Functions of the Eye Flashcards
Focal point
Point at which light rays converge after being refracted through a convex lens
Degree of Refraction
Dependent upon:
- Ratio of refractive indices
- Angle between the interface and rays of light
Focal length
distance behind a convex lens at which parallel light rays converge
Four Refractive interfaces of the eye
- Interface b/w air and anterior cornea
2 B/w posterior cornea and aqueous humor
- B/w aqueous humor and anterior lens
- B/w posterior lens and vitreous humor
Refractive power of the eye
2/3- Cornea
1/3- lens
Ciliary muscle fibers
Meridonal- extend to corneoscleral jnxn; pulls lens ligaments medially and forward
Circular- when contracted, they decrease the diameter of the circle to which the suspensory ligaments attach releasing tension on the lens
*Both funxn to give the lens a more spherical, refractive shape
Parasympathetic function on ciliary muscles
Adapts lens for focusing on nearby objects
Presbyopia
Gradual decline in the ability to accommodate the lens
-Corrected with bifocal lens
Functions of iris
- Control the amount of light entering the eye
- Control the depth of field
- Depth of field is GREATEST when pupil diameter is smallest
- light rays are allowed little chance to diverge
The Near Triad
- Convergence of the eyes
- Accommodation of the lenses
- Miosis to maximize depth of field
Emmetropia
Normal eye
Hyperopia
Light rays are not bent to a sufficient degree due to decreased refractive power of the cornea of a short eyeball
- Ciliary muscles constantly contract to correct
- Corrected with a CONVEX lens
Myopia
Light rays converge before the retina due to an eyeball that is too long or an overpowered lens system
- Ciliary muscle is constantly relaxed to decrease the refractive power of the lens
- Corrected with CONCAVE lens
Astigmatism
Uneven curvature of the cornea resulting in unequal bending of light
- Lens is unable to accommodate
- Corrected with a cylindrical lens
Keratoconus
Progressive thinning and bulging of the cornea causing chaotic refraction of light
-Corrected with a contact lens
Cataract
An opaque lens that obscures the transmission of light
-Corrected by removing the lens and replacing with an artificial, plastic lens
Normal intraocular pressure
15 +/- 2 mmHg
-Determined by the resistance to outflow of aqueous humor through the Canal of Schlemm
Glaucoma
damage to the optic nerve that results in loss of vision
- Elevated intraocular pressure=risk factor
- Treated w/ carbonic anhydrase inhibitors(decreased formation of aqueous humor) and prostaglandin analogs (increased outflow thru the Canal of Schlemm)
Tonometry
measurement of intraocular pressure
Opthalmoscopy
Visual inspection of the optic nerve/disk
Gonoiscopy
Measurement of the angle b/w the iris and cornea
*Closure of angle impedes outflow of intraocular fluid => increased pressure
Vertical Pathway
Photoreceptors and bipolar cells generate graded potentials that release glutamate
Ganglion cells receive NT and generate an AP
*Glutamate can be either excitatory or inhibitory
Lateral Pathway
Maintain inhibition b/w photoreceptors by releasing GABA (horizontal cells) or dopamine, Ach serotonin, GABA (amacrine cells)
Dark current
Continuous inward current of Na and Ca due to the opening of CNG ion channels giving photoreceptors a slightly depolarized resting potential
-Held open by increased levels of cGMP
PDE in photoreceptors
Activated by light stimulation; degrades cGMP closing CNG ion channels making the membrarne hyperpolarized
Composition of photopigment
- Retinal- light absorbing chromophore
2. Opsin- retinal binding protein
Transducin
stimulates PDE; activated by all-trans-retinal
Bleaching
Absorbance of light causing retinal to change from purple to yellow
Light adaptation
PDE becomes less effective in closing CNG channels due to a decrease in intracellular Ca causing
- Increased guanylyl cyclase activity
- Increased affinity of CNG for cGMP
Cone sensitivity
Requires at least 100 photons of light to respond
-Cannot provide specific info about wavelength of light due to overlapping sensitivities
Red/Green deficiency
Loss of red and green cones
X-linked; more commonly affects males
Scotopic vision
Vision under low-light conditions; mostly uses rods
Mesopic vision
Vision under moonlight that uses both rods and cones
Photopic vision
Vision under lighting that uses only cones
On-center bipolar cells
Stimulation of the center of these fields will hyperpolarize the photoreceptor causing decreased glutamate release
- Contain glutamate receptor receptor that closes CNG channels when stimulated
- Inhibited by off-center illumination
Off-center bipolar cells
Contains conventional ionotropic glutamate receptors that are excited (depolarized) by the glutamate from photoreceptors in the dark
-Hyperpolarize when center of field is in light
Horizontal cell function
Releases GABA onto lateral photoreceptors when stimulated by glutamate release from its own receptor
Synaptic connections of rods
Only to on-center bipolar cells
Output gets relayed by A-II amacrine cells onto cones
Ganglion cell output
Dependent upon the ratio of intensities of light
(Ratio of background light-focused light determines neuron firing rate)
-Poorly stimulated by diffuse illumination
M-type ganglion cells
“Magnocellular”
- 5% of gangilion
- Large
- Fast firing
- Detects movement
P-type ganglion cells
“Parvocellular”
- 90% of ganglion
- Small receptive field
- Slow conductors
- Sensitive to different wavelengths
- Detect fine form and detail
Red-green ganglion cells
Stimulated by red light in center; blue light on outside
-R+G-
Center excited by red; surround inhibited by green
White light=ineffective stimulus
The eye is what type of lens?
Convex cylindrical
Ora Serata
Anterior border of the retina
-Non-photosensitive portion continues over the ciliary body
Function of Pigmented Epithelium
- Synthesize Brush’s membrane
- attaches retina to the choroid
- Transport nutrients to the retina
- Remove waste products
- Phagocytosis and recycling of photoreceptors
- Metabolism of retinol
Muller Cells
Form the inner limiting membrane of the retina
-Function as glial cells
