Primary projections of the lateral line nerves in adult lampreys.

Primary lateralis projections in silver lampreys, Ichthyomyzon unicuspis, and young adult sea lampreys, Petromyzon marinus, were examined utilizing silver impregnation of degenerating fibers and transganglionic transport of horseradish peroxidase. Anterior lateral line nerve afferents terminate throughout the neuropil of the electroreceptive dorsal nucleus and in the lateral neuropil of the mechanoreceptive medial nucleus. The superficial ophthalmic, buccal and recurrent rami of the anterior lateral line nerve all project to the ipsilateral dorsal nucleus; the recurrent ramus, supplying electroreceptors on the trunk, possesses the largest terminal field. No somatotopy was apparent in the projections to the dorsal nucleus. Primary afferents in the posterior lateral line nerve project to the medial neuropil of the ipsilateral medial nucleus, continue rostrally into the cerebellum and cross to the central neuropil of the contralateral medial nucleus. Lateralis projections in lampreys resemble those in nonteleost bony fishes and elasmobranchs; however, the contralateral projection of the posterior lateral line nerve is presently recognized only in petromyzontids.