Development of habituation in the crayfish due to selective weakening of electrical synapses.

During posthatching development, transmission becomes substantially reduced at monosynaptic electrical synapses between tactile afferents and the command neuron for caudal tailflip escape responses of the crayfish. The effectiveness of a parallel disynaptic pathway to the same command neuron is unaltered during posthatching growth. In small crayfish both the monosynaptic and disynaptic sensory pathways can elicit command cell action potentials. At this stage, the characteristics of the tailflip neural circuit are evidently controlled by the shorter latency, non-labile monosynaptic pathway. Consequently, tailflips can be reliably elicited in young crayfish, even at brief (2 s) interstimulus intervals. Tailflip responses of large, older crayfish are known to habituate when tactile stimuli are repeated at intervals up to 5 min. This decline in behavioral responsiveness is presumed to be mediated by low frequency synaptic depression (LFD) at first-order synapses of the disynaptic pathway. The lability of these synapses does not change during post-hatching development. Weakening of the monosynaptic pathway may be caused by command cell growth during posthatching development.