The Development of Timing Control and Temporal Or anization in Coordinated Action J. Fagard anfP.H. Wolff (Editors) Q Elsevier Science Publishers B.V.. 1991

93

CENTRAL GENERATORS AND THE SPATIO-TEMPORAL PATIERN OF MOVEMENTS

Yu. I. Arshavsky*, S. Grillner**, G.N. Orlovsky***, Yu. V. Panchin*

*

* *

* * * Moscow State University, Moscow, USSR

Institute of Problems of Information Transmission, Acad. Sci., Moscow, USSR

Nobel Institute of Neurophysiology, Karolinska Institute, Stockholm, Sweden

Introduction

Animals and humans are capable of producing a great variety of movements with their body, limbs, eyes etc. These movements can be classified according to the role of corrections performed in the course of a movement. On one extreme, there are movements which need continuous corrections like in the case of tracing a moving target. For these movements both information concerning the external world and that concerning the current state of the own motor apparatus are necessary. On another extreme, there are movements which need no or minimal corrections during their execution either because they are very brief (like ballistic movements) or because they proceed under standard conditions (like swallowing and scratching movements). Some of these movements have rather complex spatial and temporal patterns of muscular activity.

Most movements, however, are of a mixed nature, i.e. they can be performed in a rough way without any (or with minimal) information about the external world and about the current state of the own motor apparatus, but they become perfect if such information is available. The locomotor movements belong to this "mixed class. The basic pattern of locomotion is generated within the nervous system (see below), but when executed, the movements are adapted to the environment, and various sensory inputs (visual, proprioceptive) are used for these adaptations.

Thus one can conclude that, for many movements, there is within the central nervous system (CNS) a "central representation" or a "program" of these movements. This program is realized as a result of the activity of a definite neuronal mechanism which is usually referred to as a "central pattern generator" or a "central generator" for a given movement. Deprived of sensory influences, the central pattern generator (CPG) produces the spatio-temporal pattern of activity of different muscles typical of a given movement (11,12,19,23,29,43,47,53). The neuronal structure of CPGs is genetically pre- determined. In vertebrates, the CPGs for various movements are located at rather low levels of the CNS, in the spinal cord (CPGs for locomotion and scratching) and in the brainstem (CPGs for respiration, chewing, swallowing, eye movements, etc). The activation of CPGs and the control of various aspects of their activity is usually

94 YaI. Arshuvsky et aL

A B YESENC

_ccM

S P I N A L PATTERN PENERATORS

S P I N A L PATTERN GENERATOR

YOTONEURONS

SENSORY

C

F I C T I V E LOCOYOTION IN THE I S O L A T E 0 RAT SPINAL CORO

R-G

. 1 so0

Fig. 1. Functional organization of the locomotor control system in mammals. A. Longitudinal section of the brainstem of the cat with the midbrain (mesenc) and the lower brainstem (pons and med. obl.). The midbrain locomotor region (MLR) is indicated and its connection to the lower brainstem locomotor region in Nuclei reticularis magnus and gigantocellularis (NRM & G). This area projects through the ventral quadrant of the spinal cord to the spinal pattern generators for locomotion. B. Schematic representation of the spinal network organizaiton. The spinal pattern generator circuitry is activated by the locomotor drive signal from the brainstem. This central pattern generator activates the different motoneurones and thereby muscles in sequence. During the ongoing movement, sensory signals arising from the movement can act at several different levels: the motoneurone, the spinal pattern generator, and various brain centres. Signals about activity of the spinal generator (efference copy) are also sent to the brain. Correcting signals from a variety of descending pathways can modify the activity of spinal mechanisms. c. The isolated spinal cord is capable of generating the basic locomotor pattern in all classes of vertebrates which proves the existence of locomotor CPG in the spinal cord. In this experiment, the spinal cord of a newborn rat was isolated together with two pairs of hindlimb muscles (left and right m. tibialis anterior, TA, and gastrocnemius, G). Bath application of the excitatory amino- acid (NMDA) evoked normal alternating flexor-extensor activity (recorded as muscle activity) typical of stepping, with a 180" phase shift between the two legs typical for walking (adapted from 34).

Central Generators and the Spatio-Tempoml Pattern of Movements 95

performed by brain centres at a higher level. A typical example of such a motor organization is the system controlling locomotion in mammals illustrated in Fig. 1. This system has been extensively investigated during the last two decades (5,17,20,24,25,27,38,49,52). In mammals (and in other classes of vertebrates) locomotion can be initiated by stimulation in the upper part of the brainstem, in an area referred to as the midbrain (mesencephalic) locomotor region (MLR) (Fig. 1A). A local electric or pharmacological stimulation will give rise to locomotion (16,17,30,38,49). A weak stimulation elicits slow walking whereas stronger stimulation can elicit trot and gallop. Thus, the basic locomotor synergy with hundreds of muscles coordinated in a specific way comes into operation under the effect of a very simple command. This effect is mediated by special neuronal circuits located in the spinal cord which control the stepping movement of each limb, i.e. by the spinal CPGs (Fig. 1B). The brainstem command signals that activate the spinal CPGs can be substituted by certain drugs; Fig. 1C shows the rhythmic activity generated by the isolated rat spinal cord under the effect of the excitatory amino acid NMDA (34). The isolated spinal cord can not only generate the basic locomotor pattern underlying stepping movements in a single limb (alternating flexor and extensor activity) but also establish the reciprocal relation between left and right sides necessary for walking (Fig. 1C). This interlimb co-ordination is determined by a specific interaction between the CPGs controlling stepping in individual limbs.

The spinal CPGs generate the basic locomotor pattern even in the absence of any sensory feedback, but during normal locomotion numerous sensory signals affect both motoneurones and CPG interneurones (Fig. 1B) to modify the motor pattern and adapt it to the current environmental conditions (1,20,35,40,46). The high level brain centres also participate in correcting the basic locomotor pattern. Their actions are based both on information coming from the spinal cord (sensory and efference copy signals) and on information coming from the visual, auditory and vestibular systems (Fig. 1B).

