Facilitation and depression at different branches of the same motor axon: evidence for presynaptic differences in release

This study provides evidence that a neuron can exhibit differences in activity-dependent transmitter release at two synaptic sites due to variations in the properties of its presynaptic terminals. Two muscles in the stomatogastric system of the lobster Homarus americanus are innervated by a single motor neuron but respond differently to that motor neuron's input, resulting in two different movements evoked by one motor neuron. During continued motor neuron stimulation, the gm8 muscle contracts slowly and maintains contraction, while the gm9 muscle contracts rapidly and then relaxes. These different muscle responses can be accounted for, in large part, by the properties of the respective neuromuscular synapses: the excitatory junctional potentials recorded in gm8 are initially small but summate and facilitate with repeated stimulation, while those in gm9 are initially large but depress with repeated stimulation. Presynaptic differences in neurotransmitter release contribute strongly to the divergent responses; reduction of the excitatory junction potential amplitude by partial postsynaptic receptor blockade or by desensitization does not change the amount of depression at gm9. However, reduction of neurotransmitter release with low-Ca2+, high-Mg2+ saline removes gm9 synaptic depression and reveals that both neuromuscular junctions exhibit frequency-dependent homosynaptic facilitation. Postsynaptic differences in muscle input resistance and muscle composition may enhance the effects of the divergent release properties, but are not responsible for the activity-dependent changes. Ultrastructural features of the nerve terminals on the two muscles are consistent with the differential output of the terminals; the synapses on gm9 are larger and have more presynaptic dense bars than their counterparts on gm8. These data suggest that the basis for the differences in transmitter release between the two muscles may be a higher density of release sites in the gm9 synapses that leads to a higher output of neurotransmitter, rapid depletion of transmitter stores, and synaptic 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.

Claw asymmetry in lobsters: case study in developmental neuroethology

An enduring debate in the study of development is the relative contribution of genetic and epigenetic factors in the genesis of an organism, that is, the nature vs. nurture debate. The behavior of the paired claws in the lobster offers promising material for pursuing this debate because of the way they develop. The paired claws and their closer muscles are initially symmetrical; both are slender in appearance and have a mixture of fast and slow fibers in their closer muscles. During a critical period of development, they become determined into a major (crusher) and minor (cutter) claw and during subsequent development acquire their final form and behavior: The crusher becomes a stout, molar-toothed claw capable of closing only slowly because its closer muscle has 100% slow fibers while the cutter becomes a slender, incisor-toothed claw capable of closing rapidly because its closer muscle has 90% fast fibers. Our initial hypothesis was that the more active claw became the crusher and its less active counterpart the cutter. Presumably, nerve activity would influence muscle transformation, which in turn would influence the exoskeleton to which they attach and hence claw morphology. Curtailing nerve activity to the claw prevented crusher development, while reflex activation of a claw promoted its development; both results support the notion that nerve activity directly regulates claw form and function. This is not, however, the case, for when both claws were reflexly exercised neither formed a crusher, signifying rather that bilateral differences in predominantly mechanoreceptive input to the paired claws somehow lateralized the claw ganglion [central nervous system (CNS)] into a crusher and cutter side. The side experiencing the greater activity becomes the crusher side while the contralateral side becomes the cutter and is also inhibited from ever becoming a crusher. This initial lateralization in the CNS is expressed, via as yet unknown pathways, at the periphery in claw morphology, muscle composition, and behavior. The critical period defines a time when the CNS is susceptible to being lateralized into a crusher and cutter side. Such lateralization is dependent upon experience of the environment in the form of mechanoreceptive input. In the absence of such experience, the CNS is not lateralized and paired cutter claws develop. Thus, while the critical period for crusher determination is genetically determined the actual trigger is influenced by experience.

Neuromuscular analysis of the chela-closer muscle associated with precopulatory clasping in male snow crabs, Chionoecetes opilio

Male snow crabs, Chionoecetes opilio (Majidae), use their modified chelae to retain females for weeks before copulation. Consequently, adaptations for such sustained activity were examined in the chela-closer muscle responsible for clasping. Based on an allometric increase in the ratio of chela size to carapace width, male snow crabs were categorized as morphometrically mature or immature, the former displaying precopulatory clasping more readily than the latter. However, the two types were similar in terms of the properties of the chela-closer muscle, which was examined in this study. The motor pattern during clasping consisted of low-frequency firing of one of the excitor motoneurons, which gives rise to small synaptic potentials. The other excitor motoneuron, which produces large synaptic potentials, fired only when the female struggled during the embrace. The synaptic potentials of both axons showed little if any fatigue at these low firing frequencies. The neuromuscular terminals of these motoneurons displayed areas of synaptic contact larger than most found in other tonically active crustacean muscles. The majority of these synapses had an active site for transmitter release denoted by a dense bar, with many containing more than three dense bars. The closer muscle had typically slow features, with 10 or 11 thin filaments surrounding a thick filament, and sarcomere lengths of 9 – 10 μm. Overall, the closer muscle with its slow-fiber composition, tonic motoneurons, and neuromuscular synapses is well suited to sustained, low-level activity such as precopulatory clasping.

