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A simple reflex is an automatic response to a stimulus. It does not involve any thought, but simply is an involuntary action. Humans and animals use simple reflexes to escape from danger as well as in some basic survival situations. Medical doctors often use reflexes to check for neurological disorders, paralysis, and other health issues as well.
One of the most common examples of a simple reflex is the patellar test, or the knee jerk response. When someone hits a particular spot on the knee, the sensory cells in the knee work as receptors. Those receptors send a signal to the person’s spinal cord via the sensory nerve cell. Inside the spinal cord, a reflex arc sends the signal directly back to the effectors in the leg muscles through intermediate and motor nerve cells. At that time the muscles of the leg contracts and the lower leg jerks or kicks upward.
The interesting thing about a simple reflex is that there are only three kinds of nerve cells that are involved: the sensory nerve cell, intermediate nerve cell, and motor nerve cell. The brain only becomes aware of the response after it occurs. In the case of the patellar test, the brain is only aware that the lower leg kicked upward, after the action was complete.
There are plenty of other examples of simple reflexes that occur in the human body as well. For example, if an insect, piece of dust, or other foreign object flies into a person’s eye, she will blink within milliseconds. Also, if a person has light shone into her eyes, the pupils will dilate or get smaller. Generally, a person cannot control whether her pupils will dilate in bright light. These things happen involuntarily and each one exemplifies a simple reflex.
Usually, an infant will have more simple reflexes than older humans. The simple reflexes in a newborn are designed to help the newborn survive. For example, an infant will involuntarily hold her breath when placed underwater. Also, most newborns innately know how to suckle and root for milk. It is not something that they are taught, but is a simple reflex.
Sometimes a simple reflex can help a medical doctor diagnose issues in a person’s neurological pathways. For example, if the doctor uses the plantar simple reflex test, it will show him whether a person over the age of six months has an abnormality of the spinal cord or brain. With the plantar reflex test, the outside of the sole of the person’s foot is stroked with a hard object, like a key. If the person’s big toe turns upward and the rest of the toes spread out, there may be damage. If the person’s toes have a downward curl, it is a sign that there may not be damage.
I remember the patellar reflex test became the basis for a lot of visual jokes in cartoons and comedy shows. The doctor would tap on a patient's knee with a rubber mallet and the resulting kick would send things flying through the air. When I was a kid, I didn't realize it was an involuntary reaction, and it wasn't nearly as dramatic when a real doctor performed the test.