Download The Neurons that Control Axial Movements in a Frog Embryo1

AMER. ZOOL., 29:53-63 (1989)
The Neurons that Control Axial Movements in a Frog Embryo1
ALAN ROBERTS
Department of Zoology, University of Bristol, Bristol BS8 1UG, England
SYNOPSIS. This paper reviews nineteen different classes of neuron present in the nervous
system of late embryos of the amphibian Xenopus laevis to see how far the behaviour of
these animals can be explained in terms of the properties of these neurons. Movements
can be initiated by light sensitive neurons in the pineal vesicle and touch sensitive neurons
innervating head and trunk skin. Swimming can be stopped by activity in neurons innervating head skin and the cement gland. A trigeminal pathway allows the skin impulse
access to the nervous system to initiate movement. Central pathways exist in the hindbrain
and spinal cord to carry excitation and inhibition to the opposite side following sensory
stimulation. Two classes of spinal neuron appear sufficient to coordinate motor neuron
activity in simple reflexes and the basic alternation in swimming. However, the longitudinal
coordination in swimming and struggling movements is not understood. For some of
the cell classes described there is no evidence on function. I conclude that the Xenopus
embryo nervous system and its relation to behaviour is better understood than any other
but still leaves us with many questions to answer!
cipal neuron types discussed here do not
My aim here, inspired by the early stud- change much from stage 33/34 to 37/38.
ies of Coghill (1929), is to take a broad look Throughout this period it seems that latat a very simple vertebrate nervous system eral eyes, the olfactory systems and the vesand see how far we can go in relating its tibulo-lateralis system are not yet funcstructure to the way in which it controls tional. The simplicity of the Xenopus
behaviour. The animal is the Xenopus laevis embryo nervous system makes it ideal for
embryo, where Hughes (1957) was the first this type of enquiry and already the structo study nervous organization. Its behav- ture and functioning of its nervous system
iour is entirely produced by axial trunk is probably more fully understood than that
movements, and near the time of hatching of any other animal. Despite this, it will
(stage 37/38 of Nieuwkoop and Faber, become clear that this review raises as many
1956) this limited behaviour must help it questions as it answers.
When released from their egg memsurvive. We can therefore look to see what
nervous machinery this animal has avail- branes into a dish Xenopus laevis embryos
able to control its longitudinal trunk mus- at stage 37/38 (Nieuwkoop and Faber,
cles. This paper will review current knowl- 1956) spend most of their time lying on
edge of Xenopus embryo neurons asking: the bottom or suspended from the side of
can we define different categories of neu- the dish or surface film by mucus secreted
ron, do we know what they do, and can we by their cement gland. They can make
explain their role in behaviour? The dis- occasional spontaneous movements. If
cussion will be limited to Xenopus, and to touched anywhere on the body they usually
one stage of development and, with a few swim away and continue to swim until they
exceptions, to cells in the hindbrain and bump into the side of the dish when they
spinal cord (all shown in Fig. 1). Working attach with cement gland mucus. Dimming
on a developing animal raises difficulties in the illumination can also evoke swimming.
freezing a picture that is changing hour by During swimming they can be stopped by
hour. However, the behaviour and prin- pressing on the head skin or cement gland.
If touched very gently on one side, the
embryos flex weakly on the opposite side.
If grasped, they make very vigorous strug1
From the Symposium on Axial Movement Systems: Bio- gling movements until they escape. This,
mechanics and Neural Control presented at the Annual in barest outline, is the behaviour. We can
Meeting of the American Society of Zoologists, 27- now look at the neurons responsible for it.
