bluuebunny/crossref_metadata_2025_split huggingface dataset:
The Crossbridge Mechanism of Muscular Contraction and its Implications
Authors: Huxley H. E. 
Published: April 2021

The basic features of the sliding-filament crossbridge mechanism are reviewed briefly, and some recent objections involving supposed changes in A-filament lengths are discussed. X-ray diffraction studies on live muscles show no sign of a decrease in axial spacing during contraction, and it is unlikely that a stepwise shortening or depolymerization of A-filaments would provide a plausible contraction mechanism. Thus electron microscope observations which occasionally are reported to show such length changes probably arise from experimental artefact, of which there are many sources.The factors which govern tension and speed in muscle contraction are described. Since all vertebrate striated muscles which have been studied have A-bands of at least approximately the same length, they are likely to have rather similar maximum isometric tensions. The design probably matches this tension to the strength of the filaments themselves. The large variations in shortening speeds between different muscles and different animals arise because of corresponding variations in the rates of particular steps in the crossbridge cycle and in the rate of ATP splitting by the actin-myosin complex involved. Questions concerning the nature and the speed of the activation mechanism are also discussed.
Metabolic heterogeneity of muscle fibres
Authors: Pette D. 
Published: April 2021

ABSTRACTMammalian skeletal muscle is an extremely heterogeneous tissue. Its diversity results from a spectrum of fibres which are metabolically suited to a wide range of functional demands. As judged from enzyme activity analyses of single fibres, the metabolic properties of fibres belonging to the same motor unit are similar or identical. It is likely, therefore, that the phenotype expression of muscle fibres is primarily under neural control. Differences in recruitment patterns of various motor units explain the wide range of metabolic properties as evidenced by pronounced variations in enzyme activities and enzyme activity ratios. There exist large overlaps between the activity spectra of various enzymes of anaerobic and aerobic metabolism in slow- and fast-twitch fibres. Nevertheless, these two major fibre classes can be distinguished by discriminative enzyme activity ratios (e.g. phospho-fructokinase/malate dehydrogenase, phosphofructokinase/3-hydroxyacyl-CoA dehydrogenase, fructose-1,6-diphosphatase/phosphofructokinase). Moreover, slow-twitch fibres display an H-type isozyme pattern of lactate dehydrogenase, whereas fast-twitch fibres are characterized by a predom-inance of LDH-5. No clear-cut differences exist between enzyme activity profiles and LDH isozyme patterns of the IIA and IIB subgroups of fast-twitch fibres. Comparative studies indicate that the metabolic properties of IIA and IIB fibres vary in different animal species. This observation supports the notion that metabolic and myosin-related properties of muscle fibres may be regulated independently. Due to relatively high turnover rates of enzymes of energy metabolism in muscle, changes in functional demands may be met by relatively rapid changes in metabolic properties. In view of these findings it is not surprising that muscle fibres display a spectrum of metabolic properties and represent stages within a dynamic equilibrium.
Power and efficiency of insect flight muscle
Authors: Ellington C. P. 
Published: April 2021

ABSTRACT
The efficiency and mechanical power output of insect flight muscle have been estimated from a study of hovering flight. The maximum power output, calculated from the muscle properties, is adequate for the aerodynamic power requirements. However, the power output is insufficient to oscillate the wing mass as well unless there is good elastic storage of the inertial energy, and this is consistent with reports of elastic components in the flight system. A comparison of the mechanical power output with the metabolic power input to the flight muscles suggests that the muscle efficiency is quite low : less than 10%.
Metabolic and circulatory limitations to muscular performance at the organ level
Authors: Terjung R. L. , Dudley G. A. , Meyer R. A. 
Published: April 2021

