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D- sensory system Anatomy and Phyiology Chapter 12 Lecture PowerPoint 12.1: Introduction • General senses • Receptors that are widely distributed throughout the body • Skin, various organs and joints • Special senses • Specialized receptors confined to structures in the head • Eyes, ears, nose and mouth 2 12.2: Receptors, Sensation, and Perception • Sensory receptors • Specialized cells or multicellular structures that collect information from the environment • Stimulate neurons to send impulses along sensory fibers to the brain • Sensation • A feeling that occurs when brain becomes aware of sensory impulse • Perception • A person’s view of the stimulus; the way the brain interprets the information 3 Pathways From Sensation to Perception (Example of an Apple) 4 Receptor Types • Chemoreceptors • Respond to changes in chemical concentrations • Pain receptors (nociceptors) • Respond to tissue damage • Thermoreceptors • Respond to changes in temperature • Mechanoreceptors • Respond to mechanical forces • Photoreceptors • Respond to light 5 Sensory Impulses • Stimulation of receptor causes local change in its receptor potential • A graded electrical current is generated that reflects intensity of stimulation • If receptor is part of a neuron, the membrane potential may generate an action potential • If receptor is not part of a neuron, the receptor potential must be transferred to a neuron to trigger an action potential • Peripheral nerves transmit impulses to CNS where they are analyzed and interpreted in the brain 6 Sensory Adaptation • Ability to ignore unimportant stimuli • Involves a decreased response to a particular stimulus from the receptors (peripheral adaptation) or along the CNS pathways leading to the cerebral cortex (central adaptation) • Sensory impulses become less frequent and may cease • Stronger stimulus is required to trigger impulses 7 12.3: General Senses • Senses associated with skin, muscles, joints and viscera • Three (3) groups: • Exteroceptive senses (exteroceptors) • Senses associated with body surface such as touch, pressure, temperature, and pain • Visceroceptive senses (interoceptors) • Senses associated with changes in the viscera such as blood pressure stretching blood vessels and ingestion of a meal • Proprioceptive senses • Senses associated with changes in muscles and tendons such as at joints 8 Touch and Pressure Senses Free nerve endings • Common in epithelial tissues • Simplest receptors • Sense itching Tactile (Meissner’s) corpuscles • Abundant in hairless portions of skin and lips • Detect fine touch; distinguish between two points on the skin Lamellated (Pacinian) corpuscles • Common in deeper subcutaneous tissues, tendons and ligaments • Detect heavy pressure and vibrations 9 Touch and Pressure Receptors Copyright © The McGraw-Hill Companies, Inc. 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Free nerve endings Section of skin Epithelial cells Epidermis (a) Sensory nerve fiber Epithelial cells Dermis Tactile (Meissner’s) corpuscle (touch receptor) Sensory nerve fiber (b) Lamellated (Pacinian) corpuscle (pressure receptor) Connective tissue cells Sensory nerve fiber (c) b, c: © Ed Reschke 10 Temperature Senses • Warm receptors • Sensitive to temperatures above 25oC (77o F) • Unresponsive to temperature above 45oC (113oF) • Cold receptors • Sensitive to temperatures between 10oC (50oF) and 20oC (68oF) • Pain receptors • Respond to temperatures below 10oC • Respond to temperatures above 45oC 11 Sense of Pain • Free nerve endings • Widely distributed • Nervous tissue of brain lacks pain receptors • Stimulated by tissue damage, chemical, mechanical forces, or extremes in temperature • Adapt very little, if at all 12 Visceral Pain • Pain receptors are the only receptors in viscera whose stimulation produces sensations • Pain receptors respond differently to stimulation • Pain receptors are not well localized • Pain receptors may feel as if coming from some other part of the body • Known as referred pain… 13 Referred Pain • May occur due to sensory impulses from two regions following a common nerve pathway to brain Copyright © The McGraw-Hill Companies, Inc. 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Liver and gallbladder Lung and diaphragm Liver and gallbladder Heart Stomach Pancreas Small intestine Appendix Ovary (female) Colon Kidney Ureter Urinary bladder 14 Pain Nerve Pathways • Acute pain fibers • A-delta fibers • Thin, myelinated • Conduct impulses rapidly • Associated with sharp pain • Well localized • Chronic pain fibers • C fibers • Thin, unmyelinated • Conduct impulses more slowly • Associated with dull, aching pain • Difficult to pinpoint 15 Regulation of Pain Impulses • Thalamus • Allows person to be aware of pain • Cerebral cortex • Judges intensity of pain • Locates source of pain • Produces emotional and motor responses to pain • Pain inhibiting substances: • Enkephalins • Serotonin • Endorphins 16 Proprioception • Mechanoreceptors • Send information to spinal cord and CNS about body position and length, and tension of muscles • Main kinds of proprioceptors: • Pacinian corpuscles – in joints • Muscle spindles – in skeletal muscles* • Golgi tendon organs – in tendons* *considered to be stretch receptors 17 Stretch Receptors Copyright © The McGraw-Hill Companies, Inc. 