Identified muscle fibers in a crab

A neuromechanical model exploring the role of the common inhibitor motor neuron in insect locomotion

In this work, we analyze a simplified, dynamical, closed-loop, neuromechanical simulation of insect joint control. We are specifically interested in two elements: (1) how slow muscle fibers may serve as temporal integrators of sensory feedback and (2) the role of common inhibitory (CI) motor neurons in resetting this integration when the commanded position changes, particularly during steady-state walking. Despite the simplicity of the model, we show that slow muscle fibers increase the accuracy of limb positioning, even for motions much shorter than the relaxation time of the fiber; this increase in accuracy is due to the slow dynamics of the fibers; the CI motor neuron plays a critical role in accelerating muscle relaxation when the limb moves to a new position; as in the animal, this architecture enables the control of the stance phase speed, independent of swing phase amplitude or duration, by changing the gain of sensory feedback to the stance phase muscles. We discuss how this relates to other models, and how it could be applied to robotic control.

Inhibitory motoneurons in arthropod motor control: organisation, function, evolution

Miniaturisation of somatic cells in animals is limited, for reasons ranging from the accommodation of organelles to surface-to-volume ratio. Consequently, muscle and nerve cells vary in diameters by about two orders of magnitude, in animals covering 12 orders of magnitude in body mass. Small animals thus have to control their behaviour with few muscle fibres and neurons. Hexapod leg muscles, for instance, may consist of a single to a few 100 fibres, and they are controlled by one to, rarely, 19 motoneurons. A typical mammal has thousands of fibres per muscle supplied by hundreds of motoneurons for comparable behavioural performances. Arthopods—crustaceans, hexapods, spiders, and their kin—are on average much smaller than vertebrates, and they possess inhibitory motoneurons for a motor control strategy that allows a broad performance spectrum despite necessarily small cell numbers. This arthropod motor control strategy is reviewed from functional and evolutionary perspectives and its components are described with a focus on inhibitory motoneurons. Inhibitory motoneurons are particularly interesting for a number of reasons: evolutionary and phylogenetic comparison of functional specialisations, evolutionary and developmental origin and diversification, and muscle fibre recruitment strategies.

The evolutionary transition to sideways-walking gaits in brachyurans was accompanied by a reduction in the number of motor neurons innervating proximal leg musculature

The forwards-walking portly crab, Libinia emarginata is an ancient brachyuran. Its phylogenetic position and behavioral repertoire make it an excellent candidate to reveal the adaptations, which were required for brachyuran crabs to complete their transition to sideways-walking from their forwards-walking ancestors. Previously we showed that in common with other forwards-walking (but distantly related) crustaceans, L. emarginata relies more heavily on its more numerous proximal musculature to propel itself forward than its sideways-walking closer relatives. We investigated if the proximal musculature of L. emarginata is innervated by a greater number of motor neurons than that of sideways-walking brachyurans. We found the distal musculature of spider crabs is innervated by a highly conserved number of motor neurons. However, innervation of its proximal musculature is more numerous than in closely-related (sideways-walking) species, resembling in number and morphology those described for forwards-walking crustaceans. We propose that transition from forward- to sideways-walking in crustaceans involved a decreased role for the proximal leg in favor of the more distal merus-carpus joint.

Passive resting state and history of antagonist muscle activity shape active extensions in an insect limb

Limb movements can be driven by muscle contractions, external forces, or intrinsic passive forces. For lightweight limbs like those of insects or small vertebrates, passive forces can be large enough to overcome the effects of gravity and may even generate limb movements in the absence of active muscle contractions. Understanding the sources and actions of such forces is therefore important in understanding motor control. We describe passive properties of the femur-tibia joint of the locust hind leg. The resting angle is determined primarily by passive properties of the relatively large extensor tibiae muscle and is influenced by the history of activation of the fast extensor tibiae motor neuron. The resting angle is therefore better described as a history-dependent resting state. We selectively stimulated different flexor tibiae motor neurons to generate a range of isometric contractions of the flexor tibiae muscle and then stimulated the fast extensor tibiae motor neuron to elicit active tibial extensions. Residual forces in the flexor muscle have only a small effect on subsequent active extensions, but the effect is larger for distal than for proximal flexor motor neurons and varies with the strength of flexor activation. We conclude that passive properties of a lightweight limb make substantial and complex contributions to the resting state of the limb that must be taken into account in the patterning of neuronal control signals driving its active movements. Low variability in the effects of the passive forces may permit the nervous system to accurately predict their contributions to behavior.

