This is the 2007 version. Click here for the 2017 chapter 04 table of contents.

Summary: Receptors and Stimulation

Receptors are the first link in the chain of information flowing into the sensory systems. They come in four varieties, sensitive to light, mechanical displacement, chemical shape, and magnetism. Magnetoreception is used primarily by animals during annual migrations. The senses are optimized for each type of energy in many organisms, meaning they are about as sensitive as it is possible for them to be without creating problems for the organism. Each sensory system responds most sensitively to a particular type of energy called the specific stimulus for that system.

Light is electromagnetic energy. Visible light is just a tiny portion of the overall electromagnetic spectrum that includes radio and television signals, microwave radiation, and x-rays. Within the visible spectrum, light appears brighter or more intense when it contains more energy (represented on a graph as higher waves). The hue or color of pure wavelengths depends on frequency, going from red (lowest) to violet (highest).

Sound consists of pressure waves moving through the air. Like light, it is commonly represented as a wave form. In this case, the peak of the wave represents pressure; the valley represents relative vacuum or rarefaction. Higher frequencies of sound are perceived as having a higher pitch. A louder or more energetic sound is represented as a taller wave, in the graph of a sound wave. Timbre is the word used to describe the unique quality of a sound, created by its distinctive mix of frequencies.

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