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C K Govind

Publications and source records attributed to C K Govind.

At least 91 records · Page 5Linked to original sources
PubMed1981

Comparison of fast and slow synaptic terminals in lobster muscle.

Synaptic terminals of fast (FCE) and slow (SCE) excitatory neurons were physiologically identified on separate fibres of one muscle, the closer muscle in lobster claws. The innervation by these identified fibers was demonstrated over long distances (7-21 microgram) by examining serial thin sections at periodic intervals. The ultrastructure of each type of innervation was consistent both qualitatively and quantitatively in two separate samples. The FEC innervation is relatively simple in having consistently small-diameter terminalis each forming a single long synapse, with few synaptic vesicles, and little if any postsynaptic apparatus. The SCE innervation is more complex in having larger-diameter but more variable terminals forming several short synapses, with many synaptic vesicles and an extensive postsynaptic apparatus. These differences in the size of the synapses and the number of synaptic vesicles parallel differences in transmitter release and fatigue sensitivity characteristic of the two types of innervation. The degree of elaboration of the postsynaptic apparatus may reflect differences in the amount of transmitter taken up after release. Our data reveal for the first time in a single muscle differences between FCE and SCE innervation previously reported in different muscles and in different species.

Action Potentials↗
PubMed1980

Development of excitatory innervation in the lobster claw closer muscle.

The development of excitatory innervation to the claw closer muscles of 1st (larval), 4th (juvenile), and adult stage lobster was examined by thin serial section electron microscopy. This was possible since neuromuscular terminals of the excitatory axon are distinguishable from those of the inhibitory axon by the shape of their synaptic vesicles in all three stages. In the adult cutter claw closer muscle, innervation of a dorsal and a ventral fiber which is supplied by the fast closer excitor (FCE) and slow closer excitor (SCE) axons respectively, was qualitatively as well as quantitatively similar. Consequently no attempt was made to distinguish between innervation by FCE and SCE axons in the subsequent analysis of 1st, 4th, and adult stage claw closer muscles. Excitatory innervation in the 1st larval stage is limited to four discrete locations in the entire cross-sectional area of the closer muscle. It subsequently spreads to each individual muscle fiber in the adult, thus demonstrating the tremendous proliferation of innervation during development. Concomitantly the mean size of synapses increases significantly from the 1st stage to the 4th stage to the adult lobster. This increase in synaptic size may occur by both the fusion and enlargement of existing smaller synapses. In contrast, the mean size of presynaptic dense bars and their mean number per synapse remained fairly constant in each of the 1st, 4th, and adult stages. However, a relatively greater proportion of adult synapses possessed two or more dense bars compared to their larval and juvenile counterparts. Development of innervation of the lobster claw closer muscle therefore consists of a substantial proliferation of axonal tissue and enlargement of synaptic size.

Animals↗
PubMed1980

Heterogeneity of excitatory synapses at the ends of single muscle fibers in lobster, Homarus americanus.

Crustacean neuromuscular synapses arising from a single excitor axon are known to be well differentiated among different muscle fibers but little is known about their condition along single fibers. Focal recording techniques were used to examine the quantal transmitter release and facilitation properties of synapses in the single excitatory innervated distal accessory flexor muscle of the lobster, Homarus americanus. Synapses were reliably differentiated with respect to quantal output so that those located near the tendon end were 1.15--4.12 times greater than those at the opposite, exoskeletal end (p less than 0.01, paired t-test). Regional differences were also seen in the amount of facilitation determined from twin pulse experiments. The fine structural basis for these differences was determined by serial section electron microscopy of 10-micrometer segments at each end to ensure that the area of focal recording was sampled. No quantitative differences were found in the terminals or synapses in the two regions. Instead, the physiological diversity was correlated with number and size of presynaptic dense bars. Thus, the tendon end had a greater number and larger mean surface area of dense bars compared to the exoskeletal end. This heterogeneity of excitatory multiterminal innervation is correlated with the axonal branching pattern. Thus, the main axon and the larger primary axon branches lie in close proximity to the tendon end of the muscle fibers, whereas the exoskeletal end is innervated by smaller secondary and tertiary axonal branches. This proximity to the large axonal branches of the higher quantal output synapses at the tendon end may be regulated by some neural influence including a timing of innervation and/or access to greater amounts of metabolites in the larger branches which may be conducive to forming high-output synapses.

Animals↗
PubMed1980

Presynaptic dense bars at neuromuscular synapses of the lobster, homarus americanus.

The three dimensional ultrastructure of presynaptic dense bars was examined by serial section electron microscopy in the excitatory neuromuscular synapses of the accessory flexor muscle in the limbs of larval, juvenile, and adult lobsters. The cross-sectional profile of the dense bar resembles an asymmetric hourglass, the part contacting the presynaptic membrane being larger than that projecting into the terminal. The bar has a height of 55-65 nm and varies in length from 75-600 nm. In its dimensions it resembles the dense projections in the synapses of the CNS of insects and vertebrates. The usual location of these dense bars is at well defined synapses, though a few are found at extrasynaptic sites either in the axon or terminal. In the latter case the bars are close to synapse-bearing regions, particularly in the larval terminals, suggesting that the extrasynaptic bars denote early events in synapse formation, In all cases the bars are intimately associated with electron lucent, synaptic vesicles located on either side, in the indentation of its hourglass-shaped cross sectional profile. The vesicles occur along the length of the bar and contact the presynaptic membrane. Consequently the dense bar may serve to align the vesicles at the presynaptic membrane prior to exocytosis. A similar role has been suggested for the presynaptic dense bodies at the neuromuscular junction of the frog, where synaptic vesicles form a row on either side of this structure.