Degree of neuromuscular facilitation is correlated with contribution to walking in leg muscles of two species of crab

Despite decades of work on the neuromuscular physiology of crustacean leg muscles, little is known about how physiological differences between these muscles relate to their behavioral usage. We studied a sideways walking shore crab, Carcinus maenas, and a forward walking spider crab, Libinia emarginata, as part of our work to understand the neural control of locomotion. The two species differed significantly in facilitation at neuromuscular junctions for every muscle studied. Further, these differences are correlated exactly with the walking use of the muscles. The forward walking spider crab showed more facilitation in muscles which operate joints having larger ranges of motion in forward walking. Likewise, greater facilitation was seen in muscles more active during sideways walking in the predominantly sideways walking shore crab. These differences even occur between muscles innervated by the same motor neuron, and become more evident with higher stimulus frequency. The increased presynaptic facilitation might allow selective recruitment of fibers innervated by the same motor neuron and aid in temporal filtering.

Invertebrate preparations and their contribution to neurobiology in the second half of the 20th century

This review summarized the contribution to neurobiology achieved through the use of invertebrate preparations in the second half of the 20th century. This fascinating period was preceded by pioneers who explored a wide variety of invertebrate phyla and developed various preparations appropriate for electrophysiological studies. Their work advanced general knowledge about neuronal properties (dendritic, somatic, and axonal excitability; pre- and postsynaptic mechanisms). The study of invertebrates made it possible to identify cell bodies in different ganglia, and monitor their operation in the course of behavior. In the 1970s, the details of central neural circuits in worms, molluscs, insects, and crustaceans were characterized for the first time and well before equivalent findings were made in vertebrate preparations. The concept and nature of a central pattern generator (CPG) have been studied in detail, and the stomatogastric nervous system (STNS) is a fine example, having led to many major developments since it was first examined. The final part of the review is a discussion of recent neuroethological studies that have addressed simple cognitive functions and confirmed the utility of invertebrate models. After presenting our invertebrate "mice," the worm Caenorhabditis elegans and the fruit fly Drosophila melanogaster, our conclusion, based on arguments very different from those used fifty years ago, is that invertebrate models are still essential for acquiring insight into the complexity of the brain.

Common and specific inhibitory motor neurons innervate the intersegmental muscles in the locust thorax

The inhibitory innervation of the intersegmental (body wall) muscles between the first and the second thoracic segment of the migratory locust, Locusta migratoria, was investigated using neuroanatomical, immunocytochemical and electrophysiological techniques. Three neurons located in the prothoracic ganglion show GABA-like immunoreactivity and project into the intersegmental nerve. Two are common inhibitors. One of those innervates the oblique intersegmental muscle M59 and two dorsal longitudinal muscles (M81 and M82). The second common inhibitor also innervates M59 and the ventral longitudinal muscle M60. The third neuron innervates M60 exclusively and, for that reason, has to be regarded as the first specific inhibitor ever observed in insect neuromuscular assemblies. According to their innervation pattern, we term these neurons CI(59/60), CI(59/81/82), and SI(60). CI(59/81/82) and CI(59/60) appear to be segmentally homologous to CI(a) and CI(b) neurons, respectively, in the other body segments.

Inhibitory motor neurones supply body wall muscles in the locust abdomen

Inhibitory motor neurones in the abdominal ganglia of the locust Locusta migratoria were identified by combining extra- and intracellular electrophysiology, labelling of motor neurones by peripheral nerve backfills, and immunocytochemistry directed against the inhibitory transmitter gamma-aminobutyric acid. The fifth and sixth abdominal ganglia were studied in particular detail, although general findings were verified in all other abdominal segments. In each abdominal ganglion half, there are two inhibitory motor neurones, CIa and CIb, which supply dorsal (CIa) and ventral (CIb) longitudinal muscles. Their cell bodies are located in the next anterior ganglion to where the axons leave the ventral nerve cord via nerve 1. Both inhibitors have contralateral somata in the posterior ventral soma cortex, looping primary neurites and bilateral dorsal arborisations. There are homonomous (segmentally homologous) motor neurones in the fused abdominal neuromeres, the thoracic ganglia, and at least the third subesophageal neuromere. These body wall inhibitors are distinctly different from the limb muscle inhibitors, CI(1-3), described previously. This is signified, for example, by the fact that both types of inhibitory motor neurones coexist in the prothoracic segment and innervate leg and body wall muscles, respectively.

