The impulses involved in sensations follow very pre-cise pathways, which all have the following parts:
1. Receptors—detect changes (stimuli) and generate impulses. Receptors are usually very specific with respect to the kinds of changes they respond to. Those in the retina detect light rays, those in the nasal cavities detect vapors, and so on. Once a spe-cific stimulus has affected receptors, however, they all respond in the same way by generating electri-cal nerve impulses.
2. Sensory neurons—transmit impulses from recep-tors to the central nervous system. These sensory neurons are found in both spinal nerves and cranial nerves, but each carries impulses from only one type of receptor.
3. Sensory tracts—white matter in the spinal cord or brain that transmits the impulses to a specific part of the brain.
4. Sensory areas—most are in the cerebral cortex. These areas feel and interpret the sensations. Learning to interpret sensations begins in infancy, without our awareness of it, and continues through-out life.