Absence of polyneuronal innervation in cat extraocular muscles

1. Polyneuronal innervation is known to occur in several types of vertebrate muscle, including mammalian muscle spindle (Gray, 1957). Previous work had led to the suggestion that the multiply innervated cat extraocular muscles were similarly polyneuronally innervated. 2. The presence of polyneuronal innervation in fibres that show propagated conduction was explored in the two muscles innervated by the abducens nerve: the lateral rectus (which contains multiply innervated fibres) and the retractor bulbus (which does not). 3. Under conditions of twitch and tetanic stimulation, the sum of individual tensions from the two nerve branches was compared with the tension elicited from the whole nerve. An analysis of variance indicated that the lateral rectus and the retractor bulbus did not differ significantly in terms of tension excess. The twitch condition yielded a small but statistically significant (P smaller than 0-01) amount of tension excess, whereas the tetanic condition did not. 4. Twitches elicited by stimulation of one branch of the abducens nerve showed slight potentiation when preceded by a tetanic stimulation to the other nerve branch. This effect could be ready differentiated from the post-tetanic potentiation elicited by applying both twitch and tetanic stimuli to the whole nerve. 5. Muscle fatigue produced by intensive stimulation of one nerve branch did not decrease the amplitude of the tetanic contraction elicited by stimulation of the other branch. In fact, instead of crossed fatigue, a small but significant (P smaller than 0-05) potentiation was observed. 6. Analysis of the results of the three tests led to the conclusion that polyneuronal innervation could not be demonstrated in cat extraocular muslce fibers that showed propagated conduction. Thus, the end-to-end muscle fibre junctions (Floyd, 1970) apparently do not transmit conducted impulses.