Abstract Under normal conditions crustacean muscles consist of a mixed population of muscle fibres, some show only delayed rectification upon depolarization, while others display graded or all-or-none action potentials . It is now well documented that these action potentials are due to a tetrodotoxin (TTX)-insensitive voltage-dependent Ca 2+ influx . Here we present evidence that several weeks after axonotomy of the motor nerve and removal of an abdominal ganglion in crayfish there was an increase of the number of slow flexor muscle fibres showing action potentials. Some of these action potentials were dependent on Na ions and were TTX-sensitive. These results suggest that after operation voltage-dependent Na + channels become apparent in the muscle fibre membrane of the crayfish.