Download Vision_notes

VISION NOTES
WCR May 2015
Accessory structures of eye



Eyelids (palpebrae), eyelashes.
Conjunctiva= epithelium (musous membrane); conjunctivitis.
Lacrimal apparatus: tear production & removal. Lacrimal gland & ducts (superior & lateral);
lacrimal puncta, canaliculi, sac, nasolacrimal ducts (medial & inferior).
Structure of eyeball
Layers, outer to inner:
 Fibrous layer. Sclera, cornea: Know their function & be able to identify them on a diagram.
 Pigmented vascular layer. Choroid, ciliary body, iris: Know function & be able to identify. Pupil
size changes due to iris muscles (sphincter pupillae, dilator pupillae). Sympathetic, parasymp
innervation of iris muscles: which dilates, which constricts iris?
 Sensory layer=retina. Pigmented outer layer, neural inner layer. Neural layer of retina includes,
from outer to inner, 3 cell layers: photoreceptors (rods & cones), bipolar cells, ganglion cells.
 Rods versus cones – know diffs. What’s the fovea. Optic disc (blind spot): what’s at blind spot that
causes it to be blind?
Chambers & fluids
 Posterior segment: behind lens, contains vitreous humor.
 Anterior segment: in front of lens, contains aqueous humor.
Lens
 Held in place by suspensory ligaments attached to ciliary body (ring of muscle). Lens curvature
changes depending on how much it is stretched (flattened) by suspensory ligaments.
Cataract=clouded lens.
 When light enters eye, what structures does it travel through, in what order, on its way to the retina?
Once it strikes the retina, in what order are the different cells of the neural layer activated?
Physiology of Vision
Light & optics
Light has color (i.e. wavelength or frequency) and intensity. Humans see wavelengths from about 400
nm (violet) to 700 nm (red); other colors in between. Light is bent when it travels from a material with
one refractive index to another, for example from air to water or from air to glass.
Light focusing on retina
 Light bent as it enters cornea from air (this is the greatest bending that occurs) and as it enters &
leaves lens. Lens “fine tunes” the focusing. Far objects: lens stretched relatively flat, which bends
light less. Near objects: lens bulges, more curved, light bent more. Symp and parasymp: which is
activated for near, far focusing? In which case is ciliary muscle contracted?

Nearsighted, farsighted questions: Which person (near- or farsighted) needs glasses to read, which
needs glasses to drive? Why do young people with good vision need glasses as they age? Are they
getting nearsighted or farsighted as they age?
Photoreceptors & phototransduction
 Rods, cones work same way. (Then how do they differ? Which comes in 3 types?) Outer segment
of photoreceptors have membranous discs with light-sensitive proteins (“visual pigments”) in the
disc membranes. Visual pigment includes opsin protein & retinal molecule. Different color
sensitivities or rods and of red, green, blue cones are due to different opsin proteins.
Rhodopsin=visual pigment in rods=opsin plus retinal combined. Any given cone cell expresses only
one flavor of opsin, i.e. all red, all blue, or all green. In the dark, most opsin molecules have a bent
(cis conformation) retinal molecule embedded in them. Absorption of a photon of light causes
retinal to switch from bent to straight (cis to trans conformation); an enzyme (which requires ATP)
converts retinal back to cis (bent).
 Exposure to light (absorption of a photon) causes retinal to go from cis to trans. Trans retinal
dissociates from opsin. The opsin conformation changes, and this changes the Na permeability of
photoreceptor, which changes the amount of transmitter it releases onto bipolar cell.
Details: After straight retinal dissociates from opsin, opsin binds to and activates G-protein transducin. Activated
transducin converts cyclic GMP (cGMP) to GMP. This causes drop in cellular cGMP which causes closing of
previously open cation channels in photoreceptor. This leads to a reduction in inward Na current and therefore a
more negative intracellular voltage in photoreceptor. This causes less release of inhibitory neurotransmitter from
photoreceptor onto bipolar cell (middle cell in neural layer). Since transmitter released from photoreceptor inhibits
bipolar cell, bipolar cell is “quiet” in the dark (no or few action potentials, little or no transmitter release from
bipolar cell onto ganglion cell). Opposite in light.
Questions: In the dark, are photoreceptors are depolarized or hyperpolarized? Are bipolar cells active (frequent
action potentials) or quiet? Are ganglion cells active or quiet? Repeat the questions for “in the light”. (Answers for
“in the light” will be opposite.)


Bipolar cell action potentials are detected by ganglion cells. Ganglion cell action potentials flow
along their axons, which make up optic nerve.
Color perception: Due to the different levels of excitation of R, G, and B cones.
Color blindness: Lack of one or more types of cones. Sex-linked, mainly males.
Visual pathway to brain and visual processing
 Light pathway: From retina (made of axons from what cells?), optic nerve, optic chiasm, optic tract,
synapse in LGN, then to primary visual cortex in occipital lobes.
 Thalamus: Most axons from optic nerve synapse in LGN of thalamus; those thalamic neurons relay
the info to primary visual cortex. Some optic nerve axons go to superior colliculi and other areas for
subconscious reflexes such as pupillary light reflex and target tracking reflexes.
 Would a light slightly to the left of midline, visible in both eyes, project to R or L side of each
retina? Would it generate nerve impulses in R or L or both optic nerves? Would it project mainly to
primary visual cortex on R or L or both occipital lobes?
 Cortical processing: by primary visual cortex (relatively simple detection of features in the visual
scene) and by visual association areas (more complex feature detection). Primary visual cortex has
upside down & backwards map of visual space, similar to each retina.
 Depth perception: Done by comparison of right & left eye views of the world. Each region of
primary visual cortex gets input from BOTH eyes (thanks to the crossing over in the optic chiasm),
facilitating depth perception.
“Simple” vision problems
 Myopia (nearsightedness)
 Hyperopia (farsightedness) tends to develop with age. Also called presbyopia.
 Fix with lens (eyeglasses or contacts) or by photo-refractive corneal reshaping (such as Lasik).
 Cataracts: Cloudy lens, fix by removing/replacing lens.