Calcium signal transmission in chick sensory neurones is diffusion based

In many cells, the cytosol is an excitable medium through which calcium waves propagate by calcium induced calcium release (CICR). Many labs. have reported CICR in neurones subsequent to calcium influx through voltage gated channels. However, these have used long depolarizations. We have imaged calcium within chick sensory neurones following 50 ms depolarizations. Calcium signals travelled rapidly throughout the cell, such that changes at the cell centre were delayed by 24 ms compared to regions 3 microm from the plasma membrane. The nuclear envelope imposed a delay of 9 ms. A simple diffusion model with few unknowns gave good fits to the measured data, indicating that passive diffusion is responsible for signal transmission in these neurones. Simulations run without indicator dye did not reveal markedly different spatiotemporal dynamics, although concentration changes were larger. Simulations of calcium changes during action potentials revealed that large calcium transients occurring in the cytosol close to the nucleus are significantly attenuated by the nuclear envelope. Our results indicate that for the brief depolarisations that neurones will experience during normal signal processing calcium signals are transmitted by passive diffusion only. Diffusion is perfectly capable of transmitting the calcium signal into the interior of nerve cell bodies, and into the nucleoplasm.