30 December 1986, at Nashville, Tennessee.
INTRODUCTION
53
54
ALAN ROBERTS
PP
KA
FIG. 1. Xenopus embryo neuron types at stage 37/38 shown diagrammatically in lateral and dorsal views of
the brain and rostral spinal cord. All nineteen neuron classes described in the text are shown with their
abbreviated names. A. Excitatory sensory pathways. Upper, pineal photoreceptor in pineal vesicle (pp), trigeminal skin touch receptor (Vt), "Rohon-Beard" skin receptor (RB). Lower, pineal ganglion cell (pg), hindbrain dorsolateral commissural interneuron (hdlc), "dorsolateral commissural" interneuron (die). B. Inhibitory
sensory pathways. Upper, trigeminal skin pressure receptors (Vp), trigeminal cement gland pressure receptors
(Vcg). Lower, "vestibular complex commissural" interneurons (vc), "mid-hindbrain reticular" interneurons
(mhr), "commissural" interneurons (c). C. Doubtful pathways. Central trigeminal receptor cells (Vc), extramedullary cells (em). D. Upper, motor pathways. Raphe-spinal cells (R), "descending" interneurons (d),
motorneurons (mn). Lower, doubtful pathways. Medial longitudinal fasciculus cells (mlf), "ascending" interneurons (a), "Kolmer-Agduhr" cells (KA).
NEURONS IN A FROG EMBRYO
SENSORY RECEPTORS
Head excitatory pathways
External stimuli reaching the head may
evoke swimming by at least three pathways.
Pineal photoreceptor pathway. T h e pineal
vesicle contains receptor cells with a modified ciliary outer segment (Bagnara, 1965).
Dimming the illumination leads to an
increase in the resting discharge recorded
extracellularly from the pineal, so we
assume that these pineal photoreceptors are
functional (pp in Fig. 1A; Roberts, 1978;
Foster and Roberts, 1982). They are most
sensitive to light of a wavelength near 520
nm. Since removing the pineal prevents
the embryo's swimming response to dimming the light, we have concluded that the
pineal photoreceptors are responsible and
that the lateral eyes are not yet functional.
At present we have no direct evidence on
the pattern of activity of the photoreceptor
cells and assume that the recordings made
were from pineal ganglion cells (see later).
Trigeminal skin touch pathway. A subset of
55
quently, when skin anywhere on the body
is strongly distorted an impulse (action
potential) is initiated, which then propagates from the point of stimulation over
the whole body surface of the embryo and
reliably evokes swimming. The skin impulse
therefore serves as a mechano-sensory system responding to more noxious stimuli.
The pathway for excitation of the nervous
system has been unclear, since neither trigeminal ganglion cells (Roberts, 1975) nor
Rohon-Beard cells (Roberts and Hayes,
1977; Clarke et al., 1984) are excited by
skin impulses. Recent lesion studies (Roberts, unpublished) have shown that cutting
the trigeminal nerves blocks reliable access
of the skin impulse to the central nervous
system and that the skin impulse cannot
enter the CNS via any spinal sensory neurons or cranial nerves caudal to the trigeminal. If the skin impulse can enter the CNS
to evoke swimming via the trigeminal
nerves, which neurons are involved? At
present one can only guess that central trigeminal sensory neurons, revealed in Xenopus by horseradish peroxidase backfills of
the trigeminal nerves (cV in Fig. 1C), could
be responsible. However, Rovainen and
Yan (1985) have shown similar cells in lampreys to be conventional skin pressure
receptors.
trigeminal ganglion cells, in both the
ophthalmic and the mandibular-maxillary
divisions of the trigeminal ganglia, innervate the head skin with unmyelinated free
nerve endings which are sensitive to local
touch to the skin (Vt in Fig. 1A; Roberts,
1980; Hayes and Roberts, 1983; Kitson and
Roberts, 1983). Similar cells are present in
Triturus and Rana embryos (Roberts, 1980). Head inhibitory pathways
All these cells respond with a few impulses
External stimuli reaching the head may
to rapid local indentation of the skin. They terminate swimming by two related pathshow little response to repeated stimula- ways.
tion. Cells from each fifth ganglion innerTrigeminal skin pressure pathway. Broad
vate head skin as far back as the gills on pressure to the head skin excites a subset
the same side of the head and the neurites of trigeminal ganglion cells whose neurites
of some cells also stray across to the other innervate the skin with branching, unmyside of the head. Touch sensitive cells show elinated, free nerve-endings (Vp in Fig. IB;
no spontaneous impulse activity and are Roberts, 1980; Hayes and Roberts, 1983).
not excited by the skin impulse (see below). These cells fire many impulses when the
Their central axons descend in the dorsal skin is slowly distorted in their receptive
part of the marginal zone to the caudal fields. This type of stimulus is very inefhindbrain.
