Digital dissections finesse flight muscle measurements

For anatomists, cutting things open and taking them apart is considered the gold standard for figuring out how muscles and bones work in coordination with other structures. But what if there is a less destructive and more accurate way to learn how structures work in living organisms? In their recent paper, Sullivan and colleagues at the University of Missouri set out on a two-fold mission 1) to study bird flight muscles and 2) to see if a new type of digital dissection could indeed be used to study muscle anatomy.

Bird flight is an “intricate symphony” of structures, and for this study, the team was interested in the muscular composition which makes flight possible. Muscles however, are the oboe of the locomotion orchestra. There are five main levels of muscle organization, just like there are five main pieces of the oboe that have to fit together and move in just the right order to produce the desired effect. Using traditional dissection techniques, it is impossible to measure these different levels of complexity because in order to measure one, you have to destroy another. The technique proposed in this study facilitates measurement of muscle architecture in 3D, creating a clearer image of how different muscle components move during locomotion.

The goal was to find harmony between micro-computed-tomography (CT) scans of the bird anatomy and computer model simulations of bird flight muscles. In non-technical terms, the team stained the muscles with iodine, scanned the muscles, and then used a computer program that could distinguish between muscle fascicles and the spaces around those fascicles. If you think of a muscle like a bundle of al dente spaghetti, this program traces the path of a single noodle within the bundle to see what structures it attaches to and in what direction it runs. The team then ran simulations to see if their program could reproduce the overall muscle patterns they were seeing in their scans.

To test their simulations, researchers focused on a single flight muscle, m. supracoracoideus, which helps a bird pull its wing up during a flapping cycle. Previously, the m. supracoracoideus was described as a classic bipennate muscle. This type of muscle looks similar to the structure of a bird flight feather, with all the fibers running in the same direction and connected to a central tendon. However, by examining the muscle with their new technique, the team noticed that the tendon ends in the middle of the muscle and the fibers had a wide range of fiber angles. Instead of a flight feather, the muscle actually resembles a fluffy down feather with fibers running in various directions. Finding differences like this in muscle systems could instrumentally change what we know about how muscles function and how they contribute to locomotion.

This method will help anatomists untangle how structures work together at a deeper level of organization than previously possible. Still, for those of us that love traditional techniques and took every class that had the term dissection in the course description, hope is not lost! The intricate symphony of anatomy still needs a conductor that can harmonize what we know from traditional gross-dissections with what we can learn from these new muscle models.

By Kelly Diamond – I am an integrative biologist interested in how animals move and interact within their constantly changing environments. Currently I am a PhD candidate at Clemson University studying how the environment impacts predator-prey interactions in Hawaiian stream fishes. To find out more, check out my website  or follow me on twitter @DiamondKMG

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