Modeling muscle function using experimentally determined subject-specific muscle properties.

Muscle models are commonly based on intrinsic properties pooled across a number of individuals, often from a different species, and rarely validated against directly measured muscle forces. Here we use a rich data set of rat medial gastrocnemius muscle forces recorded during in-situ and in-vivo isometric, isotonic, and cyclic contractions to test the accuracy of forces predicted using Hill-type muscle models. We identified force-length and force-velocity parameters for each individual, and used either these subject-specific intrinsic properties, or population-averaged properties within the models. The modeled forces for cyclic in-vivo and in-situ contractions matched with measured muscle-tendon forces with r2 between 0.70 and 0.86, and root-mean square errors (RMSE) of 0.10 to 0.13 (values normalized to the maximum isometric force). The modeled forces were least accurate at the highest movement and cycle frequencies and did not show an improvement in r2 when subject-specific intrinsic properties were used; however, there was a reduction in the RMSE with fewer predictions having higher errors. We additionally recorded and tested muscle models specific to proximal and distal regions of the muscle and compared them to measures and models from the whole muscle belly: there was no improvement in model performance when using data from specific anatomical regions. These results show that Hill-type muscle models can yield very good performance for cyclic contractions typical of locomotion, with small reductions in errors when subject-specific intrinsic properties are used.

Eye Movements in Frogs and Salamanders-Testing the Palatal Buccal Pump Hypothesis.

In frogs and salamanders, movements of the eyeballs in association with an open palate have often been proposed to play a functional role in lung breathing. In this "palatal buccal pump," the eyeballs are elevated during the lowering of the buccal floor to suck air in through the nares, and the eyeballs are lowered during elevation of the buccal floor to help press air into the lungs. Here, we used X-Ray Reconstruction of Moving Morphology to investigate eye movements during lung breathing and feeding in bullfrogs and axolotls. Our data do not show eye movements that would be in accordance with the palatal buccal pump. On the contrary, there is a small passive elevation of the eyeballs when the buccal floor is raised. Inward drawing of the eyeballs occurs only during body motion and for prey transport in bullfrogs, but this was not observed in axolotls. Each eye movement in bullfrogs has a vertical, a mediolateral, and an anteroposterior component. Considering the surprisingly weak posterior motion component of the eyeballs, their main role in prey transport might be fixing the prey by pressing it against the buccal floor. The retraction of the buccal floor would then contribute to the posterior push of the prey. Because our study provides no evidence for a palatal buccal pump in frogs and salamanders, there is also no experimental support for the idea of a palatal buccal pump in extinct temnospondyl amphibians, in contrast to earlier suggestions.

Movimientos oculares en ranas y salamandras: prueba de la hipótesis de la bomba bucal palatina (Eye movements in frogs and salamanders ­ testing the palatal buccal pump hypothesis) En ranas y salamandras, los movimientos oculares asociados con el paladar abierto a menudo se ha propuesto que desempeñan un papel funcional en la respiración pulmonar. En esta "bomba bucal palatina", los globos oculares se elevan durante la bajada del piso bucal para inhalar por las narinas, y los globos oculares se bajan durante la elevación del piso bucal para ayudar a presionar el aire hacia los pulmones. Aquí utilizamos la Reconstrucción de Rayos X de la Morfología en Movimiento para investigar los movimientos oculares durante la respiración pulmonar y la alimentación en ranas mugidoras y ajolotes. Nuestros datos no muestran movimientos oculares que estarían de acuerdo con la bomba bucal palatina. Por el contrario, hay una pequeña elevación pasiva de los globos oculares cuando se eleva el suelo bucal. La retracción interna de los globos oculares ocurre solo durante el movimiento del cuerpo y para el transporte de presas en las ranas mugidoras, pero esto no se observó en los ajolotes. Cada movimiento ocular en las ranas mugidoras tiene un componente vertical, mediolateral y anteroposterior. Considerando el componente de movimiento posterior sorprendentemente pequeño de los globos oculares, su función principal en el transporte de presas podría ser la fijación de la presa presionándola contra el suelo bucal. La retracción del suelo bucal contribuiría entonces al empuje posterior de la presa. Debido a que nuestro estudio no proporciona evidencia de una bomba bucal palatina en ranas y salamandras, tampoco hay apoyo experimental para la idea de una bomba bucal palatina en anfibios temnospóndilos extintos, en contraste con sugerencias anteriores. translated to Spanish by Y.E. Jimenez (yordano_jimenez@brown.edu).

