Reflex action. If you accidentally touch a hot stove, you jerk away before you have time to think what you are doing. Actions of this kind, which are not planned or decided beforehand, are called reflex actions. Each reflex involves some stimulus that causes a response. In the above example, the hot stove was the stimulus and the jerking away was the response.
Reflex actions are quite common and easy to notice. If light is directed at a person’s eye, the pupil of the eye will become smaller. When the light is removed and the person’s eye is shaded, the pupil becomes larger again. The light acts as a stimulus, and the reaction of the pupil is the eye’s response. Doctors often test a person’s reflex actions. Frequently they test the patellar reflex, or knee jerk. The patient sits with her or his knees crossed, and the doctor strikes a point just below the kneecap. This causes the patient’s foot to kick suddenly.
Scientists call these kinds of reflexes unconditioned reflexes. They occur in all normal persons and many animals. Unlike most of human behavior, unconditioned reflexes occur with no specific learning or experience. They are considered involuntary acts, because a response always occurs when a stimulus is presented.
How reflex action occurs.
Most reflex acts are very complicated. But in the simplest forms, four events are involved. Briefly, these events could be called (1) reception, (2) conduction, (3) transmission, and (4) 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 there, the nerve impulses are transmitted to the motor nerves, which control muscle action. The motor nerves conduct the impulses to the muscles and glands, causing them to respond, or act.
To illustrate the events in a reflex action, suppose a person touches the flame of a candle with her or his finger. The heat of the flame stimulates receptors in the skin of the finger. This creates 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. The nerve impulse passes from the sensory fibers to the motor fibers, which in turn relay it to the muscles, causing them to contract. When the muscles contract, the person’s hand jerks back.
Most reflex acts are much more complicated than this. They often involve other parts of the nervous system, such as the brain. Reflex acts are quicker than voluntary acts. You jerk your hand away from a hot stove before you feel pain. You do not have to take the time to decide exactly what you are going to do.
People have many reflex reactions to emotional stimuli. These include changes in blood pressure and respiration. A lie detector measures certain body reactions to emotional stimuli. A person telling a lie usually has small emotional reactions that can be detected because of these reflex reactions. See Lie detector .
Conditioned reflex,
another kind of reflex action, works by association. For example, a dog’s mouth begins to water when the animal smells food. The Russian physiologist Ivan P. Pavlov showed that the flow of saliva—though originally an automatic reaction to the smell of food—can become a conditioned reflex. Pavlov rang a bell each time he brought food to a dog. Eventually, the dog’s mouth began to water when Pavlov merely rang the bell—with no food being present. The dog associated the ringing of the bell with the food, just as it associated the odor with the food.
