POSTEXCITATORY INHIBITION OF THE CRAYFISH LATERAL GIANT-NEURON - A MECHANISM FOR SENSORY TEMPORAL FILTERING

Crayfish escape from threats by either giant neuron-mediated ''reflex' ' tail flexions that occur with very little delay but do not allow for much sensory guidance of trajectory or by ''nongiant'' tail flexion r esponses that allow for sensory guidance but occur much less promptly. Thus, when a stimulus occurs, the nervous system must make a rapid as sessment of whether to use the faster reflex system or the slower nong iant one, It does this on the basis of the abruptness of stimulus onse t; only stimuli of very abrupt onset trigger giant-mediated responses. We report here that stimuli which excite the lateral giant (LG) comma nd neurons for one form of reflex escape also produce a slightly delay ed postexcitatory inhibition (PEI) of the command neurons. As a result , only stimuli that become strong enough to excite the command neurons to firing threshold before the onset of PEI, within a few millisecond s of stimulus onset, can cause giant-mediated responses. This inhibiti on is directed to distal dendrites of the LG neurons, which allows for some location specificity of PEI within the sensory field of a single hemisegment.