rid in Great Britain © The Company of Biologists Limited 1989 /[ORE THAN ONE TYPE OF CONDUCTANCE IS ACTIVATED DURING RESPONSES OF BLOWFLY MONOPOLAR

The principal second-order neurones in the blowfly compound eye, the large monopolar neurones (LMCs), were studied using intracellular recording and discontinuous current-clamp techniques, in combination with measurement of dynamic input resistance. The LMCs had resting potentials of-35 to-45 mV and showed a linear current-voltage relationship in the lamina in the physiological voltage range. The hyperpolarizing light-on transient was associated with a drop in input resistance from 17 ± 5 to 3 ± 1MQ, and had a reversal potential between-60 and-90 mV. The dynamic input resistance of saturated responses and the properties of reversed responses suggested that more than one conductance was activated during the response of the LMCs. In lamina recordings, the input resistance increased beyond the resting level during repolarization, which can be interpreted in terms of a continuous release of transmitter by the photoreceptor terminals, even in darkness. The input resistance of LMCs in axon recordings in darkness and during the light-on response was generally higher than in the lamina recordings. The responses to light in axons also differed from those recorded in lamina by showing regenerative properties