Music spindles, specialized stretch receptors in the muscles, and this informati
ID: 3472631 • Letter: M
Question
Music spindles, specialized stretch receptors in the muscles, and this information is sent via sensory neurons to the CNS. The CNS then sends impulses down the motor neurons to the muscle that trigger a muscle contraction to counter the stretch. You can demonstrate the stretch reflex easily: Sit down with your knees bent and relaxed, and palpate (feel) the musculature of your posterior thigh. How do the muscles feel (taut or soft)? Now stand up, and bend over to touch your toes. Palpate the muscles of your posterior thigh again. How do they feel? The reason they feel different in this position is that you stretched the hamstring muscles when you bent over to touch your toes. This triggered a stretch reflex that resulted in shortening (or tightening) of those muscles. Technically, this reflex can he carried out without the help of the cerebral cortex and can be mediated solely by the spinal cord. We will see shortly, however, that the cortex is involved in even the simplest example of a stretch reflex-the patellar tendon (knee-jerk) reflex, shown in Figure 14.11. When you have completed the following activity, answer Check Your Understanding question 4 (p. 356). Have your lab partner sit in a chair with his or her legs dangling freely. Palpate your partner's patellar tendon between the tibial tuberosity and the patella. Tap this area with the flat end of a reflex hammer (sometimes a few taps are necessary to hit the right spot). What is the result? Now give your partner a difficult math problem to work (long division with decimals usually does the trick). As your partner works the problem, tap the tendon again. Is this response different from the original response? If yes, how, and why?Explanation / Answer
3. Result is sudden kick jerk (front movement) on tapping with hammer. It results due to the action of Quadriceps Muscle at knee joint which on jerking over patellar tendon contracts and lead to extension at knee joint. The following movement is also aided by the antagonist hamstrings muscles which relaxes during this movement.
This is a classical example of monosynaptic reflex arc where afferent impulse travels to the L3 level segment of spinal cord and efferent travels back to the quadriceps muscle via alpha motor neurons.
This test checks the integrity of spinal cord at L2,3,4 levels. Such reflex helps to maintain body in balance.
4. NO. The test result would not change even if the partner is asked to solve a difficult math problem.
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