bluuebunny/crossref_metadata_2025_split huggingface dataset:
Three-Dimensional Odor Tracking by Nautilus Pompilius
Authors: Basil Jennifer A. , Hanlon Roger T. , Sheikh Sarah I. , Atema Jelle 
Published: April 2021

ABSTRACT
The ‘living fossil’ Nautilus pompilius is thought to use olfaction as its primary sensory system during foraging, yet neither the organs responsible for olfaction nor the mechanisms or behaviors associated with odor tracking have been subjected to experimentation. Flume testing under dark conditions revealed that Nautilus could consistently detect and follow turbulent odor plumes to the source over distances up to 10 m, exhibiting two types of orientation behavior while sampling in three dimensions. The paired rhinophores were necessary for orientation behavior: when they were temporarily blocked either uni-or bilaterally, Nautilus detected odor but could not track the plume and locate the source. Animals that were tested post-blockage were able to track and locate the source. The role of the 90 thin tentacles remains enigmatic; they seemed to be able to detect odor, but they were not capable of guiding orientation behavior towards a distant odor source. Bilateral chemical sensing by rhinophores in three dimensions may have been the Umwelt of ammonites and belemnites before the evolution of complex eyes and fast locomotion in modern coleoids.
In Search Of Molecular Dovetails in Mussel Byssus: From The Threads to The Stem
Authors: Coyne Kathryn J. , Waite J. Herbert 
Published: April 2021

ABSTRACT
We recently described the cDNA sequence for a unique collagenous protein, preCol-P, in the byssal threads of the marine mussel Mytilus edulis. The translated amino acid sequence encodes an unprecedented block-copolymer-like sequence with a central collagenous domain flanked by elastin-like sequences. Here, we report on the presence of two additional variants of preCol-P. The distribution of these variants in M. edulis foot tissue was examined by reverse transcription followed by polymerase chain reaction (RT-PCR) and in situ hybridization techniques. One of the variants, P33, exhibits a graded distribution with decreasing concentrations along the longitudinal axis of the foot. The second variant, P22, is expressed only at the base of the mussel’s foot. In situ hybridization confirms the exclusive expression of preCol-P variant P22 in the stem gland. We propose that this variant may represent a molecular ‘dovetail’ between the proximal thread and the byssal stem, imparting extensibility and elastic recoil to the ring portion of the stem.
Way-Finding and Landmarks: The Multiple-Bearings Hypothesis
Authors: Kamil Alan C. , Cheng Ken 
Published: April 2021

ABSTRACT
Clark’s nutcrackers (Nucifraga columbiana) are capable of very precise searching using the metric relationships between a goal and multiple landmarks to relocate the goal location. They can judge the direction more accurately than the distance to a landmark when the landmark is distant from the goal. On the basis of these findings, we propose that nutcrackers use a set of bearings, each a measure of the direction from the goal to a different landmark, when searching for that goal. The results of a simulation demonstrate that increasing the number of landmarks used results in increasingly precise searching. This multiple-bearings hypothesis makes a series of detailed predictions about how the distribution of searches will vary as a function of the geometry of the locations of the relevant landmarks and the goal. It also suggests an explanation for inconsistencies in the literature on the effects of clock-shifts on searching and on homing.
Similarity in Flight Behaviour Between the Honeybee Apis Mellifera (Hymenoptera: Apidae) and its Presumed Mimic, the Dronefly Eristalis Tenax (Diptera: Syrphidae)
Authors: Golding Yvonne C. , Ennos A. Roland , Edmunds Malcolm 
Published: April 2021

