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Chapter 12 Lecture Outline
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12.1: Introduction to Sensory Function
Senses maintain homeostasis, by providing information about the outside world and the internal environment Sensory receptors collect information from the environment, and relay it to the CNS on sensory neurons Sensory receptors link nervous system to internal and external changes or events Sensory receptors can be specialized cells or multicellular structures 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
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12.2: Receptors, Sensation, and Perception
Sensory receptors: Respond to specific stimuli Particularly sensitive to a certain type of environmental change, and less sensitive to other stimuli Sensation: A feeling that occurs when brain becomes aware of sensory information Perception: The way the brain interprets the sensory information
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Receptor Types 5 types of sensory receptors in the body:
Chemoreceptors: Respond to changes in chemical concentrations (smell, taste, oxygen concentration) Pain receptors (nociceptors): Respond to tissue damage (mechanical, electrical, thermal energy) Thermoreceptors: Respond to moderate changes in temperature Mechanoreceptors: Respond to mechanical forces that distort receptor (touch, tension, blood pressure, stretch) Photoreceptors: Respond to light (eyes)
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Sensory Impulses Sensory receptors can take the form of ends of neurons or cells near extensions of the neurons Stimulation of receptor causes local change in its membrane potential, causing graded potential according to stimulus intensity 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
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Sensation and Perception
Sensation occurs when action potentials make the brain aware of a sensory event (such as pain) Perception occurs when brain interprets sensory impulses (realizing that the pain is a result of stepping on a tack) Projection: Process in which cerebral cortex interprets sensation as being derived from certain receptors Brain projects the sensation back to the apparent source It allows a person to pinpoint the region of stimulation
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Information Flow Through the Nervous System
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Sensory Adaptation Sensory Adaptation:
Ability to ignore unimportant (or continuous) 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) When sensory adaptation occurs, sensory impulses become less frequent and may cease Stronger stimulus is then required to trigger impulses Best accomplished by thermoreceptors and olfactory receptors
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12.3: General Senses General Senses:
Senses with small, widespread sensory receptors, associated with skin, muscles, joints and viscera General Senses are divided into 3 groups: Exteroceptive senses: Senses associated with body surface, such as touch, pressure, temperature, and pain Interoceptive (visceroceptive) senses: Senses associated with changes in the viscera, such as blood pressure stretching blood vessels Proprioceptive senses: Senses associated with changes in muscles, tendons, and joints, as when changing position or exercising
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Touch and Pressure Senses
3 types of mechanoreceptors provide 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 and texture Distinguish between 2 points on skin Lamellated (Pacinian) corpuscles: Large oval structures Common in deeper subcutaneous tissues, tendons and ligaments Detect heavy pressure and vibrations
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Touch and Pressure Receptors
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Temperature Senses Temperature receptors (thermoreceptors):
Free nerve endings in skin; 2 types: Warm receptors: Sensitive to temperatures above 25°C (77°F) Unresponsive to temperature above 45°C (113°F) Cold receptors: Sensitive to temperatures between 10°C (50°F) and 20°C (68°F) Pain receptors: Respond to temperatures below 10°C; produce freezing sensation Respond to temperatures above 45°C; produce burning sensation
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Sense of Pain Pain receptors/nociceptors consist of 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
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Visceral Pain Pain receptors are the only receptors in viscera whose stimulation produces sensations Pain receptors in viscera respond differently to stimulation than those of surface tissues Visceral pain may feel as if coming from some other part of the body; this is called referred pain Example of referred pain: Heart pain often feels like it is coming from the left shoulder or medial portion of left arm Referred pain results from common nerve pathways, in which sensory impulses from the visceral organ and a certain area of the skin synapse with the same neuron in the CNS
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Referred Pain Referred pain may occur due to sensory impulses from two regions following a common nerve pathway to brain:
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Referred Pain
