Download Lecture notes for Chapter 15

Special sensory receptors
Distinct, localized receptor cells in head
Vision
Taste
Smell
Hearing
Equilibrium
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Special sensory receptors
Distinct, localized receptor cells in head
Vision
Taste
Smell
Hearing
Equilibrium
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70% of body's sensory receptors in eye
Visual processing by ~ half cerebral cortex
Most of eye protected by cushion of fat and bony orbit
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Protect the eye and aid eye function
Eyebrows
Eyelids (palpebrae)
Conjunctiva
Lacrimal apparatus
Extrinsic eye muscles
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Overlie supraorbital margins
Function
Shade eye from sunlight
Prevent perspiration from reaching eye
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Protect eye anteriorly
Separated at palpebral fissure
Meet at medial and lateral commissures
Lacrimal caruncle
At medial commissure
Contains oil and sweat glands
Tarsal plates—supporting connective tissue
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Eyelashes
Nerve endings of follicles initiate reflex blinking
Lubricating glands associated with eyelids
Tarsal (Meibomian) glands
Modified sebaceous glands
Oily secretion lubricates lid and eye
Ciliary glands between hair follicles
Modified sweat glands
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Levator palpebrae superioris
Gives upper eyelid mobility
Blink reflexively every 3-7 seconds
Protection
Spread secretions to moisten eye
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Transparent mucous membrane
Produces a lubricating mucous secretion
Palpebral conjunctiva lines eyelids
Bulbar conjunctiva covers white of eyes
Conjunctival sac between palpebral and bulbar conjunctiva
Where contact lens rests
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Lacrimal gland and ducts that
drain into nasal cavity
Lacrimal gland in orbit above
lateral end of eye
Lacrimal secretion (tears)
Dilute saline solution containing
mucus, antibodies, and lysozyme
Blinking spreads tears toward
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medial commissure
Tears enter paired lacrimal canaliculi
via lacrimal puncta
Then drain into lacrimal sac and
nasolacrimal duct
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Six straplike extrinsic eye muscles
Originate from bony orbit; insert
on eyeball
Enable eye to follow moving
objects; maintain shape of eyeball;
hold in orbit
Four rectus muscles originate
from common tendinous ring;
names indicate movements
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Superior, inferior, lateral, medial rectus
muscles
Two oblique muscles move eye in
vertical plane and rotate eyeball
Superior and inferior oblique muscles
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Six straplike extrinsic eye muscles
Originate from bony orbit; insert
on eyeball
Enable eye to follow moving
objects; maintain shape of eyeball;
hold in orbit
Four rectus muscles originate
from common tendinous ring;
names indicate movements
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Superior, inferior, lateral, medial rectus
muscles
Two oblique muscles move eye in
vertical plane and rotate eyeball
Superior and inferior oblique muscles
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Wall of eyeball contains three layers
Fibrous
Vascular
Inner
Internal cavity filled with fluids called humors
Lens separates internal cavity into anterior and posterior segments (cavities)
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Outermost layer; dense avascular connective tissue
Two regions: sclera and cornea
1. Sclera
Opaque posterior region
Protects, shapes eyeball; anchors extrinsic eye muscles
Continuous with dura mater of brain posteriorly
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2. Cornea
Transparent anterior 1/6 of fibrous layer
Bends light as it enters eye
Sodium pumps of corneal endothelium on inner face help
maintain clarity of cornea
Numerous pain receptors contribute to blinking and tearing
reflexes
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Middle pigmented layer
Three regions: choroid, ciliary body, and iris
1. Choroid region
Posterior portion of uvea
Supplies blood to all layers of eyeball
Brown pigment absorbs light to prevent light scattering and
visual confusion
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3. Iris
•Colored part of eye
2. Ciliary body
Ring of tissue surrounding lens
Smooth muscle bundles (ciliary muscles) control lens shape
Capillaries of ciliary processes secrete fluid
Ciliary zonule (suspensory ligament) holds lens in position
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3. Iris
•Colored part of eye
•Pupil—central opening that regulates amount of light
entering eye
Close vision and bright
light—sphincter pupillae
(circular muscles) contract;
pupils constrict
Distant vision and dim
light—dilator pupillae
(radial muscles) contract;
pupils dilate – sympathetic
fibers
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Changes in emotional state—
pupils dilate when subject
matter is appealing or requires
problem-solving skills
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Originates as outpocketing of brain
Delicate two-layered membrane
Outer Pigmented layer
Single-cell-thick lining
Absorbs light and prevents its scattering
Phagocytize photoreceptor cell fragments
Stores vitamin A
Optic disc (blind spot)
Site where optic nerve leaves eye
Lacks photoreceptors
Quarter-billion photoreceptors of two types
Rods
Cones
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Inner Neural layer
Transparent
Composed of three main types of neurons
Photoreceptors, bipolar cells, ganglion cells
Signals spread from photoreceptors to bipolar cells to ganglion
cells
Ganglion cell axons exit eye as optic nerve
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Rods