The problem of neuronal organization and operation of CPGs in mammals is far from being solved, in spite of extensive studies in this field (8,14,17,20,23,41,51,52). In most cases we know neither the main neuronal groups constituting the CPG, nor the system of their interconnections, nor the properties of CPG neurones. This is why a question such as "How is the CPG organized?" is usually substituted by a question "How should the CPG be organized to fit some observations concerning the motor pattern that it generates?" However, if one turns to animals with a "simple" nervous system (lower vertebrates and invertebrates), the situation is much better, and a number of CPGs have been investigated at the level of "neuron-to-neuron interaction". We shall consider some of these CPGs and discuss the possible role of this knowledge for understanding the organization and operation of the CPGs in mammals. We shall start with rather simple neuronal mechanisms generating the basic locomotor pattern in the mollusc Clione and in the frog embryo. These two mechanisms have a lot features in common. Then we shall consider the locomotor generator of the lamprey. This generator, like the two mentioned above, belongs to the class of "symmetrical" networks, but it is capable of generating more complex efferent patterns. After this we shall consider the feeding rhythm generator of the snail Planorbis based on an asymmetrical neuronal network and producing an efferent pattern resembling those of respiration, visual nystagmus and scratching in mammals. We shall finish with the CPG for a defensive reaction in Planorbis which does not generate a rhythmical pattern but a single one. This pattern resembles that of rather complex arm movements like reaching.

96 Yu.1 Arshavsky et al.

Locomotor CPG of Clione

The pteropod mollusc Clione limacina is widely distributed over the northern seas. It is a plankton animal which maintains itself at a particular depth by continuous rhythmic (1-2 Hz) movements of two wing-like appendages. These "wings" move synchronously, similar to wings of a bird (Fig. 24B). The wing movements are controlled by the pedal

A

I B63 C

Fig. 2. A. Schematic drawing of Clione limacina (a ventral view). B. Successive wing positions during a locomotor cycle (a frontal view): (1) the maximal ventral flexion, (2) the movement in dorsal direction, (3) the maximal dorsal flexion, (4) the movement in ventral direction. The interval between frames is about 100 ms. C. The CNS of Clione consisting of the buccal (BUC), cerebral (CER), pedal (PED), pleural (PL) and abdominal (ABD) ganglia. NW, the wing nerves.

ganglia, each of which gives rise to the corresponding wing nerve (Fig. 2C). The basic efferent pattern of swimming can be recorded from the wing nerve. It consists of two bursts per cycle (2). One burst corresponds to the activity of motoneurones responsible for the dorsal flexion of the wing (D-phase of a swim cycle), the other reflects the activity of motoneurones responsible for the ventral flexion (V-phase). The same efferent pattern, with alternating activity of the D and V-phase motoneurones, is generated by completely isolated pedal ganglia. This rhythmical process, named "fictive swimming", is the manifestation of activity of the Clione locomotor CPG. The pedal ganglia contain about 1000 neurones, approximately 100 of which exhibit periodical activity during fictive swimming. About 50 of these "rhythmic" neurones are motoneurones sending axons into the wing nerve (Fig. 3A), others are interneurones with axons projecting to the contralateral pedal ganglion (Fig. 3B). Fig. 3C shows activity of two of the largest motoneurones, 1A and 2A, active in the D- and V-phases of a swim cycle, respectively. An EPSP and a burst of discharges in a given motoneurone is accompanied by IPSPs in its antagonist.

Two groups of interneurones (7 and 8) play a crucial role in the operation of the locomotor CPG: they generate the locomotory rhythm and drive motorneurones (2,6). Each group contains about 20 cells. These cells generate action potentials of a long

Central Generators and the Spatio-Temporal Pattern of Movements 97

&.J. Motoneurones and interneurones of the locomotor CPG of Clione. Morphology of the largest wing motoneurones, 1A and 2A (A), and of the group 7 and 8 interneurones (B) was revealed by intracellular staining with Lucifer yellow. (The dorsal view of pedal ganglia is shown in A, and the ventral view in B). NW, Wing nerve; CPC, cerebropedal connective; PC, pedal commissure. Activity of these cells during "fictive" swimming generated by the isolated pedal ganglia is shown in C and Q, where the phases of a swim cycle corresponding to the dorsal (D) and ventral (V) wing flexion are indicated.

duration (about 100 ms), one potential per swim cycle (Fig. 3D). Due to mutual electrical connections, all cells in the group fire synchronously. The interneuronal groups 7 and 8 exert a mutual inhibitory effect upon each other, and each action potential in a given group is followed by an IPSP in the antagonistic group of interneurones (Fig. 3D).

Each cell in groups 7 and 8 is a pacemaker operating at a frequency of the locomotory rhythm range (4). This was demonstrated in experiments with isolation of single neurones; the method used is shown in Fig. 44B. Fig. 4C shows initial activity of the group 7 interneurone recorded before extraction, and Fig. 4D activity of the same neurone after extraction from the ganglion. One can see that the rhythmic activity persisted in the isolated cell but the firing rate increased and the mid-cycle IPSPs (produced by the antagonistic group 8 interneurones) disappeared. Rhythmic activity of isolated cells strongly depended on the level of membrane potential. By varying the amount of current injected into the cell, one could cover the whole range of frequencies used by the intact Clione during swimming (0.55 Hz) (Fig. 4E). If the cell was inactive after extraction from the ganglion, its rhythmic activity could be restarted by depolarization (Fig. 4F, on the right).

Thus, the generation of the locomotory rhythm in the Clione CPG is based on an

98 Yu.I. Arshvsky et aL

0 nA

rOZnA 3

F - -

-0.5nA t 0 2 n A t0.5nA

m. Experiments with isolation of interneurones of Clione pedal ganglia responsible for the rhythm generation. Ganglia were treated with proteolitic enzymes to destroy the mechanical connection between cells, after which a cell could be penetrated and extracted from ganglia by means of the microelectrode as shown in A and B. Activity of the group 7 interneurone before extraction is shown in 6 and after extraction, in R. In E is shown the effect produced by injection of various direct currents (the current value indicated on the left) in the isolated group 7 interneurone hanging on the tip of a microelectrode. If the neurone was not active after extraction from ganglia, a single action potential could be evoked in it on "rebound after the pulse of hyperpolarizing current, or rhythmic generation could be evoked by injecting a depolarizing current (E).

endogenous rhythmic activity of interneurones of groups 7 and 8. The temporal efferent pattern underlying wing movements (a half-a-cycle shift between the D- and V-phase motoneurones) is determined by mutual inhibitory interactions between the two groups of interneurones: due to this interaction they Cannot fire simultaneously but only in succession.

However, the pacemaker property of group 7 and 8 interneurones is not the only basis for the pattern generation: the CPG can generate the rhythmic pattern even when the interneurones of groups 7 and 8 are "below the threshold of their pacemaker activity. In this case, the "postinhibitory rebound is of crucial importance: both group 7 and group 8 cells are capable of generating a single action potential after they have been released from inhibition (Fig. 4F, on the left). This is why, by producing an IPSP in the antagonistic group of neurones, a given group will evoke activity of these neurones on the "rebound" after termination of the IPSP.