fective in evoking swimming but often stops
Trigeminal skin impulse pathway. T h e skin ongoing swimming. Such pressure sensiof a number of amphibian embryos is excit- tive cells innervate only the side of the head
able (Alytes: Wintrebert, 1904; Cynops: Shi- on which they originate, via the ophthalmic
fan and Rongxi, 1962; Sato et al, 1981; and maxillary-mandibulary nerves. Their
Xenopus: Roberts, 1969, 1971; Roberts and field of innervation extends caudally to the
Stirling, 1971; Roberts and Smyth, 1974; gills. They are not spontaneously active and
Rana and Bufo: Roberts, 1971). Conse- their central axons have a similar distri-
56
ALAN ROBERTS
bution to the trigeminal cells described
above.
Trigeminal cement gland pressure pathway.
A subset of trigeminal cells, in the maxillary-mandibulary division, innervate the
caudal part of the cement gland (Vcg in
Fig. IB; Roberts and Blight, 1975). They
respond with many impulses to pressure on
the gland or tension in the mucus secreted.
Both these stimuli are very effective in terminating swimming. The unmyelinated
free nerve-endings in the gland are simple,
bulbous, and generally unbranched. These
cells have spontaneous activity and their
central axons descend in the same tract as
other trigeminal cells.
Conclusion
The head has a simple photoreceptor in
the pineal vesicle which is excited by light
dimming. The remaining head sensory
pathways are trigeminal and include: touch
receptors, skin pressure receptors, and an
uncharacterized pathway for the skin
impulse evoked by noxious stimuli. The
discovery of this last pathway throws doubt
on the identity of the free nerve-endings
associated with head-skin pressure receptors (see Hayes and Roberts, 1983) since
now two functions instead of one may be
served by these neurites (pressure and skin
impulse access).
Trunk excitatory pathways
External stimuli to the trunk skin may
evoke swimming by two possible pathways.
(The skin impulse, which can be evoked by
stimulation anywhere, has access to the
CNS via the brain but not via the spinal
cord and has already been considered.)
neous activity. Some have substance-P like
immunoreactivity but pharmacological
evidence suggests that "Rohon-Beard" cells
release an excitatory amino acid at their
central synapses (Roberts and Sillar, 1987).
Extramedullary cell pathway. "Extramed-
ullary" cells lie outside the spinal cord, have
central axons like Rohon-Beard cells and
appear to innervate the skin (em in Fig.
1C; Hughes, 1957; Roberts and Clarke,
1982). Like Rohon-Beard cells they arise
during gastrulation (Lamborghini, 1980).
When observed as they develop peripheral
neurites, extramedullary cells appear like
Rohon-Beard cells whose somata have
grown along their own peripheral neurite
(Taylor and Roberts, 1983) suggesting that
they may form a subclass of Rohon-Beard
cells and have similar properties. Unfortunately no relevant physiological evidence
is available but I assume they are touch
receptors.
CENTRAL SENSORY PATHWAYS
(EXCITATION)
All the skin mechanoreceptors described
above have central axons which lie on the
same side as the cell soma. However, when
the skin is stimulated on one side the first
muscle contraction is usually on the opposite side. Pathways are therefore needed to
carry excitation across the midline and then
distribute it longitudinally. The pineal
photoreceptors have no axons, so for them
to initiate movements, pathways from the
pineal to hindbrain and spinal cord motor
cells are necessary.