Dim light, peripheral vision receptors
More numerous, more sensitive to light than cones
No color vision or sharp images
Numbers greatest at periphery
Cones
Vision receptors for bright light
High-resolution color vision
Macula lutea exactly at posterior pole
Mostly cones
Fovea centralis
Tiny pit in center of macula with all cones; best vision
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Two sources of blood supply
Choroid supplies outer third (photoreceptors)
Central artery and vein of retina supply inner two-thirds
Enter/exit eye in center of optic nerve
Vessels visible in living person
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The lens and ciliary zonule separate eye into two segments
Anterior and posterior segments
Posterior segment contains
vitreous humor that
Transmits light
Supports posterior surface of lens
Holds neural layer of retina firmly
against pigmented layer
Contributes to intraocular pressure
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Forms in embryo; lasts lifetime
Anterior segment composed of two
chambers
Anterior chamber—between cornea
and iris
Posterior chamber—between iris and
lens
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Anterior segment contains
aqueous humor
Plasma like fluid continuously
formed by capillaries of ciliary
processes
Drains via scleral venous sinus
(canal of Schlemm) at scleracornea junction
Supplies nutrients and oxygen
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mainly to lens and cornea but also to
retina, and removes wastes
Glaucoma: blocked drainage of
aqueous humor increases pressure
and causes compression of retina
and optic nerve  blindness
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Biconvex, transparent, flexible,
and avascular
Changes shape to precisely focus
light on retina
Two regions
Lens epithelium anteriorly; Lens
fibers form bulk of lens
Lens fibers filled with transparent
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protein crystallin
Lens becomes more dense, convex,
less elastic with age
•cataracts (clouding of lens) consequence
of aging, diabetes mellitus, heavy
smoking, frequent exposure to intense
sunlight
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Eyes respond to visible light
Small portion of electromagnetic spectrum
Wavelengths of 400-700 nm
Light
Packets of energy (photons or quanta) that travel in wavelike fashion
at high speeds
Color of light objects reflect determines color eye perceives
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Refraction
Bending of light rays
Due to change in speed when light passes from one
transparent medium to another
Occurs when light meets surface of different medium at an
oblique angle
Curved lens can refract light
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Light passing through convex lens (as in eye) is bent so that rays converge at
focal point
Image formed at focal point is upside-down and reversed right to left
Concave lenses diverge light
Prevent light from focusing
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Pathway of light entering eye:
cornea, aqueous humor, lens,
vitreous humor, entire neural
layer of retina, photoreceptors
Light refracted three times along
pathway
Entering cornea
Entering lens
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Leaving lens
Majority of refractory power in
cornea
Change in lens curvature allows for
fine focusing
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Eyes best adapted for distant vision
Far point of vision
Distance beyond which no change in lens shape needed for focusing
20 feet for emmetropic (normal) eye
Cornea and lens focus light precisely on retina
Ciliary muscles relaxed
Lens stretched flat by tension in ciliary zonule
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Light from close objects (<6 m) diverges as approaches eye
Requires eye to make active adjustments using three simultaneous
processes
Accommodation of lenses
Constriction of pupils
Convergence of eyeballs
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Accommodation
Changing lens shape to increase
refraction
Near point of vision
Closest point on which the eye can
focus
Presbyopia—loss of
accommodation over age 50
Constriction
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Accommodation pupillary reflex
constricts pupils to prevent most
divergent light rays from entering eye
Convergence
Medial rotation of eyeballs toward
object being viewed
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Myopia (nearsightedness)
Focal point in front of retina, e.g.,
eyeball too long
Corrected with a concave lens
Hyperopia (farsightedness)
Focal point behind retina, e.g.,
eyeball too short
Corrected with a convex lens
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Astigmatism
Unequal curvatures in different parts
of cornea or lens
Corrected with cylindrically ground
lenses or laser procedures
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Rods and cones
Modified neurons
Receptive regions called outer segments
Contain visual pigments (photopigments)
Molecules change shape as absorb light
Inner segment of each joins cell body
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Vulnerable to damage
Degenerate if retina detached
Destroyed by intense light
Outer segment renewed every 24 hours
Tips fragment off and are phagocytized
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Vulnerable to damage
Degenerate if retina detached
Destroyed by intense light
Outer segment renewed every 24 hours
Tips fragment off and are phagocytized
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rods
Functional characteristics
Very sensitive to light
Best suited for night vision and peripheral vision
Contain single pigment
Perceived input in gray tones only
Pathways converge, causing fuzzy, indistinct images
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Cones
