Pattern generation Allen I. Selverston University
of California,
The study of rhythmic by preparations Using
these
modified
motor pattern generation
in which
cellular
preparations,
basic
in order
to be answered.
to generate The action
and
channel
proteins
and
behavior.
There
simulations,
San Diego,
can
and circuit questions a large
continues
has been
a
link
circuits
of patterns on second
between
a large increase
and their usefulness
can be bridged.
such as how
number
in the
as a way of studying
USA
to be dominated
mechanisms
of neuromodulators provide
California,
have
can
be
begun
messengers
molecular
studies
use of computer pattern
generation
is growing.
Current
Opinion
in Neurobiology
Introduction Rhythmic motor patterns are generated by the interactions of neurons within the central nervous systems of all animals. The properties of the individual neurons, as well as the synaptic networks they form, are the basis for systematically investigating pattern generation. It has become clear over the past few years that as a result of changing chemical factors in the immediate environment, the electrical and synaptic interactions between neurons, as well as their biophysical properties, are in a highly fluid state. Circuits can undergo major functional reorganization as a result of exposure to peptides, amines and other substances. These neuromodulators can be delivered via the bloodstream or more rapidly via synaptic terminals within the pattern generating circuits. The determination of the precise cellular mechanisms involved in producing changes in motor patterns has been an especially active area of research over the past year. It is clear that neuromodulators provide enormous flexibility to central pattern generating (CPG) circuits, suggesting that different motor patterns do not necessarily require separate circuits, but can be fashioned out of one circuit by altering cellular and synaptic properties. In addition to the neuromodulation of pattern generators, there has been progress in the development of new preparations, particularly fetal and juvenile rat spinal cords that are capable of rhythm generation. The use of such preparations, along with the tremendous progress made using the lamprey and Xenopus preparations, suggests that the cellular analysis of more complicated central nervous systems may now be possible. There has been a continued elucidation of the role sensory feedback plays in the generation of motor patterns, and several modeling studies can be used to help understand the many simultaneous non-linear computations performed by CPGs.
1992, 2~776-780
Several reviews have appeared which may be of interest to workers in the motor pattern generating field. One fairly large review of the principles of organization of motor systems proposes a mathematical theory of opti mal motor control that can be applied to many aspects of rhythmic patterns [ 11, as well as other kinds of brain function. Another reviews the role of neuromodulators in invertebrates [2]. Detailed accounts of the current state of knowledge pertaining to the crustacean stomatogastric system [3**] and the lamprey locomotory system [4] are also available. For a general review see [5].
Circuit-level
effects of neuromodulators
In the stomatogastric system, neuromodulators have already been shown to have different kinds of actions. They can move one or more neurons from one CPG to another. They can take neurons from different CPGs and combine them to form a ‘blended’ circuit, or they can combine elements in a way that produces an entirely de nouo circuit and a concomitant de nouo motor pattern (see [3-1). Previous work has shown that neuromodulatoty fibers from other ganglia in the central nervous system enter the stomatogastric ganglion via the stomatogastric nerve and presumably release modulators into the neuropile in a paracrine fashion. In a new study [ 6.1, intracellular recordings were made from the modulatory fibers close enough to the ganglion to record synaptic potentials from identified stomatogastric neurons. The firing pattern of the modulatory neuron was actually shaped by the cyclic activity of the gastric and pyloric neurons, suggesting for the first time that fibers that modulate the pattern are actually inlluenced by the pattern-generating network.
Abbreviations CPC-central 776
pattern
generator;
CABA--y-aminobutyric
@ Current
Biology
acid; NMDA-N-methyl-o-aspartate.
Ltd ISSN 0959-4388
Pattern generation
Acetylcholine, which acts both as a conventional transmitter as well as a neuromodulator in the stomatogastric system, was also shown to have strong effects on spontaneous episodes of fictive activity in Xenopus embryos 171. In both preparations [6*,7] the effects could be blocked with muscarinic antagonists.
Cellular-level
effects of neuromodulators
The action of neuromodulators occurs as a result of their binding affinity to particular neurons. Once bound, the modulators, acting via G proteins, appear to increase the concentration of specific second messengers. This increase in second messenger concentration can activate a protein kinase, which in turn leads to the phosphorylation of a receptor, channel or another protein. This alters the properties of the cell or synapse in a characteristic way and leads eventually to the generation of a new or altered pattern. There is a great deal of interest in examining the cellular mechanisms involved, and last year witnessed many excellent papers on this subject. New actions have been described for octopamine effects on interneurons of the insect ilight motor system [8-l, and serotonin effects on peripheral axons of the crab, Cancer borealis, where the modulator causes the initiation of peripheral spikes, which then propagate antidromically [9]. Octopamine produces intrinsic bursting in some neurons that is similar to that seen during wind-induced flight activity, suggesting that intrinsic bursting plays an important role in generating the normal flight motor pattern. Neuromodulatory receptor activation has been shown to have a variety of physiological effects in many different motor systems [10,110,12,13]. The peptide FMRFamide was shown to have effects on dissociated leech heart muscle cells [lo]. Superfusion of lo-6M FMRPamide elicits a persistent inward current with a Na+ component, and in addition modulates both voltage-dependent Ca2+ and K+ currents. In pyloric muscle fibers of the mantis shrimp, Squilla oratoria, it was reported that like some other crustacean stomatogastric preparations, the fibers themselves had endogenous bursting properties [ 131. These fibers use voltage- and time-dependent conductances to Ca2 + and Nat, as well as a Ca2+ -activated K+ conductance to generate the oscillatory potentials, and any of these are possible targets for modulatory substances. Activation of y-aminobutyric acid (GAI3A)n receptors in the lamprey spinal cord leads to a decrease in axonal membrane resistance and a reduction in spike duration [11-l. The effects of phaclofen, a GABAn antagonist, are mimicked by pretreatment of the spinal cord with picrotoxin. As this inactivates a certain class of G proteins, it seems likely that the receptor-mediated effects are initiated via a presynaptic population of picrotoxin-sensitive G proteins. In addition to studying modulatory effects on single cells, more is being learned about the properties of ionic than
Selverston
nels in neurons of CPGs. This information is important in providing a baseline so that phosphorylation-induced conductance changes can be measured. One excellent example of such a study concerns the role of the transient K+ current, I,, in determining cycle frequency and phasing of neurons in the pyloric rhythm in the stomatogastric ganglion [14*]. Using 4-aminopyridine to reduce I*, it was shown that the cycle period could be reduced by approximately 20 %, and that the phasing of follower cells altered relative to the onset of pacemaker activity. Voltage-clamp experiments indicated that pyloric cells differed in the amount of IA current they could express, and this could be responsible for the different rates of recovery from inhibition. A detailed description of currents in the lateral pyloric cell of the pyloric rhythm CPG has shown it to contain at least three inward and three outward currents [ 15.1. Neurons from leech ganglia can be made to burst by blocking intrinsic Cal+ currents [161; however, the role of these currents under normal conditions is not yet clear.
