A sensory receptor selectively responds to a single form of energy and converts it to receptor potentials, which change membrane potential in proportion to stimulus strength. Sensory receptors include chemoreceptors, photoreceptors, and mechanoreceptors. Invertebrates have chemoreceptors to detect food and perform other functions. Invertebrate photoreceptors contain pigments associated with membranes. Light stimulation alters the pigment and changes the charge across the membrane, which may generate an action potential.
Visual systems range from simple ocelli to compound eyes to complex lens systems.
Invertebrate hearing depends on setae and tympanal organs, and balance and equilibrium depend on statolith crystals within statocysts. Vertebrate sensory systems are similar in structure and function in different species, but they may be located in different parts of the body. Some species have senses humans do not have. Humans perceive smell when odorant molecules bind receptors in the olfactory epithelium of the nasal passages. The brain perceives a smell by evaluating the pattern of olfactory receptor cells that bind odorant molecules.