Non-linear summation of tension in motor units of toad slow muscle

Tension developed by single motor units and bundles of motor units has been recorded from the slow, non-twitch portion of the iliofibularis muscle of the toad Bufo marinus. Evidence was sought for the presence of individual muscle fibres within a motor unit which remained less than fully activated when the axon was maximally stimulated, i.e. at a rate and duration sufficient to produce maximum tension. When two motor units were stimulated separately and together, one at a low rate and the other at a maximal rate, the tension recorded on combined stimulation was greater than the sum of individual tensions, i.e. combined stimulation acted to 'facilitate' tension in a proportion of fibres. Adding the anticholinesterase eserine to the solution bathing the muscle produced a large increase in individual motor unit tensions but a rather smaller increase during combined stimulation of several axons. This suggested that in normal solution, maximally stimulating a single axon left many muscle fibres less than fully activated. When the local anaesthetic lignocaine was added to the bathing solution, tension in response to nerve stimulation fell, presumably because of impulse blockade in small terminal branches of the axon. However tension fell more steeply for single motor units than for combinations of motor units, again, we propose, because single motor units contain a larger fraction of incompletely activated fibres whose tension output depends critically on the level of activation. Overlap between two motor units is a measure of the proportion of fibres innervated by both axons. When overlap was measured over a range of different muscle lengths and using contractions of different durations, evidence was obtained in support of the notion that tension output of some muscle fibres was less than maximum not only because the fibre received too sparse an innervation to achieve a sufficient over-all level of depolarization but because they had been non-uniformly activated, leading to internal motion within the fibre and a consequent further drop in tension.