A stretch 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.

When muscle lengthens, the spindle is stretched and the activity increases. This increases alpha motorneuron activity. Therefore the muscle contracts and the length decreases as a result. The gamma co-activation is important in this reflex because this allows spindles in the muscles to remain taut therefore sensitive even during contraction.

Consider this scenario: flexing your arm to touch your nose and while doing that you encounter perturbation which reverses the movement. With Gamma co-activation, monosynaptic stretch reflex return the movement to its previous course and the arm reaches the nose, but without it, spindles are slack and reflex will not be able to correct for the perturbation and the arm will fail to reach the nose.

Function of this reflex is to maintain a constant length and has the shortest latency of all spinal reflexes including 'Reflex mediated by the GTO (Golgi Tendon Organ)' and 'Reflexes mediated by pain and cutaneous receptors.'

Contents 1 Examples 2 Supraspinal control 3 Spinal control 4 Pathology 5 See also 6 External links

[edit] Examples

A person standing in the upright position 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.

The patellar (knee-jerk) reflex is an example.

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. This reflex can be enhanced by the Jendrassik maneuver.

[edit] Supraspinal control

Inhibitory signals on gamma neurons through lateral reticulospinal tract from

• Motor Area 6
• Paleocerebellum
• Red nucleus

Facilitatory signals on gamma neurons through ventral reticulospinal tract from

• Motor Area 4
• Neocerebellum
• Vestibular nucleus

[edit] Spinal control

• Withdrawer reflex
• Damping & Loading reflex
• Jendrassik manoeuvre

[edit] Pathology

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 neurone lesion.

[edit] See also

• Reflex

[edit] External links

• MeSH Reflex,+Stretch
• Stretch reflex at Dorland's Medical Dictionary

v • d • e Nervous system physiology: neurophysiology - reflex Cranial nerve midbrain: Pupillary light reflex · Accommodation reflex pons/medulla: Jaw jerk reflex · Corneal reflex · Caloric reflex test/Vestibulo-ocular reflex/Oculocephalic reflex · Pharyngeal (gag) reflex Tendon reflexes upper limb: Biceps reflex C5/C6 · Brachioradialis reflex C6 · Extensor digitorum reflex C6/C7 · Triceps reflex C7/C8 lower limb: Patellar reflex L2-L4 · Ankle jerk reflex S1/S2 · Plantar reflex Primitive reflexes Galant · Gastrocolic · Grasp · Moro · Rooting · Stepping · Sucking · Tonic neck Lists general · alphabetical · by organ Cardiovascular Bainbridge reflex · Bezold-Jarisch reflex  · Coronary reflex  · Mammalian diving reflex  · Oculocardiac reflex Baroreflex Reflex bradycardia  · Reflex Tachycardia Respiratory Churchill-Cope reflex Other Acoustic reflex · H-reflex · Stretch reflex · Golgi tendon reflex · Optokinetic · Startle reaction · Withdrawal reflex (Crossed extensor reflex)

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