visual Flashcards
visual system
-detects and interprets light stimuli -> electromagnetic waves
-distinguishes two qualities of light -> brightness and wavelength
-2 fluids in eye are aqueous and vitreous humors (posterior chamber) and aqueous humor (anterior chamber)
layers of eye
-wall of eye consists of 3 concentric layers: outer, middle, inner
-outer- fibrous, includes cornea, corneal epithelium, conjunctiva, and sclera
-middle- vascular, includes iris and choroid
-inner- neural contains retina
macula
-central point of the retina
-visual acuity is highest
-highest amount of photoreceptors here
fovea
-light focused at depression in macula
lens
-focuses light
pigments
-absorb light and reduce scatter
fluids in eye
-aqueous humor- anterior chamber of eye
-vitreous humor- posterior chamber of eye
retina
-cover entire posterior eye
-exception of blind spot -> aka optic disc (head of optic nerve)
-retina is specialized sensory epithelium that contains photoreceptors and other cell types arranged in layers
-retinal cells include: photoreceptors, interneurons (bipolar, horizontal, and amacrine cells), and ganglion cells
sensory receptors for vision
-photoreceptors
-located on retina
-2 types: rods and cones
-information received and transduced by photoreceptors on retina -> carried to CNS via axons of retinal ganglion cells
-some optic nerves cross at optic chiasm
-others continue isilaterally
-main visual pathway is through dorsal lateral geniculate nucleus of thalamus which projects visual cortex
rods
-low threshold
-sensitive to low intensity light
-function well in darkness
-low acuity and do not participate in color vision
-connected to few bipolar and ganglion cells
cones
-higher threshold for light
-operate best in daylight
-provide higher visual acuity
-participate in color vision
-not sensitive to low intensity light
-multiple cones required to activate ganglion cell
-requires a lot more light to activate
pigment cell layer
-pigment epithelial cells absorb stray light and have tentacle like processes that extend into photoreceptor layer to prevent scatter of light between photoreceptors
photoreceptor layer
-photoreceptors (rods and cones)
-consist of cell body, outer segment, and inner segment
-outer and inner segments of photoreceptors are in this layer
outer nuclear layer
-nuceli of photoreceptors (R) are contained in the outer nuclear layer
outer plexiform layer
-synaptic layer containing presynaptic and postsynaptic elements of photoreceptors and interneurons of retina
-cell bodies of retinal interneurons are contained in the inner nuclear layer
-synapses are made between photoreceptors and interneurons and also between interneurons themselves
-synaptic terminals (on bipolar and horizontal cells) are located in outer plexiform layer
inner nuclei layer
-contains cell bodies of retinal interneurons including bipolar cells (B), horizontal cells (H), and amacrine cells (A)
-bipolar cells connect photoreceptor to ganglion cell -> sends signal out
-horizontal cells laterally inhibit-> improve visual acuity
-amacrine cells- improve complexity of vision
inner plexiform layer
-second synaptic layer
-contains presynaptic and postsynaptic elements of retinal interneurons
-synapses are made between retinal interneurons and ganglion cells
ganglion cell layer
-contains cell bodies of ganglion cells (G) -> output cells of retina
optic nerve layer
-axons of retinal ganglion cells form the optic nerve layer
-these axons pass through the retina (avoiding the macula) -> enter optic disc -> leave the eye in optic nerve
rods vs cones
-differences in retinal circuitry
-only few cones synapse on single bipolar cell, which synapses on a single ganglion cell
-this arrangement accounts for higher acuity and lower sensitivity of cones
-acuity is highest in fovea where 1 cone synapses on 1 bipolar cell which synapses on one ganglion cell
-many rods synapse on a single bipolar cell
-this accounts for lower acuity but higher sensitivity of rods -> light striking any one of the rods will activate bipolar cell
rhodopsin
-outer segments of both rods and cones
-light sensitive pigment (photopigment)
-greater amount of photopigment -> greater sensitivity to light -> accounts in part for greater light sensitivity of rods
-single photon of light can activate a rod whereas several 100 photons are required to activate a cone**
-rods shed rhodopsin
photoreception
-transduction process in rods and cones that converts light energy to electrical energy
-when light strikes photoreceptors -> retinal chemically transforms in process called photoisomerization
-photoisomerization begins the transduction process
-when light hits photoreceptors -> ALWAYS hyperpolarized and release decreased amounts of glutamate
-glutamate crosses over synapse on bipolar cell
-bipolar cell can be ionotropic -> Decrease glutamate -> hyperpolarization of center of bipolar cell -> inhibit
-bipolar cells can be metabotropic-> decreasing glutamate -> depolarization of center of bipolar cell
-if receptive field has ionotropic glutamate receptors -> bipolar cell will be inhibited
-if receptive field has metabotropic glutamate receptors -> bipolar cell is excited
-surround of bipolar cells receptive field receives input from horizontal photoreceptors -> shows opposite response of center bc horizontal cells are inhibitory
-relay to ganglion cells
outer segments of cones and rods
-rods- outer segments are long and consist of stacks of free floating double membrane discs containing large amount of rhodopsin
-cones- short cone shaped outer segments -> consist of infoldings of surface membrane
-infolded membrane also contains rhodopsin but smaller amount than present in rods
inner segments of cones and rods
-connected to outer segments by single cilium
-contain mitochondria and other organelles
visual discrimination
-neurons of visual cortex detect shape and orientation of figures
-3 cells involved: simple, complex, hypercomplex
-simple- receptive fields similar to ganglion cells and lateral geniculate cells (on-center or off-center) -> although patterns are elongated rods rather than concentric circles
-simple cells response best to bars of light that have “correct” position and orientation
-complex- respond best to moving bars of light or edges -> identifies edges of objects
-hypercomplex- respond best to lines of particular length and to curves and angles -> structure objects
optic nerves and optic tracts
-axons from retinal ganglion cells form this
-synapse in lateral geniculate body of thalamus and ascend to visual cortex in geniculocalcarine tract
nasal hemiretina
-nerve fibers from each nasal hemiretina cross at optic chiasm and ascend contralaterally
-temporal visual fields project onto nasal retina and nasal fields project onto the temporal retina
temporal hemiretina
-nerve fibers remain uncrossed and ascend ipsilaterally
optic tract
-fibers from left nasal hemiretina and fibers from right temporal hemiretina form the right optic tract and synapse on right lateral geniculate body
-fibers from right nasal hemiretina and fibers from left temporal hemiretina form left optic tract and synapse of left lateral geniculate body
geniculocalcarine tract
-fibers from lateral geniculate body form the geniculocalcarine tract -> ascends to visual cortex (area 17 of occipital lobe)
-fibers from right lateral geniculate body form the right geniculocalcarine tract
-fibers from left lateral geniculate body form the left geniculocalcarine tract
hemianopia
-loss of vision in half the visual field of one or both eyes
-cutting optic nerve causes blindness in ipsilateral eye
-all sensory information coming form that eye is lost because the cut occurs before any fibers cross at the optic chiasm
-cutting optic chiasm causes heteronymous (both eyes) bitemporal (both temporal visual fields) hemianopia -> all information is lost from fibers that cross -> information from temporal visual fields from both eyes is lost because these fibers cross at optic chiasm
-cutting the optic tract causes homonymous contralateral hemianopia -> cutting left optic tract results in loss of temporal visual field from right eye (crossed) and loss of nasal visual field from left eye (uncrossed)
photoreceptive field
-receptive field of bipolar cell: center and surround
-center- direct connections from photoreceptors and can be either excited (on) or inhibited (off) depending on type of glutamate receptor on bipolar cell
