Vision

• farsightedness
• distance vision is blurred when light rays focus behind the retina

correction with a plus lens allows light to once again focus on the retina

• nearsightedness
• distant vision is blurred when light rays from distant objects come to focus in front of the retina

correction with a minus lens allows light to once again focus on the retina

• eye divided into 3 layers/ coats
• fibrous
• vascular
• sensory tunic (retina)

• two 
• sclera
• cornea

• means hard
• white
• 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

• choroid
• ciliary body
• iris

• 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

drains humor

• 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

• 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

• ganglioin cell layer (has ganglion cells)
• bipolar cell layer (bipolar, horizontal, and amacrine cells)
• rods and cones cell layer (photoreceptors)

• sensory tunic (retina)
• neural layer
• optic disk
• fovia

• direct mediator of vision
• transparent
• outpocketing of the brain
• Has five cell types: photoreceptors, bipolar cells, ganglion cells, horizontal cells, amacrine cells

• 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)

parallel light rayes from object greater than 20 ft.

• 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

objects greater thantwenty feet away

objects nearer than twenty feet

• hyperopia
• presbyopia
• myopia
• astigmatism

eyeball too short; light focuses behind retina

normal vision

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

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

• retinal
• rhodopsin
• bleaching

• 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)

• grey (sensitive)
• light is so intense
• the rods kick in; night vision

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

• fovia (pit/ indentation)
• cones
• visual acuity

• three
• opsin
• retinal
• red, green, blue

• 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

• simple light receptor that responds to light intensity and direction without forming an image
• planaria are negative photropic to avoid predation

• e.g.: insects, crustaceans, and some polychaete
• Thousands of light detectors called ommatidia, each with its own cornea and lens
• results in mosaic image

• divider
• 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

less sensitive (need more light)

jelly-like