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Unformatted text preview: Chapter 17: Sense Organs Chapter Overview Human senses enable us to appreciate visual beauty and music, taste delicious foods, and enjoy the richness of the world around us. Senses are also crucial for tactile, visual, and auditory communication. In sum, senses provide our connection to our physical and social environment. They underlie our ability to respond to external and internal stimuli and maintain homeostasis. Receptor Types and the General Senses A receptor is any structure specialized to detect a stimulus. Receptors may be nerve endings, or dendrites, while others are elaborate sense organs. Classification of Receptors Receptors may be classified according to the type of stimulus to which they respond (modality). Thermoreceptors respond to heat and cold whereas photoreceptors respond to light. Chemoreceptors respond to chemicals such as odors. Nociceptors respond to pain. Pressure, stretch, or vibrations stimulate mechanoreceptors. Receptors may also be classified according to their distribution in the body. General (somatosensory) senses are widely distributed in the skin, muscles, tendons, joints, and viscera and sense touch, temperature, pain, etc. Special senses, in contrast, are associated with complex sense organs in the head, and include vision, smell, hearing, taste, and equilibrium. Another way to classify receptors is by where stimuli originate. Exteroceptors sense stimuli from outside the body such as light, or chemicals that stimulate taste and smell. Interoceptors detect stimuli in internal organs such as the stomach and intestines and produce sensations such as nausea. Proprioceptors sense movement and position of the body and are located in muscles, tendons, and joints. The General Senses Receptors for general senses are relatively simple in structure and physiology. Some have unencapsulated nerve endings, i.e. the sensory dendrites lack a connective tissue covering. Examples of unencapsulated nerve endings follow. Free nerve endings Free nerve endings are bare dendrites with no special association with specific accessory cells or tissues. They are most abundant in connective tissue and epithelium and include receptors for heat and cold and nociceptors. Tactile (Merkel) discs Tactile discs are flattened nerve endings associated with a tactile (Merkel) cell at the base of the epidermis. They are receptors for light touch and pressure on the skin....
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This note was uploaded on 02/09/2012 for the course BIO 102 taught by Professor William during the Spring '11 term at Harvard.
- Spring '11