A NOTE CONCERNING THE PROBABLE FUNCTION OF VARIOUS AFFERENT END-ORGANS IN SKELETAL MUSCLE

Abstract

When an extensor muscle of a decerebrate preparation is placed under slight initial passive stretch, irregular action currents are detectable in the galvanometer record. If knee-jerks are elicited from a muscle under these circumstances, though large action currents precede each jerk, the irregular vibrations of the tonic muscle are absent during the jerk but reappear in the course of its relaxation. This is interpreted as indicating that during a knee-jerk the adequate stimulus is removed from the stretch afferent end-organs. A discussion is given of the nature of an end-organ which is thus responsive to passive stretch but unresponsive to tension set up by active contraction. Arguments are set forth to show that such an end-organ must exist in parallel with the tension-yielding elements of the muscle, for if, as with the tendon organs, it were disposed in series with these elements, no distinction could be made between the tension of a passive stretch and that of active contraction. Histo-logically the most important group of end-organs existing in parallel with the striated muscle fibers are the muscle spindles. In view of this and of other evidence it is suggested that the muscle spindles are the receptors for the knee-jerk and the stretch reflex. The tendon organs are evidently the tension recorders and they may give rise to impulses which lead to reflex inhibition of the muscle itself. Whether or not they mediate pain, is left an open question.