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Abstract

1. The tonic responses of angular-position-sensitive afferents in the metathoracic chordotonal organ of the locust leg exhibit much hysteresis. For a given joint angle, the ratio of an afferent's tonic firing rate after extension to its firing rate after flexion (or vice versa) is typically between 1.2:1 and 3:1 but can be as large as 10:1. Spiking local interneurons, that receive direct inputs from these afferents, can, by contrast, exhibit much less hysteresis (between 1.1:1 and 1.2:1). We tested the hypothesis that presynaptic inhibitory interactions between afferent axons reduces the hysteresis of postsynaptic interneurons by acting as an automatic gain control mechanism. 2. We used two kinds of neural models to test this hypothesis: 1) an abstract nonspiking neural model in which a multiplicative, shunting term reduced the "firing rate" of the afferent and 2) a more realistic compartmental model in which shunting inhibition presynaptically attenuated the amplitude of the action potentials reaching the afferent terminals. 3. The abstract neural model demonstrated the automatic gain control capability of a network of laterally inhibited afferent units. A postsynaptic unit, which was connected to the competitive network of afferents, coded for joint angle without saturating as the strength of the afferent input increased by two orders of magnitude. This was possible because shunting inhibition exactly balanced the increase in the excitatory input. This compensatory mechanism required the sum of the excitatory and inhibitory conductances to be much larger than the leak conductance. This requirement suggested a graded weighting scheme in which the afferent recruited first (i.e., at a small joint angle) received the largest inhibition from each of the other afferents because of the lack of active neighbors, and the afferent recruited last (i.e., at a large joint angle) received the least inhibition because all the other afferents were active. 4. The compartmental model demonstrated that presynaptic shunting inhibition between afferents could decrease the average synaptic conductance caused by the afferents onto the spiking interneuron, thereby counterbalancing the afferents' large average firing rates after movements in the preferred direction. Therefore the total postsynaptic input per unit time did not differ much between the preferred and nonpreferred directions.(ABSTRACT TRUNCATED AT 400 WORDS)