Clasp-knife reflex

Clasp-knife reflex or 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 a upper motor neuron lesion. It gets its name from the resemblance between the motion of the limb and the sudden closing of a spring loaded claspknife after sufficient pressure is applied.

The reaction can also be seen in the limbs of decerbrate animals. If pressure is applied resistance occurs due to the stretch reflex that contracts the muscle. However if pressure is increased the resistance is overcome and the muscle becomes very relaxed.

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, and is due to the action of the golgi tendon organ. The golgi tendon organ participates in a stretch reflex, in which lower motor neurons are inhibited in response to increased tension in the muscle's tendon. Functionally, this prevents a person from applying so much force with their muscles, that they damage themselves. When an upper motor neuron is damaged, the patient typically experiences spastic paralysis, in which muscle tone and stretch reflexes are abnormally high. This causes the limb to resist movement by an examiner. If an examiner were to attempt to flex the arm of a person with spastic paralysis, the tricep would resist this movement due to the increased tone. As it is moved, the muscle is stretched. This, combined with the increased tone in the tricep, leads to higher tension in the golgi tendon organ. As it is activated, the lower motor neuron is inhibited and the muscle will suddenly relax, and there will be no more resistance.