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We’ve all heard of the five senses: sight, smell, taste, touch, and hearing. In considering sensory reception from the perspective of the nervous system, we need to examine what’s happening at the cellular level.
Sensory reception begins with specialized cells called sensory receptors. A sensory receptor is a neuron or epithelial cell that generates an electrical impulse in response to a sensory stimulus. When appropriately stimulated, ion channels within the sensory receptor cell’s membrane become altered. That changes the flow of ions across the membrane, and therefore the membrane’s electrical potential. The resulting impulse is transmitted from the sensory receptor to the neuron it contacts,and from there, it propagates into the nervous system.
A sensory receptor cell can be classified according to the type of stimulus that activates it. Let’s consider some of them.
Mechanoreceptors respond to mechanical stimuli, such as changes in pressure, or vibrations, like sound waves. A mechanical deformation of the mechanoreceptor’s membrane triggers the change in its electrical potential.
Mechanoreceptors are distributed throughout the skin, and are largely responsible for what we call the sense of touch. The hair cells of the inner ear are also mechanoreceptors. Sound waves funneled into the ear bend the hairlike projections extending from these cells, triggering an electrical impulse.
Chemoreceptors are the basis for our senses of smell and taste. A chemoreceptor becomes stimulated when a particular smell or taste molecule binds to its particular receptor protein. Binding activates the receptor protein, leading to a change in electrical potential and the generation of an impulse.
The detection of light, and the sense of vision in animals with eyes, depends on photoreceptors. These sensory receptor cells have membrane proteins with light-absorbing pigment molecules attached. When light is absorbed by the pigment, the protein molecule becomes altered. The altered protein initiates events within the cell that lead to a change in the cell’s membrane potential, and an impulse is generated.
The signals conveyed to the nervous system by sensory receptors can be refined by sense organs, the anatomical structures that contain these cells. The variety of eyes to be found in the animal kingdom is a good example. But the information produced by sensory receptors is of no use until it is processed by the nervous system. It’s not so much a photoreceptor or eye that sees, but the brain. Sensation, and our body’s response to it, requires the integrated operation of the entire nervous system, including the brain’s powerful information processing ability.
Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education