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 http://pubs.nrc-cnrc.gc.ca/rp/rppdf/z04-163.pdf Link to full length PDF, which I have. "Primary motor center The
PMC
contains
the
anteriormost
motoneurons
in the
cord and
a number
of large
premotor
interneurons.
These cell
types occur
elsewhere
in the
cord, but
not in
such a large
cluster.
The
important
cells, from
an
organizational
standpoint,
are three
pairs of
large
paired
neurons
(LPNs).
These are
extensively
innervated
by sensory
inputs,
both directly by
primary
sensory
cells in
the
periphery
and by
synapses
from the
anteriormost
RB cells
(= B
cells of
Bone 1959,
aRB cells
in Fig.
8). The
third pair,
the LPN3s,
are the
most
important
and are
cross-innervated
in a
bilaterally
symmetrical
fashion, an
indication
that they
may be
mutually
inhibitory
and hence
capable of
pacemaker
function
(Lacalli
1996;
Lacalli and
Kelly
2003b).
Their
output is
to ventral
compartment
(VC, or
fast)
motoneurons
via
synapses
and to
DC (slow)
motoneurons
via an
unusual
class of
intercellular
junctions
(juxtareticular
junctions;
see Lacalli
2002a).
Fig.
8.
(A)
Side
view
of
the
head
of
an
amphioxus
larva
showing
the
position
of
the
nerve
cord
(nc)
in
relation
to
the
notochord
(shaded).
The
head
is
highly
asymmetric,
with
the
mouth
(heavy
outline)
on
the
left
side
and
the
pharyngeal
(gill)
slits
(lighter
out-
lines)
on
the
right.
(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). The
LPN3s are
thus the
best
candidates
for neurons
exert- ing
a direct
controlling
influence
over both
fast and
slow
swimming,
which
appear to
have a
similar
neuromuscular
basis in
amphioxus
and
vertebrates
(Bone
1989). Fast
or es-
cape
swimming
occurs in
response to
sensory
inputs,
which are
a massive
and
redundant
input to
the VC
system. The
VC system
also
receives
synaptic
input from
fibers in
the
postinfundibular
neuropile
and may
be subject
to
additional
paracrine
input as
well, via
fibers
passing
through the
neuropile,
all of
which
provides an
opportunity
to modulate
the
response to
sensory
stimuli. In
contrast,
the slow
system,
which
drives
vertical
migration,
is almost
devoid of
synaptic
input.
Besides its
link via
junctions
to the
LPN3s, this
path- way
seems to be mainly under the control of the PPN2s
mentioned
above, a
class of
preinfundibular
projection
neu- rons
that make
repeated
junctional
contacts
with the
axons of
the DC
motoneurons.
What this
means in
functional
terms is
not clear,
but the
circuitry
(Fig. 9)
suggests a
switching
device of
some kind.
Perhaps the
escape
response is
sup-
pressed
during
migration,
which might
itself be
under
circa- dian
control. To
assess such
proposals,
however,
much more
information
is needed
on the
nature of
the various
types of
preinfundibular
neurons
than is
currently
available.
Fig.
9.
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. It
is not
known
whether the
larval
motoneurons
persist
through to
the adult
stage or
whether the
larval
cells are
replaced
at some
point in
development.
Lacalli
(2000) has
ar- gued
for the
former,
based on
the
measured
lengths of
the
motoneuron
apices.
These are
axially
elongated
by an
amount that
roughly
matches the
axial
expansion
of the
somites
during
development. 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. In
contrast to
the
detailed
information
now
published
on the
microanatomy
of the
anterior
cord,
nothing
comparable
is yet
available
for more
caudal
regions.
Swimming
behavior
changes as
the larva
grows, from
a phased
side-to-side
bend- ing
of the
whole body
in very
young
stages to
what looks
like a
propagated
wave of
contractions
(Stokes
1997). The
latter
implies a
locomotory
signal
propagated
from
segment to
segment,
more like
the
situation
in
vertebrates.
One inter-
pretation
is that
the
pacemaker
circuits
identified
in the
ante- rior
cord of
young
larvae are
involved in
initiating
locomotory
contractions,
but these
are
probably
propagated
through the
more caudal
segments by
a series
of local
pace-
makers.
Regardless
of details,
it seems
clear that
the control
circuits
described
from the
anterior
cord of
young
larvae
cannot
account
fully for
the
complexity
and
dynamics of
be- havior
in older
larvae." Frontal Eye Circuitry, Rostral Sensory Pathways and Brain Organization (Goog) http://rstb.royalsocietypublishing.org/content/351/1337/243.abstract 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 |
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