I have been previously told that the Patellar reflex (knee-jerk-reaction) exists to prevent the hyper-extension of the patellar tendon. Yet if the impact to the tendon is delivered slowly - i.e. by pressing down on it rather than striking it - there is no reflex response. Why is this, as surely the tendon is being extended by the same amount?


1 Answer 1


This effect you are observing has to do with the nature of the afferent neurons (Ia fibers), which carry a signal into the spinal cord and synapse onto motor neurons directly. See this text (scroll down to section 1.10) for a diagram. At their other end, these Ia fibers penetrate into the muscle and wrap themselves around the body of the muscle spindles, "[which] are specialized receptors that signal (a) the length and (b) the rate of change of length (velocity) of the muscle."

Because the main role of these spindles is to monitor the muscles for very rapid changes in length, the neurons have a static range which is optimized for these quick jerks (rather than firing over a wide range of velocities). When you are stretching the muscle slowly, the Ia fiber is not building up a sufficient depolarization to fire off an action potential.

  • $\begingroup$ +1 this explains the mechanism nicely, but it makes me wonder: if the neurons are capable of measuring the length, then why isn't there a need for a reflex based on length? To bring in some teleology: why would the system be optimized only to automatically respond to (b) and not also to (a)? $\endgroup$ Jun 17, 2012 at 0:10
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    $\begingroup$ Presumably because (a) is handled by higher, slower, functions. If nothing is happening particularly quickly the spinal cord can 'wait for instructions' from the brain. Reflexes exist because when things are happening very quickly it's too slow to contact the brain and wait for a reply. If they're happening slowly, that's not an issue. $\endgroup$
    – Resonating
    Sep 16, 2015 at 14:51

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