MORPHOLOGY AND PROJECTIONS OF NEURONS IN REMAKS NERVE OF THE DOMESTIC-FOWL REVEALED BY INTRACELLULAR INJECTION OF BIOCYTIN

Micro-injections of biocytin were made into neurons in whole-mount pre parations of Remak's nerve of the domestic fowl to visualise the morph ology and projections of Remak's neurons. Remak's neurons were classif ied into four distinct morphological types. Remak type-I and -II neuro ns had a morphology resembling that of mammalian sympathetic neurons. Type-I neurons (found only in juxta-jejunal ganglia) had numerous prim ary dendritic processes (8-14) with large dendritic fields and extensi ve dendritic arborizations. Type-Il neurons had 2-9 primary dendritic processes, large dendritic fields and sparse dendritic branching. Thes e were found in similar numbers in juxta-rectal and juxta-jejunal gang lia. Remak type-III neurons were the most numerous cell type of juxta- rectal ganglia. They had small cell somata and short dendritic process es that branched infrequently. Remak type-IV neurons (found only in ju xta-jejunal ganglia) had a morphology resembling that of in vertebrate neurons in that they possessed a prominent long tapering axon from wh ich most of the numerous long dendritic processes emerged. In juxta-je junal ganglia, all type-IV and most type-I, -II and -III neurons proje cted orally, whereas axons of juxta-rectal neurons (types II and III) projected either orally or aborally, or projected directly into a late ral nerve bundle supplying the gut. These regional differences in neur on types and axonal projections suggest that different neural circuits exist between Remak's nerve and the small and large intestine.