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Sensory nerve fiber Sensory nerve endings Golgi tendon organ Sensory nerve fiber Tendon Intrafusal fiber Skeletal muscle fiber Muscle spindle Skeletal muscle fiber Connective tissue sheath (a) (b) 18 18 Visceral Senses • Receptors in internal organs • Convey information that includes the sense of fullness after eating a meal as well as the discomfort of intestinal gas and the pain that signals a heart attack 19 Summary of Receptors of the General Senses 20 12.4: Special Senses • Sensory receptors are within large, complex sensory organs in the head • Smell in olfactory organs • Taste in taste buds • Hearing and equilibrium in ears • Sight in eyes 21 Sense of Smell • Olfactory receptors • Chemoreceptors • Respond to chemicals dissolved in liquids • Olfactory organs • Contain olfactory receptors and supporting epithelial cells • Cover parts of nasal cavity, superior nasal conchae, and a portion of the nasal septum 22 Olfactory Receptors Copyright © The McGraw-Hill Companies, Inc. 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Nerve fibers within the olfactory bulb Olfactory tract Olfactory bulb Cribriform plate Olfactory area of nasal cavity Superior nasal concha Nasal cavity Cilia (a) Olfactory receptor cells Columnar epithelial cells Cribriform plate (b) 23 Olfactory Nerve Pathways • Once olfactory receptors are stimulated, nerve impulses travel through • Olfactory nerves olfactory bulbs olfactory tracts limbic system (for emotions) and olfactory cortex (for interpretation) 24 Olfactory Stimulation • Olfactory organs located high in the nasal cavity above the usual pathway of inhaled air • Olfactory receptors undergo sensory adaptation rapidly • Sense of smell drops by 50% within a second after stimulation 25 Sense of Taste • Taste buds • Organs of taste • Located on papillae of tongue, roof of mouth, linings of cheeks and walls of pharynx • Taste receptors • Chemoreceptors • Taste cells – modified epithelial cells that function as receptors • Taste hairs –microvilli that protrude from taste cells; sensitive parts of taste cells 26 Taste Receptors Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Papillae Taste buds Epithelium of tongue Taste cell (a) Taste hair Supporting cell Taste pore (b) Connective tissue Sensory nerve fibers 27 Taste Sensations • Four primary taste sensations • Sweet – stimulated by carbohydrates • Sour – stimulated by acids • Salty – stimulated by salts • Bitter – stimulated by many organic compounds 28 Taste Nerve Pathways • Sensory impulses from taste receptors travel along: • Cranial nerves to… • Medulla oblongata to… • Thalamus to… • Gustatory cortex (for interpretation) 29 Sense of Hearing • Ear • Organ of hearing • Three (3) sections: • External ear • Middle ear • Inner ear 30 External Ear • Auricle • Collects sounds waves • External auditory meatus • Lined with ceruminous glands • Carries sound to tympanic membrane • Terminates with tympanic membrane • Tympanic membrane • Vibrates in response to sound waves Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Auricle Semicircular canals Incus Stapes Malleus Cochlea Vestibulocochlear nerve Oval window (under stapes) Round window Tympanic cavity Tympanic membrane External acoustic meatus Auditory tube Pharynx 31 Middle Ear Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. • Tympanic cavity Auricle • Air-filled space in temporal bone • Auditory ossicles • Vibrate in response to tympanic membrane • Malleus, incus and stapes • Hammer, anvil and stirrup • Oval window • Opening in wall of tympanic cavity • Stapes vibrates against it to move fluids in inner ear Semicircular canals Incus Stapes Malleus Cochlea Vestibulocochlear nerve Oval window (under stapes) Round window Tympanic cavity Tympanic membrane External acoustic meatus Auditory tube Pharynx 32 Auditory Tube Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. • Also known as the Eustachian Auricle tube • Connects middle ear to throat • Helps maintain equal pressure on both sides of tympanic membrane • Usually closed by valve-like flaps in throat Semicircular canals Incus Stapes Malleus Cochlea Vestibulocochlear nerve Oval window (under stapes) Round window Tympanic cavity Tympanic membrane External acoustic meatus Auditory tube Pharynx 33 Inner Ear Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. • Complex system of labyrinths • Osseous labyrinth • Bony