Ventral Motor Neurons

Cross references:   Amphioxus Nervous System     Amphioxus Motor Nerves    
Dorsal Motor Neurons    Primary Motor Center        Amphioxus Locomotion  
Amphioxus Muscles   Motor Neuron Evolution   

Ventral neurons in the anterior nerve cord of amphioxus larvae. I An inventory of cell types and synaptic patterns. -   

    "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.

Ventral neurons in the anterior nerve cord of amphioxus larvae. II. Further data on the pacemaker circuit. (Goog)   
    "Previous serial EM studies of the anterior nerve cord of amphioxus larvae implicate the third pair of large paired neurons (LPN3s) as key components of the pacemaker responsible for oscillatory premotor output in somites 1 and 2. Here the synaptic relationship between the LPN3s and a fourth such pair (LPN4s), located in somite 3, is examined from a second series of sections. Because of limited overlap between the two series, fiber identity can only be inferred in most instances. To act as pacemakers, the LPN3s must inhibit each other; the current data show similar patterns of synaptic contact, presumably also inhibitory, with the LPN4s. The oscillatory signal appears therefore to be relayed from one LPN pair to the next.

Acetylcholine - Wiki 
Note:  Nerves that use acetylcholine as a neurotransmitter are referred to as 'cholinergic'. 

The cholinergic gene locus in amphioxus: Molecular characterization and developmental expression patterns
The cholinergic gene locus in amphioxus: molecular characterization and developmental expression patterns.
 Related articles Free article
Free PDF on line.  Also discussed in Amphioxus NeurotransmittersAmphioxus Genomics and Dorsal Motor Neurons.   
"In chordates, cholinergic transmission is typically a characteristic
of motoneurons, although in vertebrates, in particular, the presence
of cholinergic neurons in the brain means that the VAChT/ChAT gene is also expressed in some classes of interneurons" 

"ChAT and VAChT mRNAs are present in rows of cells in the anterior nerve cord adjacent to the floor plate in amphioxus, and while we cannot link this expression with specific cell types, the expression
zones coincide with regions where larval motoneurons are known to be generated" 

" ... CGL-expressing cells contain a mixture of motoneurons and interneurons... the implication is that a substantial proportion of the cells of the locomotory control circuit, both interneurons and motoneurons, may all be using the same transmitter."  "In fact, it would be very surprising if the motoneurons were not cholinergic based on what we know of other chordates ...  but it is also clear that there could be other classes of cholinergic neurons in the nerve cord."