Analysis
of pattern generators
Reports of several new methods applicable to the study of neuronal pattern generators have become available during the past year. The use of optical recordings combined with conventional intracellular recordings will likely shed some light on the synaptic relationships of Aplysia neurons in culture [ 171. It is now possible to study the spatial patterns of neural activity associated with motor pattern generation in in vitro brainstem-cerebellum preparations with the fluorescent dye, sulforhodamine-I01 [ 181, and an electronic circuit can be used to artificially couple stomatogastric neurons in culture [19-l. Two reports have appeared demonstrating the use of a fetal rat brain-spinal cord preparation for use in studying the ontogeny of rhythmic respiratory discharges at the circuit and cellular level [ 20,211. A detailed statistical analysis of respiratory-related neurons along the brainstem midline indirectly demonstrated evidence for inhibitoty and excitatory functional connections between them, but did not provide any information on circuitry [22-241, Similarly, a study of fictive motor patterns in early and late spinal cats showed that different patterns could be elicited in chronic animals depending on the method of stimulation, but little about underlying mechanisms was presenred [25*]. Molluscan generators continue to be of importance in elucidating mechanisms of pattern formation, and the past year has seen a description of the neural basis for the somewhat irregular respiratory behavior in Lymnaea [ 26,271. The first paper describes the behavior and identifies the motor neurons. It also suggests a role for sensory feedback in the system. The second paper describes the pattern-generating circuit, the basic synaptic connectivity, and outlines a scheme for how the network operates. One pair of interneurons has been described that not only synapse onto respiratory interneurons, but
777
778
Neural
control
are also inhibited by some of them [28-l. This same pair of neurons also appears to coordinate locomotor and cardiorespiratory networks, making them extremely important in the control of cyclic motor patterns. In one particularly interesting study, an interneuron from the respiratory CPG was removed from the brain leading to a deficit [ 291, Grafting an identified neuron from an other snail to the operated brain corrected the deficit by connecting to other relevant neurons in the CPG circuit. A temporal analysis of the feeding rhythm in Lymnaea has shown that, like many cyclic behaviors, there is a frequency-dependent variable phase for some neurons and a frequency-independent fixed phase for others [30]. Command-like neurons have been found in the buccal ganglion of the snail Achatina filica, which appear to be involved in the control of rhythmic activities [31]. Similar fibers, controlling the escape reaction in the marine mollusc Clione, have been recently described [32].
Two new crustacean papers have appeared, one dealing with the relationship between the swimmeret rhythm postural and locomotor activity [33], and another very interesting paper on long-term electromyograph recording of the pyloric rhythm in vivo [34*-l. Recent studies suggest that descending modulatory input entering the stomatogastric ganglion from the commissural ganglia via the stomatogastric nerve is both necessary and sufficient for the pyloric rhythm to exist. When the stomatogastric nerve is severed or blocked in in vitro preparations, the pyloric rhythm ceases immediately. When the nerve is severed in vivo, the pyloric rhythm is unaffected initially and continues for many days. Application of lobster blood to an in vitro preparation restores the rhythm to a ganglion whose stomatogastric nerve has been cut or blocked, suggesting that modulators can be delivered humorally as well as via the nerve. Intersegmental coordination of oscillators has been reported to occur via serotonergic modulation in the lam prey [35*]. The spinal serotonin system can depress Ca2 + -dependent K + channels in some network neurons resulting in a change in phase from a rostrocaudal to a caudorostral direction, corresponding to a reversal in the direction of swimming. The role of sensory feedback in pattern generation has been described for the locust [ 361 and the crayfish [37]
Another six-conductance model has been written for the lateral pyloric cell of the pyloric CPG [38,39]. Equations were given to describe its voltage-, time- and Ca2+-dependence. The model is supposed to duplicate the steady state biological I-V curves for this cell, but major errors have been found for values of the transient IA currents, and they had to be significantly altered before the results could be duplicated. A modified Hodgkin-Huxley model of an Onchidium pacemaker neuron reproduced tonic firing, and both irregular and regular bursting activity that was very similar to the biological neuron [40*]. An andysis of the irregular bursting showed features of chaotic dynamics, and the steady state I-V curve showed a negative slope resistance. An even simpler two-conductance model for K+ and Ca2+ was able to switch between five separate stable states [ 411. AS part of a larger study, models of N-methyl-D-aspartate (NMDA)-induced oscillatory activity in lamprey spinal cord were able to account for the effects of bath-applied NMDA [ 421.
