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 ventral 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 axo‐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.

Serial sections were used to map the ventrally positioned neurons of the anterior nerve cord of a 12.5-day amphioxus larva from the infundibular region to the end of somite 2. Synaptic patterns reveal five categories of descending pathways, four of which are associated with the ventral compartment (VC) motoneurons responsible for escape swimming. 1) Pre-, para-, and postinfundibular (tegmental) neurons with large varicosities and mixed vesicle populations provide both synaptic and paracrine input to various components of the tegmental neuropile and primary motor center. Four categories of these neurons are distinguished on the basis of their vesicles. 2) Multiple anterior sensory pathways converge on the large paired neurons (LPNs) located near the junction of somites 1 and 2. LPN synaptic output is almost exclusively contralateral. This, together with the evidence for cross-innervation between the third pair of LPNs, is consistent with the latter acting as locomotory pacemakers. 3) Axons from several classes of tegmental neurons converge in the paraxial region on each side of the cord where they form distinct tracts, the upper paraxial bundles. The right bundle is larger than the left, which suggests a role during early development when myotome contractions are biased to one side. 4) Fibers in the ventral tracts from ipsilateral projection neurons, sensory neurons, and additional ascending fibers synapse repeatedly with VC motoneurons. This may be how the overall level of excitation of the latter is controlled so as to modulate their response to pacemaker input. The fifth pathway consists of fibers involved in controlling the dorsal compartment (DC) motoneurons responsible for slow swimming, which are largely isolated from inputs to the VC locomotory system. The ventral neurons of the primary motor center form a more or less continuous file on either side of the floor plate, with certain cell types showing a tendency to cluster. There are, however, few obvious patterns of the kind expected if development were controlled by a rigid, lineage-based mechanism. The evolutionary implications of the involvement of a midbrain-level pacemaker in controlling larval swimming in amphioxus is discussed.

Serial electron microscope reconstructions were used to examine the organization and cell types of the nerve plexus that surrounds the mouth in amphioxus larvae. The plexus is involved in a rejection response that occurs during feeding: a number of oral spines project across the mouth, and debris impinging on them triggers a contraction of the gill slit and pharyngeal musculature that forces water through the mouth, dislodging the debris. The oral spine cells are secondary sense cells that synapse with neurites belonging to a class of peripheral interneurons intrinsic to the oral nerve plexus. These in turn synapse with a second class of peripheral neurons with large axons that we interpret as sensory cells and which probably transmit signals to the nerve cord. The intrinsic cells also appear to synapse with each other, implying that local integrative activities of some complexity occur in the oral plexus. In comparative terms, the intrinsic neurons most closely resemble the Merkel-like accessory cells of vertebrate taste buds, and we postulate a homology between oral spine cells and taste buds despite di¡erences in function. There are also similarities between the amphioxus oral plexus and adoral nerves and ganglia of echinoderm larvae, suggesting homology of both the oral nerve plexus and the mouth itself between lower deuterostome phyla and chordates.

Lacalli, T. C. and Kelly, S. J. 2000. The infundibular balance organ in amphioxus larvae and related aspects of cerebral vesicle organization. Ð Acta Zoologica (Stockholm) 81: 37±47Serial EM reconstructions were used to examine the organization and constituent cell types of the infundibular region of the cerebral vesicle (c.v.) in a 12.5-day larva of Branchiostoma floridae. The balance organ lies just in front of the infundibular cells and consists of 10 electron-dense cells with long, bulbous cilia, each surrounded by a ring of accessory cells. The ciliary bulb cells have axons that terminate in vesicle-filled swellings that lack identifiable synapses. The accessory cells have short basal processes that are minor contributors to the adjacent neuropile. Based on morphology, we suggest a mechanosensory function for the ciliary bulb cells, possibly related to balance or motion detection. Scattered cells of similar type are found elsewhere in the cerebral vesicle, along with a variety of other neurones with caudally projecting axons and varicosities, but few synapses. Instead, nonsynaptic, paracrine secretion appears to be the predominant mode of transmitter release in the neuropile and ventral tracts of the cerebral vesicle. The closest vertebrate homologue of this part of the amphioxus brain is arguably the limbic core of the caudal diencephalon and mesencephalon, including the homeostatic control centres of the hypothalamus. We postulate that this limbic core is an ancient structure traceable at least as far back in evolution as the common ancestor of amphioxus and vertebrates.

 AbstractLacalli, T.C. and Kelly, S.J. 2002. Floor plate, glia and other support cells in the anterior nerve cord of amphioxus larvae. -Acta Zoologica (Stockholm) 83 : 87-98 Serial electron micrograph reconstructions and interval series were used to examine the support cells, including glia, of the anterior nerve cord in 6-, 8and 12.5-day larvae of Branchiostoma floridae and one newly metamorphosed juvenile. The floor plate begins immediately behind the infundibular cells. It consists for the most part of a single file of midline cells, but adjacent lateral floor plate cells occur in some places. The floor plate is interrupted at one point, in the posterior part of the cerebral vesicle above the tegmental neuropile. A class of early developing axons crosses the midline at this point, which suggests that the floor plate may have a developmental role in axon guidance. The structural integrity of the cord is maintained by ependymal and ependymoglial cells that attach to its sides. Two other glial cell types were found in larvae. Both appear to originate adjacent to the floor plate and hence are referred to here as midline glia. Those in somites 1 and 2 remain connected to the central canal; they appear to be a mixed population that may include precursors of midline support cells which are present later in the juvenile. Those caudal to somite 3 detach early from the central canal and develop an extensive network of axial processes; they are referred to here as axial glia and treated as a subcategory of midline glia. Based on their site of origin and the absence of glial filaments, their closest counterpart among vertebrate glia may well be the oligodendrocyte. To our knowledge, this is the first report of a possible amphioxus homologue of this important vertebrate cell type.