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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! REFERENCES Agduhr, E. 1922. 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