Architectural and Histochemical Analysis of the Biceps brachii Muscle of the Horse

SUMMARYMost marine mammals are hypothesized to routinely dive within their aerobic dive limit (ADL). Mammals that regularly perform deep, long-duration dives have locomotor muscles with elevated myoglobin concentrations that are composed of predominantly large, slow-twitch (Type I) fibers with low mitochondrial volume densities (V mt ). These features contribute to extending ADL by increasing oxygen stores and decreasing metabolic rate. Recent tagging studies, however, have challenged the view that two groups of extreme deep-diving cetaceans dive within their ADLs. Beaked whales (including Ziphius cavirostris and Mesoplodon densirostris) routinely perform the deepest and longest average dives of any air-breathing vertebrate, and short-finned pilot whales (Globicephala macrorhynchus) perform high-speed sprints at depth. We investigated the locomotor muscle morphology and estimated total body oxygen stores of several species within these two groups of cetaceans to determine whether they (1) shared muscle design features with other deep divers and (2) performed dives within their calculated ADLs. Muscle of both cetaceans displayed high myoglobin concentrations and large fibers, as predicted, but novel fiber profiles for diving mammals. Beaked whales possessed a sprinterʼs fiber-type profile, composed of ~80% fast-twitch (Type II) fibers with low V mt . Approximately one-third of the muscle fibers of short-finned pilot whales were slow-twitch, oxidative, glycolytic fibers, a rare fiber type for any mammal. The muscle morphology of beaked whales likely decreases the energetic cost of diving, while that of short-finned pilot whales supports high activity events. Calculated ADLs indicate that, at low metabolic rates, both beaked and short-finned pilot whales carry sufficient onboard oxygen to aerobically support their dives. Supplementary material available online at

Section: Muscle Histochemistry and Fiber Diametermentioning

confidence: 99%

Novel locomotor muscle design in extreme deep-diving whales

Velten

Dillaman

Kinsey

et al. 2013

“…Small, rectangular pieces of epaxial white muscle were excised from the frontal region parallel to the fiber's longaxis orientation and fixed for 24h in 4% paraformaldehyde in Sörensen's phosphate buffered saline (PBS), pH7.4. To determine fiber type, frozen sections of fresh unfixed tissue were stained for succinate dehydrogenase (Presnell and Schreibman, 1997;Hardy et al, 2010) and myosin ATPase (Hermanson and Hurley, 1990;Dearolf et al, 2000;Etnier et al, 2004) (data not shown). Both stains confirmed that all the fibers were anaerobic, fast glycolytic white fibers.…”

Section: Dissectionmentioning

confidence: 99%

Growth patterns and nuclear distribution in white muscle fibers from black sea bass, Centropristis striata: evidence for the influence of diffusion

Priester

Morton

Kinsey

et al. 2011

SUMMARYThis study investigated the influence of fiber size on the distribution of nuclei and fiber growth patterns in white muscle of black sea bass, Centropristis striata, ranging in body mass from 0.45 to 4840g. Nuclei were counted in 1m optical sections using confocal microscopy of DAPI-and Acridine-Orange-stained muscle fibers. Mean fiber diameter increased from 36±0.87m in the 0.45g fish to 280±5.47m in the 1885g fish. Growth beyond 2000g triggered the recruitment of smaller fibers, thus significantly reducing mean fiber diameter. Nuclei in the smaller fibers were exclusively subsarcolemmal (SS), whereas in larger fibers nuclei were more numerous and included intermyofibrillar (IM) nuclei. There was a significant effect of body mass on nuclear domain size (F118.71, d.f.3, P<0.0001), which increased to a maximum in fish of medium size (282-1885g) and then decreased in large fish (>2000g). Although an increase in the number of nuclei during fiber growth can help preserve the myonuclear domain, the appearance of IM nuclei during hypertrophic growth seems to be aimed at maintaining short effective diffusion distances for nuclear substrates and products. If only SS nuclei were present throughout growth, the diffusion distance would increase in proportion to the radius of the fibers. These observations are consistent with the hypothesis that changes in nuclear distribution and fiber growth patterns are mechanisms for avoiding diffusion limitation during animal growth.

“…It is composed of two short-fibred, multi-pennate, muscle heads, the lateral head and the medial head (Hermanson and Hurley, 1990). Muscle fibre lengths vary from 5 mm to 7.5 mm in the lateral head and 22 mm to 40 mm in the medial head and the muscle belly is approximately 400 mm long in a thoroughbred horse (Watson and Wilson, 2007).…”

Section: Introductionmentioning

confidence: 99%

Intensity of activation and timing of deactivation modulate elastic energy storage and release in a pennate muscle and account for gait-specific initiation of limb protraction in the horse

Lichtwark

Watson

Mavrommatis

et al. 2009

SUMMARYThe equine biceps brachii (biceps) initiates rapid limb protraction through a catapult mechanism. Elastic strain energy is slowly stored in an internal tendon and is then rapidly released to protract the forelimb. The muscle fibres are short, have little scope for length change and can therefore only shorten slowly compared with the speed at which the whole muscle must shorten, which makes them poor candidates for driving rapid limb protraction. We suggest that the muscle fibres in the biceps act to modulate the elastic energy output of the muscle-tendon unit (MTU) to meet the demands of locomotion under different conditions. We hypothesise that more elastic strain energy is stored and released from the biceps MTU during higher speed locomotion to accommodate the increase in energy required to protract the limb and that this can be achieved by varying the length change and activation conditions of the muscle. We examined the work performed by the biceps during trot and canter using an inverse dynamics analysis (IDA). We then used excised biceps muscles to determine how much work could be performed by the muscle in active and passive stretch-shorten cycles. A muscle model was developed to investigate the influence of changes in activation parameters on energy storage and energy return from the biceps MTU. Increased biceps MTU length change and increased work performed by the biceps MTU were found at canter compared with at trot. More work was performed by the ex vivo biceps MTU following activation of the muscle and by increasing muscle length change. However, the ratio of active to passive work diminished with increasing length change. The muscle model demonstrated that duration and timing of activation during stretch-shorten cycles could modulate the elastic energy storage and return from the biceps. We conclude that the equine biceps MTU acts as a tuneable spring and the contractile component functions to modulate the energy required for rapid forelimb protraction at different speeds. Supplementary material available online at