Muscle spindle activity and natural sleep in the cat.

1) Single spike activities of the primary ending of the muscle spindle ofankle flexors and extensors were recorded in the unanesthetized chronic cat, keeping the muscle slightly and constantly stretched, with particular referenceto the natural sleep. Three different kinds of influences attributable to the fusimotor neuron activity were differentiated during the para-sleep: tonicdepression, phasic depression and phasic facilitation.2) In awake state the muscle spindle activity changed its rate, continuously, being higher or lower. During ortho-sleep the rate was more regular andrelatively lower than in awake state.3) In the para-sleep state, compared with ortho-sleep, the rate of the spontane-ous activity of muscle spindle afferents became irregular and decreased in 12units out of tested 14 units (tonic depression).4) In addition to the tonic depression during para-sleep there was transientburst pattern of the firing with or without muscle twitching (phasic facilita-tion) and there was the transient depression without co-contractions (phasicdepression). The latter often associated with the rapid eye movement phase.