A sensation (perception) is a feeling that occurs when the brain interprets sensory impulses. Because all the nerve impulses that travel away from sensory receptors into the central nervous system are alike, the resulting sensation depends on which region of the cerebral cortex receives the impulse. For example, impulses reaching one region are always interpreted as sounds, and those reaching another portion are always sensed as touch.
It makes little difference how receptors are stimulated. Pain receptors, for example, can be stimulated by heat, cold, or pressure, but the sensation is always the same because in each case the same part of the brain interprets the resulting nerve impulses as pain. Similarly, factors other than light, such as a sharp blow to the head, may trigger nerve impulses in visual receptors. When this happens, the person may "see" lights, even though no light is entering the eye, since any impulses reaching the visual cortex are interpreted as light. Normally receptors only respond to specific stimuli, so the brain creates the correct sensation for that particular stimulus.
At the same time that a sensation forms, the cerebral cortex interprets it to seem to come from the receptors being stimulated. This process is called projection because the brain projects the sensation back to its apparent source. Projection allows a person to pinpoint the region of stimulation. Thus, we are aware that the eyes see an apple, the nose smells it, and the ears hear the teeth crunch into it.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.