Why does a medical doctor often tests the knee for reflexes? What is it really testing for ? Surely not that single area for a single purpose (reflex). What am I missing?
In: Biology
When someone tests a knee reflex they hit the patella tendon. The tendon is attached to muscles which stretched slightly when the tendon was hit. Muscles have stretch receptors which detect the sudden stretch and relay this information to the spinal cord. In the spinal cord, the sensory nerve is directly connected to a motor nerve via a relay nerve. There is just a hard wired direct connection and no actual thinking is required… so when the muscle is stretched a signal comes straight back to say “Quick! Contract!” and the muscle contracts. This muscle stretch -> muscle contraction direct connection is the basis for tendon reflexes tested by doctors.
Reflexes can be demonstrated in hitting various other tendons and watching for muscle contractions such as the achilles tendon and biceps tendon. Reflexes can also sometimes be elicited without hitting tendons but instead quickly stretching a muscle (such as pointing someones ankle downward and then quickly jerking it upwards to stretch out the calf muscles which is an often used alternative to hitting the achilles with a tendon hammer).
If theres a problem with the sensory/motor circuitry that detects the stretching, sends the signal to the spinal cord and then sends the “contract!” signal back to the muscle… you’ll see a missing or weak muscle contraction.
Interestingly our brains and spinal cords quickly learn to dampen our reflexes as we grow up. But if we have damage to our brains or spines the dampening effect will be missing and suddenly reflexes become more pronounced. So increased reflexes are a sign of a brain/spinal problem.
TLDR: Tendon reflexes happen when muscles are suddenly stretched and watching for reduced or exaggerated reflexes gives us clues as to where in the body damage might have occurred.
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