Clasp-knife response refers to a stretch reflex with a rapid decrease in resistance when attempting to flex a joint, usually during a neurological examination. It is one of the characteristic responses of an upper motor neuron lesion. It gets its name from the resemblance between the motion of the limb and the sudden closing of a claspknife after sufficient pressure is applied.
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Cause
Although seemingly a stretch reflex when flexing a joint, force from the muscle during the attempt to flex a joint is actually thought to be caused by the tendon reflex of the antagonistic muscle of that joint, which is an extensor muscle that becomes stretched. In upper motor neuron lesions, muscle tonus may increase and resistance of muscle to stretch increases. However, if sufficient force is applied, limb resistance suddenly decreases.
Mechanism
This reflex is observed in patients with upper motor neuron lesions. It is frequently attributed to the action of the golgi tendon organ, likely because of early studies showing that tendon organs are activated by strong muscle stretch and inhibit motoroneurons of the stretched muscle. It was thought that this was a protective reflex, preventing application of so much force that muscles become damaged. More recent work strongly suggests that tendon organs are not involved in the clasp knife reflex, but that other sensory receptors in muscles are responsible.
Example
passive flexion of elbow meets immediate resistance due to stretch reflex in the triceps muscle.Further stretch activates inverse stretchreflex.The resistance to flexion suddenly collapses,and the flexes.continued passive flexion stretches the muscle and the sequence may be repeated.