Flashcards in Week 5: The Eye, Visual Physiology Deck (26)
List the 3 layers of the eye and describe their function with respect to vision.
-sclera: dense irregular connective tissue with collagen and elastic fibers
-protects eye from trauma, maintains shape, site of attachment for EOMs
-cornea: transparent, avascular.
2. Uveal (iris, ciliary body, choroid)
List the chambers of the eye and the contents contained within each chamber
-filled with aqueous humor that is secreted by ciliated epithelium
-provides nutritional support to lens and cornea
2. Vitreous Chamber
-gel-like, made of collagen Type II, anchored to basal lamina of retina, ciliary body, and lens
-no blood vessels or nerves
What are the layers of the cornea?
1. corneal epithelium-stratified, non keratinized epithelium
2. Bowman's membrane-basement membrane of corneal epithelium
3. Corneal stroma- type I collagen, type V collagen, hydrated ground substance
4. Descemet's membrane - basement membrane of corneal endothelium
5. corneal endothelium - simple squamous epithelium. controls hydration of stroma-->maintains transparency (limited proliferation, damage requires transplantation)
What is the limbus of the eye?
-transitional zone between cornea and sclera
-aqueous humor leaves anterior chamber via trabecular meshwork which forms canal of Schlemm
What is the purpose of the conjunctiva?
-thin transparent mucus membrane: lateral surface of cornea, over sclera in anterior part, internal surface of eyelids
-stratified columnar or squamous epithelium w/ goblet cells and laminal propia
-mucus and tear production to lubricate eye
What are the components of the lens?
1. Lens capsule: hypertrophied basal lamina
2. subcapsular epithelium: single layer, mitotically active cuboidal cells. On anterior surface of lens. move towards equator as cells divide
3. Lens fibers: long hexagonal cells. Cells deeper to the epithelium lose their nuclei, become filled with crystallin (transparency and refractive properties)
How is the lens suspended to the ciliary body?
By the Suspensory ligament (Zonule of Zinn)
-tension of these zonule fibers can be altered by contraction or relaxation of ciliary muscle
-accommodated: ciliary muscles contracted, zonular fibers relaxed, more spherical
-unaccomodated: ciliary muscles relax, zonular fibers tense, lens flattened
What is presbyopia?
-decreased ability to focus on near objects.
-with age, loss of elasticity of lens and contractility of ciliary muscles
Describe the characteristics of the choroid.
-vascularized, pigmented connective tissue between sclera and retina
-outer layer (sclera side)-larger vessels, mostly veins
-inner layer (retina side)-choriocapillary layer, fenestrated capillaries with large irregular lumens. Supply outer layers of retina
-in between retina and choroid is Bruch's membrane=basal lamina of choroid capillary endothelium, basal lamina of pigmented epithelium cells, and elastic and collagen fibers
What is the ciliary body? What are its components?
-ciliary body is an extension of the choroid
-Stroma: 3 grps of smooth muscle (ciliary muscles) embedded in connective tissue. Rich vascular bed.
-Epithelium: non-pigmented layer (continous with neural retina) that produces aqueous humor and pigmented layer (continuous with retinal pigmented epithelium)
What are the 4 layers of the Iris?
1. Anterior surface: incomplete layer of fibroblasts
2. Stroma: connective tissue with blood vessels, nerves, melanocytes
3. Smooth muscle- at medial margin is sphincter pupillae muscle, at base of iris is dilator pupillage muscle
4. Epithelium -double layer pigmented cells to block light so that only light coming through pupil will pass through lens
Describe the basic structure of a photoreceptor cell.
-outer segment: stack of horizontal membrane discs (rods: separate from membrane, cones: discs continuous with membrane)
-connecting stalk: contain a cilium
-inner segment: organelles
-process that extends to form synaptic connections
List the layers of the neural retina (from outside inward).
1. photoreceptor layer: outer and inner segments
2. External imitating membrane: junctional complexes between photoreceptors and muller cells
3. outer nuclear layer: cell bodies and nuclei of photoreceptor cells.
4. Outer plexiform layer: processes of photoreceptors
5. Inner nuclear layer: cell bodies of bipolar, horizontal, amacrine, and muller cells
6. inner plexiform layer: processes of bipolar, horizontal, amacrine cells
7. ganglion cell layer
8. nerve fiber layer: axons of ganglion cells
9. internal limiting membrane: expanded terminal portions of muller cells with their basal lamina
What is the function of the retinal pigment epithelium?
-made of cuboidal to columnar cells, have lots of melanin granules
1. reduces backscattering of light that enters the eye
2. phagocytoses packets of outer segment membrane
3. esterifies Vitamin A derivatives
Briefly describe embryonic development of the eye.
-optic vesicle evaginates from third ventricle-->folds into optic cup-->outer layer becomes retinal pigmented epithelium, inner layer becomes neural retina
-lens placode from ectoderm-->forming lens vesicle
Where does the detachment of the retina occur and why?
-between pigment epithelium and neural retina.
-due to bi-layered nature of the optic cup
What is the macula lutea?
-lateral to optic disc
-yellow pigmented zone
-fovea is within macula
What is the fovea?
-in the center of the macula
-has highest visual acuity of retina
-contains only cones
-inner layers of retina not present
What is the ground state of photoreceptor cells? What state are the cells in when exposed to light and dark?
--ground state (dark): depolarized
-voltage dependent calcium channels are open and they enter synaptic terminus-->glutamate is released
-Ca2+/Na+ are entering the outer segment and being pumped out the inner segment (retinal cells are most metabolically demanding cells)
--in light: hyperpolarization
-voltage dependent channels close, Ca2+ levels drop, glutamate release slows
Describe the photo activation and biochemical cascade of photoreceptor cells.
-visual pigment: 11-cis retinal and opsin protein
1. Rhodopsin is activated by a photon.
-photoisomerization of 11 cis retinal to all trans retinal conformational change
2. allows receptor protein to activate transducin, G protein
3. activated transducer activates phophodiesterase enzyme
4. PDE hydrolyzes cGMP which leads to closure of cGMP gated channels
5. hyper polarization due to closure of channels
6. deactivation of photo cascade by phophorylation of rhodopsin
-RPE is needed for regeneration of 11-cis retinal from all trans form
What are the cell types in the retina and what are their functions?
1. bipolar cell: 1st synapse in visual system. 11 types that receive input from cones, 1 type from rods.
-Off bipolar cells hyper polarize in light
-On bipolar cells depolarize in light
2. horizontal cells: inhibitory feedback onto rod or cone
3. Amacrine cells: feedback to bipolar cells, synapse on ganglion cells
What is the relationship between convergence and resolution in the rod and cone system?
-convergence=ratio of photoreceptor cells:bipolar cells:ganglion cells
-low convergence-->better resolution
-cone pathways are more direct and more narrow field and convergence than rod pathways
Describe the concept of the receptive field (RF) and describe the on-ganglion and off-ganglion cells.
-receptive field of ganglion=region of space in retina in which presence of stimulus will alter the firing of the cell
-RFs detect contrast and direction of motion
-On center surround cells: increase firing rate when light is in center, slow firing rate when light is in surround
-Off center surround cells: slow firing rate when light is in center, increase firing rate when light is in surround
Define scotopic, mesopic, and photopic vision.
-scotopic: see by starlight (rods)
-mesopic: see by moonlight (rods+cones)
-photopic: anything brighter than moonlight (cones only)
How do cones give us color vision?
-three varieties of cones
-long wavelength: red
-middle wavelength: green
-short wavelength: blue