Amphioxus Nervous System

Note:  The purple link is to the original article which is no longer available to the general public for free.  The green link is to the PDF which I was able to download at the local university library. 

    This is a
29 page PDF with more than a dozen diagrams.  If you have a general idea of what you're looking for, you may be able to find it more quickly and in more detail if you look at the relevant section. 

Anterior Cerebral Vesicle

Amphioxus Sensory Nerves
    Amphioxus Touch
    Amphioxus Smell
    Amphioxus Taste
    Amphioxus Gut Nerves
    Frontal Eye
    Lamellar Body = Pineal Eye
    Amphioxus Hearing
    Amphioxus Balance Organ
    Amphioxus Tectum

Amphioxus Motor Nerves  

    Ventral Motor Neurons
    Dorsal Motor Neurons
    Primary Motor Center
What follows is an abbreviated version of the original which I abstracted in 2011 in order to be able to move through the very long paper more quickly.  For the full paper, click on:   

The nervous system of amphioxus: structure, development, and evolutionary significance     


Side View of a Fully Mature Amphioxus 

I have altered the picture in the PDF slightly by darkening the nerve cord so that it stands out more clearly. 

Abbreviations and their corresponding terms:   
atsm = atriopore sphincter muscle   
bc = buccal cavity   
    chd = notochord (not shown for most of the length) 
cJo = Joseph cells    
cRo = Rohde cell   
elm = external labial muscle   
gon = gonad   
    Hp = Hatschek's pit 
    ilm = inner labial muscle   
    io = infundibular organ 
Köp = Kölliker’s pit    
    n1, n2, ... = dorsal nerves 1, 2, etc. 
nRo = nucleus of Rohde   
oHe = organ of Hesse   
pm = pterygeal muscle   
tm = trapezius muscle of atriocoelomic funnel   
vsm = velar sphincter muscle   

Oblique View of the Larval Anterior Nervous System

Figure A shows the anterior portion of the entire larva and the position of the anterior portion of the nerve cord.  Figure B is a reconstruction of a series of electron microscope pictures of the anterior portion of the nerve cord of a 12.5 day larva.  

Abbreviations and their corresponding terms:   

    nc = nerve cord 
    n1 & n2 = dorsal nerves 1 & 2 
    fe = frontal eye 
    bo = balance organ 
    io = infundibular organ 
    lb = lamellar body (pineal) 
    pinfn = postinfindibular neuropile (tegmentum) 
    aRB = anterior Retzius bipolar cells (tectum) 
    PMC = primary motor center 
    m1 = first myotome

The three landmarks which are labeled in both the larva, Figure B, and in the side view of the adult are the infundibular organ (io) and the first two dorsal nerves (n1 & n2).  Using these three landmarks to compare the picture of the larva with the picture of the adult makes it clear that the picture of the larva represents only the very front tip of the picture of the adult.  The nervous system of the adult is mostly spinal cord with only a tiny brain at the very front end.  

Cross Section of a Fully Mature Amphioxus 

This is a cross section somewhere in the middle of the animal.  The black, unlabeled inverted heart-shaped object above the notochord is the nerve cord.  The unlabeled nerve which leaves from the upper portion of the nerve cord is the dorsal nerve which then divides into the dorsal and ventral rami. 

Abbreviations and their corresponding terms:   

ap = atrial papillae   
    chd = notochord 
enc = encapsulated nerve endings   
gd = gut diverticulum   
gon = gonad   
    m = muscle 
mpf = metapleural folds   
ph = pharynx   
pm = pterygeal muscle   
    rdors = dorsal ramus of the dorsal nerve 
    rvent = ventral ramus of the dorsal nerve 
rventc = ventral ramus, ventral cutaneous branch   
rvisc = visceral ramus   
rvisca = visceral ramus, ascending branch   
rviscd = visceral ramus, descending branch   

Cross Section through the Nerve Cord 

"A highly schematic transverse section of amphioxus through the central nervous system and the top portion of the notochord, showing various sensory and motor cell types and the composition of the dorsal nerves.

Since this diagram is quite complex, a simplified version showing only the sensory nerves is presented in Amphioxus Sensory Nerves and a version showing only the motor nerves is presented in Amphioxus Motor Nerves

Abbreviations and their corresponding terms:   

DC = dorsal compartment   
DR = dorsal root cell   
EC = Edinger cell   
enc = encapsulated nerve endings   
MC = mid-commissural cell   
Mg = Müller’s glia (Schwann cell analogues)   
mRoax = median descending Rohde axon   
ncmf = notochordal motor fibers/tract   
RB = Retzius bipolar cell   
rg = radial glia   
Sg = Schneider’s glia   
SM1 somatomotor cell type 1   
smf = somatomotor fibers/tract   
ssf = somatosensory fibers/tract   
VC = ventral compartment   
VM1, VM2 = visceromotor cell type 1, 2   
vsf = viscerosensory fibers/tract   

    I got PDF from the library. 
Also discussed in Amphioxus Motor Nerves .   
     "... the amphioxus brain consists of a much simplified version of the ventral brainstem, including a region probably homologous with the hypothalamus, and a locomotory control center roughly comparable to the vertebrate tegmentum and reticulospinal system."  

A neurochemical map of the developing amphioxus nervous system   

 Related articles     Full length HTML 



Amphioxus, representing the most basal group of living chordates, is the best available proxy for the last invertebrate ancestor of the chordates. Although the central nervous system (CNS) of amphioxus comprises only about 20,000 neurons (as compared to billions in vertebrates), the developmental genetics and neuroanatomy of amphioxus are strikingly vertebrate-like. In the present study, we mapped the distribution of amphioxus CNS cells producing distinctive neurochemicals. To this end, we cloned genes encoding biosynthetic enzymes and/or transporters of the most common neurotransmitters and assayed their developmental expression in the embryo and early larva.


By single and double in situ hybridization experiments, we identified glutamatergic, GABAergic/glycinergic, serotonergic and cholinergic neurons in developing amphioxus. In addition to characterizing the distribution of excitatory and inhibitory neurons in the developing amphioxus CNS, we observed that cholinergic and GABAergic/glycinergic neurons are segmentally arranged in the hindbrain, whereas serotonergic, glutamatergic and dopaminergic neurons are restricted to specific regions of the cerebral vesicle and the hindbrain. We were further able to identify discrete groups of GABAergic and glutamatergic interneurons and cholinergic motoneurons at the level of the primary motor center (PMC), the major integrative center of sensory and motor stimuli of the amphioxus nerve cord.


In this study, we assessed neuronal differentiation in the developing amphioxus nervous system and compiled the first neurochemical map of the amphioxus CNS. This map is a first step towards a full characterization of the neurotransmitter signature of previously described nerve cell types in the amphioxus CNS, such as motoneurons and interneurons.

Branchiostoma; Cephalochordate; Chordate evolution; Lancelet; Neural patterning; Neurotransmitter

CotA: Amph Nervous Sys 
140703 - 2338