Functional degeneration of isolated central stumps of crayfish sensory axons

In the crayfish, Procambarus clarkii, nerve 5 carries primarily sensory axons from the tail fan to the 6th abdominal ganglion where they synaptically activate interneuron A. Since the sensory neurons have their somata located at the periphery, transection of nerve 5 part way to the ganglion allowed us to examine the fate of their soma-less central stumps. Up to 3 weeks postlesion the response to stimulation of nerve 5 consisted of a brief latency spike in interneuron A, similar to that in control animals and to stimulation of the intact nerve 4. Stimulation of the lesioned nerve 5 beyond 3 weeks failed to fire interneuron A. This loss of function was correlated to loss of axons in nerve 5 deduced by comparing the numbers in the lesioned nerve 5 to its contralateral intact counterpart. The numbers are about equal in the paired nerves but rapidly decline on the lesioned side to 50% within 1 week, 20% within 3 weeks, and less than 10% in subsequent weeks. This loss affects all size classes of axons. However, in the 3 week lesioned nerve large glial infoldings subdivided some of the larger axons and single nuclei were seen in a few of the medium-sized axons. Possibly subdivision of large axons by glial infolding may introduce glial nuclei into axons.

Early innervation of abdominal swimmeret muscles in developing lobsters

The swimmerets in the abdomen of the lobster Homarus americanus are paired external appendages whose back and forth propulsive movements are brought about largely by a group of power and return stroke muscles located in the lateral abdominal cavity. We find functional innervation of these muscles by several excitatory axons and a single inhibitor in embryonic and stage 1 larval lobsters before the external appendages are even formed. This early innervation is via a few nerve bundles in which branches of the motor axons are intertwined in a complex manner. As the swimmerets develop to maturity in later larval and juvenile stages, the innervation consisting usually of several excitor and a single inhibitor synaptic terminals becomes localized to individual muscles. Patterned synaptic activity in these muscles was not seen in the embryonic and larval stages but has been shown in early juvenile stages, when it coincides with the onset of rhythmic movement of the swimmerets. Consequently, such early innervation of the swimmeret muscles may be influential in establishing the central circuitry for the generation of patterned activity, a possibility that was discounted in a previous study (Proc. Natl. Acad. Sci. USA, 70:954-958).

Age-related remodeling of lobster neuromuscular terminals

Multiterminal innervation of a lobster limb muscle by an identified excitor motoneuron was examined during primary development and adult growth. To keep pace with the growth in the target muscle, axonal branches proliferate by sprouting from synaptic terminals; an increasingly complex branching pattern results. Neuromuscular synapses multiply in number, enlarge in size, and become perforated. Concomitantly, synapses tend to appear on the more distal axonal branches and to disappear on more proximal branches, providing for continual remodeling of multiterminal innervation. This plasticity in an identified motoneuron occurs over a long life span of several decades.

Neuromuscular organization of the buccal system in Aplysia californica

The intrinsic muscles and peripheral nerves in the buccal system of the sea hare Aplysia californica were studied to build a foundation on which to base future investigations of feeding in intact animals. A detailed description of the bilaterally paired intrinsic muscles is given identifying previously unreported muscles. Each of the six buccal nerves (n1-n6) and the cerebrobuccal connective (CBC) have been characterized in several respects. Cell bodies in the buccal ganglion with projections into each of the buccal nerves have been identified via the cobalt backfilling technique. All nerves contain axons of cell bodies in the ipsilateral as well as the contralateral ganglia. For each nerve, there is a consistent pattern in the distribution of cell bodies in the paired ganglia with the number of cell bodies in the contralateral ganglion being less than or equal to the number in the ipsilateral ganglion. Although the total number of backfilled cell bodies varies among the nerves, their size ranges are similar with the majority being small. Nerves 1, 2, 4, 5, and 6 provide motor innervation to the intrinsic buccal muscles in varying degrees with nerve 4 supplying all the intrinsic muscles; nerve 2 supplies only one. The axon composition of each nerve was scrutinized and revealed large numbers of axon profiles, the majority of which were less than 2 microns in diameter. The present study provides a framework for analysis of feeding behavior in Aplysia californica.

