During a voluntary movement, gamma motor neurons fire alongside alpha motor neurons to the same muscle. The functional purpose of this alpha-gamma coactivation is:

• A To increase the force output of extrafusal fibres during contraction
• B To inhibit antagonist muscles through spinal interneurons
• C To keep spindle afferents responsive to stretch even while the muscle shortens ✓
• D To provide direct excitatory input to Golgi tendon organs

Explanation

When extrafusal fibres shorten, an unexcited spindle would go slack and fall silent, losing its ability to signal further stretch. Gamma efferents contract the intrafusal fibre poles, keeping the central sensory region taut so spindle discharge continues to encode length changes throughout the movement. Gamma activity adds negligible force because intrafusal fibres contribute almost nothing to total tension, which eliminates option A.

Reference: Ganong's Review of Medical Physiology, 26th ed.

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