Absence of action potentials in frog slow muscle fibres paralysed by botulinum toxin.

1. As shown previously, slow muscle fibres of the frog develop the ability to produce action potentials in 2 weeks after denervation.2. Normal transmission at the slow fibre neuromuscular junction gives a junction potential of 11 mV average (range 6-20 mV), caused by summation of potentials from several motor axons.3. Botulinum toxin injected intramuscularly into the iliofibularis blocks the neuromuscular junction of slow fibres in 6 days at room temperature. Single nerve stimuli give junction potentials of 0.05 mV average (range 0-1.6 mV). Contraction of slow fibres in response to tetanic stimulation is eliminated.4. No action potentials could be elicited in botulinum treated fibres, even 6(1/2) weeks after injection of toxin. This finding is discussed in relation to the possible involvement of a ;trophic' factor regulating the action potential mechanism in frog slow muscle fibres.