The skin contains numerous sensory receptors which receive information from the outside environment. The sensory receptors of the skin are concerned with at least five different senses: pain, heat, cold, touch, and pressure. The five are usually grouped together as the single sense of touch in the classification of the five senses of the whole human body. The sensory receptors vary greatly in terms of structure. For example, while pain receptors are simply unmyelinated terminal branches of neurons, touch receptors form neuronal fiber nets around the base of hairs and deep pressure receptors consist of nerve endings encapsulated by specialized connective tissues. Receptors also vary in terms of abundance relative to each other. For example, there are far more pain receptors than cold receptors in the body. Finally, receptors vary in terms of the concentration of their distribution over the surface of the body, the fingertips having far more touch receptors than the skin of the back.
Other types of receptors located throughout the whole body, including proprioceptive receptors and visceral receptors, receive information about the body's internal environment. Proprioceptive or stretch receptors, located in muscles and tendons, sense changes in the length and tension of muscles and tendons and help to inform the central nervous system of the position and movement of the various parts of the body. Each stretch receptor consists of specialized muscle fibers and the terminal branches of sensor neurons. The muscle fibers and sensor neuron endings are very closely associated and are encased in a sheath of connective tissue.