Localization of a FMRFamide-related peptide in efferent neurons and analysis of neuromuscular effects of DRNFLRFamide (DF2) in the crustacean Idotea emarginata

In the ventral nerve cord of the isopod Idotea emarginata, FMRFamide-immunoreactive efferent neurons are confined to pereion ganglion 5 where a single pair of these neurons was identified. Each neuron projects an axon into the ipsilateral ventral and dorsal lateral nerves, which run through the entire animal. The immunoreactive axons form numerous varicosities on the ventral flexor and dorsal extensor muscle fibres, and in the pericardial organs. To analyse the neuromuscular effects of a FMRFamide, we used the DRNFLRFamide (DF2). DF2 acted both pre- and postsynaptically. On the presynaptic side, DF2 increased transmitter release from neuromuscular endings. Postsynaptically, DF2 depolarized muscle fibres by approximately 10 mV. This effect was not observed in leg muscles of a crab. The depolarization required Ca2+, was blocked by substituting Ca2+ with Co2+, but not affected by nifedipine or amiloride. In Idotea, DF2 also potentiated evoked extensor muscle contractions. The amplitude of high K+ contractures was increased in a dose dependent manner with an EC50 value of 40 nm. In current-clamped fibres, DF2 strongly potentiated contractions evoked by current pulses exceeding excitation-contraction threshold. In voltage-clamped fibres, the inward current through l-type Ca2+ channels was increased by the peptide. The observed physiological effects together with the localization of FMRFamide-immunoreactive efferent neurons suggest a role for this type of peptidergic modulation for the neuromuscular performance in Idotea. The pre- and postsynaptic effects of DF2 act synergistically and, in vivo, all should increase the efficacy of motor input to muscles resulting in potentiation of contractions.

Evolution of the arthropod neuromuscular system. 2. Inhibitory innervation of the walking legs of a scorpion: Vaejovis spinigerus (Wood, 1863), Vaejovidae, Scorpiones, Arachnida

Inhibitory motoneurons which supply the leg musculature are identified and characterized in the scorpion, Vaejovis spinigerus (Wood, 1863) (Vaejovidae, Scorpiones, Arachnida). (1) Successive intracellular muscle fiber recordings from antagonists, and correlation of the monitored inhibitory postsynaptic potentials with spikes in motor nerves, suggest supply of the scorpion leg musculature by common inhibitory motoneurons. (2) Anti-GABA immunohistochemistry is combined with transmission electron microscopy to estimate the number of inhibitory motor axons present in the main leg nerve. The number of immunoreactive axons decreases toward more distal leg segments, from 14 to 18 in the basis to 6-8 in the tibia. No immunoreactive axons are detected beyond the tibia. (3) The distribution of putative inhibitory neurons in the subesophageal ganglion mass is determined by anti-GABA immunohistochemistry, revealing notable similarities to the situation in pterygote insects. This provides a framework for the characterization of the inhibitory motoneurons. (4) Backfills from leg nerves are combined with anti-GABA immunocytochemistry to identify inhibitory motoneurons in the central nervous system. Putative inhibitory motoneurons occur in three clusters per hemi-segment. Two clusters are located near the posterior edge of the neuromere, one lateral, the other more medial, and both contain ca. 8-10 cell bodies. The third cluster consists of two somata located contralaterally, just off the ganglion midline.

Motor control of the mandible closer muscle in ants

Despite their simple design, ant mandible movements cover a wide range of forces, velocities and amplitudes. The mandible is controlled by the mandible closer muscle, which is composed of two functionally distinct subpopulations of muscle fiber types: fast fibers (short sarcomeres) and slow ones (long sarcomeres). The entire muscle is controlled by 10-12 motor neurons, 4-5 of which exclusively supply fast muscle fibers. Slow muscle fibers comprise a posterior and an antero-lateral group, each of which is controlled by 1-2 motor neurons. In addition, 3-4 motor neurons control all muscle fibers together. Simultaneous recordings of muscle activity and mandible movement reveal that fast movements require rapid contractions of fast muscle fibers. Slow and subtle movements result from the activation of slow muscle fibers. Forceful movements are generated by simultaneous co-activation of all muscle fiber types. Retrograde tracing shows that most dendritic arborizations of the different sets of motor neurons share the same neuropil in the subesophageal ganglion. In addition, fast motor neurons and neurons supplying the lateral group of slow closer muscle fibers each invade specific parts of the neuropil that is not shared by the other motor neuron groups. Some bilateral overlap between the dendrites of left and right motor neurons exists, particularly in fast motor neurons. The results explain how a single muscle is able to control the different movement parameters required for the proper function of ant mandibles.

