Synonyms
- Monosynaptic reflex, myotactic reflex, deep tendon reflex, tendon jerk
Definition
- It is an automatic, monosynaptic reflex that involves a muscle and tendon, and produces a jerk.
Most commonly tested
- Biceps (C5, C6)
- Elbow flexion
- Triceps (C7,C8)
- Elbow extension
- Patella (L2 – L4)
- Knee extension
- Achilles (S1,S2)
- Foot plantarflexion
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.
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