Postembryonic development of centrally generated flight motor patterns in the hawkmoth, Manduca sexta

This study analyses the maturation of centrally generated flight motor patterns during metamorphosis of Manduca sexta. Bath application of the octopamine agonist chlordimeform to the isolated central nervous system of adult moths reliably induces fictive flight patterns in wing depressor and elevator motoneurons. Pattern maturation is investigated by chlordimeform application at different developmental stages. Chlordimeform also induces motor patterns in larval ganglia, which differ from fictive flight, indicating that in larvae and adults, octopamine affects different networks. First changes in motoneuron activity occur at the pupal stage P10. Rhythmic motor output is induced in depressor, but not in elevator motoneurons at P12. Adult-like fictive flight activity in motoneurons is observed at P16 and increases in speed and precision until emergence 2 days later. Pharmacological block of chloride channels with picrotoxin also induces fictive flight in adults, suggesting that the pattern-generating network can be activated by the removal of inhibition, and that proper network function does not rely on GABA_A receptors. Our results suggest that the flight pattern-generating network becomes gradually established between P12 and P16, and is further refined until adulthood. These findings are discussed in the context of known physiological and structural CNS development during Manduca metamorphosis.     

Steroid hormone activation of wandering in the isolated nervous system of Manduca sexta

Steroid hormones modulate motor circuits in both vertebrates and invertebrates. The insect Manduca sexta, with its well-characterized developmental and endocrinological history, is a useful model system in which to study these effects. Wandering is a stage-specific locomotor behavior triggered by the steroid hormone 20-hydroxyecdysone (20E), consisting of crawling and burrowing movements as the animal searches for a pupation site. This study was undertaken to determine whether the wandering motor pattern is activated by direct action of 20E on the CNS. 20E acts on the isolated larval nervous system to induce a fictive motor pattern showing features of crawling and burrowing. The latency of the response to 20E is long, suggestive of a genomic mechanism of action. The abdominal motoneurons or segmental pattern generating circuits are unlikely to be the primary targets of 20E action in inducing fictive wandering. Exposure of the segmental ganglia alone to hormone did not evoke fictive wandering. Therefore, as suggested by an earlier study, the likely site of 20E action is within the brain.     

Dominance of local sensory signals over inter-segmental effects in a motor system: experiments

Legged locomotion requires that information local to one leg, and inter-segmental signals coming from the other legs are processed appropriately to establish a coordinated walking pattern. However, very little is known about the relative importance of local and inter-segmental signals when they converge upon the central pattern generators (CPGs) of different leg joints. We investigated this question on the CPG of the middle leg coxa?Ctrochanter (CTr)-joint of the stick insect which is responsible for lifting and lowering the leg. We used a semi-intact preparation with an intact front leg stepping on a treadmill, and simultaneously stimulated load sensors of the middle leg. We found that middle leg load signals induce bursts in the middle leg depressor motoneurons (MNs). The same local load signals could also elicit rhythmic activity in the CPG of the middle leg CTr-joint when the stimulation of middle leg load sensors coincided with front leg stepping. However, the influence of front leg stepping was generally weak such that front leg stepping alone was only rarely accompanied by switching between middle leg levator and depressor MN activity. We therefore conclude that the impact of the local sensory signals on the levator?Cdepressor motor system is stronger than the inter-segmental influence through front leg stepping.     