Dorsolateral commissural pathway. "Dor-
solateral commissural" interneurons lie in
the dorsolateral part of the spinal cord
Rohon-Beard skin touch pathway. "Rohon- where their dendrites could be contacted
Beard" cells lie in the dorsal spinal cord, by "Rohon-Beard" cell axons. Their axons
have ascending and descending longitudi- cross the cord ventrally and branch to
nal central axons and a peripheral unmy- ascend and descend longitudinally on the
elinated neurite which innervates the skin opposite side (die in Figs. 1A and 3; Robwith free nerve-endings (RB in Figs. 1A erts and Clarke, 1982; Clarke and Roberts,
and 2; Hughes, 1957; Roberts and Hayes, 1984). These cells are excited by "Rohon1977; Roberts and Clarke, 1982; Clarke et Beard" cells (Sillar and Roberts, unpubal., 1984). These cells respond to touch like lished), and fire briefly following skin stimthe touch cells in the trigeminal ganglion. ulation. However, they show no repeated
They extend along the whole length of the firing, are silent at rest and are actively
spinal cord and innervate the body surface inhibited during swimming. Though the
caudal to the gills. They have no sponta- numbers of "dorsolateral commissural"
57
NEURONS IN A FROG EMBRYO
2O0jjm
FIG. 2. Neuron populations on one side of the nervous system. RB, "Rohon-Beard" cells (134, nuclear
features); d, "descending" interneurons (148, horseradish peroxidase staining, uncertain numbers caudal to
star); c, "commissural" interneurons (272, glycine immunocytochemistry); a, "ascending" interneurons (106,
GABA immunocytochemistry); KA, "Kolmer-Agduhr" cells (144, GABA immunocytochemistry); R, Raphespinal cells (30, serotonin immunocytochemistry); vc, "vestibular complex commissural" interneurons (68,
GABA immunocytochemistry); mhr, "mid-hindbrain reticular" interneurons (29, GABA immunocytochemistry). Brackets: the number in one typical case and the method used to reveal the cells.
interneurons are uncertain, we have concluded that a few "Rohon-Beard" impulses
travelling along one side of the spinal cord
can excite many of these interneurons
(Clarke and Roberts, 1984). This effectively amplifies the excitation before it is
transferred to the other side (see also Roberts et al, 1983).
Hindbrain dorsolateral commissural path-
way. In most parts of the hindbrain there
are interneurons with multipolar somata
and dendrites near the dorsal part of the
marginal zone, and axons which cross ventrally to descend longitudinally in the spinal
cord (hdlc in Fig. 1A; Roberts and Clarke,
1982; van Mier and ten Donkelaar, 1984;
Nordlander et al., 1985; Roberts, unpublished). All of these neurons have dendrites
sufficiently dorsal to be contacted by the
central axons of trigeminal mechanoreceptors or "Rohon-Beard" cells. The Mauthner neuron is one of this type of interneuron and in other species is known to be
excitatory (Faber and Korn, 1978). The
spinal "dorsolateral commissural" interneurons are also excitatory. In the absence
of any direct evidence it therefore seems
probable that some of these decussating
interneurons in the hindbrain are also excitatory and amplify excitation from trigeminal touch receptors before taking it
to the other side and down the spinal cord.
58
ALAN ROBERTS
Pineal commissural pathway. Multipolar
ganglion cells in the pineal vesicle send
axons ventrally to cross before ascending
into the forebrain along the optic tract (pg
in Fig. 1A; Foster and Roberts, 1983; Roberts, unpublished). Since dimming leads to
increased pineal ganglion cell discharge and
is followed by swimming, I assume these
ganglion cells are excitatory and could be
generating the recorded impulses. However, the ganglion cells have no descending
axons so they must excite more caudal
motor systems via further interneurons.
There are suitable mesencephalic neurons
with descending ipsilateral axons in the
medial longitudinal fasciculus (mlf in Fig.
ID; van Mier and ten Donkelaar, 1984;
Nordlander et al., 1985). Again, there is
no physiological evidence on these neurons.
Conclusion
For each excitatory sensory input there
are interneurons suitably placed to amplify
the signal and carry it to the opposite side
to initiate a motor response. In the
mechanosensory pathways this function is
served by "dorsolateral commissural"
interneurons in the spinal cord and Mauthner and similar reticulospinal interneurons
in the hindbrain. The parallels between
these two types of cells suggest: firstly, that
Mauthner cells are a specialized derivative
of the spinal "dorsolateral commissural"
cell and secondly, that all these cells which
have an initiation or trigger function would
be inhibited during swimming so that they
only fired impulses prior to swimming.
ways which then turn off swimming. The
immunocytochemical staining suggests two
possible pathways.
"Mid-hindbrain reticular pathway" (GABA).
