Cross references: Amphioxus Nervous System Amphioxus Motor Nerves
Dorsal Motor Neurons Ventral Motor Neurons Amphioxus Locomotion
Amphioxus Muscles Motor Neuron Evolution
The Nervous System of Amphioxus: structure, development, and evolutionary significance
Link to full length PDF, which I have.
"Primary motor center
such a large
both directly by
(B) Oblique dorsal view of the anterior nerve cord showing its main landmarks and selected cells. Asterisks indi- cate the third pair of large paired neurons (LPN3s), which are putative locomotory pacemaker neurons. Landmarks include the frontal eye (fe), infundibular organ (io), lamellar body (lb), and primary motor center (PMC); the zone of neuromuscular junctions is shaded. This view extends to just beyond the boundary between somites 1 and 2, roughly the extent of the cerebral vesicle as anatomically de- fined and coextensive with the zone of Otx expression. The io marks the junction between the anterior and posterior parts of the cere- bral vesicle. The two regions differ in the shape of the central canal and the direction in which most cilia project into it. Other abbreviations and their corresponding terms are listed in Appendix A. See text for further details. Modified from Lacalli (1996).
seems to be mainly under the control of the PPN2s
Schematic diagram of the main locomotory control cir-cuits in the anterior cord in young amphioxus larvae. The LPN3s (third pair of large paired neurons) are central control neurons that probably act as pacemakers. They receive external sensory inputs via several pathways and communicate with the two classes of motoneurons (DCm and VCm) by synapses or junctions as shown. There is extensive additional synaptic input (not shown) to the VC (fast) system, but almost none to the DC (slow) sys- tem except for junctions with a single class of preinfundibular neurons (type 2 preinfundibular projection neurons, PPN2s) and, more caudally (not shown), input from the dorsal ocelli. The function of the ipsilateral projection neurons (IPNs) is not clear, but they appear to provide some kind of link between the two systems. The postinfundibular (= tegmental) neuropile is a para- crine center, and the specific interactions among its components are not clear from the morphological data. Modified from Lacalli (2002a)
VC motoneurons in larvae resemble the somatic moto- neurons (SM cells) reported from the adult in overall mor- phology (cf. Lacalli and Kelly 1999; Bone 1959, 1960a). They are distributed rather irregularly in the anterior cord, with roughly equal numbers on each of the two sides, but there is no sign of bilateral pairing. They receive synapses on dendritic spines of varying length, located all along the axon, which confirms the supposition that the thin collaterals reported from adult motoneurons (Bone 1960a; Castro et al. 2004) are dendrites. It is useful to note that as the cord grows, and its neuropile expands, early dendrites would have to lengthen to maintain their original connections. Spines in the adult cord will thus be longer than those in the larval cord, and the longest spines are the earliest, and presumably most important, functional connections. Since the longest spines in the larva are postsynaptic to LPNs, this interpreta- tion supports the central role proposed for these cells in ini- tiating swimming.
DC motoneurons differ from VC motoneurons in being restricted to the anterior part of the cord, specifically somites 2–6. This restriction was first inferred from EM data, which showed that while axons project both rostrally and caudally from the last two members of the series, located in somites 4 and 5, none travel forward from more caudal segments (Lacalli and Kelly 1999). Confirming this, the amphioxus homolog of the estrogen-related receptor gene (ERR) selectively marks the same cells, revealing six pairs in the anterior somites and none more caudally (Bardet et al. 2005). Various molecular data support the idea of a segmen- tal or otherwise periodic repeat in the arrangement of cell types in the cord at the level of somites 2–7 (Jackman and Kimmel 2002; Mazet and Shimeld 2002), which is essen- tially the amphioxus homolog of the hindbrain. The DC motoneurons evidently form a compressed series, with more than one pair per segment. The true nature of patterning in this part of the anterior cord is still, therefore, not clear. It may be that some cell types show a strictly repeating seg- mental pattern, while others are more loosely controlled, or there may be several quasi-segmental patterns superimposed over one another. See Shimeld and Holland (2005) for further discussion.
Frontal Eye Circuitry, Rostral Sensory Pathways and Brain Organization (Goog)
32 page PDF. I was able to 'screen capture' the pictures, but I was unable to copy the text.
Enlarged View of the Primary Motor Center (PMC) Seen from the Side