Functional characteristics
Need bright light for activation (have low sensitivity)
React more quickly
Have one of three pigments for colored view
Nonconverging pathways result in detailed, high-resolution vision
Color blindness–lack of one or more cone pigments
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rods
Functional characteristics
Very sensitive to light
Best suited for night vision and peripheral vision
Contain single pigment
Perceived input in gray tones only
Pathways converge, causing fuzzy, indistinct images
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Cones
Functional characteristics
Need bright light for activation (have low sensitivity)
React more quickly
Have one of three pigments for colored view
Nonconverging pathways result in detailed, high-resolution vision
Color blindness–lack of one or more cone pigments
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Retinal
Light-absorbing molecule that combines with one of four proteins
(opsins) to form visual pigments
Synthesized from vitamin A
Retinal isomers: 11-cis-retinal (bent form) and all-trans-retinal
(straight form)
Bent form  straight form when pigment absorbs light
Conversion of bent to straight initiates reactions  electrical
impulses along optic nerve
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Deep purple pigment of rods–rhodopsin
11-cis-retinal + opsin  rhodopsin
Three steps of rhodopsin formation and breakdown
Pigment synthesis
Pigment bleaching
Pigment regeneration
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Pigment synthesis
Rhodopsin forms and accumulates in dark
Pigment bleaching
When rhodopsin absorbs light, retinal changes to all-trans isomer
Retinal and opsin separate (rhodopsin breakdown)
Pigment regeneration
All-trans retinal converted to 11-cis isomer
Rhodopsin regenerated in outer segments
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Light-activated rhodopsin activates G protein transducin
Transducin activates PDE, which breaks down cyclic GMP (cGMP)
In dark, cGMP holds channels of outer segment open  Na+ and Ca2+
depolarize cell
In light cGMP breaks down, channels close, cell hyperpolarizes
Hyperpolarization is signal!
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Similar as process in rods
Cones far less sensitive to light
Takes higher-intensity light to activate cones
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Photoreceptors and bipolar cells only generate graded potentials (EPSPs and
IPSPs)
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When light hyperpolarizes photoreceptor cells
Stop releasing inhibitory neurotransmitter glutamate
Bipolar cells (no longer inhibited) depolarize, release neurotransmitter
onto ganglion cells
Ganglion cells generate APs transmitted in optic nerve to brain
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Move from darkness into bright light
Both rods and cones strongly stimulated
Pupils constrict
Large amounts of pigments broken down instantaneously, producing
glare
Visual acuity improves over 5–10 minutes as:
Rod system turns off
Retinal sensitivity decreases
Cones and neurons rapidly adapt
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Move from bright light into darkness
Cones stop functioning in low-intensity light
Rod pigments bleached; system turned off
Rhodopsin accumulates in dark
Transducin returns to outer segments
Retinal sensitivity increases within 20–30 minutes
Pupils dilate
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Axons of retinal ganglion cells form optic nerve
Medial fibers of optic nerve decussate at optic chiasma
Most fibers of optic tracts continue to lateral geniculate body of thalamus
Fibers from thalamic neurons form optic radiation and project to primary
visual cortex in occipital lobes
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Fibers from thalamic neurons form optic radiation
Optic radiation fibers connect to primary visual cortex in occipital lobes
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Fibers from thalamic neurons form optic radiation
Optic radiation fibers connect to primary visual cortex in occipital lobes
Other optic tract fibers send branches to midbrain, ending in superior colliculi
(initiating visual reflexes)
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A small subset of ganglion cells in retina contain melanopsin (circadian
pigment), which projects to:
Pretectal nuclei (involved with pupillary reflexes)
Suprachiasmatic nucleus of hypothalamus, timer for daily biorhythms
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Both eyes view same image from slightly different angles
Depth perception (three-dimensional vision) results from cortical fusion of
slightly different images
Requires input from both eyes
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Retinal cells split input into channels
Color, brightness, angle, direction, speed of movement of edges
(sudden changes of brightness or color)
Lateral inhibition decodes "edge" information
Job of amacrine and horizontal cells
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Lateral geniculate nuclei of thalamus
Process for depth perception, cone input emphasized, contrast
sharpened
Primary visual cortex (striate cortex)
Neurons respond to dark and bright edges, and object orientation
Provide form, color, motion inputs to visual association areas
(prestriate cortices)
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Occipital lobe centers (anterior
prestriate cortices) continue
processing of form, color, and
movement
Complex visual processing
extends to other regions
"What" processing identifies
objects in visual field
Ventral temporal lobe
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"Where" processing assesses spatial
location of objects
Parietal cortex to postcentral gyrus
Output from both passes to frontal
cortex
Directs movements
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