What is hyperopia?
- distance vision is blurred when light rays focus behind the retina
How to correct hyperopia
correction with a plus lens allows light to once again focus on the retina
What is myopia?
- distant vision is blurred when light rays from distant objects come to focus in front of the retina
How to correct myopia
correction with a minus lens allows light to once again focus on the retina
What are the three tunics?
- eye divided into 3 layers/ coats
- sensory tunic (retina)
Fibrous tunic has __ different regions. What are they?
- means hard
- forms posterior portion and majority (85%) of fibrous tunic
- anterior part covered by simple squamous epithelium: conjunctiva
- very high in collagen (dense connective regular tissue that is linear to give a crystalline and clear tendency
- anterior portion of fibrous tunic (approx. 15%)
- regular arrangement of collagen fibers make it crystal clear
- many pain fibers and fibers associated with reflex blinking and lacrimal secretion
- no vessels, derives nutrients from aqueous humor
What are the parts of the vascular tunic?
- highly vascular, dark brown (contains much melanin) pigmented
- continuous anteriorly with ciliary body and iris
- has tapedum: in nocturnal animals only [must be able to see at night]= reflective surface
- - reason: minimum amount of light will reflect around the eye to allow it to see
- humans have no tapedum; they have a choroid
- contains bundles of smooth muscles (ciliary muscles)
- suspensory ligaments: connect ciliary body to lens
- tension on ligaments causes lens to flatten
- When ligaments relax, lens get thicker due to internal elasticity
- Ciliary process: highly vascularized portion of ciliary body that produces aqueous humor; has blood vessels
- makes fluid in front of lens; aqueous humor
- visible colored part of the eye (gives the color) pigmented (melanin)
- located between lens and cornea
- forms central opening, the pupil, through which light enters eye
- acts as reflexively activated diaphragm
- in bright light, smooth muscles of iris contract causing pupil to constrict, and vice versa
- constriction/ dilation also controlled by sympathetic (dilation) and parasympathetic (constriction) fibers
canal of schlem
Explain the tapedum.
- similar to the choroid of nocturnal animals only; must be able to see at night; acts as reflective surface
- in humans, we have the choroid
- the reason a minimum amount of light will reflect around the eye to allow it to see
What are pupils?
What is red eye?
- black holes since the light that goes in can't come back out.
- light reflecting off of the capillaries; there is so much light that the choroid can't absorb
How many layers of the retina are there? What are the most important three?
- ganglioin cell layer (has ganglion cells)
- bipolar cell layer (bipolar, horizontal, and amacrine cells)
- rods and cones cell layer (photoreceptors)
What is the third tunic? What does it contain?
- sensory tunic (retina)
- neural layer
- optic disk
Neural (nervous) layer
- direct mediator of vision
- outpocketing of the brain
- Has five cell types: photoreceptors, bipolar cells, ganglion cells, horizontal cells, amacrine cells
Optic disc (optic nerve head)
- small circular area in medial retina where the optic nerve exits the eye
- no photoreceptor cell present in this part of the retina (no rods or cones)
- functionally, it is the "blind spot"
- focal point for light on the retina. Center of the visual field
- point of greatest visual acuity (sharpest vision)
- light passes directly to photoreceptors since all other cells displaced, i.e., bipolar and ganglion cells off to side
- site of greatest cone concentration (nothing but cones)
Refraction by the Cornea
parallel light rayes from object greater than 20 ft.
Accommodation by the Lens
- accommodation= lens changes shape for near vision
- divergent light rays
- ciliary muscles (from the ciliary body)contract to relieve tension on lens (lens get fatter, thickening by its own elasticity)
- incraesed curvature increases refractive power
What does the cornea function for?
objects greater thantwenty feet away
What does the lens function for?
objects nearer than twenty feet
Normal vs. Abnormal:
eyeball too short; light focuses behind retina
hardening of lens that accompanies aging; lens unable to flatten sufficiently during relaxation adn unable to fatten sufficiently during accommodation; due to cross-linking of collagen fibers; loses elasticity
irregularities in the curvature of the cornea or lens that produce different amounts of refraction
Snellen eye chart
20/20 vision: numer of feet required by a gien individual to discriminate characters on a specific line in the eye chart over the number of feet average perage person requires to view the same line
- light absorbing molecule; synthesized from vitamin A (retinol)
- cis (bent), but becomes trans when the light hits it
- opsin (protein that starts the response) and retinal; red pigment called "visual purple" important for vision in dim light
- also called visual purple
- deactivation of rhodopsin by bright light; separation into opsin and retinal
- (rods are more sensitive to light than cones)
Rods are for __. Rods are not really used in daylight because what?
When there is not enough light for cones and rods are inactive, what?
- grey (sensitive)
- light is so intense
- the rods kick in; night vision
Rods and Cones
photoreceptors of the vertebrate eye
more sensitive to light than cones, but don't distinguish color
responsible for daytime color vision; visual acuity, best visual discrimination for visual acuity
The __ has nothing but __ because of the __.
- fovia (pit/ indentation)
- visual acuity
__ subclasses of cones, each with its own type of __ associated with __ to form visual pigments:
- red, green, blue
What is the mechanism?
- light causes shape change in retinal
- triggers chain of metabolic events that decrease signal to cells with which photoreceptor cells synapse
- it is a decrease in the chemical signal that serves as the message
- rods and cones synapse with bipolar neurons, which synapse with ganglion cells
- eye cup of planarians
- compound eye of invertebrates
Explain the eye cup of planarians.
- simple light receptor that responds to light intensity and direction without forming an image
- planaria are negative photropic to avoid predation
Compound eye of invertebrates
- e.g.: insects, crustaceans, and some polychaete
- Thousands of light detectors called ommatidia, each with its own cornea and lens
- results in mosaic image
The lens is a __ into what?
- space in front: anterior segment
- space behind: posterior segment
subdivision of the anterior segment in front of the iris and behind the cornea
behind iris and in front of lens
cones are __ and need more __.
less sensitive (need more light)
The vitreous humor is __.