ABSTRACT
Within working muscle, development of conditions that directly influence exercise performance is dependent on many factors, including: intensity and duration of exercise, type of skeletal muscle fibres recruited, cardiovascular support to the working fibres and the inherent metabolic characteristics of the contracting fibres. In general, it is possible to identify factors that seem to alter exercise performance only at relatively intense exercise conditions. During prolonged moderately intense exercise (e.g. 70-80% maximal oxygen consumption for at least 60—90 min) decline in performance is related to the depletion of glycogen within the working muscle. Although the cause of muscle performance decline during very intense exercise is not known, an extreme acidosis is found, especially in fast-twitch muscle, which could significantly disrupt normal metabolic and contractile processes. During fatigue caused by intense contraction conditions, ATP content decreases (by approx. 50 %) and there is a stoichiometric production of IMP and ammonia in fast-twitch muscle. This loss in adenine nucleotide content is dependent on the severity of the contraction conditions relative to the functional aerobic capacity of the muscle fibre, since fast-twitch red (high mitochondria, high blood flow) and fast-twitch white (low mitochondria, low blood flow) muscles respond differently. In contrast, during similarly intense contraction conditions, rat slow-twitch muscle fibres maintain their ATP content and do not produce significant amounts of IMP. Indirect evidence suggests that a similar contrast between fibres occurs in humans during maximal exercise. Thus, there seems to be a fundamental difference between fast- and slow-twitch muscles in the management of their adenine nucleotide contents during intense contraction conditions. Whether this is related to the known differences in the fatigue process between these fibre types is not known.
Malleability of skeletal muscle in overcoming limitations: structural elements
Authors: Hoppeler H. , Lindstedt S. L. 
Published: April 2021

ABSTRACT
Muscles allow animals to interact mechanically with their environment. The basic processes concerned with motion as well as the structure of the sliding filaments in muscle cells are found with little modification throughout the animal kingdom (Huxley, 1973). However, the energetic demand imposed on a muscle by the organism may vary greatly depending on the specific function of that muscle. Thus, both the heart and the diaphragm have to deliver high rates of mechanical energy continuously during the whole life-span of an animal, whereas other muscles, such as the gastrocnemius, are only briefly activated every once in a while. The present review analyses the malleability of the structural components of the energetic machinery of skeletal muscle cells with regard to the energetic demand placed upon them by the organism.
How flexible is the neural control of muscle properties?
Authors: Edgerton V. R. , Martin T. P. , Bodine S. C. , Roy R. R. 
Published: April 2021

ABSTRACTSince this classic report, there have been numerous studies which have essentially verified these initial findings. Many of the subsequent papers have addressed two major questions regarding the phenomenon of neural control of muscle properties. First, how does the nervous system exert its control over certain physiological and biochemical properties? Three principal means of neuronal control have been proposed: (1) the pattern (quality) of impulses; (2) the total number (quantity) of impulses; and, (3) a neurotrophic (nonactivity-related) mechanism (Buller et al. 1960). All three possibilities remain viable today. However, the most promising evidence suggests that impulse frequency (Salmons & Sreter, 1976) and quantity of impulses (Sreter, Pinter, Jolesz & Mabuchi, 1982; Jolesz & Sreter, 1981) could be responsible, at least in part, for differences in the speed-related properties of slow and fast muscles.
Adaptability of ultrastructure in the mammalian muscle
Authors: Eisenberg B. R. 
Published: April 2021

ABSTRACT
All the skeletal muscle fibres taken from an adult mammal do not look alike. The structural differences are a result of adaptations which allow gradations in mechanical output to be achieved. The anatomy is described and the amounts of the subcellular components are measured by stereological techniques from electron micrographs. A population of normal, adult fibres is classified by the Z-line width, by the amounts of the mitochondria, T-system and terminal cisternae (TC), and by the isoforms of contractile proteins present. Classification of fibres by some of these ultrastructural components gives clusters named fast-twitch and slow-twitch types, but classification by other components gives a continuum of overlapping properties. Transformation from the fast- to the slow-twitch type or vice versa follows a specific alteration in the use of the fibre. The mechanical demand on the fibre is modified by changing the frequency of stimulation in the nerve with an implanted electrode. The time course of the changes in subcellular composition in the fibre during adaptation is followed for many weeks. Changes in the membrane systems begin within hours and are complete in days. Changes in the contractile proteins and metabolic systems begin in days and are complete in weeks. During these transitional phases of adaptation the fibres have an unusual complement of components never seen in a normal adult fibre. Extreme alterations, such as myofibril disassembly or supranormal amounts of mitochondria also result during some adaptive transitions. The aberrant appearance in the transitional fibres may be a result of doing the required mechanical work with a less than optimal set of proteins. At the end of the fibre type transformation, the fibre ultrastructure is indistinguishable from normal.
Component Permeabilities and Water Contents in Periplaneta Integument: Role of the Epidermis Re-Examined
Authors: Machin J. , Lampert G. J. , O’Donnell M. J. 
Published: April 2021

