Exp Brain Res (1981) 42:23-33 E ,x _'mental Bran Research 9 Springer-Verlag 1981 Electrosensory Pathways to the Valvula Cerebelli in Mormyrid Fish T.E. Finger 1, C.C. Bell 2, C.J. Russell 2 1 Department of Anatomy, Universityof Colorado Medical Center, 4200East 9th Ave., Denver, CO 80262, USA 2 NeurologicalSciencesInstitute, Good Samaritan Hospital and MedicalCenter, 1015N. W. 22nd Ave., Portland, OR 97210, USA Summary. The valvula cerebelli of mormyrid fish has been implicated in the electroreceptive capabilities of these animals. This study uses peroxidase and triti- ated amino acid tracing techniques to examine the pathways by which electroreceptive information gains access to the valvula. Ampullary and mormyromast receptor informa- tion reaches the medial and ventrolateral portions of the valvula by means of a large, direct projection from n. lateralis, part of the midbrain electrosensory area. In addition, smaller indirect n. lateralis-valvu- lar projections travel via a pretectal nucleus and the so-called postventral thalamic nucleus. Knollenorgan receptor information ends in the dorsolateral portion of the valvula. A small, relatively direct pathway runs via n. medialis ventralis, but most Knollenorgan information appears to travel via an indirect system involving n. extrolateralis pars posterior and n. isthmi. The lateral line processing areas of the valvula project back onto many of the midbrain electrosen- sory nuclei. These results indicate that much of the cerebellum of this family is devoted to sensory processing rather than motor functions. Key words: Cerebellum - Electroreception - Lateral line - Valvula cerebelli - Mormyrid Members of the family Mormyridae have not only the usual mechanoreceptive lateral line system but also a highly elaborate and sensitive electroreceptive capability. The electrosensory system consists of three distinct classes of receptors: (1) ampullary, (2) mormyromast and (3) Knollenorgan, with each Offprint requests to: Dr. T.E. Finger (address see above) receptor class being primarily involved in one specific aspect of electroreception, respectively: (1) low fre- quency electrolocation, (2) active electrolocation, and (3) communication. The mechano- and electrosensory components of the lateral line system terminate in separate primary medullary nuclei respectively, the anterior and post- erior lateral line lobes (Maler et al. 1973a, b; Bell and Russell 1978). Within the mormyrid posterior lateral line lobe, separate areas exist for processing the input from each electroreceptor class (Enger et al. 1976; Bell and Russell 1978). Higher order projections maintain to some extent this segregation of elec- troreceptor information (Zipser and Bennett 1976a, b; Enger et al. 1976; Haugede-Carre 1979). The remarkable elaboration of the valvula cere- belli in mormyrids has been believed to be related to the electrosensory capacity in this family. Yet other electrosensitive species function quite well without an especially large valvula. Furthermore, no direct connection between the posterior lateral line lobe and the valvula has been reported. Accordingly some doubt exists as to whether the valvula is intimately related to the electrosense. A recent physiological study by Russell and Bell (1978) demonstrates that at least a portion of the valvula is involved in processing electrosensory information. Their study also sug- gested that information from each electroreceptor class is processed in different areas of the valvula. A recent anatomical study by Haugede-Carre (1979) has traced input from one class of receptor, the Knollenorgan, to a precerebellar nucleus in the isthmus. However, no experimental anatomical evi- dence exists which demonstrates the manner in which the bulk of the electroreceptive information reaches the valvula cerebelli. Accordingly, the present study seeks to demonstrate some of the pathways involved in carrying electroreceptor information to the valvula cerebelli in mormyrids. 001A 4819/81/0042/0023/$ 2.20