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.
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, gamma co-activation might keep the spindles taut when a muscle is contracted, preserving their stretch-sensitivity even as the muscle fibers become shorter. Otherwise the spindles would become slack and the reflex would cease to function.
This reflex has the shortest latency of all spinal reflexes including the Golgi tendon reflex and reflexes mediated by pain and cutaneous receptors.
A person standing upright begins to lean to one side. The postural muscles that are closely connected to the vertebral column on the side will stretch. Because of this, stretch receptors in those muscles contract to correct posture.
Other examples (followed by involved spinal nerves) are responses to stretch created by a blow upon a muscle tendon:
- Jaw jerk reflex (CN V)
- Biceps reflex C5/C6
- Brachioradialis reflex C6
- Extensor digitorum reflex C6/C7
- Triceps reflex C7/C8
- Patellar reflex L2-L4 (knee-jerk)
- Ankle jerk reflex S1/S2
Another example is the group of fibers in the calf muscle, which synapse with motor neurons supplying muscle fibers in the same muscle. A sudden stretch, such as tapping the Achilles' tendon, causes a reflex contraction in the muscle as the spindles sense the stretch and send an action potential to the motor neurons which then cause the muscle to contract; this particular reflex causes a contraction in the soleus-gastrocnemius group of muscles. Like the patellar reflex, this reflex can be enhanced by the Jendrassik maneuver.
There are basically four types of muscle fibers. This includes the slow twitch (ST) fibers, which are slow contracting and slow to fatigue. The fast twitch muscle fibers are sub-divided into several sub-classes and include fibers that are fast contracting and resistant to fatigue (FRF), fast contracting but more easily fatigued (FEF), and fast contracting fast fatiguing white fibers (FFF). 
Supraspinal control 
The central nervous system can influence the stretch reflex via the gamma motoneurons, which as described above control the sensitivity of the reflex.
Inhibitory signals arrive at gamma neurons through the lateral reticulospinal tract from Brodmann area 6, the paleocerebellum and the red nucleus. Facilitatory signals arrive through the ventral reticulospinal tract from Brodmann area 4, the neocerebellum and the vestibular nucleus.
Spinal control 
- Withdrawer reflex
- Damping & Loading reflex
See also 
- Reflex, Stretch at the US National Library of Medicine Medical Subject Headings (MeSH)
- "Stretch reflex" at Dorland's Medical Dictionary
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