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Chapter 16

Copyright (c) The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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Sense Organs

• Sensory receptors– properties and

types

• General senses

• Chemical senses

• Hearing and equilibrium

• Vision

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Properties of Receptors• Sensory transduction

– convert stimulus energy into nerve energy

• Receptor potential– local electrical change in receptor cell

• Adaptation– conscious sensation declines with continued

stimulation

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Receptors Transmit Information1. Modality - type of stimulus

2. Location– each sensory receptor receives input from its

receptive field– sensory projection - brain identifies site of

stimulation

3. Intensity – frequency, number of fibers and which fibers

4. Duration - change in firing frequency over time– phasic receptor - burst of activity and quickly adapt

(smell and hair receptors)– tonic receptor - adapt slowly, generate impulses

continually (proprioceptor)

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Receptive Fields

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Classification of Receptors• By modality:

– chemo-, thermo-, mechano-, photo- receptors and nociceptors

• By origin of stimuli– interoceptors - detect internal stimuli– proprioceptors - sense body position and

movements– exteroceptors - detect external stimuli

• By distribution– general senses - widely distributed– special senses - limited to head

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Unencapsulated Nerve Endings

• Dendrites not wrapped in connective tissue

• General sense receptors– for pain and temperature

• Tactile discs – associated with cells at

base of epidermis

• Hair receptors – monitor movement of hair

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Encapsulated Nerve Endings

• Dendrites wrapped by glial cells or connective tissue– tactile corpuscles - phasic

• light touch and texture

– krause end bulb - phasic• tactile; in mucous membranes

– lamellated corpuscles - phasic• deep pressure, stretch, tickle and

vibration

– ruffini corpuscles - tonic• heavy touch, pressure, joint

movements and skin stretching

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Somesthetic Projection Pathways• 1st order neuron (afferent neuron)

– from body, enter the dorsal horn of spinal cord via spinal nerves

– from head, enter pons and medulla via cranial nerve

– touch, pressure and proprioception on large, fast, myelinated axons

– heat and cold on small, unmyelinated, slow fibers• 2nd order neuron

– decussation to opposite side in spinal cord or medulla/pons

– end in thalamus, except for proprioception (cerebellum)

• 3rd order neuron– thalamus to primary somesthetic cortex of

cerebrum

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Pain

• Nociceptors – allow awareness of tissue injuries– found in all tissues except the brain

• Fast pain travels in myelinated fibers at 30 m/sec– sharp, localized, stabbing pain perceived with injury

• Slow pain travels unmyelinated fibers at 2 m/sec– longer-lasting, dull, diffuse feeling

• Somatic pain from skin, muscles and joints• Visceral pain from stretch, chemical irritants or

ischemia of viscera (poorly localized)• Injured tissues release chemicals that stimulate pain

fibers (bradykinin, histamine, prostaglandin)

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Projection Pathway for Pain• General pathway – conscious pain

– 1st order neuron cell bodies in dorsal root ganglion of spinal nerves or cranial nerves V, VII, IX, and X

– 2nd order neurons decussate and send fibers up spinothalamic tract or through medulla to thalamus

• gracile fasciculus carries visceral pain signals

– 3rd order neurons from thalamus reach primary somesthetic cortex as sensory homunculus

• Spinoreticular tract– pain signals reach reticular formation, hypothalamus

and limbic– trigger visceral, emotional, and behavioral reactions

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Pain Signal Destinations

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Referred Pain

• Misinterpreted pain– brain “assumes” visceral pain is coming from

skin– heart pain felt in shoulder or arm because

both send pain input to spinal cord segments T1 to T5

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Referred Pain

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CNS Modulation of Pain

• Intensity of pain - affected by state of mind

• Endogenous opiods (enkephalins, endorphins and dynorphins)– produced by CNS and other organs under

stress– in dorsal horn of spinal cord (spinal gating)– act as neuromodulators block transmission

of pain

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Spinal Gating

• Stops pain signals at dorsal horn– descending analgesic fibers from reticular

formation travel down reticulospinal tract to dorsal horn

• secrete inhibitory substances that block pain fibers from secreting substance P

• pain signals never ascend

– dorsal horn fibers inhibited by input from mechanoreceptors

• rubbing a sore arm reduces pain

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Spinal Gating of Pain Signals

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Chemical Sense - Taste

• Gustation - sensation of taste – results from action of chemicals on taste

buds

• Lingual papillae– filiform (no taste buds)

• important for texture

– foliate (no taste buds)