Morphology and Behavior of an Unusually Flexible Thoracic Limb in the Snapping Shrimp, Alpheus heterochelis

The second thoracic limb in the snapping shrimp, Alpheus heterochelis, is much thinner, more elongated and flexible, and has a larger ganglion than its serial homologs. The greater length and flexibility is largely due to one of the limb segments--viz., the carpus--which consists of five separate segments, rather than the single segment typical of the other limbs. Externally, the multi-segmented carpus is relatively free of cuticular projections except for scattered simple setae. The adjoining segments--the merus and the propus--are also smooth except for clusters of long simple setae on the pollex and dactyl. Internally, each of the carpal segments has three muscles--a bender, stretcher and rotator--all restricted to the distal half of the segment. In keeping with the sensillum-free exterior of the multisegmented carpus, only about 1000 axon profiles originate in the carpus out of a total of 6000 counted at the base of the ganglion. This total number is roughly half that found in the first thoracic limbs. Conversely, the number of axon profiles in the longitudinal connectives to the second thoracic ganglion is about 25% greater than that to the first thoracic ganglion and may partly account for the size difference between these two ganglia. In terms of their behavior, the second thoracic limbs are almost constantly active, mostly probing the substrate, and occasionally grooming various body parts. Part of the probing behavior consists of food foraging and retrieval, especially from concealed and hard-to-reach locations. Because of their flexibility, these limbs are particularly adept at such movements.

Retarded and Mosaic Phenotype in Regenerated Claw Closer Muscles of Juvenile Lobsters

The closer muscle in the paired claws of the lobster Homarus americanus become determined into their asymmetric form of a cutter and crusher type claw during the 4th and 5th juvenile stages and differentiate their fiber composition accordingly in subsequent juvenile stages. Our aim was to study the effects of claw loss during this critical juvenile period on muscle regeneration. Hence the fiber composition of the paired closer muscles in newly regenerated claws was examined histochemically following removal of both claws either in the 4th and 5th stages or in the 4th through 7th stages. The newly regenerated muscle was retarded compared to its original counterpart in both cases. In the former case, however, the retardation was temporary as the muscle composition in later stages resembled the original. Recovery in the latter was not apparent in later stages, suggesting that retardation is more permanent. Also in both protocols the newly regenerated closer muscle occasionally displayed a mosaic distribution, with slow fibers interspersed among fast fibers in a central band that is normally homogenously fast. Therefore, loss of the paired claws during a developmentally sensitive period affects the phenotype of the regenerated muscle with the change persisting for shorter or longer periods depending on how often the claws are lost.

Neuromuscular properties of the quintuply innervated flexor muscle in lobster limbs

The flexor muscle of the lobster's walking leg was shown by enzyme histochemistry and electrophysiology to display a regional segregation of fibre types: medial fibres have a higher ATPase activity, lower oxidative capacity, and shorter membrane time constant than peripheral fibres lying near the cuticle. The muscle was confirmed to receive one inhibitory and four excitatory motor axons. As judged by the properties of their output excitatory junctional potentials (ejp's), the four excitors lie along the fast-to-slow gradient defined by the two specialized excitors of dually excited muscles. The Fα axon produces initially large ejp's which facilitate weakly or antifacilitate; they are similar to those of fast axons in other muscles. The Fρ axon at the other end of the spectrum produces strongly facilitating ejp's which are initially small, resembling those of known slow axons. The Fβ and Fγ axons show intermediate properties. The inhibitor, which is the common inhibitor of all leg muscles, innervates preferentially the more tonic muscle fibres, as does Fρ. Muscle fibres were observed to receive anywhere from one to five efferents, most receiving two to four. Serial electron microscopic observations in several regions revealed a rich supply of synaptic terminals, usually comprising a single inhibitory terminal and two or three excitatory ones. The inhibitory terminal typically has a few large synapses, each with more than one active site. Excitatory terminals, on the other hand, have many more smaller synapses, each with at least one active site. Although excitatory and inhibitory terminals were often closely juxtaposed, no synaptic interactions were observed between them.

