Sensory receptors respond to stimuli and transmit data about them to the brain. In the skin, receptors detect touch, pressure, vibration, temperature, and pain. Elsewhere in the body, more specialized receptors detect light (see How the eye works), sound (see The mechanism of hearing), smell, and taste. Internal receptors called proprioceptors sense body position and the location of body parts in relation to each other.
Touch receptors are found all over the body. The most common are free nerve endings, which sense pain, pressure, and temperature in addition to touch. Other touch receptors include Merkel’s discs and Meissner’s corpuscles, which detect light touch, and Pacinian corpuscles, which sense deep pressure and vibration.
Olfactory receptors in the roof of the nasal cavity are stimulated by odours. Nerve impulses from these receptors travel to the olfactory bulb (the end of the olfactory nerve) and then to the olfactory centres in the brain. Our sense of smell is thousands of times more sensitive than our sense of taste, and we can detect more than 10,000 odours.
There are about 10,000 taste buds on the upper surface of the tongue. Each bud contains about 25 sensory receptor cells, on which tiny taste hairs are exposed to drink and food dissolved in saliva. Buds sense the five basic tastes: bitter, sour, salty, sweet, and umami (a savoury, meaty taste). A combination of odours and these basic tastes produce more subtle tastes.
Proprioceptors are types of internal sensory receptors that monitor the degree of stretch of muscles and tendons around the body. This information gives us our sense of balance and our awareness of the position of various parts of the body in relation to each other.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.
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