Anatomy and physiology of neurons composing the commissural ring nerve of the cricket, Acheta domesticus.

The commissural ring nerve (RN) of the cricket Acheta domesticus links together the two cercal motor nerves of the terminal abdominal ganglion. It contains the axons of almost 100 neurons including two bilateral clusters of eight to 13 ventrolateral neurons and approximately 75 neurons with midline somata within the terminal abdominal ganglion. The ventrolateral neurons have an ipsilateral dendritic arborization within the dorsal neuropil of the ganglion and their axons use the RN as a commissure in order to enter the contralateral nerves of the tenth ganglionic neuromere. In contrast, most midline neurons have bifurcating axons projecting bilaterally into the neuropil of the ganglion as well as into the RN where they often branch extensively before entering the contralateral tenth nerves. Most RN neurons have small, non-spiking somata with spike initiation zones distant from the soma. Many midline neurons also produce double-peaked spikes in their somata, indicative of multiple spike initiation zones. Spontaneous neuronal activity recorded extracellularly from the RN reveals several units, some with variable firing patterns, but none responding to sensory stimuli. The RN is primarily composed of small (50 nm diameter) axon profiles with a few large (0.5-1 microm diameter) profiles. Occasionally, profiles of nerve terminals containing primarily small clear vesicles and a few large dense vesicles are observed. These vesicles can sometimes be clustered about an active zone. We conclude that the primary function of the RN is to serve as a peripheral nerve commissure and that its role as a neurohemal organ is negligible. J. Exp. Zool. 286:350-366, 2000.