Tension development by isolated muscle spindles of the cat.

1. Tension developed by mammalian intrafusal muscle fibres stimulated directly with single or repetitive electrical pulses has been determined in sixteen isolated lumbrical muscle spindles of the cat's foot.2. On microscopic observation, contraction to single shocks was observed in ten preparations. In three of these preparations twitch-like records were obtained.3. In the preparations that ;twitched', the tetanic fusion frequency was 10 pulses/sec at 20 degrees C.4. In the ;twitching' intrafusal fibres the magnitude of contraction increased with increase of stimulus strength.5. In one of these spindles, only ;slow' contractions were obtained at low stimulus strengths when it was stimulated repetitively at 100 pulses/sec. With increase in stimulus strength, such ;slow' contractions gave twitch-like responses towards the end of a period of repetitive stimulation.6. Tension changes were recorded from all preparations stimulated repetitively. With supramaximal stimuli, the usual frequency of stimulation at which intrafusal tension was recorded was 10 pulses/sec at 20 degrees C. The magnitude of contraction increased progressively with increase in stimulus frequency.7. Larger intrafusal tensions and faster time courses of contraction were obtained when the electrodes were astride the capsular region than when they were over the inferior extracapsular region.