Most of us take a lot of what the body does for us for granted. That’s a good thing. It would be extremely difficult to comprehend every tiny movement we made. We could never focus on anything else.
As you read this, subtle readjustments are constantly being made between the muscles of your spine and torso to keep you in balance. Your eyes make tiny readjustments for every shift of your head. Your pupils dilate appropriately to adjust to the level of light and to focus on what’s in front of you. When you swallow, your throat automatically closes off the airway to prevent saliva from going down the wrong tube. Each breath you take automatically readjusts to provide the right balance of oxygen and carbon dioxide in your blood, as well as taking deeper breaths now and then to keep the lungs open.
These are just a few examples of the automatic responses that keep us functioning every day. Most of the functions that are critical for life are outside of our conscious control, probably so that we can’t consciously do something to mess around with it. Instead, these functions are governed by reflexes. A reflex is a relatively simple (but critical) way to relay information that never reaches conscious awareness.
The most familiar reflex is the patellar reflex, in which the knee jerks when a doctor taps it with a hammer. While this is considered a simple reflex, many pages could be filled exploring the technical details. Briefly, though, what you see is the body’s automatic attempt to correct for an imbalance that could otherwise cause it to fall over. A muscle is stretched by the hammer hitting a tendon, and an electrical signal is sent to the spinal cord, which sends out a signal to tense the muscle to return it to its proper length. The result is a brief jerking motion, and the knee kicks out.
At the same time, an electrical signal is sent to the opposing muscles in the hamstring in order to tell these muscles to relax so that they don’t interfere with the leg’s straightening. No electrical signal ever needs to reach the brain for this reflex to occur.
Neurologists use different reflexes to see how different parts of the nervous system are functioning. For example, for the knee jerk reflex to work, the nerves to and from the muscle must be intact, and the spinal cord needs to be working at that level. Similarly, a brainstem reflex, such as the pupils constricting to light, can help a neurologist know that the brainstem is working properly.
Furthermore, reflexes are moderated by many other things in the body. For example, the brain usually sends impulses down the spinal cord that keeps reflexes like the knee jerk relatively calm. After a stroke or other injury to the brain, the calming influence on the reflex is slowly lost, and this results in reflexes being hyperactive. One of the reasons neurologists check reflexes is to see if there is an imbalance between the left and right sides, which can be a clue to damage to the brain or spinal cord.
Sometimes a reflex can look a lot like conscious behavior. For example, in the “triple flexion” reflex, the knee, hip, and foot flex in such a way that the leg withdraws when a painful stimulus is applied. This can happen even if an electrical signal never reaches the brain -- it can be completely orchestrated by the spinal cord. It’s important to distinguish between a reflex and intentional movement in cases of coma or altered consciousness.
Not knowing everything that reflexes do for us saves us a lot of trouble in day-to-day life. However, knowing about reflexes and how to test for them can shed a lot of light on how the nervous system works and where the problem lies in a nervous system disorder.
Hal Blumenfeld, Neuroanatomy through Clinical Cases. Sunderland: Sinauer Associates Publishers 2002
AH Ropper, Samuels MA. Adams and Victor's Principles of Neurology, 9th ed: The McGraw-Hill Companies, Inc., 2009.