Staining for GABA reveals a group of large
cells in the mid-hindbrain, in a mid-dorsoventral position and with fairly extensive
dendrites. The most distinguishing feature
of these "mid-hindbrain reticular" interneurons is that they each have descending
axons on both sides of the nervous system
(mhrinFigs. IB and 2; Roberts^ al, 1987).
There is no physiological evidence, but if
trigeminal pressure receptors excited these
cells, they could have general inhibitory
effects on swimming if their fairly ventral
axons contacted spinal neurons active in
swimming.
"Vestibular complex commissural pathway"
(GABA). Staining for GABA shows a large
group of dorsal neurons in the rostral hindbrain in the region of the otic vesicle. These
have axons which cross to the opposite side
and may then descend or ascend longitudinally, probably in a rather dorsal position
in the marginal zone (vc in Figs. IB and 2;
Roberts et al., 1987). The somata of these
interneurons are rather dorsal but could
possibly be contacted by trigeminal axons
and provide a crossed inhibitory pathway.
MOTOR SYSTEM NEURONS
At stage 37/38 there are three main
responses to stimulation: (1) a brief flexion
on the opposite side, (2) this flexion followed by swimming, and (3) slower,
stronger flexions alternating to produce
struggling. In general, these three
responses are evoked by excitatory stimuli
CENTRAL SENSORY PATHWAYS
of increasing intensity (Kahn et al., 1982;
(INHIBITION)
Kahn and Roberts, 19826). The struggling
Swimming can reliably be stopped by movements are typically evoked by any
pressure to the head skin or cement gland. attempt to grasp the embryo. Our present
The simplest hypothesis to explain this evidence suggests that three types of spinal
would be for the central synapses of the neuron control at least the flexure and
trigeminal sensory cells involved to release swimming responses (see also Roberts et al.,
an inhibitory transmitter (Roberts, 1980). 1983, 1986): "descending" interneurons,
However, immunocytochemical staining "commissural" interneurons and motorfor glycine and GABA has not stained any neurons (d, c and mn in Fig. 3). The Raphetrigeminal ganglion cells or their axons in spinal neurons in the hindbrain are also
the hindbrain (Dale et al., 1986; Roberts et likely to be motor in function so are conal., 1987). This suggests that the trigemi- sidered here. Unlike the neurons in the
nal sensory neurons excite inhibitory path- central sensory pathways, these neurons are
NEURONS IN A FROG EMBRYO
59
neurons excite neurons belonging to the
motor system on the same side of the spinal
cord by releasing an excitatory amino acid.
In the simple flexion reflex (Fig. 3A) the
motorneurons and premotor interneurons
("descending" and "commissural") on the
same side as the stimulus are weakly excited
by "Rohon-Beard" cell axons but whether
the pathway is direct or polysynaptic is not
at present clear (? in Fig. 3A). Excitation
in "Rohon-Beard" axons would meanwhile be amplified by "dorsolateral commissural" cells and taken to the opposite
side to fire the motor system as described
above. Impulses in "descending" interneurons here could further amplify the excitation, leading to motorneuron firing and
muscle contraction (Fig. 3A).
Swimming is the most frequent response
to sensory excitation and usually starts with
a contralateral flexion. The physiological
evidence has been reviewed (Roberts et ai,
FIG. 3. Spinal circuits where each circle represents
a population of cells, labelled as in Figure 1 with inhib- 1986) and in outline our conclusions are
itory cells shaded. Open triangles are excitatory syn- as follows. Subsequent firing of motorneuapses, closed circles are inhibitory synapses, arrows rons and rhythmic premotor interneurons
indicate impulse flow and the central dashed line is occurs on rebound from inhibition (see
the longitudinal midline. (A) Circuit for the flexion below). "Descending" interneurons fire
response when skin stimulation (at star) on the left
excites RB cells. These then excite die interneurons once per cycle providing a long (200 to 300
which excite c, d and mns on the right. This leads to msec) excitation of motorneurons, "comcontraction on the right and inhibition of left mn, d missural"
interneurons and other
and c cells by right c cells. A weak excitation of left "descending" interneurons so that during
mn and d cells occurs (? and dashed connections). (B) swimming the whole longitudinal column
Circuit for rhythmic activity during swimming where
mn, c and d cells on left and right discharge alter- of each cell type fires (Fig. 3B). This nornately. On each cycle: excitation within each side comes mally occurs in a rostral to caudal sequence
from d cells; c cells inhibit c, d and mn cells on the (Kahn and Roberts, 1982a) but the mechopposite side and die cells on the same side; mns excite anism for the sequencing is not underthe swimming muscles. RB cells are silent but not
stood. It could depend in part on the more
inhibited.
rostral concentration of "descending"
interneurons.