Adaptive motor control in crayfish

This article reviews the principles that rule the organization of motor commands that have been described over the past five decades in crayfish. The adaptation of motor behaviors requires the integration of sensory cues into the motor command. The respective roles of central neural networks and sensory feedback are presented in the order of increasing complexity. The simplest circuits described are those involved in the control of a single joint during posture (negative feedback-resistance reflex) and movement (modulation of sensory feedback and reversal of the reflex into an assistance reflex). More complex integration is required to solve problems of coordination of joint movements in a pluri-segmental appendage, and coordination of different limbs and different motor systems. In addition, beyond the question of mechanical fitting, the motor command must be appropriate to the behavioral context. Therefore, sensory information is used also to select adequate motor programs. A last aspect of adaptability concerns the possibility of neural networks to change their properties either temporarily (such on-line modulation exerted, for example, by presynaptic mechanisms) or more permanently (such as plastic changes that modify the synaptic efficacy). Finally, the question of how "automatic" local component networks are controlled by descending pathways, in order to achieve behaviors, is discussed.

Allatostatin modulates skeletal muscle performance in crustaceans through pre- and postsynaptic effects

Allatostatins, originally identified in insects as peptide inhibitors of juvenile hormone biosynthesis, are regarded as potent inhibitory regulators of intestinal muscles in insects and crustaceans. However, accumulating data indicate that allatostatins might also be involved in modulation of skeletal neuromuscular events. We show that most ganglia of two isopod crustaceans (Idotea baltica and I. emarginata) contain pairs of large, allatostatin-immunoreactive motor neurons which supply several segmental muscles. Among them are the dorsal extensor muscles, of which some fibres receive immunoreactive, varicose innervation. We demonstrate, on identified muscle fibres, that allatostatin exerts a twofold inhibitory effect: it reduces contractions of single voltage-clamped fibres, and it decreases the amplitude of evoked excitatory junctional currents recorded from individual release boutons. No change in excitation-contraction threshold or in passive membrane parameters was observed. As the amplitude of miniature currents generated by spontaneously released single transmitter quanta was not changed, the inhibitory effect of the peptide on junctional currents must be of presynaptic origin. Supportive results were obtained on leg muscles of the crab Eriphia spinifrons, where allatostatin decreased evoked synaptic currents by reducing the mean number of transmitter quanta released by presynaptic depolarization without affecting the amplitudes of currents generated by single quanta. This effect of allatostatin was similar for two functionally different neurons, the slow and the fast closer excitor. The data show that allatostatin occurs in identified motor neurons of Idotea and exerts complementary pre- and postsynaptic modulatory effects which reduce muscle responses. Thus, allatostatin counteracts the effects of another neuropeptide, proctolin, which is also present in Idotea and causes potentiating effects on the same muscle fibres.

Non-uniformity of sarcomere lengths can explain the 'catch-like' effect of arthropod muscle

The 'catch-like' effect, a hysteresis phenomenon in arthropod skeletal muscle contraction thought to be related to the catch of molluscan smooth muscle, was investigated in the closer muscle of the crab Eriphia spinifrons. Several parameters were varied to determine their influence on the catch-like effect. These parameters were (1) the frequency of repetitive stimulation of the slow excitatory neuron, (2) additional stimulation of the inhibitory neuron, (3) the amount of stretch applied to the muscle and (4) the stiffness of the mechano-electrical transducer. The results show that the catch-like effect is not related to the catch of molluscan smooth muscle but rather to the 'residual force enhancement' or 'creep' phenomenon described for vertebrate muscle. A hypothesis for residual force enhancement implies that the increase in force is caused by non-uniformity of sarcomere lengths along the muscle fibre. Based on this hypothesis and the actual force-length relationship of the crab muscle studied, calculations were carried out to determine, if the observed catch-like effect can be explained by such a model. The calculations corroborate the experimental evidence. The catch-like effect of arthropod muscles can thus be explained by the same mechanism responsible for residual force enhancement and creep in vertebrate muscle. A physiological relevance of the catch-like effect in arthropod muscle is inferred.