Conclusion The new data reinforce a basic shift in thinking away from neurons with rigidly fixed properties incorporated into hard-wired pattern generating circuitry, toward the idea of increased flexibility. While this brief review has con centrated on motor patterns, the formation of patterned activity is the common currency of all neural functions including sensory and central processing. We can begin to see from the analysis of small neural circuits that by providing networks with a rich set of connections, neuromodulators (which include almost all conventional neurotransmitters) can sculpt out specialized circuitry for specific purposes. Neural modeling will be an important method for conceptualizing this process. The absence of physiological techniques for dealing with the enormous complexity of mammalian pattern generators at this time precludes the analysis of cellular interactions with the high degree of resolution seen in invertebrates. For these large networks, modeling, even with many as sumptions, may be the only way to proceed.
References Modeling
of pattern generators
Computer simulation continues to provide a useful tool for those wishing to understand rhythmic motor patterns at a more theoretical level. Usually the most interesting results have occurred when the models are well grounded in biological reality. For example, the work on the production of oscillatory activity in leech neurons by blocking Ca2+ channels has been incorporated into a conceptual model [ 161. The model, which uses neurons with six separate conductances, suggests a plausible mechanism for the induction of oscillatory activity that is consistant with the physiological data.
and recommended
Papers of particular interest, published view, have been highlighted as: . of special interest .. of outstanding interest 1.
2.
reading
within the annual period of re-
BAW KV, SHKVIANSKY YF: Principles of Organization of Neural Systems Controlling Automatic Movements in Animals. Prog Neurobiol 1992, 39~455112. HARRIS-WARRICK F&l,
works
for Behavior.
Annu
E: Modulation of Neural NetR~L’Neurosci 1991, 143957.
MARDER E, SELVERSTON Al, MOUUNS M: Dynamic Biological Networks: the Stomatogastric Nervous System. Cambridge, Massachusetts: MIT Press; 1992. The latest compendium of work on the crustacean stomatogastric system from the molecular to the behavioral level.
3. ..
HARRIS-WARRICK RM,
MARDER
Pattern
4.
GRIUNER S, MATSUSHIMA T: The Neural Network Underlying Locomotion in Lamprey-Synaptic and Cellular Mechanisms. Neuron 1991, 7:1-15.
5.
AR?HAVSKY YI, GRILLNER S, OKLOVSKYGN, PANCHINYV: Central Generators and the Spatio-Temporal Pattern of Movements. In ne Development of Timing Control and Temporal Organization in Coordinated Action. Edited by Fagard J, Wolff PH. New York: Elsevier; 933115.
6. .
NUSHALIM MP, WEIMANNJM, Go~owksc~ J, MARDER E: Presynaptic Control of Modulatory Fibers by Their Neural Network Targets. J Neurosci 1992, 12:27062714. Modulatory fibers were identified for pattern-generating networks and shown to be influenced by elements of the network. The modulation is an interaction between the input fibers and the neurons. 7.
PANCHINW, PERR~NSRJ, ROBERTSA: The Action of Acetylcholine on the Locomotor Central Pattern Generator for Swimming in Xenopus Embryos. J E.Q Biol 1991, 161:527-531.
8. .
RAMIREZJ-M, PEARSON KG: Octopaminergic Modulation of Intemeurons in the Flight System of the Locust. J Neuro pbysiol 1991, 66:1522-1537. Octopamine is shown to produce intrinsic bursting in identified neurons of the locust Ilight pattern generator. The characteristics of the cellular changes are similar to those observed during natural Ilight activity. 9.
MEYRANDP, WEIMANNJM, MARDER E: Multiple AxonaI Spike Initiation Zones in a Motor Neuron: Serotonin Activation. J Neurwzi 1992, 12:2803-2812.
10.
THOMPSON KJ, CALABRESERI,: FMRFamide Effects on Membrane Properties of Heart Cells Isolated from the Leech, Hirudo medicinalis. J Neuropbysiol 1992, 67:28&29l.
11. .
ALFORDS, GRILLNERS: The Involvement of GABAu Receptors and Coupled G-Proteins in Spinal GABAergic Presynaptic Inhibition. J Neurosci 1991, 11:37183726. A novel mechanism for decreasing the effectiveness of the presynaptic terminal by activation of GABAn receptors and G proteins in the lamprey spinal cord. 12.
QUINLANEM, MURPHYAD: Glutamate as a Putative Neurotransmitter in the BuccaI Central Pattern Generator of HeIisoma trivolvis. J Neurophysiol 1991, &:1264--l 271.
13.
TAVW K, CHIBA C: Mechanisms Underlying Burst Generation of the Pyloric Muscle in the Mantis Shrimp, Squilla oratooria. J Comp Pbysiol [A] 1991, 169~737-750.
14. .
TIEKNEYAJ, HARRIS-WARRICK Rh4: Physiological Role of the Transient Potassium Current in the Pyloric Circuit of the Lobster Stomatogastric Ganglion. J Neuropbysiol 1992, 67:59!+609. The transient I, current in neurons generating the pyloric rhythm was blocked with 4~aminopyridine. The ongoing pyloric rhythm was decreased by 20%, and there were increases in burst and spike fre quency. In addition, the phase of follower cells was altered relative to pacemaker bursts. The authors suggest that the 4-aminopyridine~sensitive current may represent a distinct channel subtype. 15. .
Gm3wAsc~
J, GUARDER E: Ionic Currents of the Lateral Pyloric Neuron of the Stomatogastric Ganglion of the Crab. J Neuroplysiol 1992, 67:318331. A definitive study of inward and outward currents .in one neuron of the crab pyloric CPG. 16.
ANGSTADT JD, FRIESENWO: Synchronized
in Leech Neurons Induced by Calcium .I Neuropbysiol 1991, 66:185&1873. 17.
Oscillatory Activity Channel Blockers.
PARSONS TD, SALZBERClBM, O~AID AL, RACCIiL&BEHLlNGF,
KIEINFEID D: Long-Term Optical Recording of of Electrical Activity in Ensembles of Cultured Neurons. J Neuropbysiol 1991, 66:31&333. 18.
