BIOL 252: CH. 13 SENSES: THE EYE AND VISION
78 Cards in this Set
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Accessory Structures of the Eye
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Eyebrows
Eyelids Conjuctiva Lacrimal Apparatus Extrinsic Eye Muscles
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Eyebrows
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Short, coarse hairs that overlie the supraorbital margins of the skull
Function: help shade the eyes from sunlight and prevent perspiration from forehead reaching the eyes
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Eyelids
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An eyelid is a thin fold of skin that covers and protects an eye.
AKA palpebrae
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Palpebral Fissure
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Palpebral fissure is the anatomic name for the separation between the upper and lower eyelids.
"eyelid shut" Meet at the medial and lateral angles of the eye
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Lateral Canthus
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Lateral angle of the eye
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Medial Canthus
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Medial angle of the eye
Sports a fleshy elevation called the Lacrimal Caruncle
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Lacrimal Caruncle
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"a bit of flesh"
Contains sebaceous and sweat glands and produces the whitish, oily secretion that sometimes collects at the medial canthus, especially during sleep (Sandman's eye-sand)
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Tarsal Plates
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Connective tissue sheets that internally support the eye-lids and anchor the orbicularis oculi and levator palpebrae superioris muscles
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Levator Palpebrae Superioris
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Muscles that raises the upper eyelid (blinking) to prevent drying of the eyes (spreads oil, mucus, and saline solution) and protect the eye from foreign objects
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Eyelashes
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Sensitive hairs on the edge of the eyelid that triggers reflex blinking
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Tarsal Glands
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Sebaceous glands embedded in the tarsal plates with ducts at the eyelid edge just posterior to the eyelashes
produce an oily secretion that lubricates the eyelid and prevents the eyelids from sticking together
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Ciliary Glands
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Smaller, more typical sebaceous glands/modified sweat glands that lie between the hair follicles found on the margin of the eye lid
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Conjunctiva
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A transparent mucous membrane
Lines the eyelids (Palpebral Conjunctiva) Reflects over the anterior surface of the eyeball (Bulbar Conjunctiva): covers only the white of the eye, not the cornea; blood vessels are visible
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Conjunctival Sac
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A slitlike space between the conjunctiva-covered eyeball and eyelids (when the eye is closed)
Function: to produce a lubricating mucus that prevents the eyes from drying out
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Lacrimal Apparatus
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Lacrimal gland and the ducts that drain the excess lacrimal secretions into the nasal cavity
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Lacrimal Gland
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Lies in the orbit about the lateral end of the eye and is visible through the conjunctiva when lid is everted
Continually releases a dilute saline solution- Lacrimal Secretion
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Lacrimal Secretion
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Tears
released by the Lacrimal Gland into the superior part of the conjunctival sac through several small excretory ducts
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Lacrimal Sac
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Drains the tears
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Nasolacrimal Duct
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The nasolacrimal duct (sometimes called tear duct) carries tears from the lacrimal sac into the nasal cavity at the inferior nasal meatus
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Lacrimal fluid
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Consists of mucus, antibodies, and lysozyme (enzyme that destroys bacteria)
Cleanses and protects the eye surface as it moistens and lubricates it
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Extraocular Muscles (6)
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Muscles that allow the eyes to follow a moving object, help maintain the shape of the eyeball, and hold it in orbit.
(4) Superior, Inferior, Lateral, Medial Rectus (2) Superior, Inferior Oblique: move the eye in the vertical plane when the eyeball is already turned medially by the rectus…
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Diplopia
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"Double Vision" Perception of two images of a single object
Can result from paralysis or weakness of certain extrinsic muscles
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Strabismus
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"cross-eyed" Congenital weakness of the external eye muscles
Affected eye rotates medially or laterally
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Innervation of External Eye Muscles
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Abducens (VI) - Lateral Rectus
Oculomotor (III)- Superior, Inferior, Medial Rectus and Inferior Oblique Trochlear (IV)- Superior Oblique
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Structure of the Eyeball
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Eyeball
Poles: Anterior and Posterior Poles Walls: 3 layers- Fibrous, Vascular, and Sensory Layers Internal Cavity: filled with Humors fluid, Lens Divided to Anterior and Posterior Segments/Cavities
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The Fibrous Layer
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Outermost coat of the eye
Composed of dense avascular connective tissue 2 regions: Sclera and Cornea
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Sclera
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white of the eye
protects, shapes the eyeball and anchors extrinsic eye muscles
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Cornea
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The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber.
