A synchrotron X-ray CT-based 3D atlas of the songbird syrinx with single muscle fibre resolution implies fine motor control of syringeal vocal folds

Avian vocalizations are produced by precisely coordinated motion of the respiratory, syringeal and upper vocal tract systems. Syringeal muscles are controlled with unprecedented resolution, down to independent control of individual muscle fibres. However, we currently lack an anatomical description of syrinx muscles at single fibre resolution. Here, we combined a micron-resolution synchrotron X-ray CT scan of the zebra finch syrinx with micro-dissections of independent specimens to resolve syrinx muscle morphology at individual muscle fibre level. We define two new, previously unknown muscles and update the fibre trajectories and attachment sites of three previously described muscles. Our new insights into the fine anatomy of syrinx muscles show that not one, but both avian vocal folds can be directly controlled by contracting syrinx muscles. Thus, our data reveal novel anatomical complexity with consequences for the biomechanics and motor control of sound production. This article is part of the theme issue 'The biology of the avian respiratory system'.