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.


The patellar tendon reflex is an example of the stretch reflex.

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.[1]

Structures

The stretch reflex is accomplished through several different structures. In the muscle, there are muscle spindles, whose extrafusal muscle fibers lie parallel to the muscle and sense changes in length and velocity. The afferent sensory neuron is the structure that carries the signal from the muscle to the spinal cord. It carries this action potential to the dorsal root ganglion of the spinal cord. The efferent sensory neuron is the structure that carries the signal from the spinal cord back to the muscle. It carries the action potential from the ventral root of the spinal cord to the muscle down the alpha motor neuron.[2] This synapsis on the first structure discussed, the extrafusal fibers of the muscle spindle.

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.

Spinal control

Spinal control of the stretch reflex means the signal travels between the muscle and spinal cord. The signal returns to the muscle from the same spinal cord segment as where it entered the spinal cord. This is the shortest distance for a reflex signal to travel, thus creating a fast response. These responses are often referred to short latency stretch reflexes. An example of a short latency stretch reflex is the Withdrawal reflex.[3]

Supraspinal control

Supraspinal control of the stretch reflex means the signal travels above the spinal cord before traveling back down to the same segment it entered the spinal cord from. The responses from these pathways are often termed medium or long latency stretch reflexes, because the time course is longer due to distance it needs to travel. [4] The central nervous system can influence the stretch reflex via the gamma motoneurons, which as described above control the sensitivity of the reflex.

Clinical significance

The knee jerk reflex is an example of the stretch reflex and it is used to determine the sensitivity of the stretch reflex. Reflexes can be tested as part of a neurological examination, often if there is an injury to the central nervous system. To test the reflex, the muscle should be in a neutral position. The patient should flex the muscle being tested for the clinician to locate the tendon. After the muscle is relaxed, the clinician strikes the tendon. The response should be contraction of the muscle. If this is the knee jerk reflex, the clinician should observe a kick. The clinician rates the response.[5]

Grading of stretch reflexes upon tapping muscle tendon[6]
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. [7]

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See also

References
  1. Spirduso, Waneen Wyrick (1978). "Hemispheric Lateralization and Orientation in Compensatory and Voluntary Movement". Information Processing in Motor Control and Learning. pp. 289–309. doi:10.1016/B978-0-12-665960-3.50019-0. ISBN 9780126659603.
  2. Dolbow, James; Bordoni, Bruno (2019), "Neuroanatomy, Spinal Cord Myotatic Reflex", StatPearls, StatPearls Publishing, PMID 31869093, retrieved 2019-12-30
  3. Feher, Joseph (2012). "Spinal Reflexes". Quantitative Human Physiology. pp. 332–340. doi:10.1016/B978-0-12-382163-8.00036-0. ISBN 9780123821638.
  4. ELDRED E, GRANIT R, MERTON PA (1953). "Supraspinal control of the muscle spindles and its significance". J Physiol. 122 (3): 498–523. doi:10.1113/jphysiol.1953.sp005017. PMC 1366137. PMID 13118557.CS1 maint: multiple names: authors list (link)
  5. Walkowski, A. D.; Munakomi, S. (2019). "StatPearls". PMID 31082072. Cite journal requires |journal= (help)
  6. Walker, H. K.; Walker, H. K.; Hall, W. D.; Hurst, J. W. (1990). "Deep Tendon Reflexes". PMID 21250237. Cite journal requires |journal= (help)
  7. Ashby P, Mailis A, Hunter J (1987). "The evaluation of "spasticity"". Can J Neurol Sci. 14 (3 Suppl): 497–500. doi:10.1017/s0317167100037987. PMID 3315151.CS1 maint: multiple names: authors list (link)
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