Connections with the Posterior Lateral Line Lobe

Abstract

The mesencephalic torus semicircularis in gymnotid electric fish has been identified as a key structure for electrosensory signal processing and control of the electric pacemaker (Scheich 1974, 1977c; Bastian and Heiligenberg 1980). In terms of relative volume, it is the largest torus in any vertebrate, and in Eigenmannia it can be subdivided into 15 cell and fiber laminae (see Sect. 2). The layers from the right and left toms are continuous across the midline. Some of the dorsal layers receive bilateral input from the posterior lateral line lobe (LLLP) of the rhombencephalon, the first target of the electroreceptive afferents of the lateral line nerve (Ebbesson and Scheich 1980; Scheich and Ebbesson 1981; Carr et al. 1981). These recent anatomical accounts have also substantiated the findings of Scheich and Maler (1976) and Scheich (1977c) that units in superficial layers of the torus represent a topographic map of the fish’s electroreceptive surface. There is a point-to-point projection to the torus from LLLP where several topographic representations of the fish’s surface are believed to be present (Maler et al. 1974, Maler 1979; Carr et al. 1981).