THE MECHANISM OF TONIC INHIBITION OF CRAYFISH ESCAPE BEHAVIOR - DISTAL INHIBITION AND ITS FUNCTIONAL-SIGNIFICANCE

The excitability of crayfish escape behavior is seldom fully predictab le. A major determinant of this fickleness is a form of descending inh ibition that is reliably evoked during restraint or feeding and is cal led ''tonic inhibition.'' Tonic inhibition was found to inhibit postsy naptically the lateral giant neurons, the command neurons for one form of escape. This inhibition is located on lateral giant dendrites that are electrotonically distant from the neuron's spike initiating zone. In contrast, the postsynaptic inhibition due to ''recurrent inhibitio n,'' which prevents new escape responses from starting while a previou sly initiated one is in process, occurs proximally, near the spike ini tiating zone. The distalness of tonic inhibition could be an adaptatio n for selective suppression of parts of the lateral giant dendritic tr ee. Consistent with this, evidence was obtained that the tonic inhibit ory system can suppress responses to specific sensory fields. An indep endent reason for targeting recurrent inhibition proximally and tonic inhibition distally was suggested by the functional requirements of ea ch inhibitory process: recurrent inhibition needs to be ''absolute'' i n the sense that the response should be absolutely prevented, whereas it must be possible to override tonic inhibition. Neuronal models demo nstrated that proximal inhibition gives recurrent inhibition the requi red property of absoluteness while distal inhibition allows tonic inhi bition to be overridden (''relativity''). It was shown that the relati vity of distal inhibition arises from its interaction with the process of saturation of excitation and that tonic inhibition does indeed int eract with excitatory saturation as predicted. It is suggested that th e property of relativity of distal inhibition is exploited in other ne rvous systems as well.