Flexibility in the patterning and control of axial locomotor networks in lamprey

In lower vertebrates, locomotor burst generators for axial muscles generally produce unitary bursts that alternate between the two sides of the body. In lamprey, a lower vertebrate, locomotor activity in the axial ventral roots of the isolated spinal cord can exhibit flexibility in the timings of bursts to dorsally-located myotomal muscle fibers versus ventrally-located myotomal muscle fibers. These episodes of decreased synchrony can occur spontaneously, especially in the rostral spinal cord where the propagating body waves of swimming originate. Application of serotonin, an endogenous spinal neurotransmitter known to presynaptically inhibit excitatory synapses in lamprey, can promote decreased synchrony of dorsal-ventral bursting. These observations suggest the possible existence of dorsal and ventral locomotor networks with modifiable coupling strength between them. Intracellular recordings of motoneurons during locomotor activity provide some support for this model. Pairs of motoneurons innervating myotomal muscle fibers of similar ipsilateral dorsoventral location tend to have higher correlations of fast synaptic activity during fictive locomotion than do pairs of motoneurons innervating myotomes of different ipsilateral dorsoventral locations, suggesting their control by different populations of premotor interneurons. Further, these different motoneuron pools receive different patterns of excitatory and inhibitory inputs from individual reticulospinal neurons, conveyed in part by different sets of premotor interneurons. Perhaps, then, the locomotor network of the lamprey is not simply a unitary burst generator on each side of the spinal cord that activates all ipsilateral body muscles simultaneously. Instead, the burst generator on each side may comprise at least two coupled burst generators, one controlling motoneurons innervating dorsal body muscles and one controlling motoneurons innervating ventral body muscles. The coupling strength between these two ipsilateral burst generators may be modifiable and weakening when greater swimming maneuverability is required. Variable coupling of intrasegmental burst generators in the lamprey may be a precursor to the variable coupling of burst generators observed in the control of locomotion in the joints of limbed vertebrates.     

Sensorimotor pathways involved in interjoint reflex action of an insect leg

Coordination of motor output between leg joints is crucial for the generation of posture and active movements in multijointed appendages of legged organisms. We investigated in the stick insect the information flow between the middle leg femoral chordotonal organ (fCO), which measures position and movement in the femur-tibia (FT) joint and the motoneuron pools supplying the next proximal leg joint, the coxa-trochanteral (CT) joint. In the inactive animal, elongation of the fCO (by flexing the FT joint) induced a depolarization in eight of nine levator trochanteris motoneurons, with a suprathreshold activation of one to three motoneurons. Motoneurons of the depressor trochanteris muscle were inhibited by fCO elongation. Relaxation signals, i.e., extension of the FT joint, activated both levator and depressor motoneurons; i.e., both antagonistic muscles were coactivated. Monosynaptic as well as polysynaptic pathways contribute to interjoint reflex actions in the stick insect leg. fCO afferents were found to induce short latency EPSPs in levator motoneurons, providing evidence for direct connections between fCO afferents and levator motoneurons. In addition, neuronal pathways via intercalated interneurons were identified that transmit sensory information from the fCO onto levator and/or depressor motoneurons. Finally, we describe two kinds of alterations in interjoint reflex action: (a) With repetitive sensory stimulation, this interjoint reflex action shows a habituation-like decrease in strength. (b) In the actively moving animal, interjoint reflex action in response to fCO elongation, mimicking joint flexion, qualitatively remained the same sign, but with a marked increase in strength, indicating an increased influence of sensory signals from the FT joint onto the adjacent CT joint in the active animal. © 1997 John Wiley & Sons, Inc. J Neurobiol 33: 891–913, 1997     

Differential effects of octopamine and tyramine on the central pattern generator for <Emphasis Type="Italic">Manduca</Emphasis> flight

The biogenic amine, octopamine, modulates a variety of aspects of insect motor behavior, including direct action on the flight central pattern generator. A number of recent studies demonstrate that tyramine, the biological precursor of octopamine, also affects invertebrate locomotor behaviors, including insect flight. However, it is not clear whether the central pattern generating networks are directly affected by both amines, octopamine and tyramine. In this study, we tested whether tyramine affected the central pattern generator for flight in the moth, Manduca sexta. Fictive flight was induced in an isolated ventral nerve cord preparation by bath application of the octopamine agonist, chlordimeform, to test potential effects of tyramine on the flight central pattern generator by pharmacological manipulations. The results demonstrate that octopamine but not tyramine is sufficient to induce fictive flight in the isolated ventral nerve cord. During chlordimeform induced fictive flight, bath application of tyramine selectively increases synaptic drive to depressor motoneurons, increases the number of depressor spikes during each cycle and decreases the depressor phase. Conversely, blocking tyramine receptors selectively reduces depressor motoneuron activity, but does not affect cycle by cycle elevator motoneuron spiking. Therefore, octopamine and tyramine exert distinct effects on the flight central pattern generating network.     

