In a signal transduction pathway, the event that initiates the sequence of events leading to receptor activation is Ligand binding.
A ligand is a molecule that binds specifically to a receptor, typically a protein on the surface of a cell or within a cell. When a ligand binds to its corresponding receptor, it causes a conformational change in the receptor, which then triggers a series of downstream events in the signal transduction pathway. This can lead to various cellular responses, such as gene expression, enzyme activation, or changes in cellular activity.
Environmental stimuli can also initiate signal transduction pathways, but they often do so indirectly through ligand binding or other mechanisms that ultimately lead to receptor activation. Signal diffusion and receptor activation are intermediate steps in the pathway, following the initial ligand binding.