Lecture 22: Physiology of the Visual System Flashcards
What is rod-mediated vision called?
What is it called when rod-vision is lost?
Scotopic vision
Night Blindness
What is cone-mediated vision called?
Photopic Vision
What is mesopic vision called?
Rods and cones are both activated
Which retinal cells are horizontally oriented?
Horizontal cells
Amacrine cells
Which retinal cells are vertically oriented?
Receptor cells (Rods and cones)
Bipolar cells
Ganglion cells
What do rods and cones converge to?
Rods/Cones –> Bipolar Cells –> Amacrine Cells
Multiple rods converge to one bipolar cell but only one cone contacts one bipolar cells
In the retina, where are rods most abundant?
Where are cones most abundant?
Rods - Just off center
Cones - Directly center (fovea)
What NT is released by rods/cones?
Glutamate
Glutamate is constantly released despite dark/light
What visual conditions cause higher glutamate release?
Dark Current
Darkness: cells are constantly depolarized, releasing GLU
What visual conditions cause lower glutamate release?
Bright light
Light causes hyperpolarization –> less release of GLU
What is the specialized glutamate receptor found on on-center bipolar cells?
When are they activated?
mGluR6: Gi GPCR
- Activated with low concentrations of GLU (light)
- High concentrations of GLU will inactivate (dark)
What is the specialized glutamate receptor found on off-center bipolar cells?
When are they activated?
AMPA (non-NMDA)
- Activated with high concentrations of GLU (dark)
- Low concentrations of GLU will inactivate (light)
What NT causes depolarization of a ganglion cell?
Glutamate
Where are ganglion cells found?
What do ganglion cell axons form?
Inner Plexiform Layer
Fibers of Optic Nerve
In activation of a rod photoreceptor, what cell acts as an interneuron between bipolar cells?
What NT is released?
Amacrine cell
GABA or glycine (inhibitory)
What is the purpose of on and off center cells?
Increase ability to detect edges and sharpen our vision
-
On Center: Where something is
- Excited by bright spot in center of receptor field
-
Off Center: Where something ends
- Excited by dark spot in center of receptor field
What does the lateral genicular body do?
- Control the motions of the eyes to converge on a point of interest
- Control the focus of the eyes based on distance
- Determine relative position of objects to map them in space
- Detect movement relative to an object.
What does the superior colliculus do?
- Creates a map of visual space to activate appropriate motor responses
- Coordinates head and eye movement to visual targets
What tract is associated with the superior colliculus?
Tectospinal
What does the pretectum do?
- Reflex control of pupil and lens
- Sends projections to Edinger-Westphal then on to ciliary ganglion
What does the hypothalamus do?
- Retinohypothalamic tract (from optic tract) terminate in nuclei in the hypothalamus
- Visual input to the hypothalamus drives the light–dark entrainment of neuroendocrine function and other circadian rhythms
What does the medial temporal area (MTA)/V5 do?
- Contains neurons that responds selectively to the direction of a moving edge.
- Tracks the motion across a scene in terms of directionality and background/foreground contex
- Ignores color
What does the accessory optic nuclei do?
- Advanced visual processing
- Important role in eye movements of compensation and pursuit particularly in alternation with saccadic-type eye movements, responding to prolonged watching of large field motion.
What is the major function of V1?
Identify edges and contours of objects
- Used to identify what and where objects are
- Retinotopic organization is maintained
- Local image: size, orientation, direction of movement
What is the main function of V2?
Depth perception
- Analyzes disparity between two eyes
What is the main function of V3a?
Identification of motion occuring
What is the main function of V4?
Complete processing of color input
What type of input do ocular dominance columns respond to?
Each column will prefer input from either right of left eye but not both
What type of input do the orientation columns respond to?
Each orientation column responds various angles
What type of input are the blobs of the primary visual cortex responsive to?
Specific wavelengths on the color absorption spectra associated with a specific cone (Red, green, blue)
What is the difference between achromatopsia and color blindness?
- Achromatopsia: cones work fine but there is damage to cortical areas
- Color Blindness: improper functioning of cones
Where do melanopsin ganglion cells project to?
What color are they sensitive to?
Project to suprachiasmaatic nucleus
Sensitive to blue
What are non-image-forming light-responsive systems important for regulating?
Circadian rhythms
What is the function of the ventral pathway?
Where does it begin and travel to?
- Interprets images and complex patterns (recognizing or copying shapes, forms, faces)
- Begins in primary visual cortex and travels to inferior temporal cortex
What is the function of the dorsal pathway?
What part of the visual cortex does it pass through?
Completes motor acts based on visual input
Passes through V3 (from primary visual cortex to parietal/frontal cortex)