ABSTRACT
It is generally accepted that the dronefly Eristalis tenax is a Batesian mimic of the honeybee Apis mellifera. Previous work has established that the foraging behaviour of droneflies is more similar to that of its model than to that of other more closely related flies, suggesting that behaviour may be important in the mimicry. Locomotor mimicry has been demonstrated in mimetic Heliconius butterflies but not in hoverflies. This study therefore investigated aspects of the flight behaviour of Eristalis tenax, Apis mellifera and two other flies, Syrphus ribesii and a Musca sp. Insects were filmed foraging on Helichrysum bracteum flowers, and flight sequences were analysed to determine flight velocities, flight trajectories and the percentage of time spent hovering. It was found that the flight behaviour of droneflies was more similar to that of honeybees than to that of the other flies. This suggests that the flight behaviour of Eristalis tenax may be mimetic.
Muscle tissue adaptations to hypoxia
Authors: Hoppeler Hans , Vogt Michael 
Published: April 2021

SUMMARYThis review reports on the effects of hypoxia on human skeletal muscle tissue. It was hypothesized in early reports that chronic hypoxia, as the main physiological stress during exposure to altitude, per se might positively affect muscle oxidative capacity and capillarity. However, it is now established that sustained exposure to severe hypoxia has detrimental effects on muscle structure. Short-term effects on skeletal muscle structure can readily be observed after 2 months of acute exposure of lowlanders to severe hypoxia, e.g. during typical mountaineering expeditions to the Himalayas. The full range of phenotypic malleability of muscle tissue is demonstrated in people living permanently at high altitude (e.g. at La Paz, 3600–4000m). In addition, there is some evidence for genetic adaptations to hypoxia in high-altitude populations such as Tibetans and Quechuas, who have been exposed to altitudes in excess of 3500m for thousands of generations. The hallmark of muscle adaptation to hypoxia in all these cases is a decrease in muscle oxidative capacity concomitant with a decrease in aerobic work capacity. It is thought that local tissue hypoxia is an important adaptive stress for muscle tissue in exercise training, so these results seem contra-intuitive. Studies have therefore been conducted in which subjects were exposed to hypoxia only during exercise sessions. In this situation, the potentially negative effects of permanent hypoxic exposure and other confounding variables related to exposure to high altitude could be avoided. Training in hypoxia results, at the molecular level, in an upregulation of the regulatory subunit of hypoxia-inducible factor-1 (HIF-1). Possibly as a consequence of this upregulation of HIF-1, the levels mRNAs for myoglobin, for vascular endothelial growth factor and for glycolytic enzymes, such as phosphofructokinase, together with mitochondrial and capillary densities, increased in a hypoxia-dependent manner. Functional analyses revealed positive effects on V̇O2max (when measured at altitude) on maximal power output and on lean body mass. In addition to the positive effects of hypoxia training on athletic performance, there is some recent indication that hypoxia training has a positive effect on the risk factors for cardiovascular disease.
Mechanisms of cell survival in hypoxia and hypothermia
Authors: Boutilier R. G. 
Published: April 2021

SUMMARYMost animals experience some degree of hypoxia and hypothermia during the course of their natural life history either as a consequence of ambient ‘exposure’ per se or through metabolic, respiratory and/or circulatory insufficiency. A prevailing experimental approach has been to probe tissues from natural models of hypoxia-tolerant and cold-tolerant vertebrates to look for common mechanisms of defence against O2 lack and hypothermia. The ability to sustain vital cellular functions in severe cases of either condition varies widely amongst the vertebrates. Like humans, the vast majority of mammals are unable to survive prolonged periods of hypothermia or O2 deprivation owing to irreversible membrane damage and loss of cellular ion homeostasis in vital organs such as the brain and heart. However, numerous hibernating endotherms, neonatal and diving mammals as well as many ectotherms can tolerate prolonged periods that would, in clinical terms, be called asphyxia or deep hypothermia. The key to their survival under such conditions lies in an inherent ability to downregulate their cellular metabolic rate to new hypometabolic steady states in a way that balances the ATP demand and ATP supply pathways.
Limits to sustainable human metabolic rate
Authors: Westerterp Klaas R. 
Published: April 2021

