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O Peripheral Nervous System consists of the nerves and nerve ganglia and its function is to connect the central nervous system to various parts of the human body.
Nerves are bundles of nerve fibers surrounded by a layer of connective tissue. In the nerves there are blood vessels, responsible for the nutrition of nerve fibers. The fibers in the nerves can be either dendrites how axons which lead, respectively, nerve impulses from the various regions of the body to the central nervous system and vice versa.
Nerve ganglia are clusters of cell bodies of neurons located outside the central nervous system. The ganglia appear as small dilations in certain nerves.
Sensory nerves, motors and mixed
Sensory nerves are those that contain only sensory fibers, which drive impulses from the sensory organs to the central nervous system. Motor nerves are those that contain only motor fibers, which drive impulses from the central nervous system to the effector organs (muscles or glands). Mixed nerves contain both sensory and motor fibers.
The autonomic nervous system
Some of the activities of the nervous system, such as thinking and controlling movement, are very obvious to us. But the nervous system is also unknowingly working to control the internal organs.
This is the responsibility of a special part of the nervous system called the autonomic nervous system, which regulates blood circulation, digestion, respiration, reproductive organs and the elimination of waste from the body. It also controls important glands that have powerful effects on the body. The autonomic nervous system works independently of most of the brain and their cells are clustered in ganglia near the spine. It operates entirely by reflexes, and although the brainstem is also involved in its activities, we are not aware of it.
This system is divided into two parts, the sympathetic nervous system and parasympathetic, who work in opposition to each other. One system stimulates an organ, a gland, for example, by making it work hard, the other system stops that work. First one begins; then the other, and the result is that the organ is kept working at the right level.
Sympathetic nervous system work can be observed when we are angry or scared; Its action makes the heart beat faster and the breathing deepens. The pupils of the eyes dilate and we become pale as blood is drained from the skin to feed the muscles we may need for any reaction. This is all because the sympathetic system was triggered, making the body ready for an emergency.
The muscles in our internal organs work automatically, but most of our muscles work only when we want to move them. These are the voluntary muscles. Voluntary movements, such as walking, moving your arms, or using your fingers, are directly controlled by the brain. A narrow strip of cortex running through the top of our brain, called the motor cortex, is in direct connection with our movements. The motor cortex collects information from other parts of the brain, including signals from the sense organs. When the decision to move a muscle or set of muscles is made, the cortex conveys its instructions to the appropriate part of the body.
Different parts of the motor cortex have special functions, each controlling the movements of certain parts of the body. Important and complex parts, such as hands and lips, require very careful control and the many neurons needed for this work occupy large areas of the cortex. Less complicated parts need less control and therefore there are smaller areas of cortex intended for them. Just as movement is controlled by the motor cortex, special parts of the sensory cortex are responsible for touch. Other parts take care of sight, hearing and all other senses.
Movement and senses occupy only two narrow transverse bands of the cerebral cortex. The rest of the cortex does not have such easily recognizable functions. It contains the areas of association, and this is where thought probably occurs. By "thinking," we mean examining and interpreting the huge number of signals that reach the brain, and the decision of any action to be taken - or sometimes the decision not to act. Some functions, including speech, are scattered throughout the cortex in small areas. Speech is also controlled by many different areas of the brain as well as a part of the cortex.
The way in which the areas of association work is not yet well understood. Sometimes large parts of the brain can be affected by illness or accident without causing too much trouble; On the other hand, damage to small parts can lead to serious disturbances. In fact, the way the brain works is much more complicated than it seems at first glance. Very large parts of the brain appear to have no apparent purpose, but since neurons are so interconnected, it is believed that all parts of the brain have some function. Perhaps part of this brain "reserve" is beginning to be used to replace dying neurons as we get older.