Neuronal connections and the function of the corpora pedunculata in the brain of the American cockroach, Periplaneta americana (L.)

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Abstract
The fiber constituents and connections of the calyces — the input-receiving regions — of the corpora pedunculata (“mushroom bodies”) were studied in reduced silver preparations from the American cockroach, Periplaneta americana (L.). In the outer synaptic layer of the calyces five fiber classes were distinguished, the first three of which arise outside the mushroom body. (1) Four highly similar neurons with somata near the optic lobe branch into different parts of the ipsiateral protocerebrum, including both calyces. Their fibers are highly constant in arrangement and position and contain small nucleus-like bodies. (2) The tractus olfactorio-globularis (sensu lato) emits fiber groups which course along the calycal walls as “calycal tracts” before ultimately dissipating into the synaptic layer. Variability within these tracts is described. (3) Fibers of undertermined origin outside the mushroom body radiate from the calycal center outwards through the synaptic layer. (4) From the inner calycal layer of neurites belonging to intrinsic mushroom-body neurons, perpendicular collaterals enter the synaptic layer. (5) Intrinsic-neuron somata near the calycal rim emit fibers which course tangentially within the synaptic layer from calycal rim to center. These fibers form a special peripheral zone in the pedunculus. The predominant presumably afferent calycal fiber class is that derived from the tractus olfactorio-globularis. No evidence was found for tracts from optic lobe to calyces. On this basis, and in light of the experimental and comparative anatomical literature, it is suggested that the corpora pedunculata of P. americana and other pterygotes are fundamentally second-order antennal sensory processing centers. Conflicting observations in earlier reports are critically discussed.