Peripheral innervation pattern in myotomal muscle of two teleosts as revealed by intracellular marking

Spinal nerves of abdominal and trunk segments from cod (Gadus morhua L.) and freshwater eel (Anguilla anguilla L.) were examined by light and electron microscopy and major differences in peripheral innervation patterns investigated.
Histochemically stained frozen sections clearly resolve the focal (terminal) innervation of eel fast fibres (one endplate/fibre) and the multiple innervation pattern of cod trunk muscle (around 20 endplates/fibre). Sub junctional folds are absent from eel and poorly developed in cod neuro-muscular junctions (NMJs). A single population of synaptic vesicles with mean diameter of22–25 nm is found in the NMJs of both species.
Cobalt chloride was used as an intracellular marker to map the branching between spinal roots and fast muscle endplates. Evidence was obtained for substantial inter-segmental branching of the ventral rami in cod but not in eel. Previous classification schemes, dividing fishes according to the presence of separate or mixed motor and sensory axon populations within the ventral rami, are shown to be dependent upon sample location. In this study, the ventral rami of both species contain mixed populations, having modal diameters around 2 μm and8–12 μm, with the proportion of small diameter (sensory) axons much higher in cod than eel. The results suggest a more coarse control of body curvature in cod but an increased importance of pro-prioreceptors in generating the locomotory wave.