canal in temporal bone • Filled with perilymph • Membranous labyrinth • Tube within osseous labyrinth • Filled with endolymph Bony labyrinth Perilymph Membranous labyrinth Endolymph Bony labyrinth (contains perilymph) Membranous labyrinth (contains endolymph) Semicircular canals Utricle Saccule Vestibular nerve Cochlear nerve Scala vestibuli (cut) Scala tympani (cut) Cochlear duct (cut) containing endolymph Ampullae (a) Oval Vestibule window Round Maculae window Cochlea 34 Inner Ear • Three (3) parts of labyrinths: • Cochlea • Functions in hearing • Semicircular canals • Functions in equilibrium • Vestibule • Functions in equilibrium Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Bony labyrinth Perilymph Membranous labyrinth Endolymph Bony labyrinth (contains perilymph) Membranous labyrinth (contains endolymph) Semicircular canals Utricle Saccule Vestibular nerve Cochlear nerve Scala vestibuli (cut) Scala tympani (cut) Cochlear duct (cut) containing endolymph Ampullae (a) Oval Vestibule window Round Maculae window Cochlea 35 Cochlea • Scala vestibuli • Upper compartment • Leads from oval window to apex of spiral • Part of bony labyrinth • Scala tympani • Lower compartment • Extends from apex of the cochlea to round window • Part of bony labyrinth Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Stapes vibrating in oval window Helicotrema Scala vestibuli filled with perilymph Vestibular membrane Basilar membrane Scala tympani filled with perilymph Round window Cochlear duct filled with endolymph Membranous labyrinth 36 Cochlea Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. • Cochlear duct • Portion of membranous labyrinth in cochlea • Vestibular membrane • Separates cochlear duct from scala vestibuli • Basilar membrane • Separates cochlear duct from scala tympani Scala vestibuli (contains perilymph) Vestibular membrane Branch of cochlear nerve Cochlear duct (contains endolymph) Spiral organ (organ of Corti) Basilar membrane Scala tympani (contains perilymph) (a) 37 Animation: Effect of Sound Waves on Cochlear Structures Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer. 38 Organ of Corti Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. • Group of hearing receptor cells (hair cells) • On upper surface of basilar membrane • Different frequencies of vibration move different parts of basilar membrane • Particular sound frequencies cause hairs of receptor cells to bend • Nerve impulse generated Scala vestibuli (contains perilymph) Vestibular membrane Cochlear duct (contains endolymph) Spiral organ (organ of Corti) Branch of cochlear nerve Basilar membrane Scala tympani (contains perilymph) (a) Tectorial membrane Hair cells 39 (b) Branch of cochlear nerve Nerve fibers Supporting cells Basilar membrane Organ of Corti Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Cochlear duct Tectorial membrane Scala tympani Hair cells Basilar membrane (a) (b) a: © John D. Cunningham/Visuals Unlimited; b: © Fred Hossler/Visuals Unlimited 40 Auditory Nerve Pathways Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Auditory cortex (temporal lobe) Thalamus Medial geniculate body of thalamus Midbrain Pons Cochlear nuclei Superior olivary nucleus Medulla oblongata Vestibulocochlear nerve 41 Summary of the Generation of Sensory Impulses from the Ear 42 Sense of Equilibrium • Static equilibrium • Vestibule • Senses position of head when body is not moving • Dynamic Equilibrium • Semicircular canals • Senses rotation and movement of head and body 43 Vestibule • Utricle Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. • Communicates with saccule and membranous portion of Ampullae of semicircular semicircular canals canals • Saccule • Communicates with cochlear duct • Macula • Hair cells of utricle and saccule Vestibulocochlear nerve Cochlea Utricle Cochlear duct Maculae Saccule Vestibule 44 Macula Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. • Responds to changes in head position • Bending of hairs results in generation of nerve impulse Hairs of hair cells bend Gelatinous material sags Otoliths Macula of utricle Hair cells Sensory nerve fiber Gravitational force Supporting cells 45 (a) Head upright (b) Head bent forward Semicircular Canals • Three (3) canals at right angles • Ampulla • Swelling of membranous labyrinth that communicates Ampullae of semicircular with the vestibule canals • Crista ampullaris • Sensory organ of ampulla • Hair cells and supporting cells • Rapid turns of head or body stimulate hair cells Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Vestibulocochlear nerve Cochlea Utricle Cochlear duct Maculae Saccule Vestibule 46 Crista Ampullaris Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Endolymph Semicircular canal Cupula Crista ampullaris (a) Head in still position Ampulla Crista ampullaris Hairs Hair cell Supporting cells Sensory nerve fibers (b) Head rotating (c) 47 Sense