Chapter 4: Visual System Flashcards
What is the function of the conjunctive?
Membrane that covers the front of the eye
Lines the inside of the eyelids
What is the function of the cornea?
Takes nutrients and fluid from the aqueous humour
What is the function of the pupil?
Opening that lets light enter the eye and reach the retina
What is the function of the iris?
Muscle that gives the eye its color and contraction lets eye adapt to changing light
What is the function of the lens?
Focuses light rays onto retina
What is the function of the retina?
Converts image formed by light rays into nerve impulses by photoreceptors
What bends light?
Aqueous and viteous humor
What do rods do?
Let you see shades of grey in low light conditions
What do cones do?
Sensitive to color in bright light conditions
What are the 3 categories of the opsins?
Blue cones
Red cones
Green cones
What is color blindness?
Lesions in part of the visual cortex
Cones containing opsins for seeing red-green absent
In what gene is the color blindness on?
x chromosome
What are the forms classified from for color blindness and what are they?
From the type of cone affected
Protanopia
Deuteranopia
Tritanopia
What is achromatopsy?
Total color blindness
What are the 3 layers of the retina?
- Photoreceptive layer
- Bipolar cell layer
- Ganglion cell layer
Describe layer 1 of the retina.
- back of eye
- contains cones and rods
Describe layer 2 of the retina.
conveys information from photoreceptors to ganglion cells
Describe layer 3 of the retina.
Receives visual information from bipolar cells and axons give rise to optic nerve
What is the dark current? And what is the membrane potential of the outer segment of rods in the dark?
- When the retina is not stimulated, the photoreceptors continuously flow sodium
- Approx. -30 mV
What is the result of dark current?
Light hyperpolarizes photoreceptor cells and causes it to release fewer neurotransmitter molecules into synapse with bipolar cells
Where is the fovea located and it’s the function?
- region of retina
- color-sensitive cones only in the fovea
- acute vision of birds and higher mammals
What is the optic disk’s location and function?
- location of exit point from retina of fibers of ganglion cells
- responsible for blind spot
What is the path of the optic nerve?
- intersection of optic nerve of eyes at optic chiasm
- majority of optic tract projects to lateral geniculate nucleus
- projection from LGN to visual cortex via optic radiation
What is the LGN?
- 6 layers
- receives input from only 1 eye (approx. 80%)
- Layer 1-4-6: receive input from contralateral eye
- Layer 2-3-5: receive input from ipsilateral eye
what is the receptive field?
Portion of visual field where presence of a visual stimulus will modify the activity of a neuron in a layer of retina or cortex
Describe what happens in presence and absence of light for bipolar cells.
Exposure of retina to light: hyperpolarizes rods and cones
Absence of light: neurons that connect rods and cones to ganglion cells actively inhibited by rods and cones
What do the ganglion cells do?
Respond by increasing or decreasing the frequency with which they discharge action potentials
What do complex cells do?
detect movement
What do hyper-complex cells do?
See object’s edges and angles
What is the role of the visual cortex (V1)?
Sends a large proportion of its connections to V2 and some directly to V5
What does the ventral pathway allow and where does it entend to?
- Temporal lobe
- Allows to consciously perceive, recognize and identify objects by processing their intrinsic visual properties (shape and color)
What does the dorsal pathway allow and where does it project to?
- Parietal lobe
- Visual-motor control over objects by processing their extrinsic properties (size, position and orientation in space)
What are the ventral (what) stream disorders?
- cerebral achromatopsia: complete or partial loss of color vision
- visual object agnosia: failure to recognize objects by the visual modality alone
- Pure Alexia:
severe - can’t read words, numbers, letters, map signs or symbols
mild - read abnormally slowly - prosopagnosia: face blindness
What are the dorsal (where) stream disorders?
- Akinetopsia: motion blindness
- Simultanagnosia: inability to sustain attention across different locations in visual field
- Contralateral neglect
- Blindsight