Major part of the light-bending apparatus of the eye Only tissue in body that can be transplanted from one person to another without rejection
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Vascular Layer (Uvea)
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Middle coat of the eyeball
3 regions: Choroid, Ciliary Body, Iris
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Choroid
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Vascular (nutrients), dark brown (melanocytes pigment to absorb light) membrane that forms the posterior 5/6 of the Uvea
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Ciliary Body
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Anterior of eye; thickened ring of tissue that encircles the lens
Consists of ciliary muscles- control lens shape; Ciliary processes- capillaries that secrete the fluid in anterior segment of cavity; Ciliary Zonule- holds lens in upright position)
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Iris
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Visible, colored part of the eye; most anterior portion of the uvea
Lies between the cornea and the lens and continuous with ciliary body Contain only BROWN pigment- just various concentrations Pupil: round, central opening 2 smooth layers: Sphincter Pupillae (contract in bright light) D…
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Inner Layer (Retina)
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Innermost layer of the eyeball
Outer pigmented layer: pigmented single epithelial cell lining absorb light, phagocytes, store Vitamin A Neural Layer: plays direct role in vision; anterior to ora serrata Ora Serrata: Posterior saw-toothed margin of the ciliary body; contains photoreceptor…
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3 Types of Neurons in the Neural Layer
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Photoreceptors: Signals from light goes to bipolar cells
Bipolar cells: sends signals to ganglion cells Ganglion Cells: generate axon potentials --> leave through optic nerve and exits the eye through the Optic Disc
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Optic Disc
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"Blind Spot" Where the optic nerve exits the eye; Lacks photoreceptors
On the Fundus (posterior wall of eye) which is a weak spot b/c lack of sclera support
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Filling In
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In vision, filling-in phenomena are those responsible for the completion of missing information across the physiological blind spot.
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Rods
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dim-light and peripheral vision receptors
Sensitive to light but do not provide sharp or color vision
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Cones
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photo receptors that operate in bright-light and provide sharp, color vision
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Macula Lutea
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The macula or macula lutea (from Latin macula, "spot" + lutea, "yellow") is an oval-shaped highly pigmented yellow spot near the center of the retina of the human eye.
Allows light to pass almost directly to the photoreceptors rather than through several retinal layers --> enhancing vis…
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Fovea Centralis
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A pit in the center of the Macula Lutea.
Only part that has sufficient cone density to provide detailed color vision.
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Blood supply of neural retina
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Outer 1/3rd containing photoreceptors - by blood vessels in the choroid.
Inner 2/3rds - by Central Artery and Central Vein of the Retina which enter and leave through the optic nerve
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Internal Chambers and Fluids in Eye
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Posterior Segment - Vitreous humor (clear gel)
Anterior Segment - Aqueous Humor (clear fluid) Anterior Chamber (between cornea and iris) Posterior Chamber (between iris and lens)
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Functions of Vitreous Humor
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1) transmits light
2) Supports the posterior surface of the lends and holds the neural retina firmly against the pigmented layer 3) contributes to intraocular pressure, helping to counteract the pulling force of the extrinsic eye muscles Forms in the embryo and lasts throughout lifetime
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Aqueous Humor
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forms/drains continuously and is always in motion
Flows through pupil into anterior chamber then drains into the venous blood 1) Supplies nutrients and oxygen to the lens and cornea and to some cells of the retina 2) Carries away their metabolic wastes
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Scleral Venous Sinus (canal of Schlemm)
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Venous channel that encircles the eye in the angle at the sclera-cornea junction.
Canal that drains the aqueous humor into the venous blood
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Glaucoma
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Glaucoma is a disease in which the optic nerve is damaged, leading to progressive, irreversible loss of vision. This can be due to increased pressure on the optic nerve from a clogged sceral venous sinus.
Late signs: halos around lights and/or blurred vision Treatments: eye drops to dra…
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Lens
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Biconvex, transparent, flexible structure that can change shape to focus light on the retina.
It is held in place by the capillary zonule. It has 2 regions: lens epithelium and lens fibers
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Lens Epithelium
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Location: anterior lens surface
Consists of cuboidal cells and then differentiate into lens fibers
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Lens Fibers
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Do NOT contain nuclei and few organelles
Contain transparent, precisely folded proteins called crystallins that form the body of the lens. Tightly packed together in layers like an onion. More layers are added as you age --> denser, more convex, and less elastic --> dec. ability to focus…
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Cataract
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"waterfall" a clouding of the lens.
Causes: congenital, age-related hardening, or diabetes mellitus --> inadequate delivery of nutrients to the deeper lens fibers Treatment: surgically removing the lens and implant artificial lens
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Visible Light Spectrum
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wavelength range of 400-700nm
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light photons/quanta
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small particles or packets of light energy
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Focal Point
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A single point that the light rays refract and converge together due to the convex lens
Thicker lens --> more refraction --> shorter focal distance Convex lens --> diverge light --> longer focal distance
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Real Image
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Image formed by a convex lens
Upside down and reversed from left to right
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Focusing for Distant Vision
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Far Point of Vision: the distance beyond which no change in lens shape is needed for focusing
Emmetropic eye: Normal eye, far point is 20 feet. Ciliary muscles are completely relaxed and lens is as thin as it gets. --> lowest refractory power When sympathetic input increases.
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Focusing for Close Vision
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To restore focus, simultaneously:
1) Accommodation- of the lenses Inc. refractory power of the lenses (ciliary muscles contract and lends bulge) 2) Constriction- of pupils Sphincter pupilae muscles of iris --> pupil toward 2mm 3) Convergence- of eye balls To focus object on retinal fovae…
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Near point of vision
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The maximum bulge the lens can achieve
Emmetropic vision - it's 10 cm and farther as we age (presbyopia)
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Accomodation Pupillary Reflex
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Mediated by the parasympathetic fibers of the oculomotor nerves
Prevents most divergent light rays from entering the eye to maintain sharper image.
