Ultrastructural Evidence for Neuromuscular Systems in Coelenterates

Abstract

Cellular interrelationships and synaptic connections in tentacles of several species of coelenterates were examined by means of electron microscopy to determine if neuromuscular pathways were present. The presence of sensory cells, ganglion cells, epitheliomuscular cells, interneuronal synapses, and neuromuscular junctions suggests that neuromuscular pathways are present in coelenterates. Naked axons without sheath cells form several synapses en passant with the same and with different epitheliomuscular cells as well as with nematocytes and other neurons. Interneuronal synapses and neuromuscular and neuronematocyte junctions have clear or dense-cored vesicles (700–1500 A in diameter) associated with a dense cytoplasmic coat on the presynaptic membrane, a cleft (100–300 A in width) with intracleft filaments, and a subsynaptic membrane with a dense cytoplasmic coat. At scyphozoan neuromuscular junctions there is a subsurface cisterna of endoplasmic reticulum, which is separated from the epitheliomuscular cell membrane by a narrow cytoplasmic gap (100–300 A in width) . Neuromuscular junctions in coelenterates resemble en passant axonal junctions with smooth muscle in higher animals. Morphological evidence is presented for a simple reflex involving a two-cell (sensory or ganglion-epitheliomuscular cell) or three-cell (sensory-ganglion-epitheliomuscular cell) pathway that may result in the coordinated contraction of the longitudinal muscle in tentacles of coelenterates.