Patterns Aplysia
KEIFEKJ, HOUK JC: Activity-Dependent Uptake of SuIforhodamine Labels Neural Circuits Engaged in Motor Pattern Generation In Vitro. J Neurophysiol 1991, 4:1-21.
generation
Selverston
19. SW AA, ABBOTT LF, MARDER E: Artificial Electrical Synapses . in Oscillatory Networks. J Neuropbysiol 1992, 67:1691&1694. An electronic circuit was used to couple stomatogastric neurons in culture. The interplay between electrical and chemical synapses could be studied with this interesting technique. 20.
GREERJJ, SMITHJC, FELDMANJL: Respiratory and Locomotor Patterns Generated in the Fetal Rat Brain Stem-Spinal Cord In Vitro. J Neurcpbysiol 1992, 67~996999.
21.
DI PA~QUALEE, MONTEAU R, HIIAIRE G: In Vitro Study of Central Respiratory-Like Activity of the Fetal Rat. Eap Brain Kes 1992, 89~459464.
22.
LINDSEY BG, HERNANDEZYM, MORRJS KF, SHANNON R: Func-
tional Connectivity Between with Respiratory-Modulated 1992, 67:890-904. 23.
Brain Stem Midline Neurons Firing Rates. J Neuropbysiol
LINDSEY BG, HERNANDEZ YM, MORRISKF, SHANNONR, GERSTEIN
GL: Respiratory-Related Neural Assemblies in Stem Midline. J Neuropbysiol 1992, 67:905-922. 24.
the
Brain
LINDSEY BG, HERNANDEZ YM, Moms KF, SHANNONR, GER~TEIN GL: Dynamic Recontiguration of Brain Stem Neural Assemblies: Respiratory Phase-Dependent Synchrony Versus Modulation of Firing Rates. J Neuropbysiol 1992, 67:923-930.
PEARSONKG, ROSSIGNOLS: Fictive Motor Patterns in Chronic 25. . Spinal Cats. J Neuropbysiol 1991, 66:18741887. Three different fictive motor patterns were found to correspond to three similar behaviors in chronic spinal cats. 26.
SYED NI, the Pond and the [A] 1991,
27.
SYEX NI, WINLOWW: Respiratory Behavior in the Pond Snail Lymnaea stagnalis. II. Neural Elements of the Central Pattern Generator (CPG). J Comp Pbysiol /A] 1991, 169:557-568.
HARRISOND, WINLOW W: Respiratory Behavior in Snail Lymnaea stagnalis I. Behavioral Analysis Identification of Motor Neurons. J Comp Pbysiol 169541-555.
28. .
SYED NI, WINLOW W: Coordination of Locomotor and Cardiorespiratory Networks of Lymnaea Stagnalis by a Pair of Identified Interneurones. J E_tp Biol 1991, 158:3762. A newly identified pair of interneurons was found to form a fast-acting network capable of coordinating motor patterns in the foot, body wall, heart and pneumostome. 29.
SYED NI, RIDGWAYRL, L~JKOWIAK K, B~IuOCH AGM: Transplantation and Functional Integration of an Identified Respiratory Interneuron in Lymnaea stagnalis. Neuron 1992, 8~767-774.
30.
Eu.ro’r CJH, ANDREW T: TemporaI Analysis of Snail Feeding Rhythms: a Three-Phase Relaxation Oscillator. J Exp Biol 1991, 157:391&408.
31.
ARSHAVSKYYI, DEUACINA TG, ORLOVSKY GN, PANCHIN W, POPOVA LB: Interneurones Mediating the Escape Reaction of the Marine Molhtsc Clione limacina. J Exp Biol 1992, 164307-314.
32.
YOSHIDA M, KORAYA~HIM: Identified Neurones the Control of Rhythmic BuccaI Motor Activity Achatina fulica. J Exp Biol 1992, 164:117-133.
33.
BARTHEJ-Y, REVENGIJT M, CLARACF: The Swimmeret and its Relationships with Postural and Locomotor in the Isolated Nervous System of the Crayfish barus Clark. II. J Ez@ Bioi 1991, 1~205-226.
Involved in in the Snail Rhythm Activity Procam-
KEZER E, MOUIJNSM: Humorai Induction of PyIoric Rhythmic Output in Lobster Stomatogastric Ganglion: In Vivo and In Vitro Studies. J Exp Biol 1992, 163:20!+230. . . An elegant demonstration that the pyloric CPG in the European lobster, Jasus, is modulated by both neuronal and hormonal substances in parallel, but that either one alone is able to maintain the rhythm.
34. ..
35. .
MATSUSHIMA T, GRILI~R S: Local Serotonergic Modulation of Calcium-Dependent Potassium Channels Control Intersegmental Coordination in the Lamprey Spinal Cord. J Neuro p&iol 1992, 67:1683-1690.
779
780
Neural
control
A study of the neural mechanisms underlying the effect of serotonin on intersegmenral coordination. The authors show that by aikcting Ca* + dependent K+ conductances, serotonin can control the phase lag value from rostrocaudal to caudorostrai, corresponding to a reversal in direction of swimming. A, PEARSON KG: Adaptive Modifications Plight System of the Locust After the Removal Proprioceptors. J Exp Biol 1991, 157~313333.
H, ISHIZUKA S: Chaotic Nature of Bursting Discharges in the Oncbidium Pacemaker Neuron. J kr Biol 1992, 156:269-291. At certain firing levels, a three-dimensional phase space reconstruction of the trajectory of irregular discharges resembles that of a strange attractor A one~dimensional Poincare map shows features of chaotic dynamics. A neuron with two fast and two slow channels replicates the authors’ experimental results quite well. 40. .
HAYASHI
COLDING-JORGENSEN M,
36.
BU~CHGES
37.
ELSON RC, SIIAR KT, BUSH BMH: Identified Proprioceptive Afferents and Motor Rhythm Entrabunent in the Crayfish Walking System. J Neuropbysiol 1992, 67:53&546.
