibers.
The cranial nerves of the peripheral nervous system come from the lower part of the brain. These nerves control many sensations and actions including sight, smell, chewing and swallowing. The spinal nerves of the peripheral nervous system come from the spinal cord and control the muscles of the body.
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