Visual Pathway Flashcards
(27 cards)
Types of photoreceptors
- Rods: sensitive to low light but do not detect color
○ Concentrated in peripheral retina
○ Detect movement - Cones: responsible for color vision and for high acuity vision
Concentrated in Fovea (central in retina)
Bipolar cells
neurons that relay signals from photoreceptors to ganglion cells
Ganglion cells
their axons form the Optic nerve
How do you see color
Color perception is based on relative activation of the three types of cones
- Different types of light activate different type of cones
Combination of cone activations allows to perceive a wide spectrum of color
Color blindness caused by deficiency or absence of a specific cone
Phototransduction + the roles of cone and rods
conversion of light to electrical signals using photochemical from rods (black and white) and cones (color)
Cones: color and central vision
Rods: really dim and rough vision
How/when is the image flipped?
- Cornea and lens work together as a convex lense system to bend/refract light
- Image of the retina is inverted (upside down) and reverse (left becomes right)
Brain the flips the image again
- Image of the retina is inverted (upside down) and reverse (left becomes right)
Optic disk vs central fovea
Optic disk - emerging retinal arteries blind spot
- No photoreceptors
- Axons from optic nerve exit here
Central fovea: only cone receptor, greatest visual acuity
Lateral geniculate nucleus
Take visual signal and categorize it to then send it to the right place in primary visual cortex
Primary relay center for visual info
Organized in multiple layers
Each layer processes aspects of vision (motion, depth, color, fine detail) before sending signals to primary visual areas
Contra lateral - Right LGN receives info from left visual field
Primary visual cortex
Located In calcarine sulcus of occipital lobe
Responsible for conscious visual perceptions
Retinotopic mapping: precise mapping of the visual field on the cortex
Analyses orientation, edges, motion and depth
- Upper visual field in the lower part of V1
- Lower visual field in upper part of V1
Contralateral - Left V1 processes info from right visual field, both eyes
Retinotopic mapping
upside down and reversed is maintained throughout the visual pathway
- The brain flips the inverted image so you the world right side up
Visual field deficits occur when there is damage along the visual pathway
From optic nerve to primary visual cortex, each lesion location results in a specific pattern of vision loss
Optic chiasm lesions
Results in bitemporal hemianopia (loss of vision in temporal fields of both eyes
Optic tract lesion (and beyond incl LGN and primary visual area)
Results in CL homonymous hemianopia (loss of same side of visual field in both eyes)
Lesions at the optic nerve
cause monocular blindness (loss of vision in the same eyes as the lesion)
Lesion in primary visual area:
cortical blindness
The 6 extraocular muscles
- Superior rectus: moves eye up and slightly in (elevation, add)
- Inferior rectus: moves eye down and slightly in (depression, add)
- Lateral rectus: moves eye out (abduction)
- Medial rectus: moves eye in (add)
- Superior oblique: down and out (depression, abd, intorsion)
- Inferior oblique: up and out (elevation, abd, extorsion)
Nerve supply to eye muscles
Oculomotor nerves
- Innervates SR, IR, MR and IO
- Controls levator palpebrae superioris (eyelid elevation) and pupil constriction
Trochlear Nerve
- Innervates SO
- Only CN that exits dorsally and crosses to CL side
Abducens Nerve: Innervates LR
Gaze
Gaze is the coordinated movement of both eyes to focus on an object, allowing for clear vision and depth perception
- coordination of head and eye movement
- Essential for tracking, reading, balance
- Key players
○ Superior colliculus: reflex
○ Frontal eye field: for voluntary movement
Horizontal and vertical gaze centers in brainstem for coordination
Superior colliculus
- In midbrain, responsible for reflexive eye movement
- Receives visual, auditory and sensory input
○ Coordinated rapid, reflexive saccades to unexpected stimuli - Reflexive saccades
○ Quick, involuntary eye movement to unexpected stimuli
Turning your head when you hear a loud sound or sudden flash
- Receives visual, auditory and sensory input
Frontal eye field
Located in frontal cortex, responsible for voluntary eye movement
Initiates voluntary saccades to target of interest even without visual stimuli
Controls attention and predictive tracking, planning where to go next
Voluntary saccades
- Deliberate, rapid eye movement to object of interest
E.g. reading, searching for a face in a crowd
Smooth pursuit
Slow continuous tracking of a moving object
Controlled by frontal eye field and cerebellum
Horizontal gaze center
- Lateral rectus and medial rectus need to move together
- LR innervated by CN VI
- MR innervated by CN III
In pons - coordinate horizontal eye movement by connecting these nerves
Horizontal saccades
Horizontal gaze centers synapses with ML, which crosses and then with synapses with CL nucleus of CNIII
Vertical gaze center
Coordination of CN III and CN IV
In mid brain - coordinated vertical eye movement
Parasympathetic eye innervation
- Pupil constriction (miosis): reduces light entry for near vision and protection from bright light
- CNIII carries PS to the eyes
Lens accommodation: contracts the ciliary muscles making the lens more spherical for near vision
- CNIII carries PS to the eyes
