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Stretch Reflex - Glossary Term Illustration

Stretch Reflex

Stretch reflex is a neuromuscular reflex in which an increase in muscle length activates receptors known as muscle spindles, which signal an increase in muscle activity.

Stretch Reflex

Stretch Reflex (a.k.a. Monosynaptic Reflex or Myotatic Reflex)
Stretch reflex is a neuromuscular reflex in which an increase in muscle length activates receptors known as muscle spindles, which signal an increase in muscle activity. Muscle spindles are sensory receptors whose neurons transmit an excitatory signal to the spinal cord, where a direct synapse with alpha motoneurons triggers a near-immediate contraction of the innervated motor units. The direct synapse with alpha motor neurons and the absence of interneurons is why this reflex is known as "monosynaptic"; there is only one synapse.

  • Example: The patellar reflex (knee-jerk reaction) is a tap on the patellar tendon, which activates muscle spindles in the quadriceps, triggering contraction to resist excessive stretch.

Function:

  • Protection: Stretch reflex is a protective mechanism against excessive stretch, reducing the risk of muscle or tendon injury
  • Muscle Tone and Posture: Because muscle spindle length and sensitivity may adapt in response to dysfunction, injury, surgery, activity, or prolonged changes in muscle length, the stretch reflex plays a significant and potentially primary role in controlling muscle tone and posture. If muscle spindle sensitivity increases, the stretch reflex will be initiated more often, increasing average motor unit activity (tone). If muscle spindle sensitivity decreases, then fewer instances of stretch reflex will be initiated, and average muscle activity (tone) will decrease.
  • Stability, Balance, and Coordinated Motion: By continuously monitoring and adjusting muscle length, the stretch reflex helps counteract unexpected perturbations, stabilize joint positions, and optimize movement efficiency.
  • Performance: Stretch reflex likely contributes to the performance of high velocity (power) activities as part of the stretch-shortening cycle (SSC) . That is, the stretch reflex is likely initiated by the quick pre-stretch of ballistic activities, resulting in an increase in motor unit recruitment during the concentric phase.

Additional neuromuscular reflexes:

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Frequently Asked Questions:

Is stretch reflex good or bad?

  • Stretch reflex is not good or bad; it is an essential component of our motor control systems. See the functions above.

What does an absent myotatic reflex indicate?

  • The absence of a myotatic reflex indicates a lesion of the sensory or motor component of the reflex arc, also known as a lower motor neuron (LMN) lesion or segmental sign. A loss of the reflex in one muscle group suggests a peripheral nerve lesion, and a loss of several muscles with neural innervations from the same or proximal nerve roots may suggest a nerve root lesion.

What is the reverse stretch reflex?

  • This is an uncommon term used to describe autogenic inhibition , a reflex action mediated by the Golgi tendon organ.

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