Spiking local interneurones in the mesothoracic ganglion of the locust: homologies with metathoracic interneurones.

Two bilaterally symmetrical groups of spiking local interneurones have been characterized in the mesothoracic ganglion of the locust. The cell bodies of one group, the "midline group," lie at the ventral midline. Their primary neurites run in the ventral loop of ventral commissure II to form extensive branches in the neuropile of one-half of the ganglion. A dorso-ventral process in the perpendicular tract links two distinct fields of branches, one ventral and consisting of numerous fine branches of a uniform texture that arise from stout secondary neurites, and the other more dorsal consisting of fewer branches of a varicose appearance. Cell bodies of the second, the "anterior-lateral" group, lie close to the lateral edge of an anterior connective. Their primary neurites run in a more anterior ventral commissure and their neuropilar branches are divided into two fields by a process in a more anterior dorso-ventral tract. Within the two groups, each interneurone has its own distinctive shape that is an elaboration on these basic plans. Each interneurone also has its own characteristic physiology, being excited by a particular array of mechanoreceptors on the middle leg on the same side of the body as its neuropilar branches. The receptive fields of the interneurones, defined in this way, can be extensive and cover a particular surface of all parts of the leg, or restricted to one surface of, for example, the tarsus. These interneurones therefore bear a striking resemblance to two groups of spiking local interneurones in the adjacent segmental ganglion of the metathorax.