title: The mormyrid mesencephalon. II. The medio-dorsal nucleus of the torus semicircularis: afferent and efferent connections studied with the HRP method.
abstract: The connections of the medio-dorsal nucleus (nMD) were established with the HRP tracing method in two mormyrid species, Brienomyrus niger and Gnathonemus petersii. The medio-dorsal nucleus (nMD) is the second largest nucleus of the mormyrid torus semicircularis which comprises 7 nuclei. According to our histological observations, the nMD is composed of three regions: anterior (a), medialis (m) and posterior (p), each has distinct connections. The nMD receives medullary, rhombencephalic inputs, and mesencephalic inputs. The rhombencephalic afferents arising from the acoustico-lateral area constitute the most extensive projection: they arise from nucleus octavius, nuclear anterior and a 'crest cell layer'. The largest part of these rhombo-mesencephalic connections is bilateral. The bifurcated axons are gathered in the pars medialis of the lemnisci laterales and end, respectively, in the three subdivisions--pars posterior, pars medialis, pars anterior--of the nMD. The contralateral projections are larger than the ipsilateral ones except for the octavo-mesencephalic projection, where the ipsilateral is predominant. Collaterals of these axons end bilaterally in the mesencephalic paralemniscal nuclei (nPL). The mesencephalic afferents originate in the contralateral nPL, while medullary afferents arise in the contralateral nucleus subfunicularis (nSF). None of these projections show topological specificity within the nMD. Concerning the efferent connections, the nMD projects to the ipsilateral tectum opticum and valvula cerebelli.
mesh_terms: D000344:Afferent Pathways / Q000502:physiology; D000818:Animals; D001931:Brain Mapping; D004525:Efferent Pathways / Q000502:physiology; D005399:Fishes / Q000502:physiology*; D006735:Horseradish Peroxidase; D008526:Medulla Oblongata / Q000502:physiology; D008636:Mesencephalon / Q000502:physiology*; D009435:Synaptic Transmission
chemical_list: D006735:Horseradish Peroxidase