Editing Muscle spindle (section)

=== Stretch reflex ===
When a muscle is stretched, primary type Ia sensory fibers of the muscle spindle respond to both changes in muscle length and velocity and transmit this activity to the [[spinal cord]] in the form of changes in the rate of [[action potentials]]. Likewise, secondary type II sensory fibers  respond to muscle length changes (but with a smaller velocity-sensitive component) and transmit this signal to the spinal cord. The Ia afferent signals are transmitted [[Reflex arc#Monosynaptic vs. polysynaptic|monosynaptically]] to many [[alpha motor neurons]] of the receptor-bearing muscle. The reflexly evoked activity in the alpha motor neurons is then transmitted via their efferent axons to the extrafusal fibers of the muscle, which generate force and thereby resist the stretch.  The Ia afferent signal is also transmitted polysynaptically through [[interneurons]] (Ia inhibitory interneurons), which inhibit alpha motorneurons of antagonist muscles, causing them to relax.<ref>{{Cite journal |last1=Mukherjee |first1=Angshuman |last2=Chakravarty |first2=Ambar |date=2010 |title=Spasticity Mechanisms – for the Clinician |journal=Frontiers in Neurology |volume=1 |page=149 |doi=10.3389/fneur.2010.00149 |doi-access=free |issn=1664-2295 |pmc=3009478 |pmid=21206767}}</ref>