Proprioceptive reflex interactions with central motor rhythms in the isolated thoracic ganglion of the shore crab

Summary Experiments were carried out on an isolated central nervous system preparation of the shore crab,Carcinus maenas, comprising the fused thoracic ganglion complex with two proprioceptors of one back leg still attached. These, the thoracic-coxal muscle receptor organ and the coxo-basal chordotonal organ, monitor movement and position of the first and second joints, respectively. Motor activity was recorded extracellularly from the central cut ends of the nerves innervating the promotor and remotor muscles of the thoracic-coxal joint, and the levator and depressor muscles of the coxal-basal joint of the same leg. Simultaneous intracellular recordings were made from central processes of individual motoneurones of each muscle.In the absence of any sensory input, the isolated ganglion exhibited rhythmic bursting in the motor nerve roots, with a slow, usually irregular cycle period of 5–50 s.Both receptor organs had both intra-joint and inter-joint effects on the rhythmically active preparation. In most cases the coxo-basal receptor organ had the greater effect.Resistance reflexes initiated by each of the joint proprioceptors were modulated by the rhythmic activity.It may be concluded that, while the isolated thoracic ganglion of the crab is capable of generating rhythmic motor output, proprioceptive feedback from the two basal joints is important in shaping the motor patterns underlying locomotion. Inappropriate reflexes which would impede active movements about these joints are modulated or reversed so as to permit and even reinforce intended locomotory movements.     

The brain can eat: Establishing the existence of a central pattern generator for feeding in third instar larvae of Drosophila virilis and Drosophila melanogaster

To establish the existence of a central pattern generator for feeding in the larval central nervous system of two Drosophila species, the gross anatomy of feeding related muscles and their innervation is described, the motor units of the muscles identified and rhythmic motor output recorded from the isolated CNS. The cibarial dilator muscles that mediate food ingestion are innervated by the frontal nerve. Their motor pathway projects from the brain through the antennal nerves, the frontal connectives and the frontal nerve junction. The mouth hook elevator and depressor system is innervated by side branches of the maxillary nerve. The motor units of the two muscle groups differ in amplitude: the elevator is always activated by a small unit, the depressor by a large one. The dorsal protractors span the cephalopharyngeal skeleton and the body wall hence mediating an extension of the CPS. These muscles are innervated by the prothoracic accessory nerve. Rhythmic motor output produced by the isolated central nervous system can simultaneously be recorded from all three nerves. The temporal pattern of the identified motor units resembles the sequence of muscle contractions deduced from natural feeding behavior and is therefore considered as fictive feeding. Phase diagrams show an almost identical fictive feeding pattern is in both species.     

Remodelling of Neuromuscular Systems During Insect Metamorphosis

During metamorphosis in the hawkmoth, Manduca sexta, the larvalthoracic legs are replaced by a new set of adult legs that includenew sensory neurons and muscles, and participate in new patternsof locomotor activity. Larval leg motoneurons persist to innervatethe new adult leg muscles, but undergo striking changes in dendriticmorphology that are regulated by the insect steroid, 20-hydroxyecdysone.In the periphery, the motor terminals regress as larval musclesdegenerate, and expand as new adult muscles form from myoblasts.Evidence obtained both in vivo and in vitro suggests that theproliferation of myoblasts during metamorphosis is dependentupon innervation.     

