Muscle Stretch Reflex

Muscle Stretch Reflex

Summary

Overview

Synonyms

Definition

It is an automatic, monosynaptic reflex that involves a muscle and tendon, and produces a jerk.

Most commonly tested

Key mediators

Muscle spindles, which activate via muscle stretch.
Spinal neurons, which receive sensory input and generate motor output.
Muscle fibers, which contract.
Interneurons, which modulate neuronal firing.
Golgi tendon organs, which activate via muscle contraction to terminate the reflex.

Activation

When the patellar tendon is activated,
the muscle spindle sends an excitatory volley along the Type 1a sensory afferent,
which excites the extensor motor neuron.
It activates the muscle extensors, which extend the knee.

Interneuronal Inhibition

Renshaw cells are interneurons that lie in the anterior horn of the gray matter of the spinal cord.
When Renshaw cells are activated, they inhibit flexor motor neurons using the inhibitory neurotransmitter glycine.

Termination

Golgi tendon organs are situated where the quadriceps tendon inserts into the patella.
Type 1b fibers project from the Golgi tendon organs to the Renshaw interneurons.
Inhibitory fibers project from the the Renshaw interneurons to the extensor motor neurons.
The Type 1a and 1b fibers fire at the same rate, but the muscle spindle fibers have a much lower threshold to fire than Golgi tendon organs, thus, the muscle spindle fibers fire first, and then later the Golgi tendon organs fire, which terminates the muscle stretch reflex.
Neurobiological influences, such as myosin ATPase and calcium re-accumulation into the endoplasmic reticulum aid in muscle contraction.

Clinical Correlation

In comatose patients, presence of the triple flexor reflex to plantar stimulations a sign of disinhibition, similar to the Babinski sign.