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1. The behaviour of nuclear bag and nuclear chain intrafusal fibres in isolated cat muscle spindles was studied by direct observation during repetitive stimulation of the muscle nerve at different stimulus strengths. Contraction of intrafusal fibres and stretch of sensory endings was recorded on film. 2. Tenuissimus spindles are usually operated by a total of four or five fusimotor axons, and the individual action of all of them was studied in many cases. 3. The great majority of fusimotor axons produce activity at one spindle pole only. 4. In about 60% of spindles nuclear bag and nuclear chain intrafusal fibres are selectively controlled by different fusimotor axons, while in one third of these spindles the individual nuclear bag fibres are themselves controlled independently. The remaining 40% of spindles, in addition to some selective innervation, receive one non-selective axon which operates both nuclear chain and nuclear bag fibres though usually only one of the nuclear bag fibres is involved. Selective control is demonstrated in photographs. 5. The thresholds of fusimotor axons selectively innervating nuclear bag and nuclear chain fibres, and of non-selective fusimotor axons are not significantly different. 6. It is suggested that in spindles in which the nuclear bag fibres are controlled by the same axon, it is a 'dynamic' gamma, or occasionally beta, axon. Where one nuclear bag fibre is operated along with the nuclear chain fibres it is controlled by 'static' gamma axon(s), and the other nuclear bag fibre is selectively controlled by 'dynamic' gamma, and perhaps beta, axon(s). Where two nuclear bag fibres are separately operated one may be controlled by 'dynamic' axon(s) and the other by 'static' gamma axon(s). Nuclear chain fibres are always controlled by 'static' gamma axons.

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