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Pain Pathways 2 types of axons/fibers that conduct impulses away from pain receptors: Fast pain (A-delta) fibers: Myelinated Conduct impulses rapidly (up to 30 m/sec) Associated with sharp (acute) pain in localized skin area Usually stops as soon as stimulus stops Slow pain (C) fibers: Unmyelinated Conduct impulses slowly (up to 2 m/sec) Associated with dull, aching (chronic) pain Difficult to localize Pain often continues after stimulus stops In the brain, most pain fibers synapse in reticular formation, and proceed to the thalamus, hypothalamus, and cerebral cortex
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Regulation of Pain Pathways
Thalamus: Begins sensation of pain Cerebral cortex: Judges intensity of pain Locates source of pain Produces emotional and motor responses to pain Gray matter in brainstem: Regulates flow of impulses from spinal cord Pain-inhibiting substances produced in the body: Enkephalins Serotonin Endorphins
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Clinical Application 12.1 Treating Pain
About 25% of people have moderate or severe pain. Treatments for pain: Nonsteroidal anti-inflammatory drugs (NSAIDs), such as aspirin, ibuprofen: can cause GI irritation Opiates: can be addicting Acetaminophen: non-addicting, no GI irritation, but can damage liver Reformulation efforts are producing smaller particles, that dissolve faster and relieve pain faster Chronic pain treatments: NSAIDs, exercises, injection of anesthetics into cramping muscles, antidepressants, transcutaneous electrical nerve stimulation (TENS), invasive nerve block
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Proprioception Proprioceptors:
Mechanoreceptors that send information to CNS about body position, and length and tension of skeletal muscles Main types of proprioceptors: Lamellated (Pacinian) corpuscles: Pressure receptors in joints Muscle spindles: Stretch receptors in skeletal muscles Initiate stretch reflexes, in which spindle stretch causes muscle contraction Golgi tendon organs: Stretch receptors in tendons Stimulate reflexes that oppose stretch reflexes Help maintain posture, protect tearing of muscles away from insertions
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Stretch Receptors
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Visceral Senses Visceral senses have receptors in internal organs
Examples of visceral receptors: lamellated corpuscles, free nerve endings 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
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Receptors Associated with General Senses
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12.4: Special Senses Special Senses:
Senses that have sensory receptors are within large, complex sensory organs in the head: Smell: olfactory organs in nasal cavity Taste: taste buds in oral cavity Hearing and equilibrium: inner ears Sight: eyes
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Mixed-Up Senses--Synesthesia
Clinical Application 12.2 Mixed-Up Senses--Synesthesia Synesthesia (joined sensation): Condition in which brain interprets a stimulus for one sense as coming from another Example: “The paint smelled blue” 1 in 1,000 people have this condition, and it lasts a lifetime Common in creative people Caused by genetic mutation; 4 genes have been identified Most common form is grapheme-color type synesthesia: Letters, numbers or time evoke certain colors Lexical-gustatory synesthesia: A name evokes perception of strong taste or smell
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Sense of Smell Olfaction: the sense of smell Olfactory receptors:
Olfactory receptor cells are chemoreceptors Respond to chemicals dissolved in liquids Sense of smell provides 75-80% of sense of taste Olfactory organs: Contain olfactory receptor cells (bipolar neurons) and supporting epithelial cells Cover upper parts of nasal cavity, superior nasal conchae, and a portion of the nasal septum Odorants may bind to any of almost 400 types of olfactory membrane receptors, resulting in depolarization and action potentials
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Olfactory Receptors
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Olfactory Pathways Once olfactory receptors are stimulated, nerve impulses travel through openings in cribriform plates of ethmoid bone Olfactory nerves → olfactory bulbs → olfactory tracts → limbic system (for emotions) and olfactory cortex (for interpretation) Olfactory bulbs analyze sensory impulses Limbic system, center for memory and emotion, provides emotional responses to certain odorant molecules
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Olfactory Stimulation
Leading hypothesis for encoding specific smells: Each olfactory receptor cell contains only 1 type of membrane protein Each type of membrane protein can bind several types of odorants Brain interprets binding as an olfactory code Olfactory organs located high in the nasal cavity, above the pathway of inhaled air Olfactory receptors undergo sensory adaptation rapidly Sense of smell drops by 50% within 1 second after stimulation Olfactory receptor neurons are the only ones in direct contact with environment, and can be damaged Receptors are replaced regularly (very unusual for neurons to be replaced in the human adult)
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Gustation: Sense of Taste
Gustation is the sense of taste. Taste buds: Organs of taste Located on papillae of tongue, roof of mouth, linings of cheeks and walls of pharynx About 10,000 taste buds, each with taste cells Taste receptors: Chemoreceptors Taste cells: modified epithelial cells that function as receptors Taste hairs: microvilli that protrude from taste cells through pores of taste buds; sensitive parts of taste cells Taste cells are replaced every 3 days
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Taste Receptors
