We all know that the nervous system, particularly the brain, in human beings and other animals controls the way we initiate behavior and respond to events in our world. The nervous system receives information about changes in the environment from sensory organs including the eyes, ears and nose and it transmits directions that tell our muscles and other internal organs how to react.
The brain also stores information being a repository for our memory of past events and provides our capability for thinking, reasoning and creating. the nervous system is constantly integrating the actions of our internal organs although we are not generally aware of it. Many of these organs such as the heart and digestive tract are made of muscle tissues that respond to commands. The nervous system provides these commands through an intricate network of billions of specialized nerve cells called neurons.
Although neurons in different parts of the nervous system have a variety of shapes and sizes, the diagram on your screen shows their general features. Projecting from the cell body are clusters of branches called dendrites. Generally dendrites function as receivers for messages from adjacent neurons. These messages then travel through a long slender projection called the axon, which splits into branches at the Far end. The tips of these branches have small swellings called synaptic knobs that connect to the dendrites of other neurons usually through a Fluid-filled gap. This junction is called a synapse.
Messages From the knobs cross the gap to adjacent neurons and in this way eventually reach their destination. What changes occur in the nervous system as a person develops? By the time the typical baby is born a basic structure has been formed for almost all the neurons this person will have. But the nervous system is still quite immature For instance the brain weighs only about 25% of the weight it will have when the child reaches adulthood. Most of the growth in brain size after birth results from an increase in the number of Glial cells and the presence of a white fatty substance called myelin. The Glial cells are thought to service and maintain the neurons.
A myelin sheath surrounds the axons of most but not all, neurons. This sheath is responsible for increasing the speed of nerve impulses and preventing them from being interfered with by adjacent nerve impulses much the way insulation is used on electrical wiring. The importance of myelin can be seen in the disease called multiple sclerosis which results when the myelin sheath degenerates and nerves become severed (Trapp, 1998). People afflicted with this disease have weak muscles that lack coordination and move spastically (AMA, 1989).
As the infant grows the network of dendrites and synaptic knobs to carry messages to and from other neurons expands dramatically, as you can see in the diagram the myelin sheath covering the neurons is better developed initially in the upper regions of the body than in the lower regions. During the first years of life the progress in myelin growth spreads down the body from the head to shoulders to the arms and hands to the upper chest and abdomen, and then the legs and feet. This sequence is reflected in the individual’s motor development: the upper parts of the body are brought under control at earlier ages than the lower parts. Studies with animals have found that chronic poor nutrition early in life impairs brain growth by retarding the development of myelin Glial cells and dendrites. Such impairment can produce long-lasting deficits in a child’s motor and intellectual. Although researchers had thought that the brain forms few if any new neurons after birth it is now known that new cells do form in some areas of the brain, but it is not yet clear how extensive this growth is.
Beginning in early adulthood, the brain slowly loses weight with age. Although the number of brain cells does not change very much the synapses do, leading to a decline in ability to send nerve impulses. These alterations in the brain are associated with the declines people often notice in their mental and physical functions after they reach 50 or 60 years of age the nervous system is enormously complex and basically has two major divisions the central nervous system and the peripheral nervous system—that connect to each other. The central nervous system consists of the brain and spinal cord. The peripheral nervous system is composed of the remaining network of neurons throughout the body. Each of these major divisions consists of interconnected lower-order divisions or structures. We will examine the nervous system, beginning at the top and working our way down.