Innervation of extrafusal and intrafusal fibres in snake muscle

1. Intrafusal fibres of snake receive motor supply from branches of axons innervating extrafusal motor units. By intramuscular stimulation of motor units by single shocks, and critical curarization of the muscle, we have identified at least some of the motor units contributing motor supply to individual intrafusal fibres. Intrafusal fibre activation was observed by visual examination of the contracting intrafusal fibre, and by recording the resulting spindle afferent discharge.2. The main finding is that in some cases the motor supply to one intrafusal fibre comes from more than one motor unit. The contributing motor units may be either dissimilar twitch units, or twitch and tonic units. Thus some of the intrafusal fibres studied showed polyneuronal motor innervation of heterogeneous origin.3. In critically curarized muscle, the time course of a spindle afferent discharge, following single-shock stimulation of a motor unit contributing motor supply to the intrafusal fibre, showed little variation with the type of motor unit being stimulated.4. The response of each spindle to a standard stretch was recorded. There was no correlation between dynamic index and type of motor unit or units contributing motor supply. However, the method limits the value of negative findings, and this is discussed.5. The contraction times and tensions of a sample of motor unit isometric twitches are described.