Receptors play a key role in cell signaling. Receptors help in recognising the signal molecule (ligand). However some receptor molecules respond to physical agents (voltage, light, etc). Receptor molecules are generally proteins. Receptors may be located at cell surface, or interior of the cell such as cytosol, the organelles and nucleus (especially the transcription factors). Usually the cell surface receptors bind membrane-impermeable signal molecules, but sometimes they also interact with membrane permeable signal molecules. A key step in signaling is removal and degradation of the signal molecule. Sometimes the receptor is also degraded. Neurotransmitter reuptake is a mechanism of signaling molecule removal that is commonly seen in nervous system, and is a target of some class of prescription psychiatric medications.