The inner ear has a need for speed. The sensory organs responsible for enabling us to walk, dance, and move our heads without feeling dizzy or losing balance are equipped with specialized synapses that process signals faster than any other in the human body. After more than a decade and a half o
Rob Raphael is an associate professor of bioengineering in Rice University’s George R. Brown School of Engineering. Credit: Rice University
In each, a signal-receiving neuron surrounds the end of its partner hair cell with a large cuplike structure called a calyx. The calyx and hair cell remain separated by a tiny gap, or cleft, measuring just a few billionths of a meter. “The mechanism turns out to be quite subtle, with dynamic interactions giving rise to fast and slow forms of nonquantal transmission,” Raphael said. “To understand all this, we made a biophysical model of the synapse based on its detailed anatomy and physiology.”
Eatock said, “The key capability was the ability to predict the potassium level and electrical potential at every location within the cleft. This allowed the team to illustrate that the size and speed of nonquantal transmission depend on the novel structure of the calyx. The study demonstrates the power of engineering approaches to elucidate fundamental biological mechanisms, one of the important but sometimes overlooked goals of bioengineering research.
Since then, Eatock, Lysakowski, and others discovered ion channels in the calyx that transformed scientists’ understanding of how ionic currents flow across hair cell and calyx membranes. Raphael said, “One of my very first grants was to develop a model of ion transport in the inner ear. It is always satisfying to achieve a unified mathematical model of a complex physiological process. For the past 30 years — since the original observation of nonquantal transmission — scientists have wondered, ‘Why is this synapse so fast?’ and, ‘Is the transmission speed related to the unique calyx structure?’ We have provided answers to both questions.