Inhibitory innervation of a lobster muscle

Inhibitory neuromuscular synapses formed by the common inhibitor (CI) neuron on the distal accessory flexor muscle (DAFM) in the lobster, Homarus americanus, were studied with electrophysiological and electron-microscopic (thin-section and freeze-fracture) techniques. Postsynaptic inhibition as indicated by inhibitory junctional potentials was several-fold stronger on distal compared to proximal muscle fibers. This difference correlated with the results of serial thin-section studies, which showed more inhibitory synapses on distal fibers than on their proximal counterparts. Effects of postsynaptic inhibition on excitatory junctional potentials via current shunting had a morphological correlate in the spatial relationship between inhibitory and excitatory synapses on the distal fibers. Inhibitory synapses were larger than their excitatory counterparts and had fewer glial processes. In freeze-fracture views, inhibitory synapses did not appear as raised plateaus in the P-face as do excitatory synapses, and their active zones were more widely scattered. The intramembrane particles in the inhibitory postsynaptic membrane - representing neurotransmitter receptors - are arranged in parallel rows in the sarcolemmal P-face and have complementary furrows in the sarcolemmal E-face. Altogether, our findings help to describe a population of inhibitory neuromuscular synapses formed by the CI neuron in lobster muscle.

Vascular supply to bilaterally asymmetric chelae in crustaceans

Blood vessels to the paired hemiganglia and chelae were examined in several crustacean species with bilaterally asymmetric, major and minor, chelae. The network of vessels was qualitatively similar between the paired chelae hemiganglia in Alpheus, Carcinus, Homarus, and Uca. The principal vessel supplying the chela was also similar in size on the two sides, except in the fiddler crab and snapping shrimp where the vessel was larger on the major side. Reversal of chela asymmetry in Alpheus entails a reversal in vessel size denoting plasticity of adult vascular tissue.

Higher mitochondrial density in slow versus fast lobster sensory neurons

The stretch-sensitive muscle receptor organ (MRO) in the abdomen of the lobster Homarus americanus contains an identifiable fast and a slow sensory neuron. Morphometric analysis of electron micrographs of areas through the somata of these neurons revealed a higher density of mitochondria in the slow versus the fast cell (19 vs 15%). Such differences in oxidative capacity are closely matched with differences in their physiological performances.

Structural plasticity at crustacean neuromuscular synapses

Crustacean motor axons innervate muscle fibers via a multiplicity of synaptic terminals which release small but variable amounts of transmitter. Differences in release performance appear to be correlated with the size of synaptic contacts and presynaptic dense bars (active zones). These structural parameters proliferate via sprouting from existing synaptic terminals and relocate to ever more distal sites during development and growth of an identified axon. Moreover, alterations in number of synaptic contacts and active zones occur in adults following stimulation or decentralization, demonstrating structural plasticity of crustacean neuromuscular synapses.

Regenerative fidelity in the paired claw closer muscles of lobsters

The paired claws in the lobster Homarus americanus are bilaterally asymmetric, consisting of a major (crusher) and a minor (cutter) claw. The fiber composition of the claw closer muscles is correspondingly asymmetric: the cutter muscle has predominantly fast fibers with a small ventral slow band, whereas the crusher muscle has 100% slow fibers. Loss of the paired claws results in regeneration of new ones, which resemble their predecessors in external morphology and in the fiber composition of the closer muscle. Such regenerative fidelity prevails even when the paired claws and closer muscles are symmetric and of the cutter type, and even when they have undergone two successive cycles of limb loss and regeneration. Therefore the type of closer muscle and the configuration of the paired claws is not altered by loss and regeneration.

Stimulation-induced changes at crayfish (Procambarus clarkii) neuromuscular terminals

The fine structure of neuromuscular terminals of the single excitor axon was examined in the limb stretcher muscle of the crayfish Procambarus clarkii. A morphometric comparison of the neuromuscular terminals of the left and right limbs of a control crayfish showed them to be similiar in qualitative as well as quantitative features. The excitor axon to the stretcher muscle of the right side was stimulated, by backfiring its branches in the adjacent opener muscle, at 20 Hz for 4-5 h per day over 4-5 days. The stretcher muscle on the left side was not stimulated and served as a control. Morphometric analysis of stimulated terminals revealed an increase in the number of dense bars and synaptic vesicles compared to their non-stimulated, contralateral counterparts. Since dense bars are regarded as active sites of transmitter release, changes in their number provide a morphological basis for synaptic plasticity.

