Amphioxus Motor Nerves

The nervous system of amphioxus: structure, development, and evolutionary significance  
Also considered in Amphioxus Nervous System and Amphioxus Sensory Nerves

A 29 page PDF with more than a dozen diagrams.  I will use it as a major reference. 

This simplified diagram only shows the motor nerves.  The original diagram, which shows all the nerves can be found in
Amphioxus Nervous System.   

It's important to understand that there are two, radically different, kinds of motor output pictured here and that one of the two is again divided into two parts. 

The arrows on the left, labeled "to non-myomeric muscles" leave the spinal cord through the dorsal (top) roots and innervate the viscera (guts).  These are roughly equivalent to the vagus nerve in humans: 

They originate from VM1 and VM2, the viceromotor cells type one and two. 

The right side of the diagram shows the innervation of the muscles. 

"The innervation of the myomeric muscles in amphioxus is highly unusual. ... The muscle cells each send long processes (= muscle tails) towards the ventrolateral margins of the nerve cord. 

These processes were formerly interpreted, mistakenly, as ventral nerve roots. ... the muscle processes are the sites where the synapses from motoneurons are received; the axons and synaptic terminals remain entirely confined within the nerve cord; and transmitter release occurs across the basal lamina.

...  The synaptic zones in each segment consist of two distinct domains, the ventral (VC) and dorsal (DC) synaptic compartments.  Both utilize acetylcholine as a transmitter. The ventral synaptic compartments are where the deep, anaerobic, fast muscle cells receive their innervation.  The dorsal compartment is where the superficial, aerobic, slow muscle cells of the myomeres receive their innervation." 

Note that the motor neurons emanate from the ventral (lower) part of the spinal cord, whereas, as depicted in Amphioxus Sensory Nerves, the sensory nerves project to a more dorsal (top) of the cord.  This configuration remains consistent throughout the chordate lineage, up to and including we humans.  

Vagus nerve (Wiki) 

Head Organization ... in Protochordates (Goog) 

Fig. 3 Amphioxus visceral innervation.

The top image is a cross section through the nerve cord. Cell bodies of the visceral motoneurons (solid) are ventral, with axons that project out the dorsal nerves. The somatic motoneurons lie just above these and innervate the myotome by means of axons confined within the cord itself. These synapse across the basal lamina, the postsynaptic targets being extensions from the cells of the myotome that cluster at this point.

The lower image is a dorsal view of the cord showing how the visceral motoneurons are positioned near junctions between the somites (numbered), a pattern that continues caudally for tens of somites. 

    See also Amphioxus Nervous System  for related articles. 
Amphioxus has direct pathways for activating its locomotory circuits in response to mechanical stimuli via epithelial sensory cells, but this response is evidently modulated by inputs from diverse sensory-type cells located in the putative hypothalamic homolog, and from the lamellar body, the pineal homolog.

In we humans, slow muscle fibers contain myoglobin.  So I tried searching for "amphioxus myoglobin"

    Google claimed to find 14,500 hits, but in the first four pages all the articles were about genes.  Although researchers seem to have found genes that code for something like myoglobin, none claimed to have found myoglobin in the amphioxus itself

Narrowing the search to "amphioxus myoglobin slow twitch" reduced the number of hits to 1,020.     

A review of the organization and evolution of motoneurons
innervating the axial musculature of vertebrates (Goog) - 1986 
Full length PDF available online for free.     
See:  Motor Nerve Organization .        


Fig. 1. Organization of the musculature and its relationship to the spinalcord in Branchiostoma.  The superficial muscle fibers (S) send processes (t) to a dorsal compartment (Do) of the spinal cord while the deep (D) and intermediate fibers (I) send processes (T) to a ventral compartment (Ve). M, myoseptum; N, nuclei.  Reproduced (ref. 50) with permission of author and Springer Verlag.

   "The cephalochordates have some features of the motor organization found in most anamniotic vertebrates.  These include a myomeric organization of musculature, functionally different superficial and deep fiber types, and comparable populations of motoneurons."   See: 
Amphioxus Muscles .   

    "Prior to 1966 the pattern of innervation of the muscle fibers in Branchiostoma was unclear. ... The confusion was resolved by Flood using light and electron microscopy in a study of the apparent ventral roots. He observed that the muscle fibers do indeed send processes medially to contact the spinal cord ... The ‘ventral roots’ observed by other workers are muscle processes, not nerves. These processes contact a specialized region of the ventral portion of the cord which lacks a glial covering and contains a large number of boutons with vesicles. 
    This region is divided into a dorsal compartment containing large boutons with relatively small vesicles (50-60 nm diameter) and a ventral compartment having smaller boutons and relatively large vesicles (100 nm diameter). The processes of the two different types of muscle fibers in the myomere contact these two compartments in the cord (Fig. 1). The superficial fibers send long thin processes to contact the dorsal compartment, while the deep fibers send relatively large processes to contact the ventral compartment.  Thus, there is a spatial segregation of  the region of contact of the two different muscle fiber types with the cord."  

Lacalli: 2002 
Somatic motoneurones in amphioxus larvae: cell types,
cell position and innervation patterns 
Link to abstract only, but I have the PDF.