One of the characteristics of a living organism is its ability to respond to stimuli. The human sensory system is highly evolved and processes thousands of incoming messages simultaneously. This complexity allows you to be aware of your surroundings and take appropriate actions. Sensory receptors are dendrites of sensory neurons specialized for receiving specific kinds of stimuli. Sensory receptors are classified by three methods: Classification by receptor complexity: Free nerve endings are dendrites whose terminal ends have little or no physical specialization. Encapsulated nerve endings are dendrites whose terminal ends are enclosed in a capsule of connective tissue. Sense organs (such as the eyes and ears) consist of sensory neurons with receptors for the special senses (vision, hearing, smell, taste, and equilibrium) together with connective, epithelial, or other tissues. Classification by location: Exteroceptors occur at or near the surface of the skin and are sensitive to stimuli occurring outside or on the surface of the body. These receptors include those for tactile sensations, such as touch, pain, and temperature, as well as those for vision, hearing, smell, and taste. Interoceptors (visceroceptors) respond to stimuli occurring in the body from visceral organs and blood vessels. These receptors are the sensory neurons associated with the autonomic nervous system. Proprioceptors respond to stimuli occurring in skeletal muscles, tendons, ligaments, and joints. These receptors collect information concerning body position and the physical conditions of these locations. Classification by type of stimulus detected: Mechanoreceptors respond to physical force such as pressure (touch or blood pressure) and stretch. Photoreceptors respond to light. Thermoreceptors respond to temperature changes. Chemoreceptors respond to dissolved chemicals during sensations of taste and smell and to changes in internal body chemistry such as variations of O 2, CO 2, or H + in the blood. Nociceptors respond to a variety of stimuli associated with tissue damage. The brain interprets the pain.