All living things can detect changes within themselves and in their environments. In the human body it is the nervous system that makes it possible to recognize and respond to those changes.
The nervous system consists of two main parts: the central nervous system and the peripheral nervous system. The brain and spinal cord make up the central nervous system. The peripheral nervous system consists of all the nerves that carry information in the form of impulses to and from the central nervous system.
The nerve cells, or neurons, make up the basic unit of the nervous system. Long and tube-shaped extensions of nerve cells form into bundles, and these make up the actual nerves. The human body contains billions of neurons, including more than 10 billion in the brain alone.
Neurons have much in common with other cells, but there are differences. Neurons are highly specialized, and they are not able reproduce in the usual manner. In the central nervous system, regrowth of cells is impossible. That is why severe damage to the brain or spinal cord is considered permanent. Such damage can result in paralysis of muscles or limbs because messages to move those muscles cannot go beyond the point of injury. Neurons in the peripheral nervous system can regrow if only slightly damaged. If severely damaged, however, those neurons are not able to regrow.
The nervous system consists of several different types of nerves. Sensory nerves handle the processing of information relating to such senses as taste, touch, and smell. They send this information from the sense organs to the central nervous system. Motor nerves, in contrast, carry messages in the other direction: from the central nervous system to muscles and glands throughout the body so it can be acted upon. Spinal nerves consist of bundles of sensory and motor nerves that come from the spinal cord in pairs. Connecting with the brain stem and the lower parts of the brain are the cranial nerves.
Finally, there are the autonomic nerves that regulate many important, though unconscious, activities of the human body. For example, autonomic nerves control the muscles that operate the digestive system, circulatory system, body temperature, and the breathing process.
The different kinds of nerves all work together to control and protect the body. For example, when someone touches a very hot surface, the sensory nerves on the fingers and hand send a signal to the brain indicating that the surface is hot. The brain then sends a message through the motor nerves to the muscles of the hand to make the hand flinch.
