Signal transduction is an extracellular mechanism that stimulates an intracellular activity or response. In more complex signalling the process involves binding signalling molecules and ligands to cell surface receptors that trigger cellular events via ionic flow into and out of the cell.
For simple signalling ionic flow results in changes in the electrical potential of the cell, promotes signal propagation and here signals can be instigated by a variety of inputs e.g. photonic impacts on the retina.