A particular nucleus in the mesencephalon of a weakly electric fish, gymnotus carapo, qymnotidae. I. Light microscopic structure.

Abstract

1. A particular area of the midbrain called magnocellular mesencephalic nucleus (MMN), characteristic of several weakly electric fishes, was examined by different histological methods in Gymnotus carapo, Gymnotidae. 2. This area contains a dense network of thick fibers in which large neurons are scattered: it contains also a great number of small neurons. 3. The large neurons (25–35 μ) are unipolar with a profusely branching axon that gives rise to the main part of the network within the nucleus. At least one branch of the axonal arborisation leaves the nucleus towards the torus semicircularis, i.e. in the ventral direction. The small neurons (5–12 μ) are uni- or bipolar and display a profuse arborisation of their axon near the soma. 4. Large and small neurons are preferentially localized in the postero-ventral part of the nucleus. 5. Large neurons are contacted by multiple club endings originating partly from the rhombencephalon. The unipolar small neurons receive a single cup-like terminal. 6. The total number of club endings extablishing synaptic contacts on the surface of a large neuron was counted on serial semithin sections. 7. The present findings provide new information about the mesencephalic nucleus, which according to previous electrophysiological observations (Szabo, 1967; Szabo and Sakata, 1967) has been shown to represent a relay in the rapid electrosensory pathway.