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Myopia
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"short vision" / nearsightedness
Distant objects are focused in front of the retina Distant objects are blurred From eyes that are too long Treatment: Concave lenses, radial keratotomy (LASIK)
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Hyperopia
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"far vision"/ farsightedness
Objects are focused behind retina From eyes that are too short Treatment: Convex lenses
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Astigmatism
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Unequal curvatures in different parts of the cornea or lens
Treatment: cylindrically ground lenses, corneal implants, or laser procedures
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Phototransduction
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Process by which light energy is converted into a graded receptor potential
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Functional Anatomy of Photoreceptors
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In Rods and Cones:
Outer segment (receptor region) -> Connecting cilium -> inner segment -> cell body -> inner fiber bearing synaptic terminals Rods: slender/rod shaped; inner segment connects to cell body by the outer fiber Cones: short/conical outer segment and inner segment joins cell…
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Chemistry of Visual Pigments
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-Retinal- light absorbing molecule derived from Vit. A
-Opsin- protein that binds with retinal and becomes... -11-cis-isomer- bent shaped -all-trans-isomer- absorbs light photon and becomes straight; activates opsin --> causes electrical impulses to be transmitted along the optic nerve (…
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Stimulation of Photoreceptors: Excitation of Rods
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1. Excitation of Rods: Rhodopsin (Retinal + Opsin) breaks down after reaching its all-trans isomer shape in a process called "Bleaching of the Pigment"
a. Breakdown triggers transduction process involving cascade of steps. b. all-trans becomes 11-cis (requires ATP) c. retinal heads "home…
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Stimulation of Photoreceptors: Excitation of Cones
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Cones are 100x less sensitive than Rods -> higher intensity of light needed to activate cones
Blue cones: 420nm Green: 530nm Red: 560nm All colors are stimulated equally: white About the same process as Rods but different forms of opsin and retinal
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Color Blindness
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Congenital lack of one or more cone types
X-linked condition Most common type: red-green color blindness
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Light Transduction Reactions
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1) cyclicGMP binds to and opens cation channels in outer segments. 2) Na+ and Ca+ enter outer segment --> dark current that maintains a transmembrane potential of -40mV. 3) Ca+ channels at the photoreceptor synaptic terminals open. 4) Continuous glutamate release at the photoreceptor's sy…
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Light and Dark Adaptation
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Retinal sensitivity adjusts to amt light present. (Bleaching deactivates rods not cones).
Light adaptation: rods and cones stimulated; visual pigments are broken down; rod system turns off, retinal sensitivity dec., cones take over, visual acuity is gained (5-10 min), pupil constricts Da…
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Night Blindness or nyctalopia
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Rod function is hampered, impairing driving ability.
Cause: Vit. A deficiency --> rod degeneration or retinitis pigmentosa- disease that destroys rods b/c they cannot recycle tips of rods
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Visual Pathway to the Brain
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Axons of retinal ganglia -> optic nerves -> optic chiasma, optic tracts (lateral of same side and medial of opposite side) -> hypothalamus -> MOST go to lateral geniculate body of thalamus (balances retinal input to) -> visual cortex -> optic radiation -> primary visual cortex (occipital …
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Melanopsin
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"Circadian pigment" a visual pigment that responds to light stimuli independent of vision
then goes to -> Pretectal nuclei (mediates pupillary light reflexes) -> suprachiasmatic nucleus of the hypothalamus (timer to set out biorhythms)
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Retinal Processing pt. 1
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1) light hyperpolarizes photoreceptors
2a) Bipolar neurons in "on" regions depolarize and excite ganglion cell when rods are hyperpolarized. 2b) Bipolar neurons in "off" regions hyperpolarize and inhibit ganglion cell when rods are stimulated *on and off regions have different receptor t…
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Retinal Processing pt. 2
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4) Bipolar neurons getting signals from rods -> excite amacrine cells via gap junctions -> modify rod inputs an excite ganglion cells (summated and subject to detour causing smeary image)
5) Inputs are modified and subjected to lateral inhibition by gap junction contacts w/ horizontal ce…
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Thalamic Processing
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Lateral Geniculate Nuclei (LGN) Functions
1) relays info on movement 2) Segregate retinal axons in preparation for depth percep 3) emphasize visual inputs from cones 4) sharpen contrast info from retina
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Cortical Processing
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2 types of areas for processing retinal inputs are found in visual cortex.
Primary Visual Cortex (Striate Cortex) and Visual Association Areas (Prestriate Cortices)
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Primary Visual Cortex
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"Striate Coretx" thick w/ fibers from LGN
Basic level of visual processing: of neurons responding to dark/bright edges and object orientation. Provides form, color, and motion inputs to the prestriate cortices.
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Prestriate Cortices
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"Visual Association Areas"
@ Occipital lobe: cont. processing form, color, movement Processing also extends to temporal, parietal, and frontal lobes
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