Many living creatures can run faster than human beings can. Some can see better in dim light. Others can hear better, and still others can smell faint odors better at a distance. What, then, has enabled humans to achieve high levels of technology and communication while other species have not? Humans have a highly developed brain that makes it possible to learn, to remember what has been learned, and, most important, to reason. The brain, however, is just part of the complex nervous system, which controls and coordinates essential body functions. The nervous system sends special signals to and receives responses from every organ and tissue of the body. These signals from the nervous system make it possible for you to play the piano, thread a needle, throw a baseball, write in your notebook, or just sit and think. Without a nervous system, the brain is completely unable to function.
The nervous system
The nervous system has two main subdivisions. One part, the central nervous system, consists of the brain and the spinal cord. The central nervous system receives stimuli from inside and outside the body and then coordinates the body’s response. The second part of the nervous system is the peripheral (pun rihf uhr uhl) nervous system. It provides the pathways to and from the central nervous system for electrochemical signals called impulses.
Three types of body structures are needed for the entire process of picking up stimuli and responding to them. They are receptors, conductors, and effectors. To understand how these different structures are coordinated, consider what happens when a doorbell rings. First, the ear acts as a receptor that picks up the sound of the ringing bell. A receptor is a cell, group of cells, or organ, that detects a stimulus. The receptor then generates impulses that travel along conductors, or nerve cells. Ultimately, the impulses reach effectors-structures that may react to the original stimulus. In this case, muscles are the effectors. The reaction to the original stimulus is to walk to the door and open it.
The basic functional unit of the nervous system is the nerve cell, called a neuron. Three types of neurons interact in the nervous svstem. Neurons that receive stimuli and transmit them to the central nervous system are sensory neurons. Neurons that carry impulses away from the central nervous system to muscles or glands are motor neurons. The third type of neuron, an inter-neuron, links sensory and motor neurons.
Every neuron consists of a cell body, which contains the nucleus and cytoplasm, and threadlike extensions of cytoplasm called nerve fibers. A neuron has two kinds of nerve fibers. Dendrites are fibers that carry impulses from other neurons or receptors toward the cell body. Dendrites are generally short, branched fibers. The second kind of nerve fiber is the axon, which carries impulses away from the cell body to other neurons or to effectors. A neuron has many dendrites but only one axon. Axons are usually longer than dendrites, and any branches axons have exist only at the end of the fiber.
An axon may be wrapped in a fatty insulating layer known as a myelin sheath. The sheath is formed by special cells called Schwann cells. The sheath supports, insulates, and nourishes the axons. It also helps maintain the chemical balance of the axon. Gaps between the Schwann cells, called the nodes of Ranvier, occur about every 1 mm (0.04 in.) along the myelinated axons.
Neurons are the largest cells in the body. Some neurons may measure almost 2 m (2.2 yd.). Bundles of nerve fibers, containing hundreds or even thousands of axons, form a nerve. Within a nerve, each fiber carries a separate impulse, just as each wire inside a telephone cable can carry a separate phone call at the same time.