The clasp-knife response in upper motor neuron lesions is characterized by initial hypertonicity followed by sudden release at peak stretch. What is the primary neural mechanism responsible for the sudden 'give'?

• A Reciprocal inhibition from antagonist Ia spindle afferents suppresses agonist motor neurons at peak stretch
• B Activation of Ib afferents from Golgi tendon organs (GTO) during high tension generates autogenic inhibition of the stretched muscle's α-motor neurons via Ib inhibitory interneurons ✓
• C Direct mechanical fatigue of the muscle fiber reduces active tension
• D The inhibitory Renshaw cell circuit terminates firing due to receptor desensitization

Explanation

The clasp-knife response has two components: initial hypertonicity from exaggerated stretch reflexes (Ia spindle overactivity due to loss of descending inhibition in UMN lesion), and then sudden yielding at peak tension. The sudden collapse ('clasp-knife') results from Golgi tendon organ Ib afferent firing at high muscle tension. GTOs detect active force (not length) and, through disynaptic Ib inhibitory interneurons (IbINs), inhibit homonymous α-motor neurons (autogenic inhibition). This response is amplified in UMN lesions because descending tracts normally modulate IbIN gain; its exaggeration is termed the clasp-knife phenomenon.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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