During a voluntary isotonic contraction, the quadriceps shortens markedly. If gamma motor neuron output did not increase simultaneously, what would happen to the muscle spindles, and what does alpha-gamma coactivation therefore accomplish?

• A Spindles would fire maximally; coactivation dampens their output to prevent hypertonia
• B Golgi tendon organs would unload; coactivation restores their baseline firing rate
• C Extrafusal fibers would shorten faster; coactivation slows extrafusal shortening to match the intended velocity
• D The equatorial regions would slacken and afferent firing would cease; coactivation keeps the spindle sensitive to stretch throughout the movement ✓

Explanation

When extrafusal fibers shorten, the parallel-lying spindles are unloaded and their central regions would go slack, silencing the afferent drive exactly when proprioceptive feedback is most needed. Gamma motor neurons contract the polar regions of the intrafusal fibers during the same command, keeping the equatorial stretch-sensitive area taut at any muscle length. This alpha-gamma coactivation preserves spindle gain over the full range of motion. It does not dampen spindle output or alter extrafusal mechanics, and it has no direct effect on Golgi tendon organs.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.