visual system I&II - Raj Flashcards
retina
encodes visual info. to optic nerve to travel to brain
optic nerve
hemidecussation at optic chiasm to project to lateral geniculate nucleus of thalamus
thalamocortical neurons
take info. from lateral geniculate nucleus to primary visual cortex
where do optic nerve and retina come from?
prosencephalic primary brain vesicle
internal refractive media
bend light rays as they pass through the photoreceptors
-refracted before reaching retina
fibrous (outer) layer of eye
consists of sclera and cornea
- sclera - maintains shape, attach external ocular muscles
- cornea - translucent, avascular
middle (vascular) layer of the eye
contains choroid or uvea
- focus light and regulate intensity
- choroid proper - vascular and pigmented
- ciliary body - produce aqueous humor (sometimes vitreous); lens held by suspensory ligament; contract to relax suspensory ligament and focus light
- iris - has pigments and forms rim of pupil; constrictor and dilator muscles to control size of pupil
inner (retinal) layer
- pigmented (external stratum) - single layer
2. cerebral (internal stratum) - 9 layers
cornea
different refractory index than air - bending of light rays
-main refractive structure of eye
aqueous humor
- posterior chamber –> anterior chamber
- drains into spaces of fontana –> canal of schlemm –> episcleral veins
- increase intraocular pressure w/o drainage –> can lead to glaucoma (leading cause of blindness)
lens
focusing bc its refractile power can be altered by suspensory ligament
- relax suspensory –> lens thickens
- presbyopia = lens becomes harder with age
- cataract = opacification of lens
reina
- non-neural - light absorbing pigmented cells
- neural - the deep layer photoreceptors
- transmit light energy to electrical energy
- more rods on periphery than cones in the middle
retinitis pigmentosa
failure of pigmented epith. to take up debris –> damage to photoreceptors
retinal detachment
separation of pigment and photoreceptor layers –> blurred vision
rods vs. cones
- rods
- sensitive to light - night vision, dim light
- peripheral part - decrease towards macula - cones
- bright light - color vision and high visual acuity
- middle part - fovea and macula
neurons of retina
- 1st order neuron –> bipolar cells in retina
- 2nd order neuron –> ganglion cells forming optic nerve
- interneurons - 1. horizontal cell (b/w photoreceptor and bipolar) 2. amacrine cell (b/w bipolar and ganglion)
central area of the eye
- fovea - contains cones, area for acute vision, foveola for light focusing to cones
- macula lutea
optic disc
- where unmyelinated nerve fibers coming from ganglion exit to head to brain
- fibers become myelinated when exiting
- blind spot - no photoreceptors
rods vs. cones pigment
- rods - rhodopsin pigment –> broken down to retinal and opsin and restored by vit. A after absorption
- cones - iodopsin pigment
- outer segment of photoreceptor cells –> transduce light rays to electrical energy
- inner segments –> contain mitochondria for energy
what causes night blindness?
vitamin A deficiency reduces nocturnal vision bc it cannot restore rhodopsin
what is the effect of intracranial pressure via CSF?
can compress optic nerve –> disc edema, papilledema, choked disc
fibers from temporal and nasal side of eye
- temporal fibers - travel to LGN w/o decussation
- nasal fibers - travel to opposite LGN after decussation
route for left visual field
left visual field –> right optic tract –> right LGN –> right visual cortex
route for right visual field
right visual field –> left optic tract –> left LGN –> left visual cortex
where do some optic nerve fibers go instead of LGN?
pretectal area - mediate the pupillary light reflex
central visual pathway of info.
LGN (contains 3rd order neuron) –> thalamocortical neurons to primary visual cortex in occipital lobe
primary visual cortex ***
- located above and below calcarine sulcus
- upper visual field –> lower retina –> fibers travel ventrally in temporal lobe to terminate BELOW calcarine sulcus (lingual gyrus)
- lower visual field –> upper retina –> fibers travel dorsally in parietal lobe to terminate ABOVE calcarine sulcus (cuneus)
- surrounded by visual association cortex (complex info.)
macula of the eye
post. in visual cortex
- center of eye - more visual acuity
- image also flipped
ex. of nasal/temporal side - visual field
right visual field
- nasal side of right eye
- temporal side of left eye
- vice versa for left
what do upper visual fields have to travel around?
around inferior horn forming loop (Meyer’s loop)
-lesion in temporal lobe –> affect UPPER visual field
field of vision
- fixation point - where vision is focused
- quadrants - projected on retina in reversed, inverted by lens
- visual pathways past optic chiasm carry fibers from contralateral halves of visual fields
anopsia
complete loss of vision
homonymous defect aka contralateral homonymous hemianopsia
affect same parts of visual fields in both eyes
- lesion distal to optic chiasm
- lose entire opposite field of vision in both eyes
heteronymous defect aka bitemporal hemianopsia
affect different parts of visual field of both eyes
- lesions of optic chiasm - usually affect crossing fibers (nasal)
- ex. affect left visual field of left eye and right visual field on right eye bc of decussation
dorsal vs. ventral fiber lesions
dorsal fiber lesion –> affect lower visual field
ventral fiber lesion –> affect upper visual field
light/pupillary constriction reflex
direct light reflex, can involve PNS
- light exposure –> pupil constricts
- consensual light reflex –> indirect pupil constriction in opposite eye
- optic tract –> superior colliculus –> pretectal area –> edinger Westphal –> oculomotor nerve –> ciliary ganglion –> short ciliary nerve –> constrictor muscle of iris
pupillary dilation reflex
passively by decreased PNS or actively by increased SNS (in hypothalamus)
- post. hypothalamus –> ciliospinal center in spinal cord –> ventral route –> white rami communicating –> superior cervical ganglion –> carotid plexus –> nasociliary and long ciliary nerves to dilator muscles of iris
- miosis if SNS is affected
accommodation reflex
maintain clear image when shifting from far to near object
- near triad: convergence, pupillary constriction, thicken lenses
- increase curvature of lens (bulge), contract ciliary body relaxing suspensory lig.
- increase visual acuity by convergence and constriction of pupil
accommodation reflex pathway
stimulated by perception of object
- occipital cortex –> corticotectal tract –> accomodation center –> oculomotor nuclei –> edinger westphal (pupil constriction –> oculomotor nerve –> ciliary ganglion –> short ciliary nerves –> iris constrictor muscles
- OR…accomodation center –> oculomotor somatic nuclei (for convergence) –> medial rectus
where is upper left visual field info. found?
found in lower right quadrant in visual cortex
retinitis pigmentosa
inherited disease of photoreceptors and retinal pigment epith. cells –> night blindness
-need to remove pigment
Horner syndrome
from tumors or vascular lesions
- miosis, ptosis, anhydrosis
- loss of smooth muscle innervation in upper eyelid and SNS innervation to sweat glands
