Central connections of the posterior lateral line lobe in mormyrid fish

Primary electroreceptor afferents terminate in the posterior lateral line lobe (PLLL) in electroreceptive teleosts. This paper examines the central connections of PLLL in fish of the family Mormyridae using horseradish peroxidase and tritiated amino acid tracing techniques. Some connections of the closely related lobus caudalis of the cerebellum are also examined. There are three zones on each side of the mormyrid PLLL cortex. Two receive input from mormyromast receptors, and one from ampullary receptors. An intrazonal projection system, intrinsic to PLLL, connects neighboring points within each zone. It also joins corresponding zones on the two sides of the body via commissural fibers. An interzonal system connects the two mormyromast zones on the same side of the midline. Central structures which project to PLLL include lobus caudalis, nucleus paratrigeminalis lateralis, and nucleus praeeminentialis. Nucleus praeeminentialis projects bilaterally and somatotopically to the lower molecular layer of PLLL. PLLL cortex projects bilaterally and somatotopically to two major mesencephalic sites: n. praeeminentialis, and n. lateralis. Somatotopically corresponding points in each zone of PLLL cortex project to the same small region of n. lateralis. Nucleus lateralis has a large and somatotopically organized projection to n. praeeminentialis. The afferent and efferent connections of lobus caudalis are similar to those of PLLL, indicating its close association with the electrosensory system. The anatomical results show that there is ample opportunity for electrosensory information arising on left and right sides of the body to interact centrally. One can suggest that comparison of afferent input from the two sides would reduce the non-significant variability which affects both of them equally. The results also show the presence of several somatotopically organized feedback lops which return the results of higher order processing of electrosensory information to earlier stages.