Each local spiking interneuron has two layers of branches (Fig. 8.6a). Studies with the electron microscope have shown that the ventral layer
Figure 8.6 Local spiking interneurons of the locust third thoracic ganglion. (a) Diagrams of an interneuron showing its dorsal and ventral fields of branches. This interneuron was excited by hairs on the ventral surface of the tibia near to the foot, and by proprioceptors that responded to extension of the tibia. The two oval shapes indicate locations of cell bodies of two different groups of spiking local interneurons. (b) Intracellular recordings of the responses by a local spiking interneuron to stimulation of trichoid sen-silla at three different locations on the femur, indicated in the diagram. Two or three recordings for each hair are superimposed. The lower trace shows a spike from the sensory neuron, and the upper trace is the intracellular recording from the interneuron. The middle recording shows the strongest connection: each sensory neuron spike evoked a spike in the interneuron, whereas spikes in the other two sensory neurons evoked subthreshold EPSPs. (a from Burrows, 1985; reprinted with permission of the American Physiological Society; b modified after Burrows, 1992b; reprinted with permission from Trends in Neuroscience; copyright © 1992 Elsevier Science.)
mostly receives input synapses, whereas the dorsal layer mostly makes output synapses (Watson & Burrows, 1985). The two layers are connected by a long, thin neurite that may be an axon, carrying spikes from the ventral to the dorsal layer, but its small size makes it hard to confirm this experimentally. One group of local spiking interneurons have their cell bodies clustered near the middle of the ventral surface of the ganglion, and these interneurons all make inhibitory output synapses (Siegler & Burrows, 1984). Another group have their cell bodies close to the anterior connective, and these interneurons make excitatory output synapses (Nagayama, 1989).
Each local spiking interneuron responds to stimulation of basiconic and trichoid sensilla in a well-defined receptive field on particular regions of the leg (Burrows & Siegler, 1985; Burrows & Newland, 1993). Some interneurons have fields restricted to particular parts of one segment of the leg, but others have broader receptive fields that extend in both directions from a joint. The interneuron from which the recordings in Fig. 8.6b were made responded to stimulation of trichoid sensilla on the ventral surface of the femur and there was a gradient in the strength of connections that declined in a direction away from the body. In the most sensitive part of the receptive field, a single spike in a sensillum will often be sufficient to elicit a spike in a local spiking interneuron. However, when two sensory neuron spikes occur in quick succession, the second may only elicit a small PSP, and this change in the effectiveness of the synapses is a mechanism that enhances responsiveness by the locust to new, rather than maintained, tactile stimuli (Burrows, 1992a).
Each spiking interneuron excites or inhibits a number of different targets, which are mainly non-spiking interneurons and motor neurons. The overall pattern of connectivity is diffuse, because each non-spiking interneuron or motor neuron receives inputs from a number of different local spiking interneurons. However, the pattern of connections preserves information about the location of tactile stimuli on the surface of the leg, so that non-spiking interneurons and motor neurons, as well as the local spiking inter-neurons, have well-defined receptive fields. When part of the leg is touched, therefore, a particular set of local spiking interneurons and another set of non-spiking interneurons are activated.
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