ABSTRACT
Improved in vivo and in vitro techniques for measuring cuticular water permeability are described. Air flowing over a cuticle disc mounted in a holder, permitted elimination of unstirred layers, or corrections for them, for the first time. Conditions inside the holder were incompatible with the long-term health of the epidermal cells. Significantly, mean permeabilities of these discs did not differ from values obtained in vivo on the same cuticular plate.
Overall cuticular permeability was apportioned between endocuticle and combined epicuticle and exocuticle on the basis of measurements made before and after solvent extraction of lipids. Under identical activity gradients, endocuticle permeability was 35 to 40 times greater than the value for the other layer. Permeability of both component layers showed strongly non-linear relationships with ambient activity, with empirical proportionality to the reciprocal of vapour pressure lowering.
Cuticle water contents measured in activity gradient conditions showed significantly higher values in vivo than in vitro. The amount of water contained in the combined epicuticle and exocuticle was too small to measure.
We conclude that neither permeability nor water content data support the existence of a significant water barrier in the region of the epidermis.
Chromatophore Motor Units in Eledone Cirrhosa (Cephalopoda: Octopoda)
Authors: Dubas F. , Boyle P. R. 
Published: April 2021

ABSTRACT
Innervation of chromatophore muscles of the octopus Eledone cirrhosa was investigated by stimulating nerve bundles in the skin with a suction electrode and monitoring chromatophore movements with a photo-cell or a video camera. Attention was focused on the organization of the chromato-phore muscle fibres into motor units. Individual muscle fibres respond to single electrical impulses with twitch-like contractions that do not facilitate with repetition, but summate to a smooth tetanus at about 10-15 Hz. At tetanic frequency, the degree of expansion of single chromatophores is always maximal. However, the number of expanded chromatophores can be graded by variations of either the stimulus voltage or frequency. Individual chromatophores and probably individual muscle fibres are part of several motor units. Chromatophores forming a given motor unit are found among chromatophores served by other motor axons. The motor units apparently form precise parts of natural patterning.
The Physiology of Wandering Behaviour in Manduca Sexta II. The Endocrine Control of Wandering Behaviour
Authors: Dominick Oliver S. , Truman James W. 
Published: April 2021

ABSTRACT
Removal of the prothoracic glands early during the 5th instar of Man-duca sexta prevented the larvae from wandering and from further development. Infusion of 20-hydroxyecdysone (20-HE) into these larvae induced wandering behaviour. In intact larvae, induction of precocious wandering behaviour required a 20-HE infusion lasting longer than 5 h. Infused 20-HE induced maximal response (90%) when delivered at a rate of 0·06μgg–1 body weighth–1. At considerably higher concentrations (0·25μgg–1h–1 larvae performed brief, erratic behaviour or omitted wandering entirely. The latency between appearance of 20-HE and the onset of wandering was dose-dependent with a minimum of 11 h following infusion at 0·1 μgg–1 h–1. Latency was not affected by the duration of 20-HE infusion. The duration of induced wandering behaviour was proportional to the duration of 20-HE infusion. Minimal wandering behaviour lasted 2h following 20-HE infusions at 5 h, while infusions lasting 11 h induced 9 h of wandering behaviour. Several lines of evidence suggest that the effects of 20-HE accumulate over time and directly determine the duration of wandering behaviour. Many larvae exhibited a series of temporally distinct locomotor periods following various 20-HE infusion protocols, suggesting that a series of separate exposures to 20-HE can result in corresponding serial bouts of locomotion. Responsiveness to 20-HE appeared to be principally modulated by juvenile hormone. Allatectomy of 2nd, 3rd and 4th instar larvae removed juvenile hormone (JH) precociously from these stages and was followed several days later by precocious wandering behaviour. Likewise, application of the JH mimic, EGS, prior to 20-HE exposure or at the start of it, could prevent the behavioural induction. During the 5th instar, 20-HE became increasingly effective in inducing wandering as larvae grew larger than 5 g, the size at which JH normally begins to disappear from the haemolymph. Allatectomized 5th instar larvae responded directly to 20-HE a day sooner than larvae with normal JH titres, but before day 2 the effects of 20-HE on wandering behaviour appear to be indirect, requiring a latency greater than 24 h. Several processes, of which the elimination of JH is the last, appear to be required before 20-HE can induce wandering behaviour. Throughout the entire period when 20-HE was acting to induce wan-dering, the appearance of JH could prevent further induction of behaviour, but the effects of 20-HE which had already been established up to that time were expressed. Thus, application of EGS progressively later during the interval of 20-HE action was progressively less effective in reducing the length of wandering behaviour. The presence of EGS during wandering had no behavioural effect, indicating that only the induction of behaviour by 20-HE is sensitive to modulation by JH.
The Ultrastructure Of The Epicuticular Interference Reflectors Of Tiger Beetles (Cicindela)
Authors: Schultz T. D. , Rankin M. A. 
Published: April 2021