A simplified scheme of the locomotor CPG of Clione is shown in Fig. 5A, and the

Central Generators and the Sptio-Temporal Pattern of Movements 99

A

V-PHASE

B

61' : p - 1 I

k L v J 10 P-

D

m. Locomotor CPG of Clione (A) and a schematic pattern of activity of various cell groups in a swim cycle (B). The locomotor rhythm is generated by two groups of interneurones (7 and 8) with mutual inhibitory connections. These interneurones produce EPSPs and IPSPs in motoneurones of D-phase (groups 1 and 3) and in those of V-phase (groups 2,4,6 and 10). Electrical connections between neurones are shown by resistor symbols, excitatory and inhibitory chemical synapses by white and black arrows correspondingly.

pattern of activity in various groups of neurones, in Fig. 5B. Group 7 and 8 interneurones generate the locomotory rhythm with the proper (180") shift between the D- and V-phases of a swim cycle. They control the motoneurones of various groups by producing phasic EPSPs and IPSPs in them. The duration of the action potential generated by interneurones determines the duration of efferent bursts.

As shown in Fig. 4E,F, the rate of rhythmic activity in the interneurones constituting the locomotor CPG strongly depends on the level of their tonic polarization. By varying the level of membrane potential in these neurones, one can easily control Clione's swimming, i.e. switch on and off the rhythm generator and regulate its frequency. Such a control is used, for instance, in a type of the motor behaviour named "vertical migrations". In the aquarium, and perhaps in the sea, Clione maintains itself at a definite depth by means of continuos oscillations of the wings. Sometimes the oscillations are terminated and in a few seconds the animal sinks by 20-30 cm. The oscillations then restart with an increased frequency and the animal returns to the initial depth. Clione repeats these short vertical migrations at intervals of about 1 min. Presumed command neurones (PC) controlling the vertical migrations are located in the pedal ganglia. They exhibit spontaneous bursts of activity, one of them is shown in Fig. 6A. The neurone gradually depolarized and its firing rate increased. Then the membrane potential reached a plateau where it remained for many seconds. Finally, it spontaneously returned to the resting potential. Under the effect of the PC neuron, the group 8 interneurone as well as other neurones of the locomotor CPG became strongly depolarized which resulted in a burst of locomotor activity. Besides PC neurons, about

100 Yu.1 Arshuvsky et aL

A

-p PC

B 5s

C

&&. Pedal "command neurone (PC) controlling the locomotor CPG of Clione. The neurone was capable of generating a plateau potential either spontaneously (A) or in response to a brief pulse of the depolarizing current (B). When excited, the PC neurone depolarized the group 8 interneurone (as well as other neurones of the locomotor CPG) and thus evoked the locomotor episode. Morphology of the PC neurone, located on the dorsal surface of the pedal ganglia is shown in C.

20 "command neurones were found in the pedal, cerebral and other ganglia; they could produce either de- or hyperpolarization of the CPG interneurons and thus mediate various types of influences on the locomotor activity.

Locomotor CPG o f Xenopus e mbrvo

In spite of the great distance between molluscs and vertebrates, there is a striking similarity in organization of some CPGs in these two types of animals. This is clearly seen when comparing the locomotor CPGs of Clione with that of the embryo of the frog Xenouus laevis (44). The Xenopus embryos can swim if released from their egg membrane shortly before they normally hatch (Fig. 7A). The propulsive force during swimming arises due to laterally directed waves propagating along the body in the caudal direction. The CPG for locomotion is located in the spinal cord. In animals paralyzed with a myorelaxant, rhythmical discharges with a period length typical of locomotion, can be recorded in ventral roots. There is a half-a-cycle shift between the efferent discharges in the left and the right ventral roots of the same segment (Fig. 7B) while discharges in caudal roots are delayed in relation to those in more rostra1 roots, this delay increases with distance between the segments.

A "local" (segmental) CPG consists of two symmetrical neuronal networks exerting mutual inhibitory action upon one another (Fig. 7D). The leading role in each of the networks is played by a group of excitatory interneurones (e) which generate one action potential per cycle (see Fig. 7C) and produce an EPSP in the inhibitory interneurone (i) and in motoneurones (m) forcing them to fire once per cycle (see Fig. 7B,C). The i-cells affect all the neurones in the contralateral network producing the mid-cycle IPSP in them (see Fig. 7B,C).

Central Generators and the Spatio-Temporal Pattern of Movements 101

C

e

I

vr - 200m

D

t left muscles

1 right muscles

w. The late embryo of the frog Xenopus moves due to the waves of bending that propagate along the body. In A are shown tracings of swimming movements of an embryo from film at 300 frames/s. Arrowheads indicate waves of bending which progress towards the tail. The CPG for swimming is located in the spinal cord. In B is shown a portion of the episode of fictive swimming evoked in an animal immobilized with d- tubocurarine. Recordings were made from the right and left ventral roots of the sixth segment (R6 and L6) as well as intracellularly from the motoneurone located on the right side of the same segment (m). Excitation of a motoneurone is determined by the EPSP produced by ipsilateral excitatory (e) interneurones, while the mid-cycle IPSP is produced by contralateral inhibitory (i) interneurones. Rhythmic activity of the e and i cells from the same side of a spinal segment is shown in C together with activity in the ipsilateral ventral root (vr); the episode of fictive swimming was evoked by dimming the light. D. The local (segmental) CPG for swimming of the Xenopus embrio. The populations of the excitatory (e) and inhibitory (i) interneurones as well as motoneurones (m) are shown. Excitatory synapses are shown as white squares, and inhibitory ones as black circles. (Adapted from 44).

Due to the inhibitory interaction between the two symmetrical networks, the segmental CPG of the Xenopus embryo produces a rhythmical efferent pattern with a half-cycle shift between symmetrical ventral roots. The left and right groups of e-cells are functionally equivalent to groups 7 and 8 in the locomotor CPG of Clione (Fig. 5A)

102 Yu.L Arshavsky et aL

except that the inhibitory action of e-cells upon the neurones of the antagonistic network is not direct but mediated by special i-cells.

CPG, there are two reasons for excitation of group 7 (or 8) interneurones: the pacemaker properties of these cells and the postinhibitory rebound Both factors seem to play a role in excitation of e-cells in the Xenopus embryo CPG as well. The postinhibitory rebound was demonstrated directly (43,45) while pacemaker properties could be suggested on the basis of experiments in which the spinal cord was split along the midline, and each half of the cord could generate a rhythmic efferent pattern (31).

Locomotor C PG of lamprey

Lampreys swim due to alternating contractions of the two sides at any given segment. The frequency of the contractions can vary between 0.3 and 10 Hz. The intersegmental coordination is characterized by a delay between consecutive segments resulting in a wave propagating along the spinal cord h d , therefore, along the body. The motor pattern underlying locomotion can be produced in the brainstem-spinal cord h &Q preparation, and even in the isolated spinal cord, and can conveniently be recorded in the ventral roots along the spinal cord (Fig. 8A). The entire frequency range and the intersegmental coordination can be retained in in vitrQ conditions.