Generation of motor patterns for walking and flight in motoneurons supplying bifunctional muscles in the locust

In the flight system of Locusta migratoria certain muscles move a wing and a leg (bifunctional muscles) and are active during the performance of walking and flight. A preparation that allowed intracellular recordings during these behaviors was developed to analyze the activity of motoneurons supplying these and other muscles. Motoneurons innervating bifunctional muscles were active during walking and flight, whereas motoneurons innervating unifunctional flight muscles were active only during flight. Both motor patterns, walking and flight, were sometimes generated simultaneously in our preparation. In bifunctional motoneurons the two patterns were superimposed, whereas in unifunctional motoneurons only the flight motor pattern was observed. All flight interneurons we examined were either inactive or tonically inhibited during walking. All interneurons that were strongly modulated during walking were either inactive, inhibited, or only weakly modulated during flight. Anatomical investigations showed that unifunctional flight motoneurons have their main processes in the extreme dorsal region of neuropil. With the exception of the second basalar motoneurons, all bifunctional motoneurons have their processes extending more ventrally in the neuropil. Flight interneurons have their processes restricted to the dorsal neuropil. Interneurons that were rhythmically active during walking had their processes distributed more ventrally. We conclude that motoneurons innervating bifunctional muscles are active during both motor patterns, walking and flight, and that these patterns are produced by two distinct interneuronal networks. The pattern-generating network for flight appears to be located in the extreme dorsal regions of the thoracic ganglia, and the network for walking is located more ventrally.     

Respiratory bursts at the midline of the rostral medulla of the lamprey

The contribution of a rostral crossed pathway to the coordination of fictive breathing was tested in isolated brains of adult lampreys, Ichthyomyzon unicuspis. Periodic bursts of small spikes were recorded at the midline at the rostral level of the V motor nuclei. These occurred prior to bursts by respiratory motoneurons in the IX-X cranial nerve roots. The bursts at the midline could be generated in the rostral half of the medulla, since they continued after isolation of the isthmic-trigeminal region by transections. Stimulation at the rostral midline excited respiratory motoneurons monosynaptically and could entrain or reset the respiratory rhythm. Sections of the midline sparing the rostral site still permitted bilateral synchronization of respiratory bursts. Alternatively, sections of the rostral midline still allowed coordination of respiratory bursts through crossed caudal pathways, although abnormal timing patterns were observed. It is concluded that the motor pattern for respiration is partly generated and coordinated in the rostral half of the medulla of the lamprey and is transmitted to respiratory motoneurons through descending pathways.     

The post-embryonic development of cell properties and synaptic drive underlying locomotor rhythm generation in Xenopus larvae.

In the first 24 h of post-embryonic development, the motor rhythm underlying swimming in Xenopus laevis tadpoles changes from brief (ca. 7 ms) ventral root discharge in each cycle to bursts of activity lasting around 20 ms (Sillar et al. 1991). Because individual motoneurons in the spinal cord of newly hatched embryos normally fire only a single impulse per cycle, two possible changes underly the transition to motor bursts seen in larval ventral roots; desynchronization of neurons in a given ventral root which continue to fire once per cycle, or the developmental acquisition of a multiple spike capability in individual motoneurons. Here we have recorded intracellularly from ventrally positioned spinal neurons, presumed to be myotomal motoneurons, in stage 37/38 embryos and 24 h later in development in stage 42 larvae. We find that (i) larval neurons are able to fire more than one impulse per cycle of fictive swimming activity; (ii) unlike in the embryo, they generally will fire multiple impulses in response to injected depolarizing current; (iii) the synaptic drive to motoneurons during swimming increases dramatically in complexity, although it still consists of alternating phases of synaptic excitation and chloride-dependent inhibition, superimposed upon tonic synaptic depolarization. The results therefore suggest a developmental change in the membrane properties of rhythmically active neurons as a major factor in the post-embryonic development of swimming in Xenopus larvae. This change appears to occur in premotor rhythm generating interneurons as well as in the motoneurons themselves and may satisfy a demand for behavioural flexibility that allows larvae to survive in a complex and changing environment.     

