Journal of Neuroscience, Vol 14, 3341-3350, Copyright © 1994 by Society for Neuroscience
Convergence of mechanosensory afferents from different classes of exteroceptors onto spiking local interneurons in the locust
M Burrows and PL Newland
Department of Zoology, University of Cambridge, United Kingdom.
Tactile stimuli to a leg of a locust are detected by two classes of
exteroceptors: trichoid sensilla (tactile hairs), which are purely
mechanoreceptors, and basiconic sensilla, which serve a dual role as
mechano- and chemoreceptors. The trichoid sensilla have the longer shafts,
but because the two types of receptors are intermingled over the surface of
a leg, both can be excited when the leg contacts an obstacle. This article
analyzes the mechanisms by which the tactile sensory information from these
two classes of receptor is collated in the CNS so that spatial information
is preserved. The mechanosensory afferents from both types of receptors on
a hind leg make convergent connections that are excitatory and apparently
direct with the same spiking local interneurons in a midline population of
the metathoracic ganglion. The anatomy of the afferent projections suggests
that the synapses from the two types of receptor are made onto the same
region of branches of an interneuron. The slower conduction velocity of the
spikes of the basiconic afferents compared to that of the trichoid
afferents means that a mechanical stimulus to the tibia will first produce
a depolarization caused by the trichoid afferents, followed up to 20 msec
later by a depolarization caused by the basiconic sensilla. Each
interneuron is excited by a contiguous and overlapping array of trichoid
and basiconic receptors that form its receptive field. Different
interneurons have different receptive fields such that the surface of the
leg is mapped as a series of overlapping receptive fields. Within a
receptive field the contribution of individual receptors can be markedly
different: some basiconic receptors evoke large-amplitude EPSPs so that
each afferent spike can cause a spike in the interneuron, whereas others
generate small EPSPs that must sum with other inputs to evoke a spike. The
amplitudes of the EPSPs generated by different receptors within a receptive
field form gradients that are distributed according to the axes of the leg.
The most effective basiconic and the most effective trichoid sensilla
within a particular receptive field can be at the same or at different
locations on the leg. Tactile sensory information from different types of
receptor on one region of a leg is therefore collated initially by
particular spiking local interneurons. This organization ensures summation
between the inputs from the different types of receptors and should enhance
sensitivity.(ABSTRACT TRUNCATED AT 400 WORDS)
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