41.
38.
BUCHHOLTZ F, G~LOWASCH J, EPSTEIN IR, MARDER E: Mathematical Model of an Identified Stomatogastric Ganglion Neuron. J Neuropbysiol 1992, 67332340.
42.
39.
C&IDWA.SCH
in the of Wing
J, BUCHHOLTI. F, EPSTEIN IR, MARDER E: Contribution of Individual Ionic Currents to Activity of a Model Stomatogastric Ganglion Neuron. J Neuropbysioll992, 67:341-349.
MALXEN HO,
BODHOIDT
B, MOSEKILDE
Minimal Model for Ca*+-Dependent Oscillations in Excitable Cells. J Theor Biol 1992, 156:309326.
E:
BRODIN
L, TIWEN HGC, LUSNER A, WALEN P, EKEBERG 0, Computer Simulations of N-Methyl-D-Aspartate Receptor-Induced Membrane Properties in a Neuron Model. J Neuropbysiol 1991, 66:473-484.
GFULLNER S:
AI Selverston, Department of Biology 0322, University of California, San Diego, Ia Jolla, California 92093, USA.
of California,
The study of rhythmic by preparations Using
these
modified
motor pattern generation
in which
cellular
preparations,
basic
in order
to be answered.
to generate The action
and
channel
proteins
and
behavior.
There
simulations,
San Diego,
can
and circuit questions a large
continues
has been
a
link
circuits
of patterns on second
between
a large increase
and their usefulness
can be bridged.
such as how
number
in the
as a way of studying
USA
to be dominated
mechanisms
of neuromodulators provide
California,
have
can
be
begun
messengers
molecular
studies
use of computer pattern
generation
is growing.
Current
Opinion
in Neurobiology
Introduction Rhythmic motor patterns are generated by the interactions of neurons within the central nervous systems of all animals. The properties of the individual neurons, as well as the synaptic networks they form, are the basis for systematically investigating pattern generation. It has become clear over the past few years that as a result of changing chemical factors in the immediate environment, the electrical and synaptic interactions between neurons, as well as their biophysical properties, are in a highly fluid state. Circuits can undergo major functional reorganization as a result of exposure to peptides, amines and other substances. These neuromodulators can be delivered via the bloodstream or more rapidly via synaptic terminals within the pattern generating circuits. The determination of the precise cellular mechanisms involved in producing changes in motor patterns has been an especially active area of research over the past year. It is clear that neuromodulators provide enormous flexibility to central pattern generating (CPG) circuits, suggesting that different motor patterns do not necessarily require separate circuits, but can be fashioned out of one circuit by altering cellular and synaptic properties. In addition to the neuromodulation of pattern generators, there has been progress in the development of new preparations, particularly fetal and juvenile rat spinal cords that are capable of rhythm generation. The use of such preparations, along with the tremendous progress made using the lamprey and Xenopus preparations, suggests that the cellular analysis of more complicated central nervous systems may now be possible. There has been a continued elucidation of the role sensory feedback plays in the generation of motor patterns, and several modeling studies can be used to help understand the many simultaneous non-linear computations performed by CPGs.
1992, 2~776-780
Several reviews have appeared which may be of interest to workers in the motor pattern generating field. One fairly large review of the principles of organization of motor systems proposes a mathematical theory of opti mal motor control that can be applied to many aspects of rhythmic patterns [ 11, as well as other kinds of brain function. Another reviews the role of neuromodulators in invertebrates [2]. Detailed accounts of the current state of knowledge pertaining to the crustacean stomatogastric system [3**] and the lamprey locomotory system [4] are also available. For a general review see [5].
Circuit-level
effects of neuromodulators
In the stomatogastric system, neuromodulators have already been shown to have different kinds of actions. They can move one or more neurons from one CPG to another. They can take neurons from different CPGs and combine them to form a ‘blended’ circuit, or they can combine elements in a way that produces an entirely de nouo circuit and a concomitant de nouo motor pattern (see [3-1). Previous work has shown that neuromodulatoty fibers from other ganglia in the central nervous system enter the stomatogastric ganglion via the stomatogastric nerve and presumably release modulators into the neuropile in a paracrine fashion. In a new study [ 6.1, intracellular recordings were made from the modulatory fibers close enough to the ganglion to record synaptic potentials from identified stomatogastric neurons. The firing pattern of the modulatory neuron was actually shaped by the cyclic activity of the gastric and pyloric neurons, suggesting for the first time that fibers that modulate the pattern are actually inlluenced by the pattern-generating network.
Abbreviations CPC-central 776
pattern
generator;
CABA--y-aminobutyric
@ Current
Biology
acid; NMDA-N-methyl-o-aspartate.
Ltd ISSN 0959-4388
Pattern generation
Acetylcholine, which acts both as a conventional transmitter as well as a neuromodulator in the stomatogastric system, was also shown to have strong effects on spontaneous episodes of fictive activity in Xenopus embryos 171. In both preparations [6*,7] the effects could be blocked with muscarinic antagonists.