Origin and clonal relationship of common inhibitory motoneurons CI1 and CI3 in the locust CNS

In the primordial thoracic ganglia of locust embryos, the bromodeoxyuridine (BrdU) technique for labelling proliferating cells and their progeny was combined with intracellular dye injection to investigate the origin and the clonal relationship of common inhibitory motoneurons. Common inhibitors 1 (CI1) and 3 (CI3) were found to be siblings, that is, they are produced by the division of one ganglion mother cell. This ganglion mother cell results from the first division of neuroblast 5-5, at about 30% of embryonic development. A large portion, at least, of the ganglion mother cells produced by subsequent divisions of neuroblast 5-5 give rise to interneurons with contralaterally ascending or descending axons and GABA-like immunoreactivity. Thus, CI1 and CI3 are more closely related to putative inhibitory interneurons than they are to other, that is, excitatory, motoneurons. Consistent with this, the CI somata are associated with cell bodies of putative inhibitory interneurons rather than with clusters of excitatory motoneuron somata. These results elicit speculations regarding the evolutionary origin of inhibitory motoneurons.

Retrograde signaling and the development of transmitter release properties in the invertebrate nervous system

The dynamics of presynaptic transmitter release are often matched to the functional properties of the postsynaptic cell. In organisms ranging from cats to crickets, evidence suggests that retrograde signaling is essential for matching these presynaptic release properties to individual postsynaptic partners. Retrograde interactions appear to control the development of presynaptic, short-term facilitation and depression.

Reciprocal axo-axonal synapses between the common inhibitor and excitor motoneurons in crustacean limb muscles

Nerve terminals of the common inhibitor motoneuron in a crab (Eriphia spiniforns) limb closer muscle and in a crayfish (Procambarus clarkii) limb accessory flexor muscle make neuromuscular synapses with the muscle membrane (postsynaptic inhibition) as well as axo-axonal synapses with the terminals of the excitatory axon (presynaptic inhibition). That transmission is from the inhibitor to the excitor terminals at these axo-axonal synapses is indicated by the occurrence on the inhibitor membrane of presynaptic dense bars denoting sites of transmitter release. Axo-axonal synapses with the opposite polarity, in which transmission is from an excitatory onto an inhibitory terminal, were occasionally seen either adjacent to or separate from the inhibitory axo-axonal synapse. Nerve terminals of the specific inhibitor in the crayfish opener muscle were seen to make numerous axo-axonal output synapses upon excitatory nerve terminals but excitor nerve terminals were not seen to make output synapses onto inhibitor terminals. Thus reciprocal axo-axonal synapses appear to be a feature of the common inhibitor but not of the specific inhibitor. The excitor-to-inhibitor component of these reciprocal synapses may serve to limit transmitter output in the common inhibitor axon by activating glutamateB receptors which facilitate efflux of K+ and hyperpolarization of the membrane.

Shortening velocity and force/pCa relationship in skinned crab muscle fibres of different types

Single fibres of three different types, which had been characterized histochemically with regard to differences in myofibrillar adenosine triphosphatase (ATPase) activity and its pH stability, were microdissected from freeze dried preparations of the closer muscle in walking legs of the crab Eriphia spinifrons. Shortening velocities were determined in slack tests and under constant load conditions in maximally Ca(2+)-activated skinned muscle fibres. Force/pCa relationships were also measured for the different types of fibres. Compared with data on vertebrate muscles, all crab muscle fibres required large length changes to reach zero force and showed low Ca2+ sensitivity for isometric force generation. The length/time relationship obtained from slack tests had a biphasic course. Maximal velocity of filament sliding differed in the three types of fibres investigated. The filament sliding of type IV fibres was about 3 times faster than that of type I fibres. The values obtained for type II fibres ranged in between. These data are positively correlated with myofibrillar ATPase activity determined histochemically. Ca2+ sensitivity of force generation was lowest in the fast type IV fibres. It was high in the slow type I and the faster contracting type II fibres. Ca2+ sensitivity in crab muscle seems not to be correlated with speed of shortening.

Motoneuronal commands during swimming behaviour in the shore crab

Neurograms of proximal leg motor nerves were obtained during swimming in the shore crab. Whereas excitor motoneurones fire in bursts, the common inhibitor motoneurone discharges tonically with simultaneous spikes in all the motor nerves. The average firing frequency of the common inhibitor increases as the period of the swimming cycle decreases. Moreover, greater fluctuations of the firing frequency of the common inhibitor occurs within long rather than short swim cycle periods.

