Why does voluntary contraction of a muscle increase gamma‑motor neuron activity and facilitate the tendon‑jerk reflex?

Mechanism of Tendon Reflex Facilitation During Voluntary Muscle Contraction

Voluntary muscle contraction facilitates the tendon-jerk reflex primarily through increased excitability at both the spinal and cortical levels, bringing alpha motor neurons closer to firing threshold, rather than through a direct gamma motor neuron-mediated increase in muscle spindle sensitivity. 1

Primary Mechanism: Spinal and Cortical Excitability Enhancement

During voluntary contraction, motor evoked potentials (MEPs) increase in size and minimal latency decreases compared to relaxation, reflecting heightened excitability at both the spinal cord and motor cortex levels. 1 This increased excitability means that:

• Alpha motor neurons are positioned closer to firing threshold due to voluntary, involuntary, and reflex inputs converging on them 1
• The evoked motor response from any stimulus (including tendon tap) is preferentially elicited in motor neurons already near threshold 1
• This creates a state where less additional input is needed to trigger action potentials, making the reflex more easily elicitable 1

The Gamma Motor Neuron Timing Issue

Contrary to the premise that gamma motor neurons increase spindle sensitivity before the reflex occurs, evidence shows that fusimotor effects actually lag behind skeletomotor activation:

• Spindle acceleration from gamma motor neuron activity occurs 10-50 milliseconds after the onset of electromyographic activity in voluntary contractions 2
• The fusimotor system does not participate in the initiation of voluntary contractions but rather follows skeletomotor activation 2
• There is approximately simultaneous onset of fusimotor and skeletomotor outflows from the spinal cord, but the mechanical effects of gamma activation are delayed 2

Clinical Reinforcement Maneuvers

The Jendrassik maneuver (voluntary contraction of distant muscles) demonstrates this principle clinically by increasing overall motor neuron pool excitability without requiring local gamma motor neuron effects. 1 This works because:

• Descending voluntary drive increases background excitability across multiple spinal segments 1
• Motor neurons throughout the cord become more responsive to any incoming stimulus 1
• The tendon tap stimulus then finds motor neurons already primed to fire 1

Practical Algorithm for Understanding Reflex Facilitation

1. Voluntary effort initiates descending corticospinal volleys 1
2. These volleys increase excitability at both cortical and spinal levels simultaneously 1
3. Alpha motor neurons receive subthreshold depolarization from voluntary drive 1
4. Tendon tap stimulus arrives at motor neurons already near threshold 1
5. The combined inputs (voluntary + reflex) exceed threshold more easily 1
6. Result: larger, more easily elicited reflex response 1

Important Caveats

At very high levels of voluntary effort, the reflex may actually show partial occlusion rather than facilitation, as motor neurons become refractory or maximally activated. 1 This creates an inverted-U relationship where:

• Moderate voluntary effort produces maximal facilitation 1
• Very high effort can reduce the reflex amplitude due to occlusion effects 1

The traditional explanation emphasizing gamma motor neuron-mediated spindle sensitization oversimplifies the mechanism and ignores the timing evidence showing that fusimotor effects lag behind the facilitation phenomenon. 2 The primary mechanism is central nervous system excitability modulation, not peripheral spindle sensitivity changes. 1