– fungiform• at tips and sides of tongue

– vallate (circumvallate)• at rear of tongue• contains 1/2 of taste buds

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Taste Bud Structure

• Taste cells– apical microvilli serve

as receptor surface – synapse with sensory

nerve fibers at their base

• Supporting cells• Basal cells

• Taste cells– apical microvilli serve

as receptor surface – synapse with sensory

nerve fibers at their base

• Supporting cells• Basal cells

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Physiology of Taste• Molecules must dissolve in saliva• 5 primary sensations - throughout tongue

1. Sweet - concentrated on tip2. Salty - lateral margins3. Sour - lateral margins4. Bitter - posterior5. Umami - taste of amino acids (MSG)

• Influenced by food texture, aroma, temperature, and appearance– mouthfeel - detected by lingual nerve in papillae

• Hot pepper stimulates free nerve endings (pain)

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Physiology of Taste

• Mechanisms of action– activate 2nd messenger systems

• sugars, alkaloids and glutamates bind to receptors

– depolarize cells directly• sodium and acids penetrate cells

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Projection Pathways for Taste

• Innervation of taste buds– facial nerve (VII) - anterior 2/3’s of tongue– glossopharyngeal nerve (IX) - posterior 1/3– vagus nerve (X) - palate, pharynx, epiglottis

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Projection Pathways for Taste

• To solitary nucleus in medulla

• To hypothalamus and amygdala– activate autonomic reflexes

• e.g. salivation, gagging and vomiting

• To thalamus, then postcentral gyrus of cerebrum– conscious sense of taste

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Chemical Sense - Smell

• Olfactory mucosa – contains receptor cells

for olfaction– highly sensitive

• up to 10,000 odors

– on 5cm2 of superior concha and nasal septum

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Olfactory Epithelial Cells

• Olfactory cells– olfactory hairs

neurons with 20 cilia

• bind odor molecules in thin layer of mucus

– axons pass through cribriform plate

– survive 60 days

• Supporting cells

• Basal cells – divide

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Physiology of Smell

• Molecules bind to receptor on olfactory hair– hydrophilic - diffuse through mucus – hydrophobic - transport by odorant-binding

protein

• Activate G protein and cAMP system• Opens ion channels for Na+ or Ca2+

– creates a receptor potential

• Action potential travels to brain• Receptors adapt quickly

– due to synaptic inhibition in olfactory bulbs

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Olfactory Pathway

• Olfactory cells synapse in olfactory bulb– on mitral and tufted cell dendrites– in spherical clusters called glomeruli

• each glomeruli dedicated to single odor

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Olfactory Pathway

• Output from bulb forms olfactory tracts– end in primary olfactory cortex and thalamus– travel to insula and frontal cortex– identify odors– integrate taste and smell into flavor– travel to hypocampus, amygdala, and

hypothalamus• memories, emotional and visceral reactions

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Olfactory Pathway

• Feedback– granule cells in olfactory cortex synapse in

glomeruli• food smells better when hungry

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Olfactory Projection Pathways

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The Nature of Sound• Sound - audible vibration of molecules

– vibrating object pushes air molecules

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Pitch and Loudness

• Pitch - frequency vibrates specific parts of ear– hearing range is 20 (low pitch) - 20,000 Hz (cycles/sec)– speech is 1500-4000 where hearing is most sensitive

• Loudness – amplitude; intensity of sound energy

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Outer Ear

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Outer Ear

• Fleshy auricle (pinna) directs air vibrations down external auditory meatus– cartilagenous and bony, S-shaped tunnel

ending at eardrum– glandular secretions and dead cells form

cerumen (earwax)

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Anatomy of Middle Ear

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Middle Ear

• Air-filled tympanic cavity in temporal bone between tympanic membrane and oval window– continuous with mastoid air cells

• Contains– auditory tube (eustachian tube) connects to

nasopharynx• equalizes air pressure on tympanic membrane

– ear ossicles• malleus • incus • stapes

– stapedius and tensor tympani muscles attach to stapes and malleus

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Anatomy of Inner Ear

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Inner Ear• Bony labyrinth - passageways in temporal bone• Membranous labyrinth - fleshy tubes lining bony

tunnels– filled with endolymph (similar to intracellular fluid)– floating in perilymph (similar to cerebrospinal fluid)

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Details of Inner Ear

Fig. 16.12c

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Details of Inner Ear

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Anatomy of Cochlea

• Scala media (cochlear duct) – separated from

• scala vestibuli by vestibular membrane• scala tympani by basilar membrane

• Spiral organ (organ of corti)

