Single cell neuro-sensory dynamics: Ca²⁺chemoreceptor-guided sea urchin sperm motility

Critical to the success of the fertilization process is the sperm's ability to swim to its targeted destination. The swimming dynamics itself represent a complicated interaction between the flagella and the fluid environment in which it is immersed. Quantifying the low Reynolds number swimming dynamics requires that the flagellar shape be accurately modeled as a function of both space and time. Despite the complex kinematics, dimensionality reduction techniques reveal low-dimensional dynamics that accurately represent the swimming dynamics with only a few parameters. Specifically, a few optimal modes can model and predict the swimming speed and trajectory of the cell [2]. In our work, we show that such a model reduction of the swimming sperm cell can be directly integrated with a model of the Ca²⁺ dynamics in the cell body which controls the beating (driving) of the flagella, thus providing a comprehensive model of the neuro-sensory input-output response of the sperm cell. The theoretical model developed is compared to experimental findings using gametes from the sea urchins Arbacia punctulata and Lytechinus pictus. Using a microfluidic laminar-flow device, a chemical gradient is established with known chemoattractants. Sperm placed within the device are simultaneously imaged for motility, orientation, and calcium responses under simulated hydrodynamic conditions. The experimental findings support the modeling efforts and highlight the underlying neuro-sensory processing responsible for driving sperm motility.