ABSTRACT
Tiger beetles of the genus Cicindela exhibit iridescent structural coloration due to the presence of a non-ideal multilayer interference reflector located in the outermost 2 μm of the integument. The reflector is composed of alternating layers of electron-lucent and electron-dense material. This series of layers was distinguished from chitinous procuticle by its position, ultrastructure and solubility in dilute KOH. The reflector appears homologous with the inner epicuticle of current models. Measurements of surface reflectance, refractive index and the dimensions of the alternating layers suggests that the dense layer has a refractive index (RI) near 2·0 and may be a melanoprotein.
Electrophysiological Characterization of Peptidergic Neurosecretory Terminals
Authors: Cooke Ian M. 
Published: April 2021

ABSTRACTElectrical activity recorded intracellularly from peptidergic neurosecretory terminal dilatations in the sinus gland of crabs (principally Cardisoma guanhumi and C. carnifex) is described. Recordings were made from the neurohaemal organ in situ on the neural tissue of the isolated eyestalk and from isolated sinus gland-sinus gland nerve preparations. Verification that electrodes penetrated terminals was obtained by dye marking. Resting potentials ranged between –30 and –80 mV. Overshooting action potentials of long duration (5-20 ms at 1 / 2 amplitude) relative to those of non-secretory axons ( &lt; 2 ms) were recorded in approximately 70 % of stable penetrations. Action potentials occurred spontaneously at slow (&lt;0·2s−1) rates in 75% of penetrations. Sequential intra-and extracellular recordings with the same microelectrode, on the same terminal, indicate impulse generation by the terminal itself.Extracellular stimulation of the axon tract evokes an all-or-none action potential at distinct threshold and latency. At rates of stimulation exceeding 5s−1, discrete fluctuations in the form of responses occur. Similar waveforms occur spontaneously and can be evoked by passing current through the electrode. They are interpreted to be electrotonically recorded activity of other parts of a complex axonal terminal arborization. Some, but not all, terminals exhibit impulse broadening (up to three-fold at 1/2 amplitude) during repetitive firing exceeding 1s−1. The same terminals show reduced impulse duration with hyperpolarization and broadened impulses with imposed depolarization. The changes are due to altered repolarization rates. Terminals sustain steady impulse firing at rates (up to 5 s−1) linearly related to the imposed depolarizing current.Regenerative potentials, though of reduced rate of rise and amplitude, can be evoked by depolarizing current passed through the electrode during perfusion with salines having 1/2 normal [Na+], or containing tetrodotoxin (10∼6mol 1−1). However, these block axonal conduction. Nominally Ca-free saline causes increased spontaneity and depolarization of about 5 mV in half the preparations examined, but reaching 20 mV in the others, with resultant inactivation of regenerative activity. Impulses in low-Ca saline show alterations of the falling phase, it being faster initially and then slower than normal. Thus, while the action potentials of neurosecretory axons are Na dependent, the terminals support regenerative impulses mediated by both Na and Ca.
A Synchronous Insect Muscle With an Operating Frequency Greater Than 500 Hz*
Authors: Josephson Robert K. , Young David 
Published: April 2021