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Spiral Organ

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Spiral Organ

• Stereocilia of hair cells attach to gelatinous tectorial membrane

• Inner hair cells – hearing

• Outer hair cells – adjust cochlear responses to different

frequencies – increase precision

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SEM of Cochlear Hair Cells

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Physiology of Hearing - Middle Ear

• Tympanic membrane– has 18 times area of oval window– ossicles transmit enough force/unit area at

oval window to vibrate endolymph in scala vestibuli

• Tympanic reflex – muscle contraction – tensor tympani m. tenses tympanic

membrane– stapedius m. reduces mobility of stapes

• best response to slowly building loud sounds• occurs while speaking

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Stimulation of Cochlear Hair Cells

• Vibration of ossicles causes vibration of basilar membrane under hair cells– as often as 20,000 times/second

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Cochlear Hair Cells

• Stereocilia of OHCs – bathed in high K+

• creating electrochemical gradient

– tips embedded in tectorial membrane– bend in response to movement of basilar

membrane• pulls on tip links and opens ion channels• K+ flows in – depolarization causes release of

neurotransmitter• stimulates sensory dendrites at base

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Potassium Gates

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Sensory Coding

• Vigorous vibrations excite more inner hair cells over a larger area – triggers higher frequency of action potentials– brain interprets this as louder sound

• Pitch depends on which part of basilar membrane vibrates– at basal end, membrane narrow and stiff

• brain interprets signals as high-pitched

– at distal end, 5 times wider and more flexible• brain interprets signals as low-pitched

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Basilar Membrane Frequency Response

Notice high and low frequency ends

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Cochlear Tuning• Increases ability of cochlea to receive

some sound frequencies

• Outer hair cells contract reducing basilar membranes freedom to vibrate– fewer signals from that area allows brain

to distinguish between more and less active areas of cochlea

• Pons has inhibitory fibers that synapse near the base of IHCs– increases contrast between regions of

cochlea

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Innervation of Internal Ear

• Vestibular ganglia - visible in vestibular nerve

• Spiral ganglia - buried in modiolus of cochlea

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Auditory Pathway

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Auditory Projection Pathway• Spiral ganglion formed by cell bodies of

sensory neurons• Axons form cochlear nerve portion of CN VIII• Synapse in cochlear nuclei• Binaural hearing

– superior olivary nucleus compares sounds from both sides to identify direction

• Inferior colliculus helps – locate origin of sound– process fluctuations in pitch during speech– produce startle response; head turning to loud

sound• Fibers from inferior colliculus go to primary

auditory cortex – temporal lobe

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Auditory Processing Centers

• Damage to either auditory cortex does not cause unilateral deafness (extensive decussation)

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Equilibrium• Control of coordination and balance

• Receptors in vestibular apparatus– semicircular ducts contain crista– saccule and utricle contain macula

• Static equilibrium – perceived by macula– perception of head orientation

• Dynamic equilibrium– perception of motion or acceleration

• linear acceleration perceived by macula• angular acceleration perceived by crista

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Saccule and Utricle

• Contain macula– hair cells with stereocilia and one kinocilium

buried in a gelatinous otolithic membrane– otoliths add to the density and inertia and

enhance the sense of gravity and motion

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Macula

• Static equilibrium - when head is tilted, weight of membrane bends the stereocilia

• Dynamic equilibrium – in car, linear acceleration detected as otoliths lag behind

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Crista ampullaris

• Consists of hair cells buried in a mound of gelatinous membrane (one in each duct)

• Orientation causes ducts to be stimulated by rotation in different planes

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Crista Ampullaris - Head Rotation

• As head turns, endolymph lags behind, pushes cupula, stimulates hair cells

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Vestibular Projection Pathways

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Equilibrium Projection Pathways

• Hair cells of macula sacculi, macula utriculi and semicircular ducts synapse on vestibular nerve

• Fibers end in vestibular nucleus in pons and medulla

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Equilibrium Projection Pathways

• Information sent to 5 targets– cerebellar control of head and eye

movements and posture– nuclei of CN III, IV, and VI to produce

vestibulo ocular reflex– reticular formation control of blood

circulation and posture– vestibulospinal tracts innervate

antigravity muscles– thalamic relay to cerebral cortex for

awareness of position and movement

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Vision and Light

• Vision - perception of light emitted or reflected from objects in the environment