The local (segmental) CPG for locomotion in lamprey is shown in Fig. 8B (21). It consists of two symmetrical networks exerting a mutual inhibitory action upon each other and, in this respect, it resembles the locomotor CPGs of Clione (Fig. 5A) and Xenopus embryo (Fig. 7D). A crucial role in the activity of this CPG is played by the excitatory interneurones (EIN). These cells generate bursts of action potentials lasting for about a half of the locomotor cycle. They produce excitation of different interneurones of the network as well as of motoneurones. An inhibitory action of EINs upon the contralateral network is mediated by commissural interneurones (CCIN) which produce inhibition of all types of neurones in that network.

There are several reasons for bursting activity of the EINs. First, the cell membrane is capable of generating oscillations in the frequency range of locomotion. In this respect, the EINs are similar to the interneurones of groups 7 and 8 in the locomotor CPG of Clione. However, there is a principal difference between these types of cells: in Clione, the capability of generating oscillations is a permanent property of the cell membrane as demonstrated in experiments with cell isolation (Fig. 4), while in lamprey the cells are not capable of generating slow (locomotor rate) oscillations until their membrane is affected by a special mediator. This is illustrated in Fig. 8C,D. In C is shown fictive locomotion generated in the spinal cord and monitored by the rhythmic activity of one of the spinal neurones and by rhythmical discharges in a ventral root (VR). The rhythmic activity of the locomotor CPG was evoked by bath application of an excitatory amino acid (NMDA). Then spinal neurones were functionally isolated from each other by bath application of tetrodotoxin (which blocks the spike discharges and, therefore, synaptic interaction). One can see that the "isolated cell generates rhythmic oscillations (Fig. 8D); these oscillations do not arise in the abscence of NMDA receptor activition. Such an action of NMDA was found in various types of spinal neurones. Thus, the capacity of the cell membrane to generate slow oscillations is one factor contributing to the bursting activity of the CPG interneurones. The second factor is determined by the structure of the CPG which consists of two symmetrical networks with mutual inhibition (Fig. 8B). Due to several reasons (summation of spike afterhyperpolarization,

In the

Central Generators and the Spatio-Temporal Pattern of Movements 103

D 10 min perf. TTX 15x 10-6M

- 4s

J l O m v 100 200 300 T I M E ( m s )

EgA. Locomotor CPG of lamprey. A. Fictive locomotion generated by the isolated spinal cord. Activity in the symmetrical (left, L and right, R) ventral roots alternates while activity in more caudal segment (19) is delayed in relation to rostral segment (7). B. The local (segmental) locomotor CPG consists of two symmetrical (left and right) networks, each of which comprises 3 groups of interneurones (excitatory, EIN; comissural, CCIN; and lateral, LIN; the two latter are inhibitory). C. Spinal neurones are "conditional" oscillators. An episode of fictive locomotion evoked in the isolated spinal cord by bath application of NMA is shown in C (VR, activity in the ventral root; INTRA-CELL, activity of a spinal neurone). In 10 min after application of tetrodotoxin (TTX) (a the action potentials in the neurone were blocked, as well as the ventral root activity, and the NMDA-evoked oscillations of the membrane potential are clearly seen. 51. The locomotory rhythm generated by the "artificial" network similar to that shown in B (realistic computer simulation).

and inhibition from LINs) the firing rate of neurones of a given network decreases with time in the course of a burst. The cessation of activity in one network will result in a "disinhibition" of the symmetrical network and, finally, in its excitation. This is why the lamprey CPG can generate the locomotory rhythm even without induction of oscillatory properties in its cells by NMDA. However, when both factors contribute to the rhythm generation, the CPG activity is much more stable as demonstrated both in experiments and in realistic computer simulations of the lamprey locomotor CPG (Fig. 8E) (21,22).

The spinal locomotor CPG is activated by reticule-spinal axons which have excitatory synapses on different types of spinal neurones (Fig. 9A) (9). The reticulo-spinal EPSPs are due to activation of NMDA and kainate receptors as demonstrated in experiments

104 Yu.L Arshavsky et aL

A B i V R

cVR

22Hz

40Hz

10 .. -

m. Reticulo-spinal system is responsible for activation of the locomotor CPG and for modifications of the locomotor pattern. A. Excitation of a reticulo-spinal neurone (from the posterior rombensephalic reticular nuclei, PRRN) evokes excitatory postsynaptic potentials in different groups of spinal neurones located in different segments (in the comissural interneurone, CC; in the unidentified cell, UIN; in the motoneurone, MN; and in the lateral interneurone, LIN). The NMA receptor blocker (KYAC) abolishes the EPSP or decreases its amplitude. B. Excitation of a reticulo- spinal neurone from the right middle rombencephalic nucleus resulted in the disturbance of symmetry in the CPG activity. Fictive locomotion was evoked in the isolated spinal cord by NMA, activity of the ipsilateral (in relation to the reticulo-spinal neurone) and contralateral ventral roots of the same segment was recorded. Duration of the train of stimuli applied to the reticulo-spinal neurone, and its frequency is indicated. (Modified from 10).

with a blockade of the NMDA receptor action (Fig. 9A). Therefore, reticulo-spinal neurones produce two effects in the CPG neurones: they increase their excitability and transfer their membrane from a stable into an unstable (i.e. oscillatory) state.

Besides activation of the CPG and regulation of the level of its activity the reticulo- spinal system may also exert more specific actions upon the CPG and considerably modify the locomotor pattern (Fig. 9B).

Feeding CPG of Planorbis

During feeding, the freshwater snail Planorbis corneus (Fig. lOA), like many other gastropod molluscs, performs rhythmic movements with the radula (tongue-like structure)

Central Generators and the Spatio-Temporal Pattern of Movements 105

A D

%

+P--*t R . Q P +

I s - Fig. 1Q. Feeding movements in the pond snail Planorbis corneus. Crawling snail is shown in A. When contacting the food, the radula performs rhythmic movements. Retracted position of the radula is shown in B, and protracted one (when the radula scratches the food object), in C, D. Preparation consisting of the buccal mass and buccal ganglia (BG) is capable of rhythmic radula (RAD) movements. These movements are shown in E together with activity in two buccal nerves ( n l and n2). Quiescence (a), protractor (P) and retractor (R) phases are indicated. The same efferent pattern can be generated in the isolated buccal ganglia (E).

and other parts of its buccal apparatus (Fig. 10B,C). The movements are controlled by the buccal ganglia. Fig. 10D shows a preparation consisting of the buccal mass with the buccal ganglia capable of generating co-ordinated rhythmic movements (Fig. 10E). A cycle of the radula movement consists of three phases: quiescence ( a ) , protraction (P) and retraction (R). In intact animals, the radula is brought into contact with a food substrate (e.g. algae) during the P-phase. Then, in the R-phase, the radula scratches food pieces and transports them to the oesophagus (see Fig. 1OC). The activity recorded in the radular nerve (nl) corresponds mainly to the P-phase, and that in the dorsobuccal nerve (n2), to the R-phase (Fig.lOE). Similar activity is observed in these nerves in completely isolated buccal ganglia (Fig. lOF), which demonstrates the existence of a central network for feeding, that is a CPG (3).

