The distribution of synapses on the two fields of neurites of spiking local interneurones in the locust.

Spiking local interneurones in the metathoracic ganglion of the locust have two fields of neuropilar branches linked by a single process. One, with numerous fine neurites of relatively uniform diameter, is in a ventral area of neuropile where the afferents from hairs on a hind leg also terminate. The other, with sparser and varicose neurites, is in a more dorsal area of neuropile where the motor neurones of the leg muscles have many neurites. Physiological studies have shown that these interneurones receive direct inputs from some hair afferents and in turn synapse directly upon some leg motor neurones (Burrows and Siegler, '82; Siegler and Burrows, '83). The distribution of synapses in these two fields was revealed by injecting HRP intracellularly into an interneurone following its physiological characterization. In the ventral field, the majority of synapses are input synapses. Many are from profiles with round agranular vesicles of similar diameter to those of known afferents. Other presynaptic profiles contain pleomorphic agranular, or large round granular, vesicles. There are also some output synapses, each with only a small population of vesicles. Most input synapses are made onto small-diameter neurites, whereas the majority of outputs are made from the larger-diameter neurites. In the dorsal field, the majority of synapses are output synapses. They are made from vesicle-filled varicosities onto postsynaptic profiles of small diameter. A single varicosity can be the site of many output synapses. Input synapses are few in number, but occur close to the output synapses. The absence of varicosities in the ventral field cannot therefore be correlated with a lack of output synapses and while the dorsal varicosities can be the sites of output, they can also receive input synapses.