Stretch reflex

The stretch reflex (myotatic reflex) is a muscle contraction in response to stretching within the muscle. A spinal reflex is a fast response that involves an afferent signal into the spinal cord and an efferent signal out to the muscle. The stretch reflex can be a monosynaptic reflex which provides automatic regulation of skeletal muscle length, whereby the signal entering the spinal cord arises from a change in muscle length or velocity.

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, 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.

This reflex has the shortest latency of all spinal reflexes including the Golgi tendon reflex and reflexes mediated by pain and cutaneous receptors.

Examples

A person standing upright begins to lean to one side. The postural muscles that are closely connected to the vertebral column on the opposite side will stretch. The muscle spindles in those muscles will detect this stretching, and the stretched muscles will contract to correct posture.

Other examples (followed by involved spinal nerves) are responses to stretch created by a blow upon a muscle tendon:

Another example is the group of sensory fibers in the calf muscle, which synapse with motor neurons innervating 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.

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

Pathology
Grading of stretch reflexes upon tapping muscle tendon[1]
GradeResponseSignificance
0no responsealways abnormal
1+slight but definitely present responsemay or may not be normal
2+brisk responsenormal
3+very brisk responsemay or may not be normal
4+clonusalways abnormal

The clasp-knife response is a stretch reflex with a rapid decrease in resistance when attempting to flex a joint. It is one of the characteristic responses of an upper motor neuron lesion.

See also

References
  1. Walker, H. K.; Walker, H. K.; Hall, W. D.; Hurst, J. W. (1990). "Deep Tendon Reflexes". PMID 21250237. Cite journal requires |journal= (help)
This article is issued from Wikipedia. The text is licensed under Creative Commons - Attribution - Sharealike. Additional terms may apply for the media files.