Sensory System Lecture Outline PDF

Chapter 07 Lecture Outline* Sensory Physiology Eric P. Widmaier Boston University Hershel Raff Medical College of Wisconsin Kevin T. Strang University of Wisconsin - Madison *See PowerPoint Image Slides for all fi...

Chapter 07 Lecture Outline* Sensory Physiology Eric P. Widmaier Boston University Hershel Raff Medical College of Wisconsin Kevin T. Strang University of Wisconsin - Madison *See PowerPoint Image Slides for all figures and tables pre-inserted into PowerPoint without notes. 1 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Sensory Receptors Sensory receptors are specialized cells that generate graded potentials called receptor potentials in response to a stimulus. There are five major divisions of these sensory receptors based on stimuli that they respond to: 1. Mechanoreceptors 2. Thermoreceptors 3. Photoreceptors 4. Chemoreceptors 5. Nociceptors. 2 Types of Sensory Receptors Fig. 7-1 3  Somatic Sensation Sensation from the skin, muscles, bones, tendons and joints, or somatic sensation, is initiated by a variety of sensory receptors collectively called somatic receptors. These receptors respond to: – Touch and pressure – Sense of posture and movement – Temperature – Pain 4 Touch and Pressure Receptors Fig. 7-15 5 Pain Transmission Fig. 7-16 6  Pain Pain differs significantly from the other somatosensory modalities. After transduction of the first noxious stimuli into action potentials in the afferent neuron, a series of changes occur in components of the pain pathway that alter the way these components respond to subsequent stimuli. Hyperalgesia is an increased sensitivity to painful stimuli. The pain can last for hours after the original stimulus is over. (This type of pain response is common with severe burn injuries.) Pain can be altered by past experiences, suggestion, emotions (particularly anxiety), and the simultaneous activation of other sensory modalities. 7  Pain Management Analgesia is the selective suppression of pain without effects on consciousness or other sensations. We use many mechanisms to achieve pain relief: – Electrical stimulation of specific areas of the central nervous system – Pharmacological agents (NSAIDs like Tylenol®) and Morphine (opioids) – Some of the neurons in these inhibitory pathways release morphine- like endogenous opioids. – Acupuncture (seems to be linked to activation of the endogenous opioid pathways) – Transcutaneous Electrical Stimulation (TEMS) – Massage 8 Referred Pain The brain is “confused” and you feel pain from an internal organ as another area of the Fig. 7-17 body. Fig. 7-18 9  Vision The eyes are composed of: – An optical component, which focuses the visual image on the receptor cells – A neural component, which transforms the visual image into a pattern of graded and action potentials 10 Light Fig. 7-21 11 Anatomy of the Human Eye Fig. 7-22 12 The Optics of Vision: Refraction Fig. 7-23 13 Fig. 7-25 14 Fig. 7-26 15 Photoreceptor cells Fig. 7-27 16 Neural Pathways for Vision Fig. 7-31 17  Color Vision The colors we perceive are related to the wavelengths of light that the pigments in the objects of our visual world reflect, absorb, or transmit. For example, an object appears red because it absorbs shorter wavelengths, which would be perceived as blue, while it reflects the longer wavelengths, perceived as red, to excite the photopigment of the retina most sensitive to red. Light perceived as white is a mixture of all wavelengths, and black is the absence of all light. Color vision begins with activation of the photopigments in the cone photoreceptor cells. Human retinas have three kinds of cones —one responding optimally at long wavelengths (“L” cones), one at medium wavelengths (“M” cones), and the other stimulated best at short wavelengths (“S” cones). 18 Color vision Fig. 7-32 19  Color Vision Although each type of cone is excited most effectively by light of one particular wavelength, there is actually a range of wavelengths within which a response will occur. Our ability to discriminate color also depends on the intensity of light striking the retina. In brightly lit conditions, the differential response of the cones allows for good color vision. In dim light, however, only the highly sensitive rods are able to respond. Though rods are activated over a range of wavelengths that overlap with those that activate the cones, there is no mechanism for distinguishing between frequencies. Thus, objects that appear vividly colored in bright daylight are perceived in shades of gray at night. 