Cellular-level
effects of neuromodulators
The action of neuromodulators occurs as a result of their binding affinity to particular neurons. Once bound, the modulators, acting via G proteins, appear to increase the concentration of specific second messengers. This increase in second messenger concentration can activate a protein kinase, which in turn leads to the phosphorylation of a receptor, channel or another protein. This alters the properties of the cell or synapse in a characteristic way and leads eventually to the generation of a new or altered pattern. There is a great deal of interest in examining the cellular mechanisms involved, and last year witnessed many excellent papers on this subject. New actions have been described for octopamine effects on interneurons of the insect ilight motor system [8-l, and serotonin effects on peripheral axons of the crab, Cancer borealis, where the modulator causes the initiation of peripheral spikes, which then propagate antidromically [9]. Octopamine produces intrinsic bursting in some neurons that is similar to that seen during wind-induced flight activity, suggesting that intrinsic bursting plays an important role in generating the normal flight motor pattern. Neuromodulatory receptor activation has been shown to have a variety of physiological effects in many different motor systems [10,110,12,13]. The peptide FMRFamide was shown to have effects on dissociated leech heart muscle cells [lo]. Superfusion of lo-6M FMRPamide elicits a persistent inward current with a Na+ component, and in addition modulates both voltage-dependent Ca2+ and K+ currents. In pyloric muscle fibers of the mantis shrimp, Squilla oratoria, it was reported that like some other crustacean stomatogastric preparations, the fibers themselves had endogenous bursting properties [ 131. These fibers use voltage- and time-dependent conductances to Ca2 + and Nat, as well as a Ca2+ -activated K+ conductance to generate the oscillatory potentials, and any of these are possible targets for modulatory substances. Activation of y-aminobutyric acid (GAI3A)n receptors in the lamprey spinal cord leads to a decrease in axonal membrane resistance and a reduction in spike duration [11-l. The effects of phaclofen, a GABAn antagonist, are mimicked by pretreatment of the spinal cord with picrotoxin. As this inactivates a certain class of G proteins, it seems likely that the receptor-mediated effects are initiated via a presynaptic population of picrotoxin-sensitive G proteins. In addition to studying modulatory effects on single cells, more is being learned about the properties of ionic than
Selverston
nels in neurons of CPGs. This information is important in providing a baseline so that phosphorylation-induced conductance changes can be measured. One excellent example of such a study concerns the role of the transient K+ current, I,, in determining cycle frequency and phasing of neurons in the pyloric rhythm in the stomatogastric ganglion [14*]. Using 4-aminopyridine to reduce I*, it was shown that the cycle period could be reduced by approximately 20 %, and that the phasing of follower cells altered relative to the onset of pacemaker activity. Voltage-clamp experiments indicated that pyloric cells differed in the amount of IA current they could express, and this could be responsible for the different rates of recovery from inhibition. A detailed description of currents in the lateral pyloric cell of the pyloric rhythm CPG has shown it to contain at least three inward and three outward currents [ 15.1. Neurons from leech ganglia can be made to burst by blocking intrinsic Cal+ currents [161; however, the role of these currents under normal conditions is not yet clear.
Analysis
of pattern generators
Reports of several new methods applicable to the study of neuronal pattern generators have become available during the past year. The use of optical recordings combined with conventional intracellular recordings will likely shed some light on the synaptic relationships of Aplysia neurons in culture [ 171. It is now possible to study the spatial patterns of neural activity associated with motor pattern generation in in vitro brainstem-cerebellum preparations with the fluorescent dye, sulforhodamine-I01 [ 181, and an electronic circuit can be used to artificially couple stomatogastric neurons in culture [19-l. Two reports have appeared demonstrating the use of a fetal rat brain-spinal cord preparation for use in studying the ontogeny of rhythmic respiratory discharges at the circuit and cellular level [ 20,211. A detailed statistical analysis of respiratory-related neurons along the brainstem midline indirectly demonstrated evidence for inhibitoty and excitatory functional connections between them, but did not provide any information on circuitry [22-241, Similarly, a study of fictive motor patterns in early and late spinal cats showed that different patterns could be elicited in chronic animals depending on the method of stimulation, but little about underlying mechanisms was presenred [25*]. Molluscan generators continue to be of importance in elucidating mechanisms of pattern formation, and the past year has seen a description of the neural basis for the somewhat irregular respiratory behavior in Lymnaea [ 26,271. The first paper describes the behavior and identifies the motor neurons. It also suggests a role for sensory feedback in the system. The second paper describes the pattern-generating circuit, the basic synaptic connectivity, and outlines a scheme for how the network operates. One pair of interneurons has been described that not only synapse onto respiratory interneurons, but
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control
are also inhibited by some of them [28-l. This same pair of neurons also appears to coordinate locomotor and cardiorespiratory networks, making them extremely important in the control of cyclic motor patterns. In one particularly interesting study, an interneuron from the respiratory CPG was removed from the brain leading to a deficit [ 291, Grafting an identified neuron from an other snail to the operated brain corrected the deficit by connecting to other relevant neurons in the CPG circuit. A temporal analysis of the feeding rhythm in Lymnaea has shown that, like many cyclic behaviors, there is a frequency-dependent variable phase for some neurons and a frequency-independent fixed phase for others [30]. Command-like neurons have been found in the buccal ganglion of the snail Achatina filica, which appear to be involved in the control of rhythmic activities [31]. Similar fibers, controlling the escape reaction in the marine mollusc Clione, have been recently described [32].
Two new crustacean papers have appeared, one dealing with the relationship between the swimmeret rhythm postural and locomotor activity [33], and another very interesting paper on long-term electromyograph recording of the pyloric rhythm in vivo [34*-l. Recent studies suggest that descending modulatory input entering the stomatogastric ganglion from the commissural ganglia via the stomatogastric nerve is both necessary and sufficient for the pyloric rhythm to exist. When the stomatogastric nerve is severed or blocked in in vitro preparations, the pyloric rhythm ceases immediately. When the nerve is severed in vivo, the pyloric rhythm is unaffected initially and continues for many days. Application of lobster blood to an in vitro preparation restores the rhythm to a ganglion whose stomatogastric nerve has been cut or blocked, suggesting that modulators can be delivered humorally as well as via the nerve. Intersegmental coordination of oscillators has been reported to occur via serotonergic modulation in the lam prey [35*]. The spinal serotonin system can depress Ca2 + -dependent K + channels in some network neurons resulting in a change in phase from a rostrocaudal to a caudorostral direction, corresponding to a reversal in the direction of swimming. The role of sensory feedback in pattern generation has been described for the locust [ 361 and the crayfish [37]
Another six-conductance model has been written for the lateral pyloric cell of the pyloric CPG [38,39]. Equations were given to describe its voltage-, time- and Ca2+-dependence. The model is supposed to duplicate the steady state biological I-V curves for this cell, but major errors have been found for values of the transient IA currents, and they had to be significantly altered before the results could be duplicated. A modified Hodgkin-Huxley model of an Onchidium pacemaker neuron reproduced tonic firing, and both irregular and regular bursting activity that was very similar to the biological neuron [40*]. An andysis of the irregular bursting showed features of chaotic dynamics, and the steady state I-V curve showed a negative slope resistance. An even simpler two-conductance model for K+ and Ca2+ was able to switch between five separate stable states [ 411. AS part of a larger study, models of N-methyl-D-aspartate (NMDA)-induced oscillatory activity in lamprey spinal cord were able to account for the effects of bath-applied NMDA [ 421.