Different Segments within Vertebrate Muscles Can Operate on Different Regions of Their Force-Length Relationships.

To relate in vivo behavior of fascicle segments within a muscle to their in vitro force-length relationships, we examined the strain behavior of paired segments within each of three vertebrate muscles. After determining in vivo muscle activity patterns and length changes of in-series segments within the semimembranosus muscle (SM) in the American Toad (Bufo americanus) during hopping and within the sternohyoid (SH) muscle in the rat (Rattus rattus) during swallowing, and of spatially separated fascicles within the medial gastrocnemius (MG) muscle in the rat during trotting, we measured their corresponding in vitro (toad) or in situ (rat) force-length relationships (FLRs). For all three muscles, in vivo strain heterogeneity lasted for about 36-57% of the behavior cycle, during which one segment or fascicle shortened while the other segment or fascicle simultaneously lengthened. In the toad SM, the proximal segment shortened from the descending limb across the plateau of its FLR from 1.12 to 0.91 of its optimal length (Lo), while the distal segment lengthened (by 0.04 ± 0.04 Lo) before shortening down the ascending limb from 0.94 to 0.83 Lo. In the rat SH muscle, the proximal segment tended to shorten on its ascending limb from 0.90 to 0.85 Lo while the distal segment tended to lengthen across Lo (0.96-1.12 Lo). In the rat MG muscle, in vivo strains of proximal fascicles ranged from 0.72 to 1.02 Lo, while the distal fascicles ranged from 0.88 to 1.11 Lo. Even though the timing of muscle activation patterns were similar between segments, the heterogeneous strain patterns of fascicle segments measured in vivo coincided with different operating ranges across their FLRs simultaneously, implying differences in force-velocity behavior as well. The three vertebrate skeletal muscles represent a diversity of fiber architectures and functions and suggest that patterns of in vivo contractile strain and the operating range over the FLR in one muscle region does not necessarily represent other regions within the same muscle.

Membrane muscle function in the compliant wings of bats.

Unlike flapping birds and insects, bats possess membrane wings that are more similar to many gliding mammals. The vast majority of the wing is composed of a thin compliant skin membrane stretched between the limbs, hand, and body. Membrane wings are of particular interest because they may offer many advantages to micro air vehicles. One critical feature of membrane wings is that they camber passively in response to aerodynamic load, potentially allowing for simplified wing control. However, for maximum membrane wing performance, tuning of the membrane structure to aerodynamic conditions is necessary. Bats possess an array of muscles, the plagiopatagiales proprii, embedded within the wing membrane that could serve to tune membrane stiffness, or may have alternative functions. We recorded the electromyogram from the plagiopatagiales proprii muscles of Artibeus jamaicensis, the Jamaican fruit bat, in flight at two different speeds and found that these muscles were active during downstroke. For both low- and high-speed flight, muscle activity increased between late upstroke and early downstroke and decreased at late downstroke. Thus, the array of plagiopatagiales may provide a mechanism for bats to increase wing stiffness and thereby reduce passive membrane deformation. These muscles also activate in synchrony, presumably as a means to maximize force generation, because each muscle is small and, by estimation, weak. Small differences in activation timing were observed when comparing low- and high-speed flight, which may indicate that bats modulate membrane stiffness differently depending on flight speed.

Evolutionary history of the butterflyfishes (f: Chaetodontidae) and the rise of coral feeding fishes.

Of the 5000 fish species on coral reefs, corals dominate the diet of just 41 species. Most (61%) belong to a single family, the butterflyfishes (Chaetodontidae). We examine the evolutionary origins of chaetodontid corallivory using a new molecular phylogeny incorporating all 11 genera. A 1759-bp sequence of nuclear (S7I1 and ETS2) and mitochondrial (cytochrome b) data yielded a fully resolved tree with strong support for all major nodes. A chronogram, constructed using Bayesian inference with multiple parametric priors, and recent ecological data reveal that corallivory has arisen at least five times over a period of 12 Ma, from 15.7 to 3 Ma. A move onto coral reefs in the Miocene foreshadowed rapid cladogenesis within Chaetodon and the origins of corallivory, coinciding with a global reorganization of coral reefs and the expansion of fast-growing corals. This historical association underpins the sensitivity of specific butterflyfish clades to global coral decline.