"Commissural" interneurons. These interall active and fire spikes during motor
neurons stain for glycine and form a lonresponses such as swimming.
"Descending" interneurons. The somata of gitudinal column from caudal hindbrain
these interneurons form a column from into the tail spinal cord. Their unipolar
the mid-hindbrain well down into the spinal somata give rise to a stout initial segment
cord. They have dendrites spanning the with lateral dendrites. Some have ipsilatmarginal zone dorsoventrally. Their main eral axons but all have ventral commissural
axon descends longitudinally but they can axons which ascend or T branch on the
also have a short ascending axon (d in Figs. opposite side. "Commissural" interneu1D and 2; Roberts and Clarke, 1982; Nord- rons are inhibitory, producing hyperpolarlander, 1984; Dale and Roberts, 1985; izing potentials (blocked by strychnine) in
Roberts and Alford, 1986). The physio- motorneurons and interneurons (c in Figs.
logical evidence, while still incomplete, IB and 2; Roberts and Clarke, 1982; Soffe
indicates that these "descending" inter- etal., 1984; Dale, 1985; Daleetal., 1986).
60
ALAN ROBERTS
The most important role of "commissural"
interneurons is to provide reciprocal inhibition between left and right sides of the
animal. In the simple flexion reflex they
produce inhibition on the stimulated side
(Fig. 3A; Roberts et al, 1985). In swimming (Fig. 3B) they have two very distinct
roles. The first is to produce strongly
hyperpolarizing inhibition of rhythmic
neurons on the opposite side during the
long lasting excitation from descending
interneurons. The most important effect
of this is to lead to delayed, rebound excitation and firing of the inhibited neurons
(cf. Perkel and Mulloney, 1974; Roberts et
al, 1986). The second role is that they turn
off the sensory "dorsolateral commissural"
interneurons so that they are silent during
swimming (Fig. 3B). This is probably
effected via a sub-group of "commissural"
interneurons which has ipsilateral as well
as contralateral axons (Dale, 1985).
Motorneurons. Despite considerable variation in size and form we have not subdivided motorneurons into primary and
secondary either anatomically or physiologically at the stage of development being
considered. Their ventral somata have
mainly dorsal dendrites and a descending
longitudinal central axon giving off one or
two peripheral branches to the myotomes,
which are innervated at their ends (mn in
Fig. ID). The distribution of motorneurons has not been described but it is clear
that they form a ventral longitudinal column (Hughes, 1957; Roberts and Clarke,
1982; Roberts and Kahn, 1982; Soffe and
Roberts, 1982a, b; van Mier et al, 1985).
There is at present no evidence for central
synaptic effects mediated by motorneurons. Their role is therefore to convey
impulses to the muscles primarily in
response to the excitatory and inhibitory
input which they receive from "descending" and "commissural" interneurons
respectively (Fig. 3). Like these rhythmic
interneurons, motorneurons fire one
impulse per cycle in swimming and groups
of impulses during struggling.
Raphe-spinal interneurons. These lie in the
ventral part of the rostral hindbrain, have
descending axons on the same side and
contain serotonin (R in Figs. ID and 2; van
Mier et al., 1986). We can only guess from
their ventral position that these are a part
of the motor system. Nothing is known yet
about their activity or role.
UNCERTAINTIES
Two classes of spinal neuron remain
enigmatic in the absence of physiological
information.
"Ascending" interneurons. These stain for
GABA and form a fairly dorsal column of
somata with dorsal dendrites extending well
into the spinal cord from the caudal hindbrain. Their axons are dorsal and ascend
longitudinally. "Dorsolateral ascending"
interneurons are now lumped in this class
(a in Figs. ID and 2; Roberts and Clarke,
1982; Roberts et al, 1987).
