Christopher Simpson, age 14, of McAllen, Texas, for his question:
HOW DOES A REFLEX ACTION OCCUR?
When your finger touches a hot object, the finger is withdrawn instantaneously because the sensory impulse has stimulated the motor impulse in a reflex arc long before the sensory impulse could reach the brain. This is known as involuntary reflex action.
Actions like the one in which you quickly jerk back your finger are not planned or decided before hand. This type of reflex action involves some stimulus that causes a 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 become 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 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 also have some of them.
Most reflex acts are very complicated. But in the simplest forms, four events are involved. These are 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 charged 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.
People also have many reflex reactions to emotional stimuli. These include changes in blood pressure and respiration. A lie detector measures body reactions to emotional stimuli.
A person telling a lie while being tested with lie detector equipment usually has small emotional reactions that can be detected because of reflex reactions.
Here's an illustration of a reflex action. Suppose a person touches a hot pot on the stove with his finger. The heat from the pot 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 quickly jerks back.
Most reflex acts are much more complicated than this. They often involve other parts of the nervous system, including the brain.
Reflex acts are quicker then voluntary acts. You do not have to take the time to decide exactly what you are going to do.