Printed in Great Britain EXCITABILITY CHANGES IN CRAYFISH MOTOR NEURONE TERMINALS

1. Changes in the post-activation excitability of crayfish motor nerve terminals were used to measure afterpotentials that might be related to facilitation of transmitter release. 2. The refractory period is followed by a period of supernormal excit-ability in which the threshold of nerve terminals drops to about 70 % of its pre-activation level at about 15 msec following an impulse. The thre-shold returns exponentially to its pre-activation level with a time constant of about 25 msec at 130 C. Such a supernormal excitability is rarely seen in pre-terminal nerve branches or in the main axon. 3. Following a brief high-frequency tetanus the peak of the super-normal excitability is greater than that following a single impulse. At low temperature this peak is reduced and delayed, and the decay rate of the supernormal excitability is prolonged with a Q10 of about 2-5. 4. Depolarization of nerve terminals decreases, and hyperpolarization increases, the magnitude of the post-activation supernormal excitability. 5. The magnitude of the supernormal excitability depends on the ex-ternal potassium concentration, but not on sodium. In low calcium the peak supernormal excitability is often reduced. High calcium concentra-tion and manganese ions have no effect, but cobalt abolishes the super-normal excitability, and its effects are only slightly reversible. Both cobalt and manganese reversibly block neuromuscular transmission. 6. Strophanthidin has no effect on the post-activation supernormal excitability, but proteolytic enzymes reduce or abolish it, and hyper-osmotic solutions also affect it. 7. It is suggested that the action potential is followed by a depolarizing afterpotential in nerve terminals which is caused by a transient increase in the potassium concentration around the terminals. There is no evidence * Present address: Laboratoire de Neurobiologie Cellulaire, Centre National de l