SUMMARYThere is a limit to the performance of an organism set by energy intake and energy mobilization. Here, the focus is on humans with unlimited access to food and for whom physical activity can be limited by energy mobilization. The physical activity level (PAL) in the general population, calculated as doubly-labelled-water-assessed average daily metabolic rate as a multiple of basal metabolic rate, has an upper limit of 2.2–2.5. The upper limit of sustainable metabolic rate is approximately twice as high in endurance athletes, mainly because of long-term exercise training with simultaneous consumption of carbohydrate-rich food during exercise. Endurance athletes have an increased fat-free mass and can maintain energy balance at a PAL value of 4.0–5.0. High altitude limits exercise performance as a result of combined effects on nutrient supply and the capacity to process nutrients. Thus, trained subjects climbing Mount Everest reached PAL values of 2.0–2.7, well below the observed upper limit at sea level.
Human aerobic performance: too much ado about limits to V̇O2
Authors: Lindstedt S. L. , Conley K. E. 
Published: April 2021

SUMMARYHuman endurance performance is often evaluated on the basis of the maximal rate of oxygen uptake during exercise (V̇O2max). Methods for overcoming limits to V̇O2max are touted as means for increasing athletic endurance performance. Here, we argue that the respiratory system is well designed for delivering O2 to meet O2 demands and that no single factor is rate-determining for O2 uptake. We show that V̇O2max can vary 5000-fold among mammals, while any limitation to O2 delivery by a single component of the respiratory system affects V̇O2max by 10% or less. Attempts to increase O2 delivery by enhancing one step in the respiratory system are shown to have little effect. Blood doping, hyperoxia and O2 supplementation of high-altitude natives all raise O2 availability substantially to the working muscles, but these treatments increase V̇O2max only minimally. Finally, we argue that O2 uptake is only one of a number of properties important to human aerobic performance.
Functional and structural adaptations of skeletal muscle to microgravity
Authors: Fitts Robert H. , Riley Danny R. , Widrick Jeffrey J. 
Published: April 2021

SUMMARYOur purpose is to summarize the major effects of space travel on skeletal muscle with particular emphasis on factors that alter function. The primary deleterious changes are muscle atrophy and the associated decline in peak force and power. Studies on both rats and humans demonstrate a rapid loss of cell mass with microgravity. In rats, a reduction in muscle mass of up to 37% was observed within 1 week. For both species, the antigravity soleus muscle showed greater atrophy than the fast-twitch gastrocnemius. However, in the rat, the slow type I fibers atrophied more than the fast type II fibers, while in humans, the fast type II fibers were at least as susceptible to space-induced atrophy as the slow fiber type. Space flight also resulted in a significant decline in peak force. For example, the maximal voluntary contraction of the human plantar flexor muscles declined by 20–48% following 6 months in space, while a 21% decline in the peak force of the soleus type I fibers was observed after a 17-day shuttle flight. The reduced force can be attributed both to muscle atrophy and to a selective loss of contractile protein. The former was the primary cause because, when force was expressed per cross-sectional area (kNm−2), the human fast type II and slow type I fibers of the soleus showed no change and a 4% decrease in force, respectively. Microgravity has been shown to increase the shortening velocity of the plantar flexors. This increase can be attributed both to an elevated maximal shortening velocity (V0) of the individual slow and fast fibers and to an increased expression of fibers containing fast myosin. Although the cause of the former is unknown, it might result from the selective loss of the thin filament actin and an associated decline in the internal drag during cross-bridge cycling. Despite the increase in fiber V0, peak power of the slow type I fiber was reduced following space flight. The decreased power was a direct result of the reduced force caused by the fiber atrophy. In addition to fiber atrophy and the loss of force and power, weightlessness reduces the ability of the slow soleus to oxidize fats and increases the utilization of muscle glycogen, at least in rats. This substrate change leads to an increased rate of fatigue. Finally, with return to the 1g environment of earth, rat studies have shown an increased occurrence of eccentric contraction-induced fiber damage. The damage occurs with re-loading and not in-flight, but the etiology has not been established.
Evidence that a central governor regulates exercise performance during acute hypoxia and hyperoxia
Authors: Noakes Timothy D. , Peltonen Juha E. , Rusko Heikki K. 
Published: April 2021

