MadSci Network: Neuroscience |
This question stumped me for a little while! As for me, I kind of like to be puzzled...I guess I'm in scientific research for a reason. Your question of reflex enhancement used the specific example of clenching your teeth. I looked and looked through both my texts and some medical texts for some reason why clenching your teeth might enhance the reflex at your kneecap, but I couldn't find any good evidence for how this could occur. Then I tried it myself. When all else fails -- repeat the experiment! Maybe anyone who's curious about the answer should try it too. Clench your teeth -- if this was done in front of a classroom, chances are that the person put on a good show for the class and did it really hard, so you clench your teeth hard, too. What do you feel? Well, when I did it I noticed that my whole upper body tensed -- especially my neck and shoulders, and my legs tensed, too. Clenching your teeth hard is difficult to do without tensing other body parts. So in order for me to give you a satisfactory answer to your question, I'm going to assume that the person's leg muscles were somewhat tensed when the experiment was repeated. I've seen this same experiment, and the same result was gotten when the person was told to try to prevent the reflex -- the reflex got bigger (presumably because they were flexing their leg muscles to try to prevent the "kick"). The patellar reflex is a stretch reflex. It isn't caused by hitting your patella (kneecap), it's caused by hitting the tendon just below the patella. When this tendon is hit, it and the muscle it is attached to is quickly stretched, and the nervous system responds by contracting the muscle. The trigger for the reflex is the sudden change in muscle length that is caused when you hit the tendon attached to the muscle. There is a special sensory organ in the muscle called a muscle spindle which tells the spinal cord and brain how long the muscle is (whether is is long -- relaxed -- or short -- contracted). It is not very active when the muscle is relaxed. When the tendon is hit, it senses the muscle's change in length. It sends that information to the spinal cord as a sudden increase in activity. This information is sent directly to a motor neuron which contracts the muscle to try to compensate for this sudden change. That contraction makes your leg kick. When the muscle is already slightly flexed, the muscle spindle is already more active. So when the tendon is hit, the signal sent from the muscle spindle to the motor neuron is bigger, and therefore, the motor neuron's reaction to the signal is bigger, the muscle contracts more, and the kick you see is increased. I realize that I based my answer on a question you only indirectly asked, but I think it is the most likely explanation for what you described. I am fairly confident that clenching your teeth alone, without tensing your body, will not increase this reflex. Or, I could be wrong. Try it again, but try to use better controls for your experiment. Make an effort not to tense your whole body when you clench your teeth. As another control, try it while clenching your fist instead. Make sure, also, that you hit the tendon with the same force every time. Let us know what you find, and if you still see this increase in the patellar stretch reflex, I will look into it further. Good luck, Brenda
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