Structure predicts synaptic function of two classes of interneurons in the thoracic ganglia of Locusta migratoria

The relationship between synaptic function and structure was examined for 32 spiking interneurons (13 inhibitory and 19 excitatory) in the meso- and metathoracic ganglia of the locust, Locusta migratoria. In no instance was the structure of an excitatory interneuron similar to that of an inhibitory interneuron. However, 12 of the 13 inhibitory interneurons shared a number of structural features, namely a ventromedially located soma, axon(s) projecting into contralateral connective(s), and a laterally bowed primary neurite. Structurally the excitatory interneurons formed a more heterogeneous group. Even so, 12 of the 19 had a combination of structural features in common, namely laterally located somata and axon(s) projecting into contralateral connective(s). The clear differences in structure of the two main groups of inhibitory and excitatory interneurons suggest that other neurons with structures similar to members of these two groups can be classified as inhibitory and excitatory, respectively. Thus we propose that structure predicts synaptic function for two distinct groups of interneurons in the thoracic ganglia of locusts.