Proprioreceptors respond to tension, movement in muscles and joints. Coordinate muscle contraction, maintain balance. Vestibular apparatus for equilibrium. All vertebrates have inner ear. Chemical binding causes action potential from depolarization, open channels Sweet, sour, salty, bitter Concentrated in different areas of tongue, but bud has combination of receptors.
Light causes chemical change inpigment, receptor fires, neuron stimulated.
Planaria eyespots, also cnidarians. Bowl-shaped, detect light direction, intensity. Lens concentrates light on group of photoreceptors. Light enters, fixed lens. Concave lends focus further back Farsighted eyeball too short, light focuses behind retina.