Studies on the neuronal organization of cockroach corpora pedunculata

Abstract

Continuing an electron-microscopic study of the corpora pedunculata of Periplaneta americana, we have found that the alpha lobe is ultrastructurally very similar to the beta lobe, both with regard to the characteristics and the polarity of the synapses. Both neuropiles are essentially formed by nerve fibres and nerve endings coming into close contact with each other. There are at least two types of nerve fibre. Type 1 fibres (which are interpreted as axonic processes of the globuli cells) are uniformly thin and often roughly parallel to each other; they give rise to an enormous number of nerve endings containing small clear vesicles and slightly larger semi-dense granules. These fibres sometimes make synaptic contacts between themselves (axo-axonic synapses). More often, however, they synapse on type 2 fibres, which are characterized by the lack of synaptic vesicles. Type 2 fibres, which are therefore considered as postsynaptic, are in all probability the dendrites of neurons which are extrinsic to the corpora pedunculata. Two types of neuropile are therefore present in the corpora pedunculata. In the calices the large nerve endings of the extrinsic fibres each make numerous synaptic contacts with thin dendritic branches of the globuli cells. In the alpha and beta lobes, however, numerous nerve endings of the globuli cell axones converge to synapse on the thin dendritic branches of extrinsic neurons.