Peculiarities of ultrastructural organization of the metathoracic ganglion in the locustLocusta migratoria larva

In electron microscopic study of structural organization of the thoracic ganglion of the locust larva of the 1st age (1–2 days after hatching), the data on the structure of motoneurons of the 1st nerve, basal and motor neuropil of the larva were obtained. The effector elements of the larval locust CNS are formed rather early and have the structural plan similar to that in adult insects. However, in the larval motoneurons innervating the flight muscles (longitudinal dorsal muscles, wing depressors) the clearly seen features of immaturity of these nervous elements are revealed. Study of the larval ganglion neuropil has shown that the basal neuropil is morphologically formed sufficiently completely as early as in larvae of the first days after hatching. There are shown longitudinal contacts between axons of the ventral neuropil zone, the presence of axons forming theen-passant contacts as well as the synapses with a heterogeneous set of vesicles in the presynaptic area. The presence of the great number of granular vesicles in the basal neuropil of the locust larva may indicate an important role of catecholamines in the early development of the nervous system in the locust larva.     

Cholinergic control of the walking network in the crayfish Procambarus clarkii

The output of a neuronal network results generally from both the properties of the component neurons and their synaptic relationships. This article aims at synthesizing various results obtained on the neural network generating locomotion in vitro. In the preparation used, consisting of the last three thoracic ganglia (3–5) along with motor nerves from the 5th leg ganglion to the promotor, remotor, levator and depressor muscles, motor nerve recordings generally revealed only tonic activity in several different motoneurons (MNs). However, rhythmic activity can be obtained by the use of cholinergic agents such as the oxotremorine (Oxo) superfused in the bath (5 × 10⁻⁵ M). If Oxo is pressure-ejected locally in the ganglion, it is possible, depending upon the locus where the drug is applied, to elicit a rhythmic activity restricted to a group of antagonistic MNs. To analyze how cholinergic agents are able to induce such rhythmic activity, very small volumes of drug (50–200 pl), were applied close to the recording electrode. Two types of depolarizing response occurred: a fast large amplitude depolarization (5–20 mV) and a long lasting (10 s to several minutes) low amplitude depolarization (1–3 mV). These responses persisted in the presence of TTX and Co²⁺. The transient initial depolarization is a mixed nicotinic and muscarinic voltage-independent response during which the input resistance decreases by 20 to 40%. In contrast, the long lasting component is voltage-dependent, exclusively muscarinic and associated to a 5–10% increase of input resistance due to the closing of a K⁺ conductance that is active at the resting Vm, and totally suppressed at holding potentials below −70 mV. More generally, K⁺ currents activated at resting potential are responsible for membrane potential stability. The injection of TEA, a blocker of the K⁺ currents, through the recording electrode is able to unmask plateaus above a threshold depolarization. These plateaus are TTX-sensitive but persist in the presence of Ca²⁺ channel blockers. Moreover, in 10% of TEA-filled MNs a spontaneous pacemaker activity was revealed. The organization of the locomotor network is also based upon connections between MNs and INs. Within a MN pool, connections are only loosely established, appearing to consist mainly of electrical coupling. Inhibitory synaptic connections between MNs of opposite pools are mediated by chloride channels. However, the neurotransmitter involved could be either GABA or glutamate. Therefore, at the level of a given joint, a basic rhythm occurs due to both motoneuronal membrane properties and motoneuronal connectivity. However, the coordination of all MNs of an entire leg during fictive walking activity requires the involvement of INs. Based upon these data, we propose a two-stage model of the locomotor network organization: a joint motoneuronal level and a whole leg interneuronal level.     

