The cercus-to-giant interneuron system of crickets
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The pattern of functional connections between the receptors on the surface of the cerci of crickets and the two largest interneurons (MGI and LGI) of their ventral nerve cord was studied, with special attention given to groups of receptors defined by theirlocation on the cercal surface.
Selectively blocking the transversely vibrating T-hairs on the dorsal and ventral surfaces of the cercus severely reduces the response of both interneurons in the ipsilateral connective to sound stimuli.
Selectively blocking the longitudinally vibrating L-hairs on the medial and lateral surfaces leaves the response of the MGI unaffected and enhances the response of the LGI.
Selectively stimulating the T-hairs of the contralateral cercus markedly reduces the response of both giant fibers to ipsilateral stimulation. This effect is tonic.
We interpret these results to mean that ipsilateral T-hairs provide the main excitatory input to both the MGI and LGI, and thus determine the transversely oriented directional sensitivity curves of these cells. The contralateral T-hairs provide inhibition to both cells. Ipsilateral L-hairs excite the MGI weakly and apparently inhibit the LGI; there is also indirect evidence that contralateral L-hairs are weakly excitatory.
Progressively blocking both groups of hairs starting at the tip of the ipsilateral cercus has very little effect on the giant fiber response until the proximal 1 or 2 mm are reached. Blocking the receptors starting at the base of the cercus drastically reduces the response and elevates the threshold even when the great majority of receptors are still free.
Thus, there is variation in the effectiveness of inputs to the MGI and LGI both along the circumference of the cercus and along its length. There are several possible bases for such variation; we present one explicit model based on position-dependent probability of connection between receptors and interneurons. Finally, we suggest a causal link between the position of a receptor cell body on the surface of the cercus and the nature of the connections formed by its axon within the central nervous system.
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