Local feedback mediated via amacrine cells in the insect optic lobe

Abstract

1. The proximal part of the medulla of the locust (Locusta migratoria) optic lobe contains a small number of tangential amacrine cells. Using the recently developed intracellular label, Neurobiotin, we have combined physiological characterisations with structural descriptions of the cells at light and at electron microscopic levels. 2. Each of these tangential medulla amacrine (TMA) cells arborises over a large portion of the visual field (Fig. 1), with strongly beaded dendrites restricted to the layer of the medulla immediately proximal to the large serpentine layer that divides the ganglion. There is a second, more sparse and finer arborisation in the most proximal layer of the medulla. 3. Using our own modification of Neurobiotin histochemistry for transmission electron microscopy, we investigated the synaptology of the TMA cells. In the principal layer of dendrites, TMA cells make both input and output synapses with the same cells. Thus the TMA cells might act to connect one (or more) classes of columnar cells, providing a substrate for lateral interactions between retinotopic afferent pathways. The “beads” seen at the LM level are due to aggregations of mitochondria and not to synaptic terminals. 4. Physiologically these TMA cells are transient (on/ off) units. Whilst their arborisation is extensive, the “receptive field” measured in a single recording is only about 20° across. 5. These TMA cells appear suited to mediating the inhibitory interaction between the columnar inputs to the lobula giant movement detector (LGMD) that accounts for the preference of the latter cell for small targets.