Lisa Marie Lenski, age 14, of Vancouver, Wash., for her question:
WHAT IS THE PERIPHERAL NERVOUS SYSTEM?
A human's nervous system can be separated into three divisions: the central nervous system, the peripheral nervous system and the automatic nervous system.
The central nervous system consists of the brain and the spinal cord while the automatic nervous system regulates the internal organs and blood vessels. The peripheral system carries messages to and from the central nervous system.
The third division, the peripheral nervous system, consists of 12 pairs of cranial nerves and 31 pairs of spinal nerves.
The cranial nerves come from the lower part of the brain. These nerves control many sensations and actions including sight, smell, chewing and swallowing.
The 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. There are five pair of lumbar nerves, which leave the spinal cord at the small of the back, and five pair of sacral nerves and the one pair of coccygeal nerves, which leave the spinal cord between the lowest vertebrae in the spinal cord.
Impulses in our largest nerves travel at a speed of more than 300 feet per second. In the smaller fibers of the peripheral and automatic nervous system, impulses may travel as slowly as 1 1/2 to 6 feet per second.
After a sensory nerve has been stimulated, the impulses may pass up to the brain. The brain then decides how to respond.
As an example, you may see a pencil on your desk and decide to pick it up. Impulses from the nerves in the eye pass along the sensory nerve to the brain. The brain then sends impulses down the motor nerves of the peripheral nervous system and to the muscles of the hand and arm.
The nervous system, like other parts of the body, is made up of cells. Nerve cells are called neurons. Like the other parts of the body, neurons have a nucleus surrounded by cytoplasm. The nucleus and the cytoplasm make up the cell body.
The cytoplasm surrounding the nucleus of a neuron grows out into one or more fine threads or fibers. Most neurons have a single long fiber called an axon and several branched extensions of the cell body called dendrites.
A nerve may be a single nerve fiber or a bundle of fibers. Neurons vary in shape and size depending on their function. Some neurons have short axons and dendrites and others may have axons that are several feet long.
The place where an axon connects to a nerve cell or dendrite is called a synapse. The place where the ends of a motor axon and a muscle meet is called a neuromuscular junction.