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Taste Sensations 5 primary taste sensations:
Sweet: stimulated by carbohydrates Sour: stimulated by acids (H+) Salty: stimulated by salts (N a+ or K+) Bitter: stimulated by many organic compounds, Mg and C a salts Umami: stimulated by some amino acids, MSG Each flavor results from 1 primary taste sensation or a combination Spicy foods activate pain receptors Taste receptors undergo rapid adaptation
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Taste Pathways Sensory impulses from taste receptor cells travel on fibers of 3 different cranial nerves, according to the location of the taste cells: Facial nerve (VII) Glossopharyngeal nerve (IX) Vagus nerve (X) Cranial nerves conduct impulses into medulla oblongata Impulses then proceed to the thalamus Impulses are interpreted in the gustatory cortex in the insula
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Smell and Taste Disorders
Clinical Application 12.3 Smell and Taste Disorders Disorders of smell and taste can be caused by colds, flu, allergies, nasal polyps, swollen mucous membranes in the nose, head injury, toxic chemical exposure, nutritional or metabolic problem, a disease Often, cause cannot be identified Drugs and medications can also alter smell or taste
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Sense of Hearing Ear: Organ of hearing 3 sections of the ear:
Outer/external ear Middle ear Inner/internal ear
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Outer (External) Ear Parts of the Outer Ear: Auricle (Pinna):
Funnel-shaped Collects sounds waves External acoustic meatus: S-shaped tube Lined with ceruminous glands Carries sound to tympanic membrane Terminates at tympanic membrane Tympanic membrane (Eardrum): Vibrates in response to sound waves
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Middle Ear Parts of the Middle Ear: Tympanic cavity:
Air-filled space in temporal bone Auditory ossicles: 3 tiny bones Vibrate in response to tympanic membrane vibrations; amplify force 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
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Middle Ear: Tympanic Reflex
Muscle contractions that occur during loud sounds, to lessen the transfer of sound vibrations to inner ear, and prevent damage to hearing receptors Muscles involved are tensor tympani and stapedius
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Middle Ear: Auditory Tube
Auditory (eustachian) tube: Connects middle ear to throat Helps maintain equal pressure on both sides of tympanic membrane Usually closed by valve-like flaps in throat
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Inner (Internal) Ear Inner ear is a complex system of labyrinths:
Osseous (bony) labyrinth: Bony canal in temporal bone Filled with perilymph fluid Membranous labyrinth: Tube within osseous labyrinth Filled with endolymph fluid
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Inner Ear 3 parts of labyrinths: Cochlea: Functions in hearing
Semicircular canals: Function in dynamic equilibrium Vestibule: Functions in static equilibrium
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Cochlea Cochlea: Spiral, snail-shaped tube
Coiled around bony core, the modiolus Spiral lamina is a bony shelf that coils around the cochlea
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Windows of the Inner Ear
There are 2 membrane-covered “windows” in the wall of the bony labyrinth: Oval window: Opening in the wall of the tympanic cavity, through which the stapes transfers vibrations to the fluid of the inner ear; these vibrations stimulate hearing receptors Round window: Window in the wall of the inner ear facing the tympanic cavity, through which excess vibrations dissipate into the tympanic cavity
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Cochlea The cochlea contains 3 compartments: 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 Cochlear duct: Middle compartment Portion of membranous labyrinth in cochlea
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Cochlea The cochlea contains these membranes: Vestibular membrane:
Separates scala vestibuli from cochlear duct Basilar membrane: Separates cochlear duct from scala tympani Tectorial membrane: Extends partially into cochlear duct; part of the hearing receptor organ, the Spiral Organ
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Getting a Cochlear Implant
Clinical Application 12.4 Getting a Cochlear Implant Hearing aids amplify sound, but cochlear implants directly stimulate auditory nerve Enables a person to hear certain sounds Best time is before age 3, as brain is rapidly processing speech and hearing
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Spiral Organ (Organ of Corti)
Organ for sense of hearing Sits on upper surface of basilar membrane Contains hearing receptor cells, called hair cells Hair cells contain stereovilli (or stereocilia) Tectorial membrane passes like a roof over the hair cell stereovilli Sound vibrations cause stereocilia to contact and bend against the tectorial membrane Different frequencies of vibration move different parts of basilar membrane Nerve impulses are generated
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Spiral Organ (Organ of Corti)
Sound reception occurs as the basilar membrane vibrates, vibrating the hair cells of the spiral organ, and putting them in contact with the tectorial membrane.
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Spiral Organ (Organ of Corti)
This is a “straightened out” view of the cochlea. Receptor cells in different regions of the cochlear duct detect different frequencies of vibration (cycles/sec or cps).