"Kolmer-Agduhr" cells. These also stain
for GABA and have somata forming a ventral column with one surface exposed in
the spinal canal. This apical end has microvilli and one or two cilia, while the basal
end has an axon which ascends ventrally
in the marginal zone. The name derives
from authors who described these cells in
all groups of vertebrates. In Xenopus we
had previously called them "ciliated ependymal" cells (KA in Figs. ID and 2; Kolmer, 1921; Agduhr, 1922; Roberts and
Clarke, 1982; Dale et al, 1987a, b). "Kolmer-Agduhr" cells are probably receptors
and look similar to vomeronasal receptors
in lower vertebrates. They could therefore
be chemoreceptors but, if so, seem in a
curious position. A mechanoreceptor
responding to tail flexion seems a more
probable role but at present there is no
evidence on function.
GENERAL CONCLUSIONS
Is it ridiculous to take a whole animal
and ask: how do the neurons in this animal's nervous system allow it to behave?
Despite some obvious shortcomings, I hope
that for Xenopus embryos this review shows
that it is not. The listing of neuron classes
attempts to evaluate our present level of
understanding. Many questions were
raised, but for the spinal cord and hindbrain it seems likely that many of these will
be resolved in a few years. By combining
anatomical, immunocytochemical, physio-
61
NEURONS IN A FROG EMBRYO
logical and behavioural information we
should be able to define more classes of
neurons with greater confidence. It is this
definition of neuron classes which is desperately needed before we can unravel the
organization of the vertebrate spinal cord
and hindbrain. If neuron classes are conserved during evolution, then classes
defined in a very simple nervous system like
that of the Xenopus embryo will also be
present in more developed and advanced
forms. We should then be able to use conclusions from the embryos to help explain
function in the adult. What is surprising is
how few classes of spinal neuron have been
defined anatomically and physiologically in
advanced animals such as mammals, despite
many years of effort. This suggests that
new approaches are needed, and perhaps
various lower vertebrates may provide
these (see this volume).
The main emphasis of this paper has been
the definition of classes of neurons but this
is always as a prelude to study of their physiology and behavioural role. However, I
have said little about the physiology of these
cells here because the Xenopus work has
been reviewed recently elsewhere (Roberts
et ai, 1983, 1986; Roberts, 1987). Evidence for the circuit diagrams in Figure 3
is presented in these reviews which carry
the discussion down to details of synapses
and cell membrane properties. For swimming we now have a hypothesis for how
the basic alternating pattern is generated
in the spinal cord, but the behaviour still
presents some major unsolved problems.
How is the caudal progression of waves of
bending coordinated during swimming?
What accounts for slowing-down, speeding-up, turning and other irregularities
seen as an embryo swims? What starts
"spontaneous" swimming which is only
seen when the mid- and fore-brain are
intact? How do struggling movements arise
where the pattern of motorneuron activity
is so different from that during the much
quicker swimming? Fortunately many of
these behaviours are present in "fictive"
form in paralysed embryos so we should
be able to study them.
For most of the neuron classes outlined
in this paper, members of a population act
in concert. The rhythmic spinal cord neurons controlling swimming all fire nearly
synchronously along the whole of one side
of the nervous system. The neurons in central sensory pathways relaying excitation
to the other side are all active together to
provide a strong excitation. However, it is
also clear that activity in some individual
neurons can change the behaviour of the
whole animal. Stimulating a very small area
of skin, or even exciting a single RohonBeard cell by injection of current, can initiate swimming because of the amplifiers
built into the sensory pathways. Therefore
it is not only in invertebrates or in exceptional cells like Mauthner neurons that
individual spikes in individual neurons can
determine what an animal will do!
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Clarke, J. D. W., B. P. Hayes, S. P. Hunt, and A.
Roberts. 1984. Sensory physiology anatomy and
immunohistochemistry of Rohon Beard neurones in embryos of Xenopus laevis. J. Physiol. 348:
511-525.
Clarke, J. D. W. and A. Roberts. 1984. Interneurones in the Xenopus embryo spinal cord: Sensory
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