Phototransduction and Visual System Physiology Flashcards
Achromatopsia
The three types of cones in the retina are functioning normally
-Complete or partial loss of color vision
It is damage to the specific extrastriate cortical areas that renders the patient unable to see information supplied by the retina (V4 affected in this disease)
Ventral pathway
Primarily involved in interpreting images and complex patterns
Involves V4
Activated by shape, color, texture and object recognition
Facial recognition is a different area
Projects to temporal lobe
Dorsal pathway
Passes through V2 (depth perception), V3 and the MT(medial temporal)/V5 area before going to parietal lobe
Associates vision with movement
Agnosia
Can see object but unable to identify it or assign meaning
Damage to inferior temporal lobe (ventral path) causes this
Prosopagnosia
Ability to identify a face as a face, recognize its parts and detect expressions but cannot recognize it belonging to a specific person
Damage to inferior temporal lobe (ventral path) causes this
Explain what is going on in photoreceptors in the dark vs light with cGMP channels, polarization and glutamate and the effects on bipolar cells
Dark- cGMP gated Na channels are open and the cell is depolarized, rhodopsin molecules are inactive- glutamate is continuously released (causing inhibition of the cone bipolar cells and activation of rod bipolars)
Light- Light bleaches the rhodopsin molecules, causing sodium channels to close, and the rods become hyperpolarized- glutamate release is reduced (causing depolarization of rod and cone on-center bipolar cells and hyperpolarization of the cone off-center bipolar cells)
Rhodopsin
7 TM alpha helices
Homologous to B-adrenergic receptor
Lysine-296 in 7th TM span is bound covalently to 11-Retinal (retinal is derived from vitamin A), which turns the OPSIN into RHODOPSIN and increases its absorption to 500nm
Modulation of cGMP phosphodiesterase by Gt
Lower concentration of cGMP causes hyperpolarization of visual cells –> visual signal transduction
Gt is activated by light and activates cGMP PDE
Photon comes in–>conversion of cis to trans–>GDP exchanged for GTP–> GTP bound alpha subunit of transducin–> interacts with phosphodiesterase–> promotes conversion of cGMP to GMP
Calcium’s role
In light conditions there is low cGMP which closes Na/Ca channels and calcium decreases, this will lead to increased guanylate cyclase activity which will increase cGMP levels and restore cell to inactive status
Signal termination methods
Rhodopsin can be blocked from activating transducin by binding Arrestin
Rhodopsin will hydrolyze GTP to GDP and cause signal termination
Guanylate cyclase produces cGMP
Vitamin A deficiency symptoms in eyes
Night blindness, visual impairment, xerophthalmia and Bitots spots Night blindness (XN) is a disorder of the rod cells, often associated with vitamin A deficiency which leads to insufficient rhodopsin
RPE65 and LRAT
RPE65 converts all trans retinyl to 11-cis-retinol
LRAT converts all trans retinal to all-trans retinyl ester
Dysfunctions in these can lead to retinitis pigmentosa
RPE- retinal pigmented epithelium and macular degeneration
Degeneration of RPE can damage the macula
Buildup of all-trans-retinol in the photoreceptor can form aggregates that lead to macular degeneration
This is caused by defects in the ABC transporter that clears all trans retinol from photoreceptor
Nonexudative/dry AMD
Atrophy of the macula RPE
Wet AMD
Vascular endothelial growth factor VEGF is main mediator of this
Injections of VEGF inhibitors help this
Macular carotenoids
Play role in protection of the retina against damage
Lutein
Zeaxanthin
Play direct role in protection of retina against damage
Oxidative and inflammatory changes in RPE
Impaired phagocytosis and autophagy in RPE initiate inflammatory response
Make sure to consume enough anti-oxidants
V1, V3a and V4 functions
V1- Identify edges and contours of objects
V3a- Identify whether a motion is occurring (does not track the motion, that is done by MT/V5)
V4- complete processing of color
Amacrine cells and other horizontal cells
Generally inhibitory
Important in allowing us to see in low light conditions to enhance shadows, edges etc..
Lateral geniculate body
Controls motions of the eyes to converge on a point of interest
Control focus of eyes based on distance
Determine relative positions of objects to map them in space
Detect movement relative to an object
Superior colliculus
Create a map of visual space to activate appropriate motor responses required to move eyes into their intended position within the orbits
Coordinate head/eye movement to visual targets
Pretectum
Reflex control of pupil and lens, sends projections to Edinger-Westphal and the ciliary ganglion
Hypothalamus
From optic tract, some fibers form retinohypothalmic tract which goes to hypothalamic nuclei to help drive light/dark cycle and circadian rhythms
Accessory optic system
Advanced visual processing
Important role in eye movements of compensation and pursuit, particularly in alteration with saccadic type eye movements responding to prolonged watching of large field motion
Orientation columns
Organized region of neurons that are excited by visual line stimuli of varying angles
Each column may respond to different angle of lines- vertical, diagonal, horizontal etc…
Ocular dominance columns
A slab of cells that preferentially respond to input from one eye or another
i.e. 6 columns - R eye, L eye, R eye, L eye, R eye, L eye
Blobs
Organized region of neurons that are sensitive to color assemble into cylindrical shapes - respond to the three primary colors, all 3 color coding genes must be functional for accurate representation
V1 primary vs V4
Butterfly example- butterfly nor color detected if V1 is dysfunctional
Butterfly is detected but color is not if V4 is affected (achromatopsia)