Conclusion The new data reinforce a basic shift in thinking away from neurons with rigidly fixed properties incorporated into hard-wired pattern generating circuitry, toward the idea of increased flexibility. While this brief review has con centrated on motor patterns, the formation of patterned activity is the common currency of all neural functions including sensory and central processing. We can begin to see from the analysis of small neural circuits that by providing networks with a rich set of connections, neuromodulators (which include almost all conventional neurotransmitters) can sculpt out specialized circuitry for specific purposes. Neural modeling will be an important method for conceptualizing this process. The absence of physiological techniques for dealing with the enormous complexity of mammalian pattern generators at this time precludes the analysis of cellular interactions with the high degree of resolution seen in invertebrates. For these large networks, modeling, even with many as sumptions, may be the only way to proceed.
References Modeling
of pattern generators
Computer simulation continues to provide a useful tool for those wishing to understand rhythmic motor patterns at a more theoretical level. Usually the most interesting results have occurred when the models are well grounded in biological reality. For example, the work on the production of oscillatory activity in leech neurons by blocking Ca2+ channels has been incorporated into a conceptual model [ 161. The model, which uses neurons with six separate conductances, suggests a plausible mechanism for the induction of oscillatory activity that is consistant with the physiological data.
and recommended
Papers of particular interest, published view, have been highlighted as: . of special interest .. of outstanding interest 1.
2.
reading
within the annual period of re-
BAW KV, SHKVIANSKY YF: Principles of Organization of Neural Systems Controlling Automatic Movements in Animals. Prog Neurobiol 1992, 39~455112. HARRIS-WARRICK F&l,
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for Behavior.
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E: Modulation of Neural NetR~L’Neurosci 1991, 143957.
MARDER E, SELVERSTON Al, MOUUNS M: Dynamic Biological Networks: the Stomatogastric Nervous System. Cambridge, Massachusetts: MIT Press; 1992. The latest compendium of work on the crustacean stomatogastric system from the molecular to the behavioral level.
3. ..
HARRIS-WARRICK RM,
MARDER
Pattern
4.
GRIUNER S, MATSUSHIMA T: The Neural Network Underlying Locomotion in Lamprey-Synaptic and Cellular Mechanisms. Neuron 1991, 7:1-15.
5.
AR?HAVSKY YI, GRILLNER S, OKLOVSKYGN, PANCHINYV: Central Generators and the Spatio-Temporal Pattern of Movements. In ne Development of Timing Control and Temporal Organization in Coordinated Action. Edited by Fagard J, Wolff PH. New York: Elsevier; 933115.
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NUSHALIM MP, WEIMANNJM, Go~owksc~ J, MARDER E: Presynaptic Control of Modulatory Fibers by Their Neural Network Targets. J Neurosci 1992, 12:27062714. Modulatory fibers were identified for pattern-generating networks and shown to be influenced by elements of the network. The modulation is an interaction between the input fibers and the neurons. 7.
PANCHINW, PERR~NSRJ, ROBERTSA: The Action of Acetylcholine on the Locomotor Central Pattern Generator for Swimming in Xenopus Embryos. J E.Q Biol 1991, 161:527-531.
8. .
RAMIREZJ-M, PEARSON KG: Octopaminergic Modulation of Intemeurons in the Flight System of the Locust. J Neuro pbysiol 1991, 66:1522-1537. Octopamine is shown to produce intrinsic bursting in identified neurons of the locust Ilight pattern generator. The characteristics of the cellular changes are similar to those observed during natural Ilight activity. 9.
MEYRANDP, WEIMANNJM, MARDER E: Multiple AxonaI Spike Initiation Zones in a Motor Neuron: Serotonin Activation. J Neurwzi 1992, 12:2803-2812.
10.
THOMPSON KJ, CALABRESERI,: FMRFamide Effects on Membrane Properties of Heart Cells Isolated from the Leech, Hirudo medicinalis. J Neuropbysiol 1992, 67:28&29l.
11. .
ALFORDS, GRILLNERS: The Involvement of GABAu Receptors and Coupled G-Proteins in Spinal GABAergic Presynaptic Inhibition. J Neurosci 1991, 11:37183726. A novel mechanism for decreasing the effectiveness of the presynaptic terminal by activation of GABAn receptors and G proteins in the lamprey spinal cord. 12.
QUINLANEM, MURPHYAD: Glutamate as a Putative Neurotransmitter in the BuccaI Central Pattern Generator of HeIisoma trivolvis. J Neurophysiol 1991, &:1264--l 271.
13.
TAVW K, CHIBA C: Mechanisms Underlying Burst Generation of the Pyloric Muscle in the Mantis Shrimp, Squilla oratooria. J Comp Pbysiol [A] 1991, 169~737-750.
14. .
