Figure 45.2 Sensory Cell Membrane Receptor Proteins Respond to Stimuli Mechanoreceptors and thermoreceptors have transmembrane proteins that are also cation channels. Changes in their respective stimuli, pressure or heat, alter the permeability of the channels. Electroreceptors are ion channels responsive to electric currents. Chemoreceptors and photoreceptors influence ion channels through G proteins and second messengers.