![]() ![]() Its role in finding food and warning of danger is vital for survival. Smell is the most primitive of the senses. The receptors of the chemical sensory systems send electrical signals to the brain when they bind to airborne (for smell) and dissolved (taste) molecules. By 2010, some 219,000 people had received cochlear implants, which restore functional hearing by directly stimulating the auditory nerve. Hearing is the first sense for which a device was developed to replace non-working receptors. The latter include Wernicke’s area in the dominant hemisphere, essential for language comprehension, and a right-hemisphere area that processes emotional aspects of speech. Past the primary auditory cortex, signals divide into streams that locate and analyze what we hear. Another pathway permits processing en route-coordinating signals from both ears, for example. Direct connections facilitate quick reaction to loud sounds. The frequencies to which each hair cell responds depends on its position in the cochlea.Īuditory signals pass through intermediate brain regions to the primary auditory cortex in the temporal lobe. Movement of the membrane lining the cochlea stretches the tips of hair cells to allow an influx of K+ and Ca+2 ions-charged particles that build up to generate electrical signals in acoustic nerve fibers. Hair cells within the cochlea are the receptors of the auditory system. A series of tiny bones (ossicles) in the middle ear amplify and transmit these vibrations to the cochlea of the inner ear, a fluid-filled tube rolled up like a snail shell. Sounds enter the system via the inch-long ear canal, which terminates at the tympanic membrane (eardrum) whose vibrations represent an exquisitely complex response to pressure variations. The auditory system transforms mechanical energy-sound waves-into nerve impulses. All told, some 30 cortical areas participate in visual processing. Scientists have identified brain structures specializing in faces, places, and words, among other things. In the brain, visual input passes through intermediate areas to the primary, or striate, visual cortex in the occipital lobe, and then to a secondary visual cortex where the information separates into streams: dorsal pathways, which analyze image size and position and ventral pathways that discriminate color and shape, allowing us to recognize what we see. Processing, sorting, and condensing of signals actually starts here in the retina. Light striking the surface of a photoreceptor activates a protein, rhodopsin, that initiates a chemical cascade of signals through intricate cellular wiring that ultimately converge in ganglion cells, neurons whose axons form the optic nerve. Cone cells come in three subtypes that respond to wavelengths roughly corresponding to red, green, and blue light. Rods are sensitive enough to respond to dim light, but don’t process colors. The retina contains several layers of cells, but the actual sensory work is done by two kinds of photoreceptor cells in the deepest layer. The shape of the lens changes to adjust for near and distant vision. The cornea and lens, transparent structures at the front of the eye, focus light onto the retina, the photosensitive area that covers three-fourths of the back. The brain devotes more space to processing and storing visual information than all other senses combined. While all the senses are important, we tend to rely most on sight. We might think of the principal senses as variations on a common theme. Although sensory systems share basic features of organization, each is uniquely designed to respond to a particular aspect of the world. ![]()
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