We receive information about the outside world through our senses, specifically our eyes, ears, nose, tongue, and skin. Specialized nerve cells, which have developed to respond to particular external stimuli, such as light or sound, give rise to the raw material of our perception. Stimulated receptors, also known as afferent neurons, send signals down the connecting pathways to the central nervous system (CNS) and the brain. Information travels on axons to the CNS where it activates other neurons, creating connections between them. These interneuron communications lead to muscular contractions and eventually to actions.
Normally we are not conscious of this process. When the brain is damaged, however, for example by a heart attack, it can affect the function of healthy areas of the body. For instance, if a patient loses his arm after suffering a stroke, he may feel sensations in his phantom hand, believing that it belongs to him. On the contrary, amputees often suffer from ‘phantom limb syndrome’, where they believe that a part of their body is moving, although it really is no longer intact.