Who can forget being a kid at the doctor’s office and watching the pediatrician pound on your knees with a rubber hammer? I always tried to humor the doctor by voluntarily kicking my knee toward his groin, but somehow he always saw through that ruse and persevered until the natural reflex kicked in. Checking reflexes isn’t just another way for doctors to cause pain, and as cool as it might be to see a part of the body jerk or spasm without any conscious effort, they’re not just amusing themselves, either. Our reflexes are among the most primitive and basic elements of our nervous system, and they provide a window into our neurological and physical health.
This Is Your Brain on Bypass
Reflexes are quick, involuntary actions that occur in response to a stimulus: the jerk of our knees after a painful thwack, or the way our fingers recoil when we touch something hot. What makes these movements so special is that they are deeply-ingrained actions that aren’t processed by our conscious mind. Normal movement is preceded by a thought and processed in the brain. Reflexes, on the other hand, bypass the brain entirely. Involuntary reflexes are a direct loop between a muscle and the spinal cord. When a doctor hits that certain spot on your kneecap, the involuntary response is a jerking of the leg, and not only can you not help but do it, you can’t not do it, either.
Some actions can become a so-called conditioned reflex. When people are able to quickly catch a dropped glass or kill a fly with their bare hands, we applaud their “fast reflexes,” but those actions aren’t truly reflexes, because they’re voluntary. People who are good at performing quick actions aren’t using real reflexes; they just have good reaction time. However, it is possible to be so well-practiced at an action that it eventually becomes involuntary. When you type on a keyboard, your fingers can strike the correct keys without requiring conscious thought, because your body has become conditioned to the act of typing, and can process the action in the spinal cord. Conditioned reflexes are developed and learned, and it’s only through much practice that they can become involuntary.
Reflexes from Nose to Toes
The patellar reflex (the knee-jerk response) may be the most well-known, but the body actually contains dozens of reflexes. The same hammer that abuses your knees can be used to strike the Achilles tendon, which causes an ankle-jerk reflex. If a doctor were to strike one of your forearm tendons, it would activate your brachioradialis reflex, causing your arm to jerk. The biceps brachii tendon, which sits in the crook of your arm, is the site of another reflex, as is the triceps tendon right above the elbow on the back of your forearm. Have you ever noticed that when one hand touches something hot, both hands recoil? That’s the crossed extensor reflex.
Not all reflexes produce spastic muscle movements. Shivering in response to cold is a reflex to help us regulate body temperature, and our pupils’ dilation or constriction in response to light is a reflex, too. Blushing, coughing, and sneezing are all reflex actions, including the peculiar “photic sneeze” which causes sneezing in response to sudden bright lights, and affects about 35 percent of people.
A Test You Can’t Study For
When doctors poke and pound on our limbs or shine bright lights in our eyes, they’re checking on the state of our central nervous system. Irregular or absent reflexes could not only indicate neurological damage, but also clue doctors in to the exact location of the problem, since reflexes in different parts of the body are processed by different nerves which are located on different areas of the spinal cord. Patients with no patellar reflex might have damage in their lumbar area, and patients without a biceps or brachioradialis reflex could have damage in the area of their cervical vertebrae. When evaluating injuries, doctors can use reflex checks to rule out nerve damage, and they can also use absent reflexes as a symptom of disease. Conditions like hyperthyroidism, multiple sclerosis, Lou Gehrig’s disease, and meningitis can all result in lessened or irregular reflexes.
Babies have many reflexes that disappear by early childhood, and monitoring them is a way for pediatricians to check on their neurological development. Infants start exhibiting reflexes as early as seven weeks after conception, when stimulation near their mouths causes the fetus to turn its lips toward the stimulus in a rooting reflex. By the time it’s born, an infant can demonstrate reflexive sucking and swallowing, too. Newborns also exhibit a palmar grasp reflex, where they reflexively grab any object placed in their hands, as well as a plantar reflex, a curling of the toes when the bottom of their feet are stroked. Pediatricians check all these reflexes to make sure that the infant is developing normally, and most of them disappear before the baby is a year old. Sometimes, absent or irregular reflexes can be an early sign of neurological or developmental problems such as cerebral palsy.
Doctors believe that many of our reflexes serve an evolutionary purpose. Infants’ instinctual rooting and sucking enable them to find food and feed before they can consciously perform the actions or even know that they need to eat, and other infant reflexes enable babies to lay foundations for more mature controlled movement. Reflexes such as coughing and sneezing help us to expel irritants from our respiratory tract, and our ocular reflexes help us to protect our eyes and see better. Obviously, having reflexes that force our bodies to flinch and recoil from harmful objects keeps us safer, too. Newborns even have a swimming reflex that causes them to kick and paddle if submerged in water, helping them to stay alive in time for help to arrive.
Our reflexes are like canaries in a coal mine, indicating trouble before we even recognize a problem. They aren’t just parlor tricks; they’re non-invasive and useful ways to check for very real damage to the central nervous system. So the next time your doctor approaches you with the dreaded hammer, let her give it a tap. And just hope that your body responds.