Responses to Ca2+ of two types of excitatory junctional potentials in cricket muscles

Cricket (Gryllus bimaculatus) abdominal muscle 202 produced both small and large excitatory junctional potentials (EJPs) depending on the intensity of the stimulation to the nerve. Abdominal muscle 203 generated only small EJPs. The large EJPs, the threshold of which was always slightly higher than the small EJPs, were more sensitive to the Ca2+ concentration at which they were evoked and to concentration changes. There were no marked differences between the two muscles regarding the electrical properties of the muscle fibre membrane and the quantal size of the transmitter. The causes of the differences between the two types of EJPs in these muscles are discussed.