Monosynaptic connections mediate resistance reflex in crayfish (Procambarus clarkii) walking legs

Summary In crustacean walking legs, the coxo-basipodite chordotonal organ (CB) composed of about 50 sensory cells, evokes a resistance reflex in the levator (Lev) and depressor (Dep) muscles responsible for the movements of the coxo-basipodite joint where it is located. Mechanical stimulation of the CB strand and electrical stimulation of its sensory nerve have been performed along with systematic intracellular recordings from CB terminals (CB T) and levator (Lev) or depressor (Dep) motoneurons (MNs) in order to study their connections. Measurements of conduction times in the CB nerve demonstrated different pools of sensory fibres, the fastest of which reach the ganglion in 2.5 ms. During imposed movements to the CB strand, intracellularly recorded Lev or Dep MN display EPSPs that are correlated to spikes in the CB nerve, their delays are incompatible with a polysynaptic pathway. Systematic stimulation of the CB nerve demonstrates that about 4 to 8 CB fibres are connected with each Lev or Dep MN. Classical tests for monosynaptic connections indicate that EPSPs occurring between 3 ms and 6 ms correspond mainly to monosynaptic connections with CB T, whereas IPSPs (the latencies of which are above 12 ms) are polysynaptic. In spite of the high selectivity of the CB T onto MNs, eight simultaneous intracellular recordings of coupled CB T and MN (out of more than 300 MNs penetrated) have allowed a direct measurement of synaptic delays (less than 1 ms). The functional significance of these results is discussed in relation to the proprioceptive control of locomotor movements.Abbreviations CB Coxo-basipodite chordotonal organ - CB n CB sensory nerve - CB T CB sensory terminal - Dep depressor - Lev levator - MN motoneuron     

Comparison of identified leg motoneuron structure and function between larval and adult Manduca sexta

Persistent leg motoneurons of the moth Manduca sexta were investigated in larval and adult animals to compare their dendritic structures, intrinsic electrical properties and pattern of target innervation. The study focused on two identified motoneurons of the prothoracic leg. Despite the complete remodeling of leg muscles, the motoneurons innervated pretarsal flexor muscles in both larval and adult legs. Similarly, although the central dendrites regress and regrow, the branching pattern was similar with the exception of a prominent midline branch that was not present in the adult stage. The intrinsic electrical properties of the motoneurons differed between larval and adult stages. Larval motoneurons had significantly higher membrane input resistances and more depolarized resting membrane potentials than did motoneurons in pharate adults or adults. In all stages, one motoneuron had a low maximal firing frequency, whereas the second motoneuron, which innervated the other half of the muscle, had a high maximum firing frequency. Although the two motoneurons continued to innervate the same halves of the target muscle, their relative effects on muscular contraction were reversed during metamorphosis along with concomitant changes in intrinsic properties. Pretarsal flexor motoneurons in pharate adults (just prior to emergence) displayed properties similar to those in emerged adults. Accepted: 8 January 2000     

Locomotion in the pulmonate snail Melampus--I. The motor pattern

1. The patterned neural activity that drives muscular locomotor movements in Melampus is generated within the central nervous system. 2. In the transition from quiescent state to crawling, the pattern recorded in nerves and connectives changes from short-duration bursts in many units to the 60-100 sec cycle of events recorded during tethered crawling in the semi-intact snail. 3. Extracellularly recorded bursts and individually recognizable spikes in pedal nerves are correlated with movements that occur at each stage of the cyclically repeated crawl-steps. 4. Intracellularly recorded pedal neurons involved in locomotion receive excitatory drive, inhibitory drive, or alternating excitatory and inhibitory drive during the step cycle.     