Animals↗
PubMed1979

Spatial distribution of excitatory innervation on a muscle fiber of the lobster (Homarus americanus).

The distribution of nerve terminals from a single excitatory motor axon has been followed along a fiber from the proximal accessory flexor muscle of the lobster by thin serial sectioning at periodic intervals. The excitatory motor axon provides small axonal branches (diameter 5--12 micron) of varying lengths that travel along the surface of the muscle fiber. Each of these branches gives rise to discrete synaptic terminals which are not uniformly distributed. Individual terminals vary in length from 10--60 micron and the majority possess neuromuscular synapses. The greatest distance between synapses is slightly over 1 mm which is well within the length constant (2.6 mm) of this muscle fiber. Thus the spatail distribution of synapses is such as to ensure adequate depolarization along the entire length of the muscle fiber.

Animals↗
PubMed1979

Correlation between presynaptic dense bodies and transmitter output at lobster neuromuscular terminals by serial section electron microscopy.

Lobster neuromuscular terminals releasing comparatively small (low-output type) and large (high-output type) amounts of transmitter but arising from the single excitatory motor axon to the proximal accessory flexor muscle were serially sectioned for electron microscopy. The three-dimensional reconstruction showed the two types of terminals to have a complex branching pattern in which thin branches of the motor axon often enlarged into synapse bearing terminal regions. Quantitative comparison showed that the mean surface area of a synapse is similar in the two types of terminals. However, the low-output terminal has a higher synaptic density and devotes a greater part of its surface area to synapses compared to its high-output counterpart suggesting that transmitter output is not directly related to synaptic area. The mean surface area of a presynaptic dense body is not significantly different between low- and high-output synapses, but there is a significantly greater density of these active zones in the high-output terminal. This results in the ratio of mean dense body area to mean synaptic area being approximately 3 X greater in the high-output synapses than the low-output ones. This significant difference in the surface area of presynaptic dense bodies between low- and high-output synapses correlates with the difference in transmitter output at these two synapses, and implicates the dense bodies in the mechanism of transmitter release at lobster neuromuscular synapses.

Animals↗
PubMed1979

Lobster claw motorneurons respond to contralateral sensory stimuli.

Unilateral sensory stimuli in an isolated lobster claw--ganglion preparation elicits soma and axon spikes in homologous motorneurons in the ipsilateral as well as the contralateral hemiganglion. This cross excitation of the motorneurons provides a basis for bilateral reflexes in the claw and is likely mediated through interneuron(s).

Animals↗
PubMed1978

Experimental transformation of muscle fiber properties in lobster.

Like the chelipeds, the claw closer muscles of the adult lobster are asymmetric (dipmorphic). In the crusher claw the closer muscle is composed entirely of slow fibers, and in the cutter claw it has 65 to 75 percent fast fibers and 25 to 35 percent slow fibers. While claw placement in the adult is essentially random, it can be demonstrated in two ways that the muscle fiber properties are not genetically fixed: (i) if one claw is removed in the fourth and early fifth stages, the remaining closer muscle develops all slow muscle fibers, and (ii) if the animals are raised in smooth-bottomed containers, both claws can become cutter types, having closer muscles with more than 50 percent fast fibers. Thus, as in vertebrate skeletal muscle, the properties of lobster closer muscle fibers can be transformed by various experimental manipulations.

Animals↗
PubMed1978

Nonhomogeneous excitatory synapses of a crab stomach muscle.

Neuromuscular synapses of pyloric muscle P1 in the blue crab Callinectes sapidus were examined using electrophysiological and electron microscopic methods. The muscle is innervated by a single excitatory axon of the stomatogastric ganglion. Excitatory postsynaptic potentials show striking facilitation at very low frequencies of stimulation, indicating very slow decay of the facilitation process after a single nerve impulse. Quantal content of transmitter release at a low frequency of stimulation averaged 1.5. Evidence was obtained that not all synapses on a muscle fiber are equivalent. This was particularly evident at the morphological level in serially sectioned nerve terminals. On each nerve terminal examined, a wide range of synapse sizes was found. Synaptic contact areas ranged from less than 0.5 micron2 to almost 10 micron2; the latter value is large compared with those obtained for other crustacean neuromuscular synapses. Most of the smaller synapses lacked the presynaptic dense bodies which are putative release sites for the transmitter substance. The larger synapses all had presynaptic dense bodies, and some showed evidence of splitting apart into smaller subunits. It is postulated that about half the morphologically identified synapses are relatively inactive.

Animals↗
PubMed1977

Excitatory synapses of blue crab gastric mill muscles.

Physiological and ultrastructural studies were made of neuromuscular synapses in stomach muscles, especially two gastric mill muscles of the blue crab innervated by neurons of the stomatogastric ganglion. These muscles depolarized and contracted with application of glutamate, but not acetylcholine, whereas the dorsal dilator muscles of the pyloric region depolarized and contracted in acetylcholine, but not in glutamate. Large excitatory postsynaptic potentials (EPSP's) of 5-20 mV were recorded in the gastric mill muscles. At low frequencies of activation, individual synapses released on average about 2 quanta of transmitter for each nerve impulse. Facilitation of EPSP's after a single nerve impulse could be detected for at least 10 s. Synapses were found on enlarged terminals of the motor axon; their contact areas ranged from 0.2 mum2 up to 3mum2. Both electron-lucent, round synaptic vesicles and dense-cored vesicles occurred near these synapses. A possible correlation between contact area of a synapse and output of transmitter, is discussed.

Acetylcholine↗