A patient with chronic upper motor neuron lesion shows sudden give-way of resistance when the spastic limb is passively stretched briskly. The reflex responsible originates from receptors located in which structure?
- A Collagen strands of the tendon, sensed by Ib afferent terminals ✓
- B Nuclear bag fibres within the muscle spindle
- C Joint capsule mechanoreceptors
- D Extrafusal fibre endings sensed by group III afferents
Correct answer: A. Collagen strands of the tendon, sensed by Ib afferent terminals
Explanation
The clasp-knife response reflects the inverse stretch reflex: strong tension activates Golgi tendon organs embedded among tendon collagen fibres, whose Ib afferents excite inhibitory interneurons acting on the same muscle's alpha motor neurons, causing abrupt relaxation. Spindle afferents (group Ia from bag fibres) mediate the opposite, excitatory stretch reflex. This autogenic inhibition normally guards against excessive tension and becomes prominent when descending supraspinal inhibition is lost.
Reference: Ganong's Review of Medical Physiology, 26th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.