A peculiar mode of muscular innervation in amphioxus. Light and electron microscopic studies of the so‐called ventral roots

Abstract

Abstract The present study demonstrates a neuromuscular junction between the so‐called ventral root fibers and the surface of the spinal cord of amphioxus. The ventral root fibers, interconnected by numerous zonulae occludentes , appear to be projections from muscle fibers of the myotome and end in conical expansions at the surface of the spinal cord, separated from an extensive layer of axon endings or boutons by an extracellular cleft containing a basement membrane. Within each ventral root two kinds of ventral root fibers can be identified. Each corresponds to a compartment of the bouton layer. The so‐called thick ventral root fibers are opposed to a long ventral compartment of small boutons tightly packed with vesicles about 1,000 Ä in diameter, but without a dense core. The so‐called thin ventral root fibers are opposed to a short dorsal compartment of large boutons filled with synaptic vesicles about 600 Ä in diameter.