Ligand:

A ligand is a signaling molecule that binds to a specific sensory receptor, initiating a cascade of events — converting a physical stimulus like light, sound, or taste into an electrical signal — allowing the transmission of sensory information to the brain. Different types of ligands are designed for each type of sensory receptor and cause changes in the receptor’s structure. These changes trigger intracellular events: opening ion channels, activating enzymes, or releasing second messengers. When we eat something sweet, sugar molecules (the ligand) bind to sweet taste receptors on the tongue, sending a sweet signal to the brain. Also see Transmembrane Protein Receptors. By sense:

• Vision: Light-sensitive molecules in the retina act as ligands for photoreceptor cells.
• Hearing: Sound vibrations trigger the release of ligands that activate hair cells in the inner ear.
• Taste: Chemical compounds in food act as ligands for taste receptors on the tongue.
• Smell: Odorant molecules bind to olfactory receptors in the nose