of Sight • Visual accessory organs • Eyelids • Lacrimal apparatus • Extrinsic eye muscles 48 Eyelid Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. • Palpebra • Composed of four (4) layers: • Skin • Muscle • Connective tissue • Conjunctiva • Orbicularis oculi – closes eyelid • Levator palpebrae superioris – opens eyelid • Tarsal glands – secrete oil onto eyelashes • Conjunctiva – mucous membrane; lines eyelid and covers portion of eyeball Tendon of levator palpebrae superioris Superior rectus Orbicularis oculi Eyelid Tarsal glands Eyelash Cornea Conjunctiva Inferior rectus 49 Lacrimal Apparatus • Lacrimal gland • Lateral to eye • Secretes tears • Canaliculi • Collect tears • Lacrimal sac • Collects from canaliculi • Nasolacrimal duct • Collects from lacrimal sac • Empties tears into nasal cavity Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Lacrimal gland Superior and inferior canaliculi Lacrimal sac Nasolacrimal duct 50 Extrinsic Eye Muscles • Superior rectus • Rotates eye up and medially Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Medial Superior rectus rectus Superior oblique • Inferior rectus • Rotates eye down and medially • Medial rectus • Rotates eye medially Lateral rectus (cut) Inferior rectus Inferior oblique 51 Extrinsic Eye Muscles Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. • Lateral rectus • Rotates eye laterally Medial Superior rectus rectus Superior oblique • Superior oblique • Rotates eye down and laterally • Inferior oblique • Rotates eye up and laterally Lateral rectus (cut) Inferior rectus Inferior oblique 52 Structure of the Eye • Wall has three (3) layers: • Outer fibrous tunic • Middle vascular tunic • Inner nervous tunic Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Lateral rectus Retina Ciliary body Choroid coat Sclera Suspensory ligaments Vitreous humor Iris Lens Fovea centralis Pupil Cornea Aqueous humor Anterior cavity Anterior chamber Posterior chamber Optic nerve Optic disc Posterior cavity Medial rectus 53 Outer Tunic • Cornea • Anterior portion • Transparent • Light transmission • Light refraction Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Lateral rectus Retina Ciliary body Choroid coat Sclera Suspensory ligaments Vitreous humor Iris Lens Fovea centralis Pupil • Sclera • Posterior portion • Opaque • Protection Cornea Aqueous humor Anterior cavity Anterior chamber Posterior chamber Optic nerve Optic disc Posterior cavity Medial rectus 54 Middle Tunic • Iris • Anterior portion • Pigmented • Controls light intensity • Ciliary body • Anterior portion • Pigmented • Holds lens • Moves lens for focusing • Choroid coat • Provides blood supply • Pigments absorb extra light Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Lateral rectus Retina Ciliary body Choroid coat Sclera Suspensory ligaments Vitreous humor Iris Lens Fovea centralis Pupil Cornea Aqueous humor Anterior cavity Anterior chamber Posterior chamber Optic nerve Optic disc Posterior cavity Medial rectus 55 Anterior Portion of Eye • Filled with aqueous humor Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Cornea Anterior chamber Iris Posterior chamber Ciliary process Suspensory ligaments Ciliary muscles Ciliary body Conjunctiva Vitreous humor Lens Sclera 56 Lens • Transparent • Biconvex • Lies behind iris • Largely composed of lens fibers • Elastic • Held in place by suspensory ligaments of ciliary body Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Cornea Anterior chamber Iris Posterior chamber Ciliary process Suspensory ligaments Ciliary muscles Ciliary body Conjunctiva Vitreous humor Lens Sclera 57 Ciliary Body • Forms internal ring around the front of the eye • Ciliary processes – radiating folds • Ciliary muscles – contract and relax to move lens Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Ciliary processes of ciliary body Suspensory ligaments Lens Retina Choroid coat Sclera 58 Accommodation • Changing of lens shape to view objects Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Ciliary muscle fibers contracted Suspensory ligaments relaxed Lens thick (a) Ciliary muscle fibers relaxed Suspensory ligaments taut Lens thin 59 (b) Iris • Composed of connective tissue and smooth muscle • Pupil is hole in iris • Dim light stimulates radial muscles and pupil dilates • Bright light stimulates circular muscles and pupil constricts Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Sympathetic motor nerve fiber In dim light Radially arranged Smooth muscle fibers of the iris Parasympathetic ganglion Circularly arranged smooth muscle fibers of the iris Pupil In normal light Parasympathetic motor nerve fiber In bright light 60 Aqueous Humor • Fluid in anterior cavity of eye • Secreted by epithelium on inner surface of the ciliary body • Provides nutrients • Maintains shape of anterior portion of eye • Leaves cavity through Canal of Schlemm Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Cornea