Chapter 39 The Senses Lecture Outlines by Gregory Ahearn University of North Florida Copyright 2011 Pearson Education Inc Chapter 39 At a Glance 39 1 How Does the Nervous System Sense the Environment 39 2 How Are Mechanical Stimuli Detected 39 5 How Are Chemicals Sensed Biology Life on Earth 9e Copyright 2011 Pearson Education Inc 39 1 How Does the Nervous System Sense the Environment A receptor is a structure that changes when it is acted on by a stimulus and then produces some sort of response A sensory receptor is an entire specialized cell that produces an electrical signal in response to specific stimuli The receptor translates environmental stimuli into the language of the nervous system Sensory receptors are grouped into categories according to the stimulus to which they respond Biology Life on Earth 9e Copyright 2011 Pearson Education Inc 39 1 How Does the Nervous System Sense the Environment The senses inform the brain about the nature and intensity of environmental stimuli Encoding the nature of stimulus light sound odor or touch begins with sensory receptor cells each of which contains receptor molecules that respond to certain stimuli and not others Each type of sensory receptor is linked to a specific set of axons that connect to particular places in the brain or spinal cord The first principle of sensory perception is The type of stimulus is encoded by the specific set of neurons in the brain that are activated Biology Life on Earth 9e Copyright 2011 Pearson Education Inc 39 1 How Does the Nervous System Sense the Environment Many sensory receptors are surrounded by accessory structures Some sensory receptors called free nerve endings consist of branching dendrites of sensory neurons These may respond directly to touch heat cold or pain Many other receptors are contained within structures that help them respond to a specific stimulus and include receptors for light sound taste olfaction pressure and vibration Biology Life on Earth 9e Copyright 2011 Pearson Education Inc 39 2 How Are Mechanical Stimuli Detected Mechanoreceptors are found throughout the human body They include or pressure Receptors in the skin that respond to touch vibration Stretch receptors in many internal organs Position receptors in the joints Receptors in the inner ear that allow us to detect sound Embedded within our skin are several types of mechanoreceptor neurons each with a dendrite that produces a receptor potential when its membrane is stretched or dented Biology Life on Earth 9e Copyright 2011 Pearson Education Inc 39 2 How Are Mechanical Stimuli Detected Mechanoreceptors are found throughout the human body continued The dendrites of some touch receptors are free nerve endings that can produce sensations of touch itching or tickling Endings of other receptors are enclosed in layers of connective tissue and include Pacinian corpuscles which respond to changes in pressure such as rapid vibrations or a sharp poke Meissner s corpuscles which respond to light touch or slow vibrations Ruffini corpuscles which respond to steady pressure The density of mechanoreceptors in the skin varies over the surface of the body Biology Life on Earth 9e Copyright 2011 Pearson Education Inc 39 2 How Are Mechanical Stimuli Detected Mechanoreceptors are found throughout the human body continued Mechanoreceptors in hollow organs such as the stomach rectum and urinary bladder signal fullness by responding to stretch Mechanoreceptors in the joints also respond to stretch and indicate when the joint is straight or bent Receptors in the muscles called muscle spindles indicate if the muscle is contracted or stretched Another type called the Golgi tendon organ is located where a muscle attaches to a tendon and signals how much force the muscle is exerting Biology Life on Earth 9e Copyright 2011 Pearson Education Inc 39 5 How Are Chemicals Sensed Many odors are complex mixtures of molecules that stimulate several receptor proteins Our perception of odors arises as a result of the brain interpreting signals from many different olfactory receptor neurons Dogs have 20 to 40 times more olfactory receptor neurons than humans have and twice as many different types of receptor proteins giving them a far superior sense of smell than ours Biology Life on Earth 9e Copyright 2011 Pearson Education Inc 39 5 How Are Chemicals Sensed Taste receptors detect chemicals dissolved in liquids The human tongue bears about 5 000 taste buds embedded in small bumps called papillae A few taste buds are also found in the back of the mouth and in the pharynx Each taste bud is a small pit in the epithelium opening into the oral cavity through a taste pore A taste bud contains a cluster of 50 to 150 cells of several types supporting cells stem cells and taste receptor cells Biology Life on Earth 9e Copyright 2011 Pearson Education Inc 39 5 How Are Chemicals Sensed Taste receptors detect chemicals dissolved in liquids continued Supporting cells act much like glial cells in the nervous system regulating the composition of the extracellular fluid and helping the receptor cells to function properly The stem cells divide to produce new receptor and support cells following normal wear and tear or a close encounter with scalding hot coffee The taste receptor cells bear microvilli that protrude into the taste pore Dissolved chemicals enter the pore and contract these Taste buds for specific tastes are evenly distributed on microvilli the tongue Biology Life on Earth 9e Copyright 2011 Pearson Education Inc 39 5 How Are Chemicals Sensed There are five known tastes sour salty sweet bitter And one known as umami delicious a receptor that responds to the amino acid glutamate Monosodium glutamate MSG is used in Asian food seasoning to enhance flavor Biology Life on Earth 9e Copyright 2011 Pearson Education Inc 39 5 How Are Chemicals Sensed There are five known tastes continued Sour and salty sensations are caused by hydrogen ions or sodium ions respectively entering certain taste receptor cells through ion channels in the plasma membranes of the microvilli Sweet bitter and umami sensations are caused by specific organic molecules binding to receptor proteins on the surface of the microvilli of other taste receptor cells setting off a chain of intracellular reactions In all cases the taste receptor cells respond by producing a receptor potential Biology Life on Earth 9e Copyright 2011 Pearson Education Inc 39 5 How Are Chemicals Sensed To prove that what we
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