<Tr> <Td> </Td> <Td> This article includes a list of references, related reading or external links, but its sources remain unclear because it lacks inline citations . Please help to improve this article by introducing more precise citations . (May 2014) (Learn how and when to remove this template message) </Td> </Tr> <P> The stretch reflex (myotatic reflex) is a muscle contraction in response to stretching within the muscle . It is a monosynaptic reflex which provides automatic regulation of skeletal muscle length . </P> <P> When a muscle lengthens, the muscle spindle is stretched and its nerve activity increases . This increases alpha motor neuron activity, causing the muscle fibers to contract and thus resist the stretching . A secondary set of neurons also causes the opposing muscle to relax . The reflex functions to maintain the muscle at a constant length . </P> <P> Gamma motoneurons regulate how sensitive the stretch reflex is by tightening or relaxing the fibers within the spindle . There are several theories as to what may trigger gamma motoneurons to increase the reflex's sensitivity . For example, alpha - gamma co-activation might keep the spindles taut when a muscle is contracted, preserving stretch reflex sensitivity even as the muscle fibers become shorter . Otherwise the spindles would become slack and the reflex would cease to function . </P>

When does the activation of a motor neuron occur in a stretch reflex
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