• Visible light – electromagnetic radiation with wavelengths

from 400 to 750 nm– must cause a photochemical reaction to

produce a nerve signal• radiation below 400 nm; energetic, kills cells• radiation above 750 nm; too little energy to cause

photochemical reaction

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External Anatomy of Eye

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Eyebrows and Eyelids

• Eyebrows provide facial expression

• Eyelids (palpebrae)– block foreign objects, help with sleep,

blink to moisten– meet at corners (commissures)– consist of orbicularis oculi muscle and

tarsal plate covered with skin outside and conjunctiva inside

– tarsal glands secrete oil that reduces tear evaporation

– eyelashes help keep debris from eye

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Conjunctiva

• Transparent mucous membrane lines eyelids and covers anterior surface of eyeball except cornea

• Richly innervated and vascular (heals quickly)

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Lacrimal Apparatus

• Tears flow across eyeball help to wash away foreign particles, help with diffusion of O2 and CO2 and contain bactericidal enzyme

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Extrinsic Eyes Muscles

• 6 muscles inserting on eyeball– 4 rectus, superior and inferior oblique muscles

• Innervated by cranial nerves III, IV and VI

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Innervation of Extrinsic Eye Muscles

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Tunics of the Eyeball

• Fibrous layer - sclera and cornea • Vascular layer - choroid, ciliary body and iris• Internal layer - retina and optic nerve

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Optical Components

• Structures refract light to focus on retina– cornea

• transparent cover on anterior surface of eyeball

– aqueous humor• serous fluid posterior to cornea, anterior to lens

– lens • changes shape to help focus light

– rounded with no tension– flattened due to pull of suspensory ligaments

– vitreous humor • jelly fills space between lens and retina

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Aqueous Humor

• Produced by ciliary body, flows to posterior chamber through pupil to anterior chamber - reabsorbed into canal of Schlemm

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Cataracts and Glaucoma

• Cataract - clouding of lens– aging, diabetes, smoking, and UV light

• Glaucoma– death of retinal cells due to elevated

pressure within the eye• obstruction of scleral venous sinus• colored halos and dimness of vision

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Neural Components

• Includes retina and optic nerve

• Retina – forms as an outgrowth of the diencephalon– attached at optic disc and at ora serrata– pressed against rear of eyeball by vitreous

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Detached retina

• Blow to head or lack of vitreous

• Blurry areas in field of vision

• Disrupts blood supply, leads to blindness

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Ophthalmoscopic Exam of Eye

• Macula lutea - cells on visual axis of eye (3 mm)– fovea centralis - center of macula; finely detailed

images due to packed receptor cells

• Direct evaluation of blood vessels

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Test for Blind Spot

• Optic disk = blind spot – optic nerve exits posterior surface of eyeball– no receptor cells

• Blind spot - use test illustration above– close eye, stare at X and red dot disappears

• Visual filling - brain fills in green bar across blind spot area

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Formation of an Image

• Light passes through lens to form inverted image on retina

• Pupillary constrictor - smooth muscle encircling the pupil– parasympathetic stimulation narrows pupil

• Pupillary dilator - spokelike myoepithelial cells– sympathetic stimulation widens pupil

• Active when light intensity changes or gaze shifts from distant object to nearby object– photopupillary reflex -- both pupils constrict if one

eye is illuminated (type of consensual reflex)

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Principle of Refraction

Light striking the lens or cornea at a 90 degree angle is not bent.

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Refraction

• Bending of light rays occurs when light passes through substance with different refractive index at any angle other than 90 degrees– refractive index of air

is arbitrarily set to n = 1– refractive index

• cornea is n = 1.38• lens is n = 1.40

• Cornea refracts light more than lens does– due to shape of cornea– lens becomes rounder to

increase refraction for near vision

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Near Response

• Allows eyes to focus on nearby object (that sends oblique light waves to eyes)

1. convergence of eyes• eyes orient their visual axis towards object

2. constriction of pupil• blocks peripheral light rays and reduces

spherical aberration (blurry edges)

3. accomodation of lens• ciliary muscle contracts, lens takes convex

shape– light refracted more strongly and focused onto retina

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Emmetropia and Near Response

Distant object Close object

Fig. 16.31a

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Emmetropia and Near Response

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Accommodation of Lens

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Effects of Corrected Lenses

• Hyperopia - farsighted (eyeball too short)– correct with convex lenses

• Myopia - nearsighted (eyeball too long)– correct with concave lenses

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Photoreceptor Cells

• Posterior layer of retina - pigment epithelium– purpose is to absorb stray light

and prevent reflections

• Photoreceptors– rod cells (night - scotopic vision)

• outer segment - stack of coinlike membranous discs studded with rhodopsin pigment molecules

– cone cells (color - photopic vision)

• outer segment tapers to a point

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Histology - Layers of Retina

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Location of Visual Pigments

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Nonreceptor Retinal Cells

• Bipolar cells (1st order neurons)– synapse on ganglion cells– large amount of convergence

