What is Transduction, coding and adaptation?

A sensory receptor cell contains a transduction mechanism — a protein or cascade — that converts a stimulus (a photon, a molecule, a displacement, heat, an electric field) into a change of membrane conductance and hence a receptor potential, graded with the stimulus; the cell, or the neuron it synapses on, turns this into a train of action potentials whose rate encodes intensity. The modality of a signal is given not by the spikes, which are identical in every nerve, but by the wire they travel on and where it ends in the brain: a labelled line. A sensory neuron responds to stimuli within a receptive field — a patch of skin, a region of the visual field, a band of frequencies — and neighbouring fields sharpen one another by lateral inhibition, each neuron suppressing its neighbours so that edges and contrasts are exaggerated. Most receptors adapt: their response to a maintained stimulus declines, so that they report change rather than level, and their working range shifts to sit around the prevailing background.