Gamma-Aminobutyric Acid-Inhibitory Motor Innervation of Leg Muscles of the Shore Crab

The inhibitory motor innervation of a crustacean leg was studied in the crab, Carcinus maenas. In in vitro preparations of the central nervous system and the proximal leg nerves, motor nerve recordings demonstrate the presence of a single common inhibitory motor neuron which elicits picrotoxin-sensitive inhibitory junction potentials in a distal leg muscle, the accessory flexor. This inhibitor is the common inhibitor (CI). Immunohistochemical detection of the inhibitory motor neuron neurotransmitter, gamma-aminobutyric acid (GABA), allows us to identify three immunoreactive motor neuron axons in sections of the distal leg nerves and of proximal leg nerves. One corresponds to the CI whereas the other two are the specific inhibitors, one to the stretcher and one to the opener muscles. After nickel chloride backfills of the CI in proximal leg nerves, GABA immunodetection fails and thus confirms that CI is the single inhibitor having branches in proximal leg nerves. These results demonstrate that the inhibitory motor innervation of a crab leg comprises three and only three inhibitors: the common inhibitor innervating all leg muscles and the two specific inhibitors, each innervating a single distal leg muscle. Further conclusions can be drawn: first, a muscle innervated by more than one excitatory axon has no specific inhibitor; second, sensory afferents are not mediated by GABA. Finally, during locomotion, the leg muscles receive two very distinct types of motor input: (1) one common to all the muscles coming from the common inhibitor which was previously shown by other authors to prevent build-up of tension in the muscles, thus allowing each muscle to contract according to (2) the specific motor input it receives from its own excitors.

Common and specific inhibition in leg muscles of decapods: sharpened distinctions

Crustaceans are characteristically parsimonious in their neuromuscular innervation. In extreme instances, a single efferent axon, excitatory or inhibitory, may innervate two or more muscles that have totally different actions. In particular, the inhibitory axons of the reptantian decapod leg have been reported, in various studies within four different infraorders, to innervate anywhere from one to all seven of the leg's distal muscles and to vary in number from two to four. These axons' often inexplicable combinations of target muscles have in many cases precluded interpretation of their behavioral significance. Recent findings reviewed in this paper suggest that in fact all reptants share the same three inhibitory axons: one is a universal common inhibitor, making synaptic connections within all leg muscles; the other two are specific (single-target) inhibitors of the opener and stretcher muscles, respectively (muscles which share a single excitatory axon as their sole source of activation even though they act on different joints). The literature suggests two distinct roles in the control of limb movement for these two classes of inhibitors.

Intracellular Na+, K+ and Cl- activity in tonic and phasic muscle fibers of the crab Eriphia

(1) Intracellular activities of K+, Na+ and Cl- were measured with ion-sensitive microelectrodes in four different types of muscle fibers in the closer muscle of the crab Eriphia. (2) The membrane resting potentials of the tonic fibers were 9-15 mV more positive than those of phasic muscle fibers. This was due to higher permeability of the membranes of tonic fibers for Na+. (3) The intracellular Na+-activity of tonic fibers was 35-40% higher than that of phasic fibers. Also intracellular Cl- -activity was about 15-33% higher in tonic fibers. (4) No significant differences in K+ -activities were found between physiologically different muscle fiber types. The K+ -equilibrium potentials were always more negative than the resting potentials. In muscle fibers with inhibitory innervation, Cl- -equilibrium potentials were close to (phasic fibers) or slightly more negative (tonic fibers) than resting potentials.

Disruption of muscle reorganization by lesions of the peripheral nerve in transforming claws of snapping shrimps

We have performed surgical transections on nerves in the transforming claws of snapping shrimps. In normal transformation muscle restructuring occurs, involving degeneration of some fibers and biochemical changes in others. Surgical section of the entire second limb nerve root or of its distal, dorsal branch--both of which contain the motor axons to the closer muscle--prevents muscle restructuring, even though transformation of external claw morphology proceeds. Furthermore, nerve lesions must be performed within a specific time period after transformation has been triggered in order for the effects to be observed. We suggest that transformation involves an early sensitization of the targeted muscle and that this process depends upon an intact nervous pathway within the second nerve root.