• Ganglion cells (2nd order neurons)– axons of these form optic nerve– more convergence occurs (114 receptors

to one optic nerve fiber)

• Horizontal and amacrine cells form connections between other cells– enhance perception of contrast, edges of

objects and changes in light intensity

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Schematic Layers of the Retina

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Visual Pigments

• Rod cells have rhodopsin – has absorption peak at wavelength of 500 nm– 2 major parts of molecule

• opsin - protein portion• retinal - a vitamin A derivative

• Cones contain photopsin (iodopsin)– opsin moieties contain different amino acids

that determine wavelengths of light absorbed– 3 kinds of cones absorbing different

wavelengths of light produce color vision

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Rhodopsin Bleaching/Regeneration

• Rhodopsin absorbs light, converted from bent shape (cis-retinal) to straight (trans-retinal) – retinal dissociates from opsin (bleaching)– 5 minutes to regenerate 50% of bleached rhodopsin

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Generating Visual Signals

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Generating Nerve Signals - Rods

• In the dark, rods exhibit a dark signal– flow of Na+ and release of neurotransmitter

(glutamate)• depolarization by Na+ stimulates glutamate release

• In the light, dark current and glutamate release stops– bleached rhodopsin molecule acts like an

enzyme and breaks down cGMP molecules– Na+ gates close and dark current ceases

(inhibition stops), nerve signal results

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Bipolar Cell Function

• Two kinds of bipolar cells– inhibited (hyperpolarized) by glutamate

• excited by rising light intensity

– excited (depolarized) by glutamate • excited by falling light intensity

• As your eye scans a scene, areas of light and dark cause a changing pattern of bipolar cell responses

• Variable pattern of stimulation of ganglion cells and nerve signals sent to the brain

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Light and Dark Adaptation

• Light adaptation (walk out into sunlight)– pupil constriction and pain from over

stimulated retinas– color vision and acuity below normal for 5 to

10 minutes

• Dark adaptation (turn lights off)– dilation of pupils occurs– 20 to 30 minutes required for regeneration of

rhodopsin

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Duplicity Theory

• Explains why we have both rods and cones

• Single type of receptor cell incapable of providing high sensitivity and high resolution– sensitive night vision = one type of cell

and neural circuitry– high resolution daytime vision = different

cell type and neuronal circuitry

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Duplicity Theory

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• Rods sensitive – react even in dim light– extensive neuronal convergence– 600 rods converge on 1 bipolar cell– many bipolar converge on each ganglion cell– results in high degree of spatial summation

• one ganglion cells receives information from 1 mm2 of retina producing only a coarse image

• Edges of retina have widely-spaced rod cells, act as motion detectors

Scotopic System (Night Vision)

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Photopic System (Day Vision)

• Fovea contains only 4000 tiny cone cells (no rods)– no neuronal convergence– each foveal cone cell has “private line to

brain”

• High-resolution color vision– little spatial summation so less sensitivity to

dim light

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Color Vision• Primates have well

developed color vision– nocturnal vertebrates

have only rods• Cones named for

absorption peaks of photopsins– blue cones peak sensitivity

at 420 nm– green cones peak at 531 nm– red cones peak at 558 nm

(orange-yellow)• Color perception based on

mixture of nerve signals

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Color Blindness

• Hereditary lack of one photopsin– red-green is common (lack either red or

green cones)• incapable of distinguishing red from green• sex-linked recessive (8% of males)

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Stereoscopic Vision (Stereopsis)

• Depth perception - ability to judge distance to objects– requires 2 eyes with

overlapping visual fields– panoramic vision has eyes

on sides of head (horse)

• Fixation point– farther away requires image

focus medial to fovea– closer results in image focus lateral to fovea

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Visual Projection Pathway

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Visual Projection Pathway

• Bipolar and ganglion cells in retina - 1st and 2nd order neurons (ganglion cell axons of form CN II)

• Hemidecussation in optic chiasm– 1/2 of fibers decussate so that images of all objects in left

visual field fall on right half of each retina– each side of brain sees what is on side where it has motor

control over limbs

• 3rd order neurons in lateral geniculate nucleus of thalamus form optic radiation to 1 visual cortex where conscious visual sensation occurs

• Few fibers project to superior colliculi and midbrain for visual reflexes (photopupillary and accomodation)

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Visual Information Processing

• Some processing occurs in retina– adjustments for contrast, brightness, motion

and stereopsis

• Visual association areas in parietal and temporal lobes process visual data– object location, motion, color, shape,

boundaries– store visual memories (recognize printed

words)