Biology  5/e   Raven/Johnson
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Beta-carotene: Beta-carotene is a type of carotenoid pigment. Carotenoids are composed of carbon rings linked to chains with alternating single and double bonds. They serve as accessory pigments in photosynthesis and absorb a wide range of wavelengths of light. Beta-carotene can be split to form vitamin A, which can be oxidized to form retinal. Retinal is the pigment used in vertebrate vision, as well as in annelid, mollusc, and arthropod vision.

Depolarization: When a nerve is stimulated by an appropriate stimulus, the voltage-gated Na⁺ ion channels in the membrane open and Na⁺ floods into the cell. This destroys the polarized resting potential and is called depolarization. The depolarization spreads in a wave and becomes the nerve impulse or action potential. Depolarization is the basis for the functioning of neurons and the entire nervous system. It is thus the mechanism by which the central nervous system receives information from the sensory neurons.

Sensory neuron: Neurons are cells that are specialized for conducting electric signals (i.e., waves of depolarization). Sensory neurons are part of the peripheral nervous system and conduct electric impulses from the body's internal or external environment to the central nervous system. They are stimulated to depolarize and send impulses by various sensory stimuli, either directly, or via specialized sensory receptor cells.

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