Growth of inhibitory innervation in a lobster muscle

The fine structure of inhibitory innervation to a limb muscle was examined in larval, juvenile, and adult lobsters. The innervation is essentially similar in qualitative features among these different stages, although there are some marked quantitative changes associated with growth. From being localized to discrete regions in the larval muscle, the inhibitory innervation spreads to groups of muscle fibers in the early juvenile muscle and to single fibers in the late juvenile and adult muscles. Concurrently, its neuromuscular synapses enlarge in area, become perforated, and acquire more active sites of transmitter release. Inhibitory nerve terminals occur in close proximity to their excitatory counterparts in the muscles of larval and early juvenile stages, although in later stages this juxtaposition occurs preferentially in some muscle fibers but not others. The inhibitory innervation is, nevertheless, much more restricted in occurrence than is the excitatory innervation.

Neural attrition following limb loss and regeneration in juvenile lobsters

Lobsters have considerable regenerative capacity, being able to regrow an entire, albeit smaller, limb in one intermolt. Whether there is a corresponding downscaling in the hemiganglion and its nerves to the regenerate side compared with its contralateral intact side was examined in juvenile lobsters which had undergone single or multiple (2, 4, and 6) cycles of limb loss and regeneration on the one side. The limbs studied were the enlarged thoracic chelipeds or claws which appeared as paired symmetrical cutter-type claws. The size of the regenerate limb, as indicated by its propus length, was approximately 30% smaller than its intact counterpart. Correspondingly, the total number of axons in the nerves to the regenerate side was smaller than on the intact, contralateral side. Such attrition was also by about 30% in lobsters experiencing a single cycle of limb loss and regeneration, but was considerably greater with multiple cycles. Tissue degeneration was occasionally seen in the nerves to the regenerate side but not in the ganglion. The paired hemiganglia were equivalent in all respects except in the size of the neuropil, which was smaller on the regenerate side compared with the contralateral intact side. Neuropil attrition was most marked with multiple cycles of limb loss and regeneration. Such attrition in nerve and neuropil are most likely due to the reduced number of sensory elements in the newly regenerated, but smaller, limb.

Early experience influences the development of bilateral asymmetry in a lobster motoneuron

The development of functional asymmetry between a pair of homologous motoneurons of the claw closer muscles in lobsters, Homarus americanus, was studied. In juvenile lobsters, 3-5 years old, where the paired claws are highly specialized into a major (crusher) and minor (cutter) type, the fast closer excitor (FCE) motoneuron fired longer bursts of spikes in the crusher claw compared to those in its cutter counterpart. The intraburst impulse frequency was greater for the cutter FCE and its neuromuscular synapses showed greater facilitation at these high impulse frequencies compared to that of the crusher claw. However, such asymmetry in firing patterns and synaptic facilitation was absent in lobsters raised without a substrate and having paired cutter claws. In the earliest juvenile stage, synaptic facilitation was similar between the paired claws and then developed in either an asymmetric or symmetric manner depending on whether the lobsters experienced a substrate or not. In a substrate-free environment asymmetry could be produced by exercising one of the claws during development, implicating bilateral differences in the reflexive activity of the claws as a control mechanism.

Highly active neuromuscular system in developing lobsters with programmed obsolescence

The primary locomotory apparatus in the three larval stages of the lobster, Homarus americanus, are paddlelike structures on the thoracic appendages called exopodites, which beat almost continuously. Consequently their power and return-stroke muscles are examples of highly active but short-lived neuromuscular systems. The muscles, which are well vascularized, are of the fast type with 2-3-micron sarcomere lengths and 6 thin filaments surrounding a thick one. The most striking feature, however, is the large volume of mitochondria making up 40-50% of the fiber. They appear as simple cylinders packed several layers deep along the periphery of the fiber and as large, multibranched forms distributed throughout the fiber and subdividing it into smaller units. The motor innervation to the return-stroke muscle is via 3 excitatory axons, which generate large junctional potentials and twitch contractions. The muscle is densely populated with large neuromuscular synapses, most of which have a well-defined active site or dense bar denoting the site of transmitter release. Altogether this motor system is specialized for prolonged activity. Atrophy of the neuromuscular system occurs by the late larval third stage. The muscle fibers lose their identity, fuse, and become vacuolated. The myofibrils condense and erode and the mitochondria are lost. Atrophy of motor innervation is gradual with individual axons dropping out. The largest axon providing most of the innervation is the first to degenerate. Early degenerative changes affect the axon and neuromuscular terminals but not the synaptic contacts, dense bars, and vesicles, which appear intact. Continued atrophy in the postlarval fourth stage reduces the exopodites to vestiges. Thus the return-stroke muscle of the larval exopodites in which muscle fiber and motoneurons are identifiable permits study of the interaction between a neuron and its target muscle undergoing programmed obsolescence.