106 Yu.L Arshuvsky et al.

C . .

zoourn

mot oneurons

Fie. 11. A. Schematic pattern of activity of different neurone groups in a feeding cycle. Interneurones are marked by squares, other cells are motoneurones. Designations as in Fig. 10E,F. B, C. Morphology of interneurones of l e and 2 groups and their location in buccal ganglia. CBC, cerebro-buccal connective. 12. The feeding CPG of Planorbis. Designations as in Fig. 5A.

About 100 neurones in the buccal ganglia are rhythmically active during the operation of the feeding CPG. Depending on their morphology, on the pattern of electrical activity and the phase of this activity in the feeding cycle, these neurones can be classified into 7 groups (Fig. 11A). Cells of group 1 (which consists of subgroups le and Id) and of group 2 are interneurones (Fig. 11B,C), other cells are motoneurones controlling the muscles of the buccal apparatus. Group 1 and 2 interneurones play a crucial role in the operation of the feeding CPG of Planorbis: they generate the feeding rhythm and drive motoneurones by producing rhythmic EPSPs and IPSPs in them. The neurones of both group 1 and group 2 are endogenous oscillators which was proved in experiments with cell isolation (3) (for methods see Fig. 4). The main features of the rhythmical pattern observed in these neurones before isolation persisted after they were extracted from the buccal ganglia. The subgroup le neurones periodically generated a ramp depolarization

Central Genemtors and the Spatio-Temporal Pattern of Movements 107

C D

-0.5 nA

F

Fig. 12. The main features of the firing pattern of interneurones from the feeding CPG of Planorbis persisted in isolated cells. A. Activity of le-cell in ganglia. B-D. Activity of the same cell after extraction from ganglia. E. Activity of the group 2 cell in the ganglia. E, G. Activity of the same cell after extraction. The value of a current injected into a cell is indicated in B-Q and E, 8.

accompanied by accelerating spike discharges (Fig. 12A-D). Group 2 neurones periodically generated a rectangular wave of depolarizing potential with oscillations on the top (Fig. 12E-G). The rhythmic activity in isolated cells depended strongly on the amount of current injected (Fig. 12B-D; F,G), which suggests that the CPG can be easily controlled by a tonic synaptic inflow to the generatory neurones.

In contrast to all the CPGs considered above the feeding CPG of Planorbis is essentially asymmetrical (see Fig. 11D): group 1 neurones exert an excitatory action upon group 2 neurones, while group 2 neurones inhibit group 1 neurones. During the feeding cycle, the following events take place in the CPG. Due to the intrinsic properties of l e cells, their activity gradually increases in the Q and P phases. Because of the mutual electrical connections within group 1, le-cells provide an excitatory drive to ld-

108 Yu.1 Arshavsky et al.

cells which get excited in the P-phase. The Id-cells exert an excitatory action upon group 2 neurones. Having reached the threshold, type 2 cells generate a rectangular wave of depolarizing potential and inhibit the group 1 cells, thus terminating the P-phase The duration of the wave generated by the group 2 cells determines the R-phase of the cycle. After termination of this wave, a ramp depolarization starts again in le-cells giving rise to a new cycle. Thus, the duration of a feeding cycle is determined by (i) the slope of depolarization in le-cells, (ii) the threshold of excitationof type 2 cells, and (iii) the duration of the rectangular wave generated by type2 cells. By varying these parameters, the duration of the entire feeding cycle and of the differentphases constituting the cycle can be regulated.

All synaptic potentials in motoneurones in the P-phase (both EPSPs and IPSPs) are produced by Id-cells, while group 2 cells areresponsible for the PSPs produced in motoneurones in the R-phase. Due to combinationsof the two inputs, a variety of rhythmical patterns can be produced in the different groups of motoneurones (Fig. 11A).

CPG for defensive reaction in Planorbis

The locomotor and feeding CPGs considered above generate rhythmical patterns, each cycle giving rise to a new cycle. The CPG for adefensivereaction in the snail Planorbis corneus is an example of a mechanismgenerating a single pattern. The snail Planorbis, like other gastropod molluscs (see e.g. 7), exhibits a defensive reaction to various external stimuli. With a weak stimulus applied, for instance, to the head, the snail stops crawling and pulls the shell towards the head. This is the 1st phase of the defensive reaction. With stronger stimulation, the 1st phase is followed by the 2nd phase in which the whole body of the snail is pulled into the shell. Essential features of this movement can be observed in a simple preparation consisting of the CNS and the muscle pulling the shell. As shown in Fig. 13A, a weak stimulus applied to a peripheral nerve evoked a muscle contraction lasting for several seconds (the 1st phase, or component of the defensive reaction). With stronger stimulation (Fig. 13B), the 1st component increased considerably in amplitude, and it is now followed by an additional contraction which lasted for about 1 min (this 2nd phase corresponds to pulling the body of the snail into the shell). Both phases are graded, depending on the strength of the stimulus.

Muscle contractions during this defensive reaction are controlled by numerous motoneurones. Different groups of motoneurones receive different inputs (excitatory and inhibitory) during the 1st and 2nd phases of the reaction (Fig. 13C), which ensures a coordinated movement of different parts of the body. The pattern of motoneurone activity typical for the defensive reaction can also be observed in the isolated nervous system of the snail. This pattern can either be obtained in response to stimulation of the skin or a peripheral nerve, or it can appear spontaneously. In the latter case, each of the components of the fictive defensive reaction appears either independently (Fig. 13D,E) or in a normal succession, i.e. the 1st component is followed by the second one (Fig. 13F). Thus, the essential features of the defensive reaction in the snail are determined by the CPG. This CPG is supposed to contain two groups of neurones generating the 1st and the 2nd phases of the reaction (Fig. 135). Numerous afferents eliciting a defensive reaction converge upon group 1 and 2 neurones and exert an excitatory action upon them. The proper sequence of phases (the 1st followed by the 2nd) in response to an afferent input is caused by the following conditions: (1) group 1 cells exert an