SUMMARYAn enduring hypothesis in exercise physiology holds that a limiting cardiorespiratory function determines maximal exercise performance as a result of specific metabolic changes in the exercising skeletal muscle, so-called peripheral fatigue. The origins of this classical hypothesis can be traced to work undertaken by Nobel Laureate A. V. Hill and his colleagues in London between 1923 and 1925. According to their classical model, peripheral fatigue occurs only after the onset of heart fatigue or failure. Thus, correctly interpreted, the Hill hypothesis predicts that it is the heart, not the skeletal muscle, that is at risk of anaerobiosis or ischaemia during maximal exercise. To prevent myocardial damage during maximal exercise, Hill proposed the existence of a ‘governor’ in either the heart or brain to limit heart work when myocardial ischaemia developed. Cardiorespiratory function during maximal exercise at different altitudes or at different oxygen fractions of inspired air provides a definitive test for the presence of a governor and its function. If skeletal muscle anaerobiosis is the protected variable then, under conditions in which arterial oxygen content is reduced, maximal exercise should terminate with peak cardiovascular function to ensure maximum delivery of oxygen to the active muscle. In contrast, if the function of the heart or some other oxygen-sensitive organ is to be protected, then peak cardiovascular function will be higher during hyperoxia and reduced during hypoxia compared with normoxia. This paper reviews the evidence that peak cardiovascular function is reduced during maximal exercise in both acute and chronic hypoxia with no evidence for any primary alterations in myocardial function. Since peak skeletal muscle electromyographic activity is also reduced during hypoxia, these data support a model in which a central, neural governor constrains the cardiac output by regulating the mass of skeletal muscle that can be activated during maximal exercise in both acute and chronic hypoxia.
Effects of High Extracellular [K+] and Adrenaline on Force Development, Relaxation and Membrane Potential in Cardiac Muscle From Freshwater Turtle And Rainbow Trout
Authors: Nielsen J. S. , Gesser H. 
Published: April 2021

ABSTRACT
Increases in extracellular K+ concentrations reduced the twitch force amplitude of heart muscle from the freshwater turtle (Trachemys scripta elegans) and rainbow trout (Oncorhynchus mykiss). Adrenaline augmented twitch force amplitude and reduced the relative influence of [K+]. In the absence of adrenaline, high [K+] had less effect in reducing twitch force in turtle than in trout, whereas the reverse was true in the presence of adrenaline. Under anoxic conditions, twitch force was lower in 10 mmol l−1 than in 2.5 mmol l−1 K+ in both preparations, but adrenaline removed this difference. A further analysis of turtle myocardium showed that action potential duration was shorter and resting potential more positive in high [K+] than in low [K+]. Adrenaline restored the duration of the action potential, but did not affect the depolarisation, which may attenuate Na+/Ca2+ exchange, participating in excitation/contraction coupling. The contractile responses in the presence of adrenaline were, however, similar in both high and low K+ concentrations when increases in extracellular Ca2+ were applied to increase the demand on excitation/contraction coupling. The possibilities that adrenaline counteracts the effects of high [K+] via the sarcoplasmic reticulum or sarcolemmal Na+/K+-ATPase were examined by inhibiting the sarcoplasmic reticulum with ryanodine (10 μmol l−1) or Na+/K+-ATPase with ouabain (0.25 or 3 mmol l−1). No evidence to support either of these possibilities was found. Adrenaline did not protect all aspects of excitation/contraction coupling because the maximal frequency giving regular twitches was lower at 10 mmol l−1 K+ than at 2.5 mmol l−1 K+.
Hydrobia Ulvae (Gastropoda: Prosobranchia): a New Model for Regeneration Studies
Authors: Gorbushin Alexander M. , Levakin Ivan A. , Panchina Nadejda A. , Panchin Yuri V. 
Published: April 2021

