Humans have five sense organs: the eyes, ears, taste buds, nasal mucosa, and skin. Each sense organ is specialized to intercept a particular kind of environmental energy and then to convert that energy into a message that the brain can interpret. Together, these two processes are called sensation.
The first step of sensation, the interception of external energy, is done by the part of the sense organ that is in direct contact with the environment. Each sense organ has a specialized shape and structure designed to intercept a particular form of energy. The second step, conversion of the captured energy into signals the brain can understand, is done by cells inside the sense organ called receptors. Receptors are structures to which physicists and engineers refer as transducers: They convert one form of energy into another. Artificial transducers are common. Hydroelectric plants, for example, intercept flowing water and convert it to electricity; then appliances convert the electricity into heat, moving parts, sound, or light displays. Receptors are biological transducers which convert environmental energy intercepted by the sense organ into neural signals. These signals are then sent to the brain, where they are interpreted through a process called perception.
The eye, the best understood of all the sense organs, consists of a lens which focuses light (a kind of electromagnetic energy) through a small hole (the pupil) onto a sheet of cells (the retina). The retina contains the eye's receptor cells: the rods, which are sensitive to all wavelengths of light in the visible spectrum, and three kinds of cones, which are sensitive to those wavelengths that the brain perceives as blue, green, and yellow.
The ear funnels air pressure waves onto the tympanic membrane (more commonly known as the eardrum), where vibrations are transmitted to the inner ear. In the inner ear, receptors called hair cells are stimulated by dif ferent frequency vibrations; they then send signals to the brain, which interprets them as different pitches and harmonics.
Taste buds are small bumps on the tongue and parts of the throat which are continuously bathed in liquid. Receptors in the taste buds intercept any chemicals which have been dissolved in the liquid. Molecules of different shapes trigger messages from different receptors. Humans have several kinds of taste receptors which send signals the brain interprets as bitter, at least two kinds of receptors which send signals interpreted as sweet, and one kind of receptor each that sends messages interpreted as salty and sour.
The nasal mucosa, the sense organ for smell, is a layer of cells lining parts of the nasal passageways and throat; it intercepts chemicals directly from inhaled air. Apparently, cells in the nasal mucosa can produce receptor cells (called olfactory receptors) throughout life. This way, people can develop the capacity to smell "new" chemicals which they could not smell before. New olfactory receptors seem to be created in response to exposure to novel chemicals, analogous to the production of antibodies when the immune system is exposed to foreign material. Because of this ability to create new olfactory receptors, it is not possible to list and categorize all the different types of smells.
The skin is the largest sense organ in the human body; its sense, touch, actually consists of several different senses, collectively referred to as the cutaneous senses. Receptors called mechanoreceptors are triggered by mechan-
Anatomy of the Human Eye
Central retinal artery and vein
ical movements of the skin and send signals that the brain interprets as vibration, light or deep pressure, and stretching. Thermoreceptors intercept heat passing in or out of the body through the skin; their signals are interpreted by the brain as warmth and cold, respectively. Receptors which are triggered when skin cells are damaged are called nociceptors; their signals to the brain are interpreted as pain.
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