Central Generators and the Spatio-Temporal Pattern of Movements 109

M

B u h a s e 2

A -

M N l I

M N 2 1""" A 1

25s 10s

G MN

PLIN & bmv 4P 6P BP ep

H

MN2- A

F

J a f f e r e n t i n p u t s

93 m o t o n e u r o n s

Fig. 13. Organization of the defensive reaction CPG in Planorbis. A, B. In preparation consisting of the CNS and of the muscle pulling the shell, stimulation of the lip nerve (arrowhead) evoked the 1st phase of the muscle contraction (M). With stronger stimulation (B), the 2nd, long-lasting phase appeared. C. Activity of two motoneurones (MNl and MN2) during defensive reaction evoked in the same preparation. D-E. Spontaneous defensive pattern in two motoneurones (isolated CNS). The 1st and the 2nd components appeared either separately (QE) or in succession (E). G,H. Responses of the group 1 interneurone from the pleural ganglion (PLIN) to stimulation of the lip nerve in the isolated CNS. The lower trace shows the stimuli applied to the lip nerve, the number of pulses being indicated. When excited, the neurone generates a prolonged action potential. The upper trace shows that PLIN produces EPSP in the motoneurone (MN). As shown in H, excitation of PLIN by the current injection also evoked EPSP in the follower cell (MN). 1. Morphology of PLIN from the right pleural ganglion. (CerG, cerebral ganglia). J. Defensive reaction CPG of Planorbis. Designations as in Fig. 5A.

excitatory action upon group 2 cells, (2) group 2 cells have a higher threshold for excitation, and (3) activity in group 2 cells increases slower than in group 1 cells. A gradual increase of the reaction with stronger stimuli is supposed to be caused by a recruitment of new neurones of the groups. Cells of groups 1 and 2 drive motoneurones by producing EPSPs and IPSPs in them. Fig. 13G-I shows an interneurone of group 1. When excited, it generates an action potential lasting for a few seconds (Fig. 13G,H). The duration of this potential determines the duration of excitation in motoneurones

I10 Yu.1 Arshavsky et aL

and, therefore, the duration of the 1st phase of the defensive reaction. The cellular properties of this neurone also determine some other essential features of the defensive reaction: the existence of a threshold and a long refractory period following excitation (Fig. 13H). Due to an extensive branching of its processes (Fig. 131) the neurone of group 1 will synapse on a great number of motoneurones responsible for the defensive reaction.

Thus, the defensive reaction CPG of Planorbis, generating a single pattern, is based on the same principles as the rhythmic CPGs considered above. It consists of two groups of neurones, the cell properties of which determine the time course of the 1st and 2nd phases of the reaction. The sequential activity of the groups is mainly due to their made of interaction: the 1st group excites the 2nd one. This CPG differs from rhythmical ones in that there is no back action of the 2nd group upon the 1st group. An important feature of this CPG is that its pattern strongly depends on the initial stimulus. With stronger stimulus, not only the amplitude of the muscle contraction increases but also the 2nd phase of contraction appears. In this respect the defensive reaction cannot be considered as "a fixed action" (see 32).

Discussion and conclusions

A central pattern generator is a neuronal machinery capable of producing complicated time-dependent signals on its outputs (e.g. in motoneurones). In the motor system, such signals will determine the basic temporal and spatial characteristics of a given movement. The examples presented above have shown several common features in the organization of the CPG for various movements: 1) The output signal of a CPG can be divided into sequential phases. Within a given phase, the time course of a signal is determined, to a great extent, by specific membrane properties of a certain group of neurones. (2) A transition from one phase to another phase is determined mainly by an interaction between corresponding groups of neurones. (3) The pattern produced by a network of interneurones can be regulated in various aspects: the CPG can be switched on and off, the level of its activity can be changed, numerous afferent systems usually affect the CPG and its parts, etc. Below we shall consider these points in greater detail.

Role of specific membrane uroperties for CPG activity. The main features of the temporal patterns produced by different CPGs are determined by the patterns of activity of generatory interneurones which, in their turn, depend on the membrane properties of these cells. Thus, interneurones in the locomotor CPGs of Clione and of Xenopus embryo generate one action potential per swim cycle, and the duration of this potential (which is a specific property of the cell membrane) is the main determinant of the temporal characteristics of the motor pattern of swimming. The excitatory interneurones in the locomotor CPG of the lamprey, on the other hand, generate not a single action potential per cycle but a burst of such potentials, and the duration of this burst determines the duration of activity in each of the two symmetrical segmental networks. The main characteristic of the feeding CPG of Planorbis is a ramp-shaped and a rectangular-shaped waves observed in the P- and R-phases of a cycle in many CPG neurones. These waves are generated by interneurones of groups 1 and 2 due to specific properties of their membrane. Finally, the duration of locomotor episodes in Clione is determined by the duration of plateau potentials generated by "command PC neurones.

These examples clearly show that both the duration of a definite phase of the movement and the temporal efferent pattern throughout this phase strongly depend on

Central Generators and the Spatio-Temporal Pattern of Movements 1 1 1

the characteristics of potentials generated by the membrane of the corresponding group of the interneurones. The capacity to generate potentials specifically depending on time is, in some cases, a permanent membrane property. For example, isolated interneurones of the locomotor CPG of Clione and of the feeding CPG of Planorbis generate patterns similar to those before extraction from ganglia. On the other hand, interneurones in the locomotor CPG of lamprey, for transition to the oscillatory state, are modified by the reticulo-spinal system. This system produces activation of certain types of receptors (NMDA), which induces oscillatory properties in the spinal neurones. Similarly, in the CPG controlling the pyloric rhythm in lobster, the "command neurones evoke changes in the membrane properties of some cells thus transferring them into oscillators (28,37,48).

Types o f eenerator networks. Both in the CPGs producing a single pattern (like the defensive reaction CPG of Planorbis) and in those producing a rhythmical pattern (e.g. locomotion), co-ordinated activity of generatory neurones is due to their interaction. First of all, the "synergistic" neurones are united into groups due to their mutual excitatory interaction. In molluscs, this action is mediated by electrical connections in many cases, while in vertebrates excitatory chemical synapses are more common. A proper sequence of phases of the movement is determined by a system of connections between the networks (interneurone groups) responsible for each of the phases. Inhibitory, excitatory and mixed interactions between the groups can be observed in different CPGs. In the feeding CPG of Planorbis, the group 1 neurones exert an excitatory action upon group 2, while group 2 inhibits group 1 neurones (Fig. llD). However, a mixed interaction is more common. Thus, in the locomotor CPGs of Clione, of Xenopus embryo and of lamprey the symmetrical (left and right) networks inhibit each other, but they produce a mutual excitatory action as well (in part, due to the rebound phenomenon). In some CPGs, consisting of several neuronal groups, the intergroup connections are rather complex (for review of CPG organization, see 18).

