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Mariann Culver, age 13, of Topeka, Kan., for her question:


Body movements that are not planned or decided beforehand are called reflex actions. It is a reflex action when you accidentally touch a very hot pan on the stove and you jerk away your hand before you even have time to think what you are doing.

Every reflex involves some stimulus that causes a response. The hot pan provided the stimulus and the jerking away was the response.

Most reflex acts are very complicated. In the simplest forms, however, four events are involved: a reception, a conduction, a transmission and a response.

The stimulation is received by receptors, or sensitive nerve endings. These may be in the eye, ear, nose, tongue or skin. Energy from the stimulus is changed into nerve impulses and conducted from the receptor to the central nervous system.

From the central nervous system, the nerve impulses are transmitted to motor nerves, which control muscle action. The motor nerves conduct the impulses to the muscles and glands, causing them to respond, or act.

Heat from that hot pan stimulates receptors in the skin of the finger and this in turn causes a nerve impulse that travels along a sensory nerve to the spinal cord. In the spinal cord, the sensory nerve fibers interlace with motor nerve fibers.

As the never fibers interlace with motor nerve fibers, the impulse is almost immediately relayed to the muscles, causing them to contract. When the muscles contract, the person's hand is jerked back.

Actually, most reflex acts are much more complicated than this. They often involve other parts of the nervous system, such as the brain. But reflex acts are quicker than voluntary acts. You do not have to take the time to decide exactly what you are going to do.

People also have many reflex actions to emotional stimuli.

Changes in blood pressure and respiration are reflex actions that might be found as a result of emotional stimuli. A lie detector measures body reaction.

A conditioned reflex is another type of reflex action that works by association. Your mouth might start to water, for example, when you are hungry and you smell one of your favorite types of food. And sometimes your mouth can water just thinking about the food.

One of the most famous examples of conditioned reflex was demonstrated by a Russian physiologist named Ivan Pavlov. Pavlov rang a bell each time he brought food to his dog. Eventually, the dog's mouth began to water when Pavlov rang the bell    with no food being present.

It's also a reflex action when your doctor strikes a point just below your kneecap when your legs are crossed, and your foot kicks out suddenly.



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