Serial transmission electron microscopy and 3D reconstruction were used to document cell morphology and position of the motoneurones innervating somites 1 and 2 of a 12.5-day amphioxus larva, of Branchiostoma floridae, and also those innervating the dorsal compartment of somites 3 through 6 of an 8-day larva. Motoneurones supplying the veneral and dorsal compartments can be distinguished from one another on a number of morphological criteria. The ventral compartment motoneurones are neither symmetrical nor particularly ordered in arrangement. Their cilia are short and point forward or obliquely across the central canal; their axons run along the basal lamina adjacent to processes from muscle fibres, with which they make extended linear series of synapses containing 45-60 nm synaptic vesicles. The dorsal compartment motoneurones are paired and tend to be positioned at or near the junctions between somites. Their cilia are longer and project caudally; their axons are large, filled with mitochondria and 30-45 nm synaptic vesicles, and make synapses only at specific, segmentally repeated sites. An unusual feature of both cell types is that synaptic input occurs all along the axon, either by direct axe-axonal synapses or via slender dendritic processes. This allows for redundancy and multiple inputs, and is possible only because amphioxus somatic motor axons lie entirely within the nerve cord, which is itself an unusual feature among chordates. The possible significance of dual somatic innervation is discussed in relation to the dual innervation of the head in vertebrates, which has separate sets of somatic and visceral/branchiomotor nerves.