There are a few examples of "re-organization'' of the CPG structure which allows the CPG to generate different efferent patterns. In the CPG controlling the pyloric rhythm in lobster, the efficiency of a particular synapse and therefore, timing of the output pattern can be affected by a special "command" neurone (48). In the locomotor generator of Clione, the principal network shown in Fig. 5A can be markedly modified during intense swimming when two additional groups of interneurones are involved (2). Finally, the spinal cord of lamprey usually generates forward swimming (there is a rostro-caudal delay between the segmental CPGs). However, the phase coupling between the CPGs can be modifed in such a way that backward swimming is generated (19).

Organization of CPGs in mammals. The different CPGs considered above exhibit a number of similarities despite the fact that they are from widely different groups of animals. It may therefore be relevant to consider the possible mechanisms in mammals. The biphasic pattern with gradually increasing activity in the first phase and with a rather short burst in the second phase is a characteristic of some motor patterns in mammals, e.g. of respiratory movements (Fig. 14A), of chewing movements (Fig. 14B), and of the scratch reflex (Fig. 14C). One can suggest that, for producing this pattern, a CPG similar to the feeding CPG of Planorbis (Fig. 11D) can be used (CPG with a "switch-off' mechanism, see 13). It seems very likely that interneurones generating the ramp depolarization (like le-cells in Planorbis, Fig. 12A-D) are responsible for the first phase of a movement and interneurones generating a short burst or a rectangular-shape

112 Yu. I. Arshvsky et al.

A

- 0.3 s

0.1 s

Fie. 14. A number of CPGs in mammals generate biphasic rhythmic pattern, with the activity gradually increasing in the 1st phase and rapidly falling in the 2nd phase. Three examples are illustrated: A. Integrated EMG of the phrenic nerve during the respiratory cycle (cat) (15). B. Integrated EMG of the digastric muscle during rhythmical chewing (monkey) (33). c. Total activity in two groups of spinal interneurones durin, the cycle of fictive scratching (cat). The two groups are supposed to reflect the activity of a CPG with the "switch-off mechanism" (8) similar to that of Planorbis (Fig. 11D).

depolarization wave (like group 2 cells in Planorbis, Fig. 12E-G) are responsible for the second phase.

The CPG located in the spinal cord and controlling stepping movements of a limb produces a rather complex efferent pattern (26,4). As a first, very rough approximation, this pattern can be considered as a biphasic one, i.e. as the alternating activities of the two main groups of muscles, i.e. extensors (the support phase of the step) and flexors (the swing phase). The pattern of stepping is asymmetrical in the sense that the stance phase is usually longer and varies directly with speed of locomotion but the swing phase remains fairly constant. Such a pattern could be produced basically by two groups of interneurones. Recordings from non-identified spinal interneurones during locomotion have shown that the total number of active neurones increases in the course of the swing phase and decreases towards the end of the stance phase (39). This finding suggests that the activity of CPG neurones responsible for the swing phase increases towards the end of the phase. On the contrary, the efferent activity in the stance phase seems to be controlled by interneurones generating a gradually decreasing activity, like that in the locomotor CPG of lamprey.

In the lamprey, activation of excitatory amino acid receptors of the NMDA type (produced by the reticulo-spinal neurones) transforms the spinal interneurones into oscillators operating within the frequency range characteristic of swimming (Fig. 8C,D). The fact that application of NMDA to the isolated spinal cord also evokes fictive

Central Generators and the Spatio-Temporal Pattern of Movements 113

locomotion in mammals (34), (see Fig. 1C) may suggest that some of the neurones constituting the CPG for stepping may be "conditional" oscillators activated by the reticulo-spinal fibres coming from the brainstem locomotor centres.

There is some data indicating a possible inhibitory interaction between the interneuronal networks responsible for the swing and stance phases of a step cycle (36). An excitatory interaction probably exists as well, at least in the form of postinhibitory rebound which is a property of many types of neurones in mammals. Though the biphasic pattern is usually considered as a basis for stepping and scratching limb movements in mammals, in real movements there is some phase shift between, for example, flexor muscles of different joints of a limb (26,41). For explaining this finding, an idea that each of the joints is controlled by its own CPG, and that the CPGs of different joints are interconnected, was advanced (20). Examination of this problem can be easily carried out on the lamprey which has numerous segmental CPGs working with some phase shift in relation to one another.

Besides such "automatic" movements as locomotion, respiration, etc, the central programming is also extensively used in those movements which are traditionally called voluntary (e.g. reaching or pointing arm movements). Though these movements are characterized by a great variety of motor patterns, each given pattern is determined, to a great extent, by initial information concerning the relative positions of the hand and the target (SO). One can thus suggest that these movements are also controlled by CPGs like that for defensive reaction in Planorbis (Fig. 13J), in which the output pattern strongly depends on the initial stimulus.

In this chapter we did not deal with the sensory regulation of the central networks. It is of crucial inportance, however, to recall that in most instances in vertebrates and invertebrates, the sensory input can modify the duration of a given phase of a movement and the amplitude of the output signal.

References

1 Andersson O., Grillner S. Acta Physiol. Sand., 113 (1981) 89. 2 Arshavsky Yu.I., Beloozerova I.N., Orlovsky G.N., Panchin Yu.V., Pavlova G.A. Exp. Brain

Res., (1985) 255. 3 Arshavsky Yu.I., Deliagina T.G., Orlovsky G.N., Panchin Yu.V. Exp. Brain Res., 70 (1988)

310. 4 Arshavsky Yu.I., Deliagina T.G., Orlovsky G.N., Panchin Yu.V., Pavlova G.A., Popova L.B.

Exp. Brain Res., 63 (1986) 106. 5 Arshavsky Yu.I., Gelfand I.M., Orlovsky G.N. Cerebellum and Rhythmical Movements.

Springer-Verlag, Berlin, 1986. 6 Arshavsky Yu.I., Orlovsky G.N., Panchin Yu.V. Exp. Brain Res., 3 (1985) 203. 7 Benjamin P.R., Elliot C.J.H., Ferguson G.P. In: Model Neural Networks and Behaviour,

A.I. Selverston (ed.), Plenum Press, N.Y., (1985) 87. 8 Berkinblit M.B., Deliagina T.G., Feldman, A.G., Gelfand I.M., Orlovsky G.N. J.

