As was stated before, the fusimotor system is rendered excessively active. The sensitivity of the stretch receptors is excessively high to both slow and quick stretch stimuli. In its milder from the sensitivity to quick stretch is most moticeable, when a ‘clasp-knife’ phenomenon may be demonstrated. In this situation the muscles respond to quick stretch in a phasic manner when there is synchronous firing of the primary receptors, which is turn gives a synchronous contraction of extrafusal muscle in response.
The primary receptors fire synchronously because their thershold has been made low due to the excessive activity of the fusimotor system. The term ‘clasp-knife’ phenomenon is occasionally used because the opposition of the muscle to stretch seems to build up to a climax and then to subside suddenly. The sudden reduction in opposition may beattributed to inhibitory influence exerted…
