Calcium and the tonic release of transmitter at a non-impulsive synapse in the crab

Depolarization-transmitter release coupling was studied in the promotor stretch receptor/motoneuron synapse of the crab. Callinectes sapidus, a preparation in which presynaptic action potentials do not occur. Intracellular microelectrode recordings were made from the presynaptic terminal and from the somata of postsynaptic motoneurons while injecting current pulses into the peripheral stretch receptor dendrite with the aid of the sucrose-gap. 1. For short current pulses, the relationship between presynaptic potential and postsynaptic response was found to be similar to that demonstrated in the giant synapse of the squid stellate ganglion, indicating a common reliance on the properties of voltage-dependent calcium channels. 2. The crab synapse was found to be capable of continuous transmission in the range of seconds and minutes without the pronounced depletion of transmitter seen in the squid, and without inactivation of the release process (i.e., the calcium conductance is non-inactivating). 3. A graded, transient response to depolarising current in the presynaptic fibre was found to be calcium-dependent, and probably to reflect the presence of a separate, inactivating calcium conductance. 4. It was concluded that the graded response of the presynaptic membrane could function in helping to compensate for capacitative distortion of receptor potentials decrementally conducted in the sensory dendrite, and was therefore a specialisation for non-impulsive transmission.