TIEKNEYAJ, HARRIS-WARRICK Rh4: Physiological Role of the Transient Potassium Current in the Pyloric Circuit of the Lobster Stomatogastric Ganglion. J Neuropbysiol 1992, 67:59!+609. The transient I, current in neurons generating the pyloric rhythm was blocked with 4~aminopyridine. The ongoing pyloric rhythm was decreased by 20%, and there were increases in burst and spike fre quency. In addition, the phase of follower cells was altered relative to pacemaker bursts. The authors suggest that the 4-aminopyridine~sensitive current may represent a distinct channel subtype. 15. .
Gm3wAsc~
J, GUARDER E: Ionic Currents of the Lateral Pyloric Neuron of the Stomatogastric Ganglion of the Crab. J Neuroplysiol 1992, 67:318331. A definitive study of inward and outward currents .in one neuron of the crab pyloric CPG. 16.
ANGSTADT JD, FRIESENWO: Synchronized
in Leech Neurons Induced by Calcium .I Neuropbysiol 1991, 66:185&1873. 17.
Oscillatory Activity Channel Blockers.
PARSONS TD, SALZBERClBM, O~AID AL, RACCIiL&BEHLlNGF,
KIEINFEID D: Long-Term Optical Recording of of Electrical Activity in Ensembles of Cultured Neurons. J Neuropbysiol 1991, 66:31&333. 18.
Patterns Aplysia
KEIFEKJ, HOUK JC: Activity-Dependent Uptake of SuIforhodamine Labels Neural Circuits Engaged in Motor Pattern Generation In Vitro. J Neurophysiol 1991, 4:1-21.
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Selverston
19. SW AA, ABBOTT LF, MARDER E: Artificial Electrical Synapses . in Oscillatory Networks. J Neuropbysiol 1992, 67:1691&1694. An electronic circuit was used to couple stomatogastric neurons in culture. The interplay between electrical and chemical synapses could be studied with this interesting technique. 20.
GREERJJ, SMITHJC, FELDMANJL: Respiratory and Locomotor Patterns Generated in the Fetal Rat Brain Stem-Spinal Cord In Vitro. J Neurcpbysiol 1992, 67~996999.
21.
DI PA~QUALEE, MONTEAU R, HIIAIRE G: In Vitro Study of Central Respiratory-Like Activity of the Fetal Rat. Eap Brain Kes 1992, 89~459464.
22.
LINDSEY BG, HERNANDEZYM, MORRJS KF, SHANNON R: Func-
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LINDSEY BG, HERNANDEZ YM, MORRISKF, SHANNONR, GERSTEIN
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the
Brain
LINDSEY BG, HERNANDEZ YM, Moms KF, SHANNONR, GER~TEIN GL: Dynamic Recontiguration of Brain Stem Neural Assemblies: Respiratory Phase-Dependent Synchrony Versus Modulation of Firing Rates. J Neuropbysiol 1992, 67:923-930.
PEARSONKG, ROSSIGNOLS: Fictive Motor Patterns in Chronic 25. . Spinal Cats. J Neuropbysiol 1991, 66:18741887. Three different fictive motor patterns were found to correspond to three similar behaviors in chronic spinal cats. 26.
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28. .
SYED NI, WINLOW W: Coordination of Locomotor and Cardiorespiratory Networks of Lymnaea Stagnalis by a Pair of Identified Interneurones. J E_tp Biol 1991, 158:3762. A newly identified pair of interneurons was found to form a fast-acting network capable of coordinating motor patterns in the foot, body wall, heart and pneumostome. 29.
SYED NI, RIDGWAYRL, L~JKOWIAK K, B~IuOCH AGM: Transplantation and Functional Integration of an Identified Respiratory Interneuron in Lymnaea stagnalis. Neuron 1992, 8~767-774.
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ARSHAVSKYYI, DEUACINA TG, ORLOVSKY GN, PANCHIN W, POPOVA LB: Interneurones Mediating the Escape Reaction of the Marine Molhtsc Clione limacina. J Exp Biol 1992, 164307-314.
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BARTHEJ-Y, REVENGIJT M, CLARACF: The Swimmeret and its Relationships with Postural and Locomotor in the Isolated Nervous System of the Crayfish barus Clark. II. J Ez@ Bioi 1991, 1~205-226.
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KEZER E, MOUIJNSM: Humorai Induction of PyIoric Rhythmic Output in Lobster Stomatogastric Ganglion: In Vivo and In Vitro Studies. J Exp Biol 1992, 163:20!+230. . . An elegant demonstration that the pyloric CPG in the European lobster, Jasus, is modulated by both neuronal and hormonal substances in parallel, but that either one alone is able to maintain the rhythm.
34. ..
35. .
MATSUSHIMA T, GRILI~R S: Local Serotonergic Modulation of Calcium-Dependent Potassium Channels Control Intersegmental Coordination in the Lamprey Spinal Cord. J Neuro p&iol 1992, 67:1683-1690.
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A study of the neural mechanisms underlying the effect of serotonin on intersegmenral coordination. The authors show that by aikcting Ca* + dependent K+ conductances, serotonin can control the phase lag value from rostrocaudal to caudorostrai, corresponding to a reversal in direction of swimming. A, PEARSON KG: Adaptive Modifications Plight System of the Locust After the Removal Proprioceptors. J Exp Biol 1991, 157~313333.
H, ISHIZUKA S: Chaotic Nature of Bursting Discharges in the Oncbidium Pacemaker Neuron. J kr Biol 1992, 156:269-291. At certain firing levels, a three-dimensional phase space reconstruction of the trajectory of irregular discharges resembles that of a strange attractor A one~dimensional Poincare map shows features of chaotic dynamics. A neuron with two fast and two slow channels replicates the authors’ experimental results quite well. 40. .
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COLDING-JORGENSEN M,
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ELSON RC, SIIAR KT, BUSH BMH: Identified Proprioceptive Afferents and Motor Rhythm Entrabunent in the Crayfish Walking System. J Neuropbysiol 1992, 67:53&546.
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GFULLNER S:
AI Selverston, Department of Biology 0322, University of California, San Diego, Ia Jolla, California 92093, USA.