Walking in the American cockroach: the timing of motor activity in the legs during straight walking

The timing of bursts of motor activity in extensor muscles in the coxae of pairs of legs in intact freely walking American cockroaches was studied. The timing of bursts in adjacent and non-adjacent leg pairs generally reflected the common alternating tripod gait of these insects. Detailed study of the timing further revealed two previously unreported features. (1) The timing of extensor bursts in the middle legs relative to bursts in the rear legs was more variable than it was relative to those in the front legs. This difference in variability was statistically significant for the means of bursts when all insects were considered together as well as for bursts in individual insects. An apparent difference in variability of the timing of burst starts compared to burst ends for any one leg pair was not significant. (2) There was a shift in the timing of motor bursts relative to one another when an insect walked fast such that motor bursts in the middle legs tended to lag farther behind those in the front legs, and those in the rear legs tended to lag farther behind those in the middle legs compared to the timing during slow walking. This shift was apparent in both burst starts and burst ends, although more obvious in the former. It occurred in both ipsilateral and contralateral leg pairs, and in both the mean data and the data for individual insects. The implications of these characteristics of the timing data are discussed in terms of the neural organization of insect walking.     

The antennal motor system of the stick insect Carausius morosus: anatomy and antennal movement pattern during walking

The stick insect Carausius morosus continuously moves its antennae during locomotion. Active antennal movements may reflect employment of antennae as tactile probes. Therefore, this study treats two basic aspects of the antennal motor system: First, the anatomy of antennal joints, muscles, nerves and motoneurons is described and discussed in comparison with other species. Second, the typical movement pattern of the antennae is analysed, and its spatio-temporal coordination with leg movements described. Each antenna is moved by two single-axis hinge joints. The proximal head-scape joint is controlled by two levator muscles and a three-partite depressor muscle. The distal scape-pedicel joint is controlled by an antagonistic abductor/ adductor pair. Three nerves innervate the antennal musculature, containing axons of 14-17 motoneurons, including one common inhibitor. During walking, the pattern of antennal movement is rhythmic and spatiotemporally coupled with leg movements. The antennal abduction/adduction cycle leads the protraction/retraction cycle of the ipsilateral front leg with a stable phase shift. During one abduction/adduction cycle there are typically two levation/depression cycles, however, with less strict temporal coupling than the horizontal component. Predictions of antennal contacts with square obstacles to occur before leg contacts match behavioural performance, indicating a potential role of active antennal movements in obstacle detection.     

Distribution and morphology of motoneurons innervating different muscle fiber types in an amphibian muscle complex

The distribution and morphology of motoneurons innervating specific types of muscle fibers in the levator scapulae superior (LSS) muscle complex of the bullfrog (Rana catesbeiana) and tiger salamander (Ambystoma tigrinum) were studied by retrograde labelling with cholera toxin-conjugated horseradish peroxidase (CT-HRP). The LSS muscle complex in both of these amphibians has a segregated pattern of muscle-fiber types (tonic; fast oxidative-glycolytic twitch [FOG]; fast glycolytic twitch [FG]) along an anteroposterior axis. The entire motor pool was labelled by injection of CT-HRP into the whole LSS muscle complex. The motoneurons innervating specific fiber types were labelled by injection of CT-HRP into certain muscle regions. The organization of the motoneuron pool of the LSS complex of both species was arranged in two columns—one ventrolateral and one medial. In bullfrogs, the ventrolateral column contains motoneurons innervating FG and tonic fiber types and the medial column contains motoneurons innervating FOG fiber types. In tiger salamanders, the ventrolateral column contains motoneurons innervating FG fiber types and the medial column contains motoneurons innervating FOG and tonic fiber types. The different motoneuron types also have different soma sizes and patterns of dendritic arborization. In both species, FG motoneurons are the largest, whereas FOG motoneurons are intermediate in size and tonic motoneurons are the smallest. In bullfrogs, the main dendrites of FG motoneurons extend into the dorsolateral and the ventrolateral gray matter of the spinal cord, whereas the dendrites of FOG motoneurons extend into the ventral and medial cord. In the tiger salamander, dendrites of FG motoneurons extend into the ventrolateral spinal cord and dendrites of the FOG motoneurons extend more generally into the ventral cord. Dendrites of tonic motoneurons in both amphibians were small and short, and difficult to observe. These results establish that motoneurons innervating different types of muscle fibers in the LSS muscle complex are segregated spatially and display consistent morphological differences. © 1993 Wiley-Liss, Inc.