Chapter 57. Touch, Movement, and Pain

1. How many senses do humans have? If you say “five,” you are lumping several distinctively different senses under the label touch. It would be better to call the sense of touch something like “body-sense,” to include a group of systems that detect the following: a) external objects pressing or vibrating against our skin, b) temperature, c) pain signals indicating damage to our skin or internal organs, and d) changes to our body position.

2. The body sensations we call touch originate in specialized sensory receptor cells buried in the skin. Most of these cells are mechanoreceptors, because they generate neural impulses when the skin around them is squeezed or moved. Some receptors respond primarily to light touch or vibration, while others are specialized for heavy, continuous pressure. Other cells called thermoreceptors detect warmth or cold.

3. Scattered throughout the body, in both the skin and internal organs, are special “harm detectors” called nociceptors. These sensory receptors generate neural impulses when they detect potentially damaging events, such as extreme heat or a cut on the skin. The brain uses these signals to create the perception of pain, which helps us avoid further damage.

4. In addition to detecting external objects, our “body-sense” detects the position and movement of the body. Mechanoreceptors in the muscles, tendons, and joints generate neural impulses when the arms or legs are moved—a sense called kinesthesia. Highly specialized cells in the semicircular canals of the inner ear are responsible for our vestibular sense, detecting rotation of the head or forward-backward movement of the head (and the whole body).

5. All the neural impulses from these various “body-sense” receptors travel to the brain’s parietal lobe, where they are processed in the somatosensory cortex, creating perceptions of events happening outside or inside the body.