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Landmarks in the Anterior Central Nervous System of Amphioxus Larvae (Goog)
Only the Abstract is available online. I got the PDF from the library.
"The larval nerve cord is divisible, at approximately the level of the first somite, into a short anterior region, the cerebral vesicle (c.v.), and an extended posterior region that is thought to include homologues of the vertebrate hindbrain and spinal cord.
rn: rostral nerves
p: pigment spot
c: anterior cerebral vesicle and central canal
lam: dorsal lamellar body - pineal
cm: commissure - part of the tegmental neuropile
*: cell bodies of the anterior-most motor neurons with the anterior extent of their synaptic fields indicated.
The small dorsal roots in this region are shown but not labeled.
The arrow on the left side pointing to the front part of the lamellar body indicates the approximate position of the infundibular body where the anterior cerebral vesicle narrows to become the central canal.
"Pre-infundibular" means forward of the infundibulum, in the cerebral vesicle. "Post-infundibular" means behind the infundibulum, in the central canal.
Where probable homologues of c.v. structures can be identified in vertebrate brain, they are found in the diencephalon, which suggests the c.v. and the vertebrate diencephalon are, to a degree, homologous.
The infundibular balance organ in amphioxus larvae (Goog)
Only abstract available online. I got the PDF through the library.
Also discussed in Amphioxus Balance Organ .
Cut-away side view of the anterior two-thirds of the cerebral vesicle.
Labels, from left to right:
R1-R4: frontal eye cells
p: pigment spot
PPN: preinfundiblar projection neurons
cb: ciliary bulb (balance organ) cell
in: infundiblar cell
lam: lamellar body (pineal)
tn: tegmental neuropile which includes a commissure
"... 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."
"The physical dimensions of the neuropile are such that any transmitter released in quantity could potentially affect targets anywhere within the neuropile. Flooding the neuropile and ventrolateral tracts with an appropriate mix of neuroactive substances may well be the way the response to more specific synaptic inputs from rostral sensory cells is modulated, and this could control which of several possible locomotory responses, if any, is evoked."
"It is certainly the case that many neurones in the larval cerebral vesicle in amphioxus are primitive in appearance and difficult to interpret in conventional, vertebrate terms. They typically lack clearly distinguishable axons and dendrites."
"This apparently very primitive type of neural organization does, however, have a counterpart in vertebrates, in the cells and fibre tracts of the core components of the limbic system. Nieuwenhuys has written extensively on the concept of a limbic core within the vertebrate brain, essentially an axial periventricular continuum of diffusely organized neural centres responsible for initiating the most basic motor activities. At the cellular level, the system is characterized by a predominance of thin, unmyelinated fibres, open fibre tracts, nonsynaptic (paracrine) transmission and a diversity of transmitter substances and modulators, especially neuropeptides. Anatomically, the system consists of a series of basal diencephalic centres, extending from the preoptic region through the hypothalamus and mesencephalic tegmentum to the reticulospinal tracts of the rhombencephalon."
Paracrine signalling (Wiki)
The Nervous System of the Amphioxus: structure, development and evolutionary significance (NRC):
Also considered in Amphioxus Sensory Nerves and Amphioxus Motor Nerves. A 29 page PDF with more than a dozen diagrams. Since the entire PDF is available online for free, I will use it as a major reference.
"... cells of essentially anterior character are found from the preinfundibular region to the beginning of the primary motor center (PMC). “Anterior” here refers to cells with irregular basal neurites that form repeated varicosities containing mixed vesicle types and few, if any, synapses. These are features that are generally associated with slow transmission, often involving neuropeptides."
"... three classes of preinfundibular projection neurons (PPN1–3) with mixed clear and dense-core vesicles and comparatively short axons four cells, the PPN2s, have clear vesicles and axons that travel at least to somite 7 and possibly farther. In general, from the paucity of synaptic specializations within the postinfundibular neuropile, it appears that paracrine release is the predominant mode of transmission, suggesting that this region is mainly a modulatory center."
"Beginning in the PMC, most of the neurons have well-defined axons and separate dendritic structures, either arbors or spines (both occur), and synaptic junctions, often with clear vesicles, predominate. This implies fast transmission and aminergic or amino acid transmitters, which is perhaps logical for neurons directly involved in the locomotory control circuits."