Roles of the mushroom body in locomotory control in an insect

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The mushroom body (MB) is the most conspicuous neuropil structure in the insect brain, critical to various forms of associative memory. In order to elucidate the role of the MB in the higher locomotory control, activities of output neurons and the cellular organization of the MB in the cockroach, Periplaneta americana, were studied.1. Activities of MB output neurons of freely behaving cockroaches were examined. Enamel-coated copper wires of 17 mum in diameter were implanted chronically into the MB output neuropil, and unit activity was recorded while the cockroach walked freely in an arena. After the recording, copper ions were impregnated by passing a positive current to reveal profiles of neurons in the vicinity of the electrode tip. The recorded neurons were classified into (1) sensory units which respond to various sensory stimuli, (2) motor-related neurons which are active during locomotory, (3) sensory-motor neurons which are active during sensory stimulation and also during motor action, and (4) neurons whose activity preceded the initiation of specific locomotory action.2. Cellular organization of the cockroach MB was studied by means of reduced-silver and Golgi stainings. I found that two types of repetitive modular subunits with different sizes exist in the MB output neuropil. The first, which I refer to as a sheet, is a thin (1 mum or less) meshwork formed by 200-1000 axons of Kenyon cells. The sheets are further organized into a larger structural grouping, referred to here as a slab, with a width of 3 mum or more. A total of about 30 dark and pale slabs of more or less evenly spaced alternate throughout the length of the output neuropil. I conclude that the MB,insect memory center, consists of two types of repetitive modular subunits, as does the mammalian cerebral cortex.