20  Color Blindness There are several types of defects in color vision that result from mutations in the cone pigments. The most common form of color blindness, red-green color blindness, is present predominantly in men, affecting 1 out of 12. Men with red-green color blindness either lack the red or the green cone pigments entirely or have them in an abnormal form. Because of this, the discrimination between shades of these colors is poor. Color blindness results from a recessive mutation in one or more genes encoding the cone pigments. 21 Fig. 7-34 22  Hearing The sense of hearing is based on the physics of sound and the physiology of the external, middle, and inner ear, the nerves to the brain, and the brain regions involved in processing acoustic information. Sound energy is transmitted through gaseous, liquid, or solid medium by setting up a vibration of the medium’s molecules, air being the most common medium. When there are no molecules, as in a vacuum, there can be no sound. 23 Hearing: Sound Fig. 7-36 24 Anatomy of the Human Ear Fig. 7-37 25 Sound Transmission in the Ear Fig. 7-39 26  Cochlea & the Organ of Corti Fig. 7-40 27 Hair Cells of the Organ of Corti Fig. 7-41 28  Chemical Senses Chemicals binding to specific chemoreceptors are responsible for the detection of taste and smell. 29  Taste Receptors Taste (gustation) works via the taste buds. Taste buds are small groups of cells arranged like orange slices around a hollow pore. Microvilli increase the surface area of taste receptor cells, and contain integral membrane proteins that transduce the presence of a given chemical into a receptor potential. At the bottom of taste buds are basal cells, which divide and differentiate to continually replace taste receptor cells damaged in the occasionally harsh environment of the mouth. To enter the pores of the taste buds and come into contact with taste-receptor cells, food molecules must be dissolved in liquid, either ingested or provided by secretions of the salivary glands. 30  Types of Taste Receptors Many different chemicals can generate the sensation of taste by differentially activating a few basic types of taste receptors: – Sweet – Sour – Salty – Bitter – Umami (pronounced “oo-MAH-mee”). This latter category is named after a Japanese word that can be roughly translated as “delicious.” This taste is associated with the taste of glutamate and similar amino acids, and is sometimes described as conveying the sense of savoryness or flavorfulness. 31  Signaling of Taste Receptors Each group of tastes has a distinct signal transduction mechanism. Salt taste = sodium ions Sour taste = hydrogen ions Sweet taste = glucose (G protein-coupled) Bitter flavor is associated with many poisonous substances, especially plant alkaloids like strychnine and arsenic (G protein-coupled). Umami receptor cells also depolarize via a G protein- coupled receptor mechanism. 32 Taste Receptors Fig. 7-47 33  Olfactory Receptors The olfactory receptor neurons lie in the olfactory epithelium, in the upper part of the nasal cavity. Olfactory receptor neurons last for only about two months, so they are constantly being replaced by new cells produced from stem cells in the olfactory epithelium. The cilia contain the receptor proteins that provide the binding sites for odor molecules. The axons of the neurons form the olfactory nerve, which is cranial nerve I. 34  Smell Proteins in the mucus may interact with the odorant molecules, transport them to the receptors, and facilitate their binding to the receptors. Stimulated odorant receptors activate a G protein-mediated pathway that increases cAMP, which in turn opens nonselective cation channels and depolarizes the cell. 35 Olfactory receptors Fig. 7-48 36 Factors that Affect the Sense of Smell Olfactory discrimination varies with: – Attentiveness, – Hunger (sensitivity is greater in hungry subjects), – Gender (women in general have keener olfactory sensitivities than men), – Smoking (decreased sensitivity has been repeatedly associated with smoking), – Age (the ability to identify odors decreases with age, and a large percentage of elderly persons cannot detect odors at all), – State of the olfactory mucosa (as we have mentioned, the sense of smell decreases when the mucosa is congested, as in a head cold). Some individuals are born with genetic defects resulting in a total lack of the ability to smell (anosmia). 37