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Auditory Pathways Cochlear branch of vestibulocochlear nerve ↓
Medulla oblongata Midbrain Thalamus Auditory cortex in temporal lobe of cerebrum
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Clinical Application 12.5 Hearing Loss
About 8% of people have some decree of hearing loss 2 major types of hearing loss: Conductive and Sensorineural Conductive Deafness: Interference with conduction of sound vibrations to inner ear 95% of cases of hearing loss Caused by accumulation of ear wax, hardening or injury of tympanic membrane, injury to auditory ossicles, otosclerosis Diagnostic tests: Rinne test and Weber test Sensorineural Deafness: Damage to cochlea, auditory (vestibulocochlear) nerve, or nerve pathways Can be caused by long-term exposure to very loud sounds, such as factory noise, loud music, explosions Also caused by CNS tumors, brain damage resulting from a stroke, use of certain drugs
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Steps in Generation of Sensory Impulses from the Ear
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Sense of Equilibrium Feeling of equilibrium/balance is derived from 2 senses: Static equilibrium: Senses position of head when body is not moving Receptors are found in vestibule of inner ear Dynamic Equilibrium: Senses rotation and movement of head and body Receptors are found in semicircular canals
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Static Equilibrium Utricle and saccule are expanded chambers of the membranous labyrinth of the vestibule Each contains a Macula, an organ of static equilibrium A macula is a patch of hair cells and supporting cells
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Static Equilibrium A macula has hair cells embedded in gelatinous material, with otoliths (calcium carbonate crystals) on its surface Gravity pulls on gelatinous mass when head changes position Otoliths shift position, and pull on gelatinous mass and cilia of hair cells Bending of hairs results in generation of nerve impulses in vestibular branch of the vestibulocochlear nerve
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Dynamic Equilibrium Receptors for dynamic equilibrium are found in 3 semicircular canals 3 canals sit at right angles to each other Each canal contains an ampulla: a swelling of the membranous labyrinth that communicates with the vestibule Crista ampullaris: Sensory organ for dynamic equilibrium Hair cells and supporting cells Located in ampulla of each semicircular canal Consists of hair cells whose hairs extend upward into dome-shaped gelatinous mass (cupula) Rotation of head or body bends cupula, stimulates hair cells Nerve impulses are sent over vestibular branch of vestibulocochlear n.
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Dynamic Equilibrium: Crista Ampullaris
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Sense of Sight Visual receptors are found in the eye
Accessory organs for sense of sight: Eyelids (palpebrae, protection) Eyelashes (protection) Lacrimal apparatus (tear production) Extrinsic eye muscles (eye movement)
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Visual Accessory Organs: Eyelids
Eyelids = Palpebrae Composed of 4 layers: Skin Muscle Connective tissue Conjunctiva Orbicularis oculi muscle closes eyelid Levator palpebrae superioris muscle opens eyelid Tarsal glands secrete oil onto eyelashes Conjunctiva: mucous membrane; lines eyelid and covers portion of eyeball
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Visual Accessory Organs: Lacrimal Apparatus
Lacrimal gland: In orbit, lateral to eye Secretes tears Canaliculi: 2 ducts that collect tears Lacrimal sac: Collects tears from canaliculi Lies in groove in lacrimal bone Nasolacrimal duct: Collects from lacrimal sac Empties tears into nasal cavity Lysozyme: Antibacterial component of tears
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Visual Accessory Organs: Extrinsic Eye Muscles
Superior rectus: Rotates eye up and medially Inferior rectus: Rotates eye down and medially Medial rectus: Rotates eye medially Lateral rectus: Rotates eye laterally Superior oblique: Rotates eye down and laterally Inferior oblique: Rotates eye up and laterally
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Structure of the Eye Hollow, spherical organ of sight
Wall has 3 layers: Outer (fibrous) tunic Middle (vascular) tunic Inner (nervous) tunic
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The Outer (Fibrous) Tunic
Cornea + Sclera Cornea: Anterior sixth Transparent Helps focus light rays Transmits and refracts light Sclera: Posterior five sixths White, opaque Protects eye, attaches muscles Pierced by optic nerve and blood vessels
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Middle (Vascular) Tunic
Middle (vascular) tunic: Choroid coat + Ciliary body + Iris Choroid coat: Posterior five-sixths Provides blood supply Contains melanocytes Melanin absorbs extra light Ciliary body: Anterior portion Thickest portion, pigmented Forms ring to hold lens Changes lens shape for focusing Iris: Anterior to ciliary body In front of lens Pigmented Controls light entering eye
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Anterior Portion of the Eye
Anterior cavity of eye, between cornea and lens, is filled with a watery fluid, aqueous humor Lens: Transparent, biconvex, lies behind iris, elastic, held in place by suspensory ligaments of ciliary body; helps focus light rays, and changes shape for long-distance or close vision
