The central distribution of afferents from individual eighth nerve branches (N VIII) and mechanical lateral line end organs in mormyrid fish are described. Afferents were labeled with horseradish peroxidase (HRP) placed on the cut ends of the different N VIII branches and the anterior and posterior lateral line nerves.
Descending, tangential, and magnocellular nuclei receive input almost exclusively from the utriculus and canals. Nucleus octavius receives afferents from the lateral line nerves and all N VIII branches, with one part receiving exclusive and bilateral input from the sacculus. Afferents from both lateral line nerves and all N VIII branches, except the sacculus, end in eminentia granularis. Afferents from each of the two lateral line nerves and from each of the three otolith branches of N VIII end in different regions of the anterior lateral line lobe, with some areas of overlap.
Behavioral studies in other families of fish indicate that the utriculus and canals are critical for postural control, whereas the sacculus and possibly the lagena are concerned with hearing. Such findings, together with the results of this study, suggest that mormyrids and perhaps other fish possess separate auditory and vestibular centers within the octavolateral area.
The HRP method also shows the cell bodies and axons of octavolateral efferents. N VIII and lateral line efferents arise from a common nucleus, and the central course of their axons parallels that of facial motoneurons. Axons of efferent cells divide to supply two or more branches of N VIII and some axons supply both lateral line and N VIII end organs.