Aqueous humor Anterior chamber Iris Scleral venous sinus (canal of Schlemm) Lens Sclera Ciliary process Ciliary body Ciliary muscles Vitreous humor Posterior chamber 61 Inner Tunic • Retina • Contains visual receptors • Continuous with optic nerve • Ends just behind margin of the ciliary body • Composed of several layers • Macula lutea – yellowish spot in retina • Fovea centralis – center of macula lutea; produces sharpest vision • Optic disc – blind spot; contains no visual receptors • Vitreous humor – thick gel that holds retina flat against choroid coat 62 Posterior Cavity • Contains vitreous humor – thick gel that holds retina flat against choroid coat Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Lateral rectus Retina Ciliary body Choroid coat Sclera Suspensory ligaments Vitreous humor Iris Lens Fovea centralis Pupil Cornea Aqueous humor Anterior chamber Anterior cavity Posterior chamber Optic nerve Optic disc Posterior cavity Medial rectus 63 Major Groups of Retinal Neurons • Receptor cells, bipolar cells, and ganglion cells - provide pathway for impulses triggered by photoreceptors to reach the optic nerve • Horizontal cells and amacrine cells – modify impulses Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Sclera Pigmented choroid coat Retinal pigment epithelium Rod Cone Receptor cells Horizontal cell Retina Bipolar neuron Amacrine cell Layer of connecting neurons Ganglion cell Nerve fibers Vitreous humor Impulses to optic nerve 64 Light waves Layers of the Eye 65 Light Refraction • Refraction • Bending of light • Occurs when light waves pass at an oblique angle into mediums of different densities Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Perpendicular line Air Light wave Glass Refracted light wave 66 Types of Lenses • Convex lenses cause light waves to converge • Concave lenses cause light waves to diverge Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Light wave Air Convex surface Concave surface Glass Converging light waves (a) Diverging light waves (b) 67 Focusing On Retina • As light enters eye, it is refracted by: • Convex surface of cornea • Convex surface of lens • Image focused on retina is upside down and reversed from left to right Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Light waves Image Retina Object Cornea 68 Visual Receptors • Rods • Cones • Long, thin projections • Short, blunt projections • Contain light sensitive pigment called • Contain light sensitive pigments rhodopsin called erythrolabe, chlorolabe, and • Hundred times more sensitive to light cyanolabe than cones • Provide vision in bright light • Provide vision in dim light • Produce sharp images • Produce colorless vision • Produce color vision • Produce outlines of objects 69 Rods and Cones Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Retinal pigment epithelium Cones Rods Single sensory nerve fiber (a) Many sensory nerve fibers (b) Rod Cone 70 (c) c: © Frank S. Werblin, PhD. 12.6 Clinical Application Refraction Disorders • Concave lens corrects nearsightedness • Convex lens corrects farsightedness Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Lens Point of focus Uncorrected point of focus Cornea Light waves Light waves (a) Eye too long (myopia) Point of focus Concave lens (a) Corrected point of focus Light waves Retina Uncorrected point of focus (b) Normal eye Point of focus Light waves Convex lens (c) Eye too short (hyperopia) (b) 71 Corrected point of focus Visual Pigments • Rhodopsin • Pigments on cones • Light-sensitive pigment in rods • Each set contains different light• Decomposes in presence of light sensitive pigment • Triggers a complex series of • Each set is sensitive to different reactions that initiate nerve wavelengths impulses • Color perceived depends on • Impulses travel along optic nerve which sets of cones are stimulated • Erythrolabe – responds to red • Chlorolabe – responds to green • Cyanolabe – responds to blue 72 Rod Cells Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Discs of membrane within cell Mitochondria Nucleus Synaptic ending Rod cell 73 Stereoscopic Vision • Provides perception of distance and depth • Results from formation of two slightly different retinal images Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Light waves Left eye Right eye 74 Visual Nerve Pathway Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Eye Optic nerve Fibers from nasal (medial) half of each retina crossing over Optic chiasma Optic tract Lateral geniculate body of thalamus Optic radiations 75 Visual cortex of occipital lobe 12.5: Lifespan Changes • Age related hearing loss due to: • Damage of hair cells in organ of Corti • Degeneration of nerve pathways to the brain • Tinnitus • Age-related visual problems include: • Dry eyes • Floaters (crystals in vitreous humor) • Loss of elasticity of lens • Glaucoma • Cataracts • Macular degeneration 76