Neurophysiol., 41 (1978) 1040. 9 Brodin L., Grillner S., Dubuc R., Ohta Y., Kasicki S., Hokfelt T. Arch. Ital. Biol., 126 (1988)

317. 10 Buchanan J.T., Cohen A.H. J. Neurophysiol., 47 (1982) 948. 11 Cohen A.H., Rossignol S., Grillner S. (eds.) Neuronal Control of Rhythmic Movements in

Vertebrates. John Wiley and Sons, New York, 1988. 12 Delcomyn F. Science, 210 (1980) 492. 13 Euler C., von In: Neural Origin of Rhythmic Movements, A. Roberts, B. Roberts (eds),

Cambridge University Press, (1983) 469.

114 Yu. I. Arshavsky et al.

14 Feldman J.L., Grillner S. The Physiologist, a (1983) 310. 15 Feldman J.L., Speck D.F. J. Neurophysiol., (1983) 472. 16 Garcia-Rill E., Skinner R.D. In: Neurobiology of Vertebrate Locomotion, S. Grillner,

P.S.G. Stein, D.G. Stuart, H. Forssberg, R.M. Herman (eds.), Macmillan, London, 77 (1986). 17 Gelfand I.M., Orlovsky G.N., Shik M.L. In: Neural Control of Rhythmic Movements in

Vertebrates, A.H. Cohen, S. Rossignol, S. Grillner (eds.), J.Wiley and Sons, New York, 167 (1988).

18 Getting P.A. In: Neural Control of Rhythmic Movements in Vertebrates, A.H. Cohen, S. Rossignol, S. GriUner (eds.), J. Wiley and Sons, New York, 101 (1988).

19 Grillner S. In: Function and Formation of Neural Systems, G.S. Stent (ed.) Dahlem Konferenzen, Berlin, 197 (1977).

20 GriUner S. In: Handbook of Physiology, Sect. 1. The Nervous System Vol. II. Motor Control. V.B. Brooks (ed.) Waverly Press, Maryland, 1179 (1981).

21 Grillner S., Buchanan J.T., WdBn P., Brodin L. In. Neural Control of Rhythmic Movements in Vertebrates. A.H. Cohen, S. Rossignol, S. Grillner (eds.) J. Wiley and Sons, New York, 1 (1988).

22 Grillner, S., Lansner A., Walldn P., Brodin L., Ekeberg O., TravBn H., Christenson, J. In: Cell to Cell Signalling: From Experiments to Theoretical Models. Academic Press, 77 (1989).

23 Grillner S., Science, 228 (1985) 143. 24 Grillner S., Stein P.S.G., Stuart D.G., Forssberg H., Herman R.M. (eds.) Neurobiologyd

Vertebrate Locomotion. Macmillan, London, 1986. 25 Grillner S., Wallen P., Ann. Rev. Neurosci., (1985) 233. 26 GriUner S., Zangger P., Brain Res., & (1975) 367. 27 Herman R.M., GriUner S., Stein P.S.G., Stuart D.G. (eds.) Neural Control of Locomotion.

Plenum Press, New York, 1976. 28 Hinzel H-G. In: Neural Mechanisms of Behavior. Erber J. et al. (eds.), N.Y., 61 (1989). 29 Holst E. Brit. J. h i m . Behav., 2 (1954) 89. 30 Jordan L.M. In: Neurobiology of Vertebrate Locomotion. S. GriUner, P.S.G. Stein, D.G.

Stuart, H. Forssberg, R.M. Herman (eds.) Macmillan, London, 21 (1986). 31 Kahn J.A., Roberts A., Phil. Trans. R. SOC. B., 296 (1982) 229. 32 Kandel, E.R. The cellular bases of behavior. Freeman, San Fransisco, 1976. 33 Kubota K., Niki H. In: Oral-Facial Sensory and Motor Mechanisms. R. Duber and Y.

Kawamura (eds.) Appleton-Century-Crofts, N.Y., 365 (1971). 34 Kudo N., Yamada T. Neuroscience Letters, 75 (1987) 43. 35 Loeb G.E. In: Neurobiology of Vertebrate Locomotion. S. Grillner, P.S.G. Stein, D.G.

Stuart, H. Forssberg, R.M. Herman (eds.), MacMillan, London 547 (1986). 36 Lundberg A. In: Symp. lect. of XXVIII Int. Congr. Physiol. Sci., AkadBmiai Kiado,

Budapest, 155 (1981). 37 Marder E. In: Neural Mechanisms of Behaviour. J. Erber et al. (eds.), N.Y., 55 (1989). 38 Orlovsky G.N., Shik M.L. In: International Review of Physiology. Neurophysiology II, vol.

10, 281 (1976). 39 Orlovsky G.N., Feldman A.G. Nejrofiiilogia, 4 (1972) 410. 40 Pearson KG., Duysens J. In: Neural Control of Locomotion. R.M. Herman, S. Grillner,

P.S.G. Stein, D.G. Stuart (eds.), Plenum Press, New York, 519 (1976). 41 Perret C. In: Neural Origin of Rhythmic Movements. A. Roberts, B.L. Roberts (eds.), SOC.

Exp. Biol. Symp. 42 Roberts A. In: The Computing Neuron. R. Durbm, C. M i d , G. Mitchison (eds.), Addison-

Wesley, Woringharn, 228 (1989). 43 Roberts A., Roberts B.L. (eds.) Neural Origin of Rhythmic Movements. SOC. Exp. Biol.

Symp. 22. Cambridge University Press, Cambridge, (1983).

Cambridge University Press, 405 (1983).

Central Generators and the Spatio-Temporal Pattern of Movements 115

44 Roberts A., Soffe S.R., Dale N. In: Neurobiology of Vertebrate Locomotion. S. Grillner, P.S.G. Stein, D.G. Stuart, H. Forssberg, R.M. Herman (eds.), Macmillan, London, 279 (1986).

45 Roberts, A,, Tunstall M.J., Europ. J. Neurosci., 2 (1990) 11. 46 Rossignol S., Lund J.P, Drew T. In: Neural Control of Rhythmic Movements in Vertebrates.

A.H. Cohen, S. Rossignol, S. Grillner (eds.), J. Wiley and Sons, New York, 201 (1988). 47 Selverston A.I., Behavioral Brain Sci., 2 (1980) 535. 48 Selverston A.J., Moulins M. The crustacean stomatogastric system. Springer, Berlin, (1987). 49 Shik M.L., Orlovsky G.N., Physiol. Rev., 3 (1976) 465. 50 Soechting J.F., Flandres M., J. Neurophysiol., 62 (1989) 582. 51 Stein P.S.G. Ann. Rev. Neurosci., l(1978) 61. 52 Stein P.S.G. In: Neural Origin of Rhythmic Movements. A. Roberts, B.L. Roberts (eds.),

Exp. Biol. Symp., 22, Cambridge University Press, 383 (1983). 53 Wilson D.M., J. Exp. Biol., (1961) 283.