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Biomedical subjects

C K Govind

Publications and source records attributed to C K Govind.

At least 19 recordsLinked to original sources
PubMed1992

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.

Afferent Pathways↗
PubMed1992

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).

Abdomen↗
PubMed1992

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.

Animals↗
PubMed1992

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.

Aging↗
PubMed1991

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.

Animals↗
PubMed1990

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.

Action Potentials↗
PubMed1989

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.

Animals↗
PubMed1989

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.

Aging↗
PubMed1989

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.

Animals↗
PubMed1989

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.

Animals↗
PubMed1988

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.

Animals↗
PubMed1988

Remodeling of nerves during claw reversal in adult snapping shrimps.

Adult snapping shrimps, Alpheus heterochelis, undergo a reversal of their claw laterality following removal of the major claw, a process in which the existing minor claw transforms into a major and a new minor regenerates at the old major site. During such reversals the nerves to the ganglion are remodeled from one claw type to the other. Conversion of the nerves from the minor to the major type occurs within several days after removal of the contralateral major claw and involves the rapid addition of large numbers of sensory axons together with deletion of a few. Thus modeling of the nerves is essentially complete within the first intermolt in tandem with changes in the motoneurons but well ahead of changes in the muscle and external morphology. Conversion of the major nerves to the minor type is via massive degeneration of sensory axons during the first and second intermolts because of the loss of their peripherally located cell bodies. This is followed by proliferation of largely unmyelinated axons in the third intermolt, some of which become myelinated in the subsequent intermolts. Thus remodeling of the major nerves to minor, which is associated with the loss of a claw and the regeneration of a new minor claw, is a more traumatic and prolonged process compared to the remodeling of the minor nerves to major which is associated with the transformation of an existing claw.

Animals↗
PubMed1988

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.

Animals↗
PubMed1988

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.

Action Potentials↗
PubMed1987

Development of bilateral asymmetry in sensory innervation to lobster claws.

Sensory innervation to the paired claws of the lobster. Homarus americanus, was examined during their differentiation from a bilaterally symmetric state to an asymmetric state of a slender cutter and a stout crusher claw. This was done by estimating the total number and size distribution of axons in the second nerve root which provides most (approximately 90%) of the innervation to the claws and has few, bilaterally distributed motor axons. The paired claws which are undifferentiated and resemble each other in the 1st larval stage correspondingly have nerve roots that are bilaterally symmetric. In early juvenile (4th and 5th) stages when claw type is determined, as well as in subsequent (6th, 7th, 8th, 16th) juvenile stages when the claws gradually differentiate into cutter and crusher types, the paired homologous roots are also similar. It is only in adults that asymmetry in sensory innervation is seen with more axons in the crusher root than in its cutter counterpart. The difference in number of axons between the dimorphic claws is related to differences in surface area between the claws. Thus, bilateral asymmetry in sensory innervation is acquired by the continual but differential addition of axons to the paired claws.

Animals↗
PubMed1986

Intramembranous organization of lobster excitatory neuromuscular synapses.

The fine structure of identified neuromuscular synapses of the single excitatory axon to the distal accessory flexor muscle in lobster limbs was examined with freeze-fracture and serial thin-section electron microscopy. The latter technique reveals presynaptic dense bars with synaptic vesicles aligned on either side of these bars and often fused to the membrane, suggesting exocytosis and confirming our previous contention that these bars are active zones of transmitter release. The intramembranous organization of these active zones, as revealed in freeze-etched tissue, is a ridge-like elevation of the P-face of the axolemma with a matching trough on the complementary E-face. The ridge on the P-face has rows of large scattered intramembranous particles along the apex and is often bordered by a series of small, circular depressions which are presumed to represent exocytotic vesicles attached to the presynaptic membrane. Complementing these depressions are a few volcano-like protuberances seen occasionally on the E-face membrane. Because such evidence for transmitter release occurred in both stimulated and non-stimulated preparations, it demonstrates that chemical fixatives employing aldehydes induce transmitter release. The postsynaptic receptor sites of these excitatory synapses are characterized by oval-shaped patches of densely packed particles on the E-face, arranged in a random pattern on the sarcolemma. The complementary P-face view exhibits a regular square array of particle imprints or pits.

Aminopyridines↗
PubMed1986

Reorganization of synaptic ultrastructure at facilitated lobster neuromuscular terminals.

Prolonged stimulation of the single excitor axon to the lobster distal accessory flexor muscle in the presence of ouabain caused long-term facilitation at its neuromuscular synapses. Hence the extracellularly recorded synaptic potentials failed less frequently and increased their mean amplitude, compared to the non-facilitated (control) potentials from homologous sites in the contralateral muscle. The fine structure of synaptic terminals between matched pairs of facilitated and control preparations was compared with the aid of serial section electron microscopy. Differences between facilitated and control preparations were similar both when the latter were bathed in normal saline or ouabain-containing saline, suggesting that the changes were related to the electrical stimulation rather than to the presence of ouabain. First, the facilitated terminals were smaller in surface area than the control. Second, the number and size of synaptic contacts in the facilitated terminals resembled those in the control. Third, presynaptic dense bodies or active sites increased in number although their sizes remained unaltered in the facilitated terminal. This increase is attributed to the addition of dense bodies at existing synaptic contacts since synaptic contacts remained constant in number between facilitated and control preparations. Fourth, the number and size of synaptic vesicles were unaffected by prolonged stimulation although there was a redistribution of vesicles such that they appeared to be channelled in distinct streams to synaptic contacts. Fifth, mitochondria increased in number and were situated closer to the dense bodies at facilitated nerve terminals than at control terminals. Overall, these changes denote considerable reorganization of the synaptic terminals associated with elevated transmitter release.

Animals↗