Helen Myers, age 13, of Astoria, Ore., for her question:
WHAT MAKES UP THE NERVOUS SYSTEM?
A human nervous system is made up of the brain, the spinal cord and the nerve cells and fibers that extend throughout the body. The nervous system receives messages from the sense organs, such as the ears and eyes. It interprets these messages and lets us react.
Actually, the nervous system enables all parts of our body, including the internal organs, to work together to keep up alive and well.
Three separate divisions make up the nervous system: the central nervous system, the peripheral nervous system and the automatic nervous system
Making up the central nervous system is the brain and spinal cord.
The 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 nerve fibers covered with a white myelin sheath, called white matter, and cell bodies and dendrites of neurons, called gray matter.
Some nerve impulses 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.
The peripheral nervous system is made up of 12 pairs of cranial nerves and the 31 pair of spinal nerves. Cranial nerves come from the lower part of the brain. These nerves control many sensations and actions, including sight, smell, chewing and swallowing.
Spinal nerves come from the spinal cord and control the muscles of the body. There are eight pair of cervical nerves that leave the spinal column from the first seven vertebrae. Below them are 12 pair of thoracic nerves. The five pair of lumbar nerves leave the spinal cord at the small of the back.
The automatic nervous system regulates the internal organs and blood vessels. This control is largely automatic and is strongly affected by an individual's emotional state.
Nerves of the automatic nervous system are connected to the central nervous system and are normally regulated at a subconscious level.
The automatic nervous system has two main parts; the sympathetic, or thoracolumbar system, and the parasympathetic, or craniosacral system.
Such nerves produce opposite reactions in the organs to which they go. A nerve from one system speeds up the heartbeat, for example, and a nerve from the other system slows it down.
Impulses in man's largest nerves travel at a speed of more than 300 feet per second. In the smaller fibers of the automatic nervous system, impulses may travel as slowly as one and a half to six feet per second.
The strength and speed of the impulse or action potential in a nerve do not vary, regardless of the strength or nature of the stimulus.
A nerve may, however, send impulses more often when a strong stimulus is applied.