Histochemical and biochemical characterization of two slow fiber types in decapod crustacean muscles

Myofibrillar proteins in muscles of the claws and abdomen of lobster, Homarus americanus, and the claws of fiddler crab, Uca pugnax, and land crab, Gecarcinus lateralis, have been analyzed with sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Fibers contained numerous isoforms of structural and regulatory proteins in assemblages correlated with fiber type. One fast (F) and two slow (S1 and S2) fibers were identified. All F fibers possessed two isoforms of paramyosin (P1 and P2), while all slow fibers, with the exception of Uca major claw, contained only the P2 variant. S1 and S2 fibers were distinguished by the distribution of a large isoform of troponin-T (T1; Mr = 55,000); S2 fibers in all three species contained T1 in addition to one or two smaller-molecular-weight variants usually associated with S1 fibers. In order to determine whether the slow fibers differed in histochemical properties, land crab claw closer muscle was cryosectioned and stained for myofibrillar ATPase and NADH diaphorase activities. Most S2 fibers had lower ATPase and higher NADH diaphorase activities than S1 fibers, which indicated that S2 fibers had a lower rate of contraction and were more fatigue-resistant than S1 fibers. It is proposed that the S1 and S2 fibers defined by biochemical and histochemical criteria are identical to the slow-twitch and tonic fibers, respectively characterized physiologically.

Octopamine action on the contractile system of crustacean skeletal muscle

1. In the opener muscle of walking legs of crayfish (Astacus leptodactylus) octopamine (OA) greatly enhances the contractions resulting from brief applications of L-glutamate or of elevated K-concentrations. Synephrine is as effective as OA. 2. In the case of potentiation of responses to high-K applications a presynaptic component of the OA action was excluded by first desensitising the muscle fibres to the action of the natural transmitter, using a high concentration (1 mM) of glutamate. 3. The Ca-antagonists Co, Ni and Mn (1 mM) reduced the effects of glutamate and of elevated K to about one-half. In preparations treated with OA, the same Ca-antagonists also depressed the potentiated contractural responses to glutamate and to elevated K, again to about one-half. 4. OA also enhanced contractions resulting from the application of caffeine. 5. With 5-hydroxytryptamine (5-HT) application, the same postsynaptic effects were obtained as described for OA, except that the 5-HT actions were much weaker. 6. With OA, maximal effects were obtained with concentrations of 5 x 10(-6)-10(-5) M; maximally effective concentrations of 5-HT were around 10(-5) M. 7. The lowest effective concentrations of OA were around 10(-8) M; those of 5-HT were around 10(-7) M. 8. In the same preparation, 5-HT is far more effective in enhancing transmitter release (presynaptic action) than OA, the lowest effective concentration being around 10(-11) M while no presynaptic effects of OA were seen at concentrations below 10(-8) M, in some cases even below 10(-5) M.

Innervation of the limb accessory flexor muscle in several decapod crustaceans. II. Electrophysiology

The innervation of the distal and proximal heads of the accessory flexor muscle in three portunid crabs and two non-portunid decapods was studied electrophysiologically. In all species studied, the proximal head received only the two previously reported accessory flexor axons, an excitor and an inhibitor. The same two axons also innervated the distal head in all species, but in the portunids the distal head also received excitation from at least three, and probably sometimes four, of the main flexor excitor efferents. The accessory inhibitor exerted very strong effects in the tonic muscle fibers found in the proximal head and in the most proximal bundle of the distal head. The newly described inhibitory and excitatory distributions may have important implications for locomotory behavior.

Triple innervation of the crayfish opener muscle: the astacuran common inhibitor

Part of the much-studied crayfish opener muscle receives a second inhibitory input in addition to its well known specific excitatory and inhibitory innervation. This second inhibitor, formerly thought to innervate only four of the seven peripheral leg muscles, is in fact a common inhibitor of all seven. This has significance both for previous findings in this muscle and for the role of the common inhibitor in decapods.

pH lability of myosin ATPase activity permits discrimination of different muscle fibre types in crustaceans

Myofibrillar actomyosin ATPase activity has been studied histochemically in the closer muscle of the crab Eriphia spinifrons. Preincubation at pH 4.6 and 5.0 reveals differences in the lability of the ATPase. This permits the discrimination of four fibre types. Of these, three represent subgroups of rapidly contracting fibres. The histochemically defined fibre types correspond well with four groups defined according to electrophysiological criteria.