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Ciliary Body and Lens Ciliary body forms internal ring around the front of the eye Ciliary processes are the radiating folds Ciliary muscles contract and relax to move lens Suspensory ligaments hold lens in position Lens lies just behind iris and pupil
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Accommodation Accommodation:
A change in the shape of the lens, to view close objects Lens thickens and becomes more convex when focusing on close object Lens thins and becomes flatter when focusing on distant objects
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Iris Iris controls amount of light entering the eye
Iris consists of connective tissue and smooth muscle (colored portion of eye) Pupil is window or opening in center of iris Dim light stimulates radial muscles and pupil dilates Bright light stimulates circular muscles and pupil constricts Amount and distribution of melanin determines eye color
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Aqueous Humor Aqueous Humor: Fluid in anterior cavity of eye
Secreted by epithelium on inner surface of the ciliary body Provides nutrients and maintains shape of anterior portion of eye Leaves cavity through scleral venous sinus
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Inner (Nervous) Tunic Inner tunic consists of retina
Retina contains visual receptors (photoreceptors) Continuous with optic nerve in back of eye Ends just behind margin of the ciliary body toward front of eye 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; found where nerve fibers from retina leave eye to become optic nerve Vitreous humor: thick gel that holds retina flat against choroid coat
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Posterior Cavity Posterior cavity: space enclosed by lens, ciliary body, and retina Contains vitreous humor: thick gel that supports internal structures and maintains shape of eye
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Major Groups of Retinal Neurons
Photoreceptor cells, bipolar cells, and ganglion cells: provide pathway for impulses triggered by photoreceptors to reach the optic nerve Horizontal cells and amacrine cells: modify, integrate impulses
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Summary: Layers of the Eye
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Light Refraction Refraction:
Bending of light, which occurs when light waves pass at an angle between mediums of different densities
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Types of Lenses Convex lenses cause light waves to converge
Concave lenses cause light waves to diverge
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Focusing on the 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 Visual cortex corrects the reversals
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Clinical Application 12.6 Refraction Disorders
Concave lens corrects nearsightedness Convex lens corrects farsightedness
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Photoreceptors Photoreceptors are modified neurons of retina that sense light: Rods: Long, thin projections Contain light sensitive pigment, called rhodopsin Hundreds of times more sensitive to light than cones Provide vision in dim light Produce colorless vision Produce outlines of objects Cones: Short, blunt projections Contain light sensitive pigments, called erythrolabe, chlorolabe, and cyanolabe Provide vision in bright light Produce sharp images Produce color vision Fovea centralis contains only cones
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Rods and Cones Rods and cones are named for shape of receptive ends: rods are cylindrical and cones are conical
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Visual Pigments Rods and cones contain light-sensitive pigments that decompose upon absorption of light: Rhodopsin (Visual purple): Light-sensitive pigment in rods In presence of light, decomposes into Opsin and Retinal Triggers a complex series of reactions that initiates nerve impulses Impulses travel along optic nerve Iodopsins (pigments in cones): Each type of cone contains different light-sensitive pigment Each type of cone is sensitive to different wavelengths Color perceived depends on which types of cones are stimulated Erythrolabe: responds to red light Chlorolabe: responds to green light Cyanolabe: responds to blue light
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Rhodopsin in a Rod Rhodopsin is embedded in the many discs of membrane at the end of the rod.
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Stereoscopic Vision Provides perception of distance, depth, height and width of objects Results from formation of two slightly different retinal images from eyes
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Visual Pathways The visual pathway proceeds from the ganglion cells of the retina to the optic nerve, optic chiasma, optic tracts, the thalamus, optic radiations, and visual cortex in occipital lobe of cerebrum. A few fibers branch off before reaching the thalamus, and enter nuclei for visual reflexes
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12.5: Life-Span Changes Age-related hearing loss due to:
Damage to hair cells in spiral organ 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, decreasing accommodation (presbyopia) Glaucoma Cataracts Macular degeneration Age-related smell and taste problems due to: Loss of olfactory receptors (anosmia)
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