ABSTRACT
Within 2 weeks of decapitation, Hydrobia ulvae was able to regenerate new head structures including buccal ganglia. It was also capable of regenerating propodial ganglia after anterior foot amputation. The functional regeneration of the buccal ganglia was demonstrated by behavioural observations and by electrophysiological experiments. The presence of the oesophagus was shown to be important for regeneration of the buccal complex. H. ulvae provides a new model for regeneration studies, so details of the topographic anatomy and biology of this species are described. To standardize experimental animals in future studies, the effects of age, sex and trematode infestation on the regeneration capacity of H. ulvae have been evaluated. The high capacity for regeneration together with the possibility of using electrophysiological techniques makes H. ulvae a favourable model in which to study neurogenesis in adult animals.
Structural Flexibility of the Intestine of Burmese Python in Response to Feeding
Authors: Starck J. Matthias , Beese Kathleen 
Published: April 2021

ABSTRACT
The small intestine of Burmese pythons, Python molurus bivittatus, undergoes a remarkable size increase shortly after feeding. We studied the dynamics, reversibility and repeatability of organ size changes using noninvasive imaging techniques. We employed light and electron microscopy, flow cytometry and immunohistology to study the cytological mechanisms that drive the size changes of the small intestine. Within 2 days of feeding, the size of the small intestine increased to up to three times the fasting value. The size changes were fully reversible and could be elicited repeatedly by feeding. These enormous size changes were possible because the mucosal epithelium of the small intestine is a transitional epithelium that allows for considerable size changes without cell proliferation. Histological evidence suggested that a fluid pressure-pump system (lymphatic, blood pressure) was the driving force that inflated the intestinal villi. The rates of cell proliferation were not elevated immediately after feeding but peaked 1 week later when small intestine size was already declining. In contrast to the current paradigm, we suggest that the small intestine is not part of the previously proposed ‘pay-before-pumping’ model. Instead, the size of the python’s small intestine may be upregulated without major metabolic investment. It can occur even if the individual is energetically exhausted. An evolutionary perspective of the transitional epithelium mechanism suggests superior functionality compared with the pay-before-pumping model because it allows for long periods of fasting and depletion of energy reserves, while still enabling the snake to digest prey and absorb nutrients.
The kinematics and neural control of high-speed kicking movements in the locust
Authors: Burrows Malcolm , Morris Genevieve 
Published: April 2021

SUMMARYThe tibiae of locust hind legs can be extended fully in a kick in 3 ms with peak angular velocities of at least 80° ms–1. If the mass of the distal part of the leg is halved, then the extension is complete in less than 1 ms with angular velocities of more than 200° ms–1. The high velocities and the associated power are generated by a preceding storage of energy and its sudden release produced by a specific motor pattern and specialisations of the femoro-tibial joints. To understand the dynamics of these rapid movements and the interrelations between joint mechanics and the motor pattern, kicks were analysed with high-speed video images coupled to simultaneous intracellular recordings from identified leg motor neurones. The first movement is a full tibial flexion followed by co-contraction of the extensor and flexor tibiae muscles for 0.3–1 s, during which the distal end of the femur is flattened dorso-ventrally and expanded laterally. The two semi-lunar processes on the distal femur are bent when the fast extensor tibiae motor neurone spikes so that their tips move ventrally by up to 0.6 mm. The inward projections of these processes into the femur form the proximal part of the hinge joint with the tibia, so that the pivot of the joint also changes and the tibia therefore moves proximally and ventrally, widening the gap between it and the femur. Extension of the tibia begins on average 34 ms after the flexor motor neurones are inhibited at the end of the co-contraction phase. The tibia then begins to extend slowly, reaching peak velocities only when it has extended by 60–70°. The semi-lunar processes do not start to unfurl until the tibia has extended by 55°, so they cannot provide the initial energy for extension. An audible click is produced when the semi-lunar processes unfurl. The peak velocity of tibial extension is correlated with the amount of bending of the semi-lunar processes and with the number of fast extensor motor spikes, but the same amount of semi-lunar bending can be produced by both short and long co-contractions. When the tibia reaches full extension, inertial forces may cause it to bend by as much as 33° at a plane of weakness in the proximal tibia, thus allowing further extension of the distal end.