ABSTRACTThe male cicada, Okanagana vanduzeei, produces a calling song with a pulse repetition frequency of 550 Hz. This sound is produced by a pair of tymbals, each of which is buckled by a large tymbal muscle. Males sing in full sun and the operating temperature of the tymbal muscles is 40–45 °C. Analysis of the songs of animals with the tymbal mechanism destroyed on one side, and of the sounds produced by directly manipula-ting a tymbal, indicates that the two tymbals normally buckle synchro-nously and that only one sound pulse is produced per tymbal muscle contraction. This implies that the contraction frequency of each tymbal muscle is 550 Hz. Recordings of calling songs from animals with implanted electrodes show that there is usually synchrony between left and right tymbal muscle contractions and that each tymbal muscle can operate at a frequency of about 550 Hz. The recordings also show that there is a 1:1 correlation between muscle electrical and mechanical activity, i.e. these muscles are synchronous and not asynchronous muscles. The ultrastructure of the tymbal muscle is clearly that of a very fast, synchronous muscle: the myofibrils are small, the sarcoplasmic reticulum is extraordinarily well developed, and the T-tubules lie at the 1/4 and 3/4 positions along the sarcomere. When set up for isometric recording, with their nerve supply severed, the tymbal muscles often show spontaneous electrical and mechanical oscillations. The frequency of these oscillations is strongly temperature dependent, and at higher temperatures approaches the normal operating frequency of the muscle. The tendency to oscillate is so strong that single twitches could only very rarely be evoked by electrical stimulation. A twitch duration of under 6ms was observed at 30°C. We conclude that the tymbal muscle of O. vanduzeei is indeed a synchronous muscle, albeit a highly modified one, capable of operating at 550 Hz. We suggest that the advantages of asynchronous muscles lie chiefly in their greater economy of structure and operation, rather than in an ability to operate at higher frequencies than synchronous muscles.
Uptake and Loss of Potassium by Rainbow Trout (Salmo Gairdneri) in Fresh Water and Dilute Sea Water
Authors: Eddy F. B. 
Published: April 2021

ABSTRACT
Potassium turnover was studied in rainbow trout, Salmo gairdneri Richardson, adapted to fresh water or 22 %o sea water using 42K and 86Rb. Potassium space of the whole body increased with time and was about 5 mmol kg−1 after 20 h, while Rb+ space under the same conditions was only about 0·5 mmol kg−1, indicating slow penetration of body K+ by Rb +, especially in muscle and red blood cells. Potassium influx, measured by decrease in specific activity of the medium, was 0·07 mmol kg 1 h−1 in fresh water and 0·48 mmol kg−1 h−1 in 22 %o sea water; the values for eflflux were comparable, indicating that unfed fish are able to maintain K + balance. In both fresh water and dilute sea water, K+ fluxes are 5 % or less of the simultaneous Na+ and Cl− fluxes. The mechanism for K + fluxes is discussed in terms in K + -ATPases.
Behaviour and Motor Output for an Insect Walking on a Slippery Surface:II. Backward Walking
Authors: Graham D. , Epstein S. 
Published: April 2021

ABSTRACTCoordination of the legs and the motor activity of four muscles in a middle leg were recorded in adult stick insects walking on a slippery glass surface. Backward walking was not achieved by a simple phase shift of levators and depressors. In all muscles examined, there was a considerable disturbance of motor activity during backward walking when compared with that found in forward walking. In backward walking, recovery was performed, in the middle leg, by strong fast unit activity in the retractor muscle and all muscles showed weak activity at inappropriate times. Fast motor output appeared to be superimposed on the forward walking motor pattern to produce the movements required for backward walking in this insect.
Further Studies of Crayfish Escape Behaviour:II. Giant Axon-Mediated Neural Activity in the Appendages
Authors: Cooke Ian R. C. 
Published: April 2021

ABSTRACTStereotyped responses were evoked in a number of motoneurones in the appendages of semi-intact crayfish when the command neurones for escape behaviour were activated. The medial giant neurones mediated short latency responses in pereiopod common inhibitor, promotor and extensor motoneurones, several abdominal first root neurones and one uropod exopodite promotor motoneurone. The lateral giant neurones mediated short latency responses in the pereiopod common inhibitor neurones, the same abdominal first root neurones and one uropod protopodite promotor motoneurone. These responses can be correlated with stereotyped movements of the appendages which occur in the normal escape behaviour of crayfish.