HNS43 Vision Flashcards
Pathways for pupil constriction + dilatation
Pupil constriction (Pupillary light reflex): - Useful clinical diagnostic tool - Atropine blocks parasympathetic activity - Light —> Retina —> Pretectal region (midbrain) —> Edinger-Westphal nucleus —> Parasympathetic fibres —> ***Ciliary ganglion —> Sphincter muscle (Circular muscle)
Pupil dilatation: - Autonomic regions in brainstem (Fight/Flight) —> Ciliospinal centre (C8, T1) —> Sympathetic fibres —> ***Superior cervical ganglion —> Postganglionic via CNV1 —> Dilator muscle (Radial muscle)
***Visual pathway overview
Cone / Rod cells (Receptor cells)
—> Retinal Bipolar cells (Retina) (1st relay station)
—> Retinal ganglion cells
—> CN2
—> Lateral geniculate nucleus (Thalamus) (2nd relay station)
—> Primary visual cortex
—> Higher visual cortex
Functions of visual system
- Visual acuity
- Form recognition (Spatial orientation)
- Motion perception
Clinical relevance - diseases of visual system
- Retinal disease
—> Retinal detachment
- light flashes, watery vision, curtain obstructing vision, sudden decrease of vision
- Retinal tear, Vitreous gel separation
—> Macular degeneration (wet / dry)
- Wet degeneration: leakage of fluid from blood vessels
- Dry degeneration: Drusen (collagen) deposit
- genetics, age, nutrition, smoking
- Amsler grid
- Lens disease
—> Cataract (clouding of lens, protein aggregates)
Cataract
Clouding of lens (Protein aggregates)
Symptoms:
- Blurred vision
- Poor night vision
- Double vision (1 eye)
- Colour fading
Causes:
- Aging
- Family history
- Diabetes
- Medication
- Trauma
Lens accommodation
Changing of optical power
Innervation: ***Ciliary ganglion
Far accommodation: Ciliary muscle relaxed —> thinner lens
Near accommodation: Ciliary muscle contracted —> thicker lens
***Neural structures in retina
Receptor cells: base of retina
- Rod —> Night-time vision (Rhodopsin)
- Cone —> Day-time vision + Colour (3 types of Photopigment: Red, Green, Blue)
Central retina: High density of Cone cells
- Fovea (Yellow spot): central retina —> Avascular zone
- 1 Cone cell —> 1 Bipolar cell —> 1 Ganglion cell
—> ***Small receptive field, High visual acuity
Peripheral retina: High density of Rod cells
- Multiple Cone / Rod cells —> Multiple Bipolar cells —> 1 Ganglion cell
—> ***Large receptive field, Low visual acuity
Clinical relevance:
- Colour blindness: Hereditary (male > female)
Sensory transduction
Electromagnetic signals (Light / Dark environment) —> Rhodopsin (in rod) —> Biochemical process —> Open / Closure channel —> Electrical signals
In-folding of Rod / Cone cells:
- packed with Rhodopsin / Photopigments
- place of biochemical process:
Rhodopsin —(Light, Ca presence)—> Opsin + Retinol
Opsin + Retinol —(Dark)—> Rhodopsin
***Light and Dark environment
Dark:
Rhodopsin
—> open cGMP-gated Na Channels (on surface of receptors)
—> Na entry
—> ***Continuous Depolarisation of receptor cell (cone/rod)
—> NT release (tonic release)
—> Bipolar cell
Light: Bleaching of photopigment —> Activates G protein-coupled biochemical processes —> Closing of cGMP-gated Na Channels —> ***Hyperpolarisation of receptor cell —> no NT release
5 Retina cell types
- Rod / Cone cells (receptor cell) —> at base of retina (最入)
- Bipolar cell (linking cell)
- Retinal ganglion cells (output cell) —> outermost layer (最出)
- Horizontal cell (lateral pathway)
- Amacrine cell
Direction of information processing
Rods / Cones
—> Bipolar cell
—> Retinal ganglion cells (On-centre + Off-centre —> Different responsiveness to Light/Dark) (Concentric receptive field: RF centre + RF surround)
—> CN2
Bipolar cells
Concentric centre-surround receptive field
- RF of centre and surround —> Antagonistic
- Illumination of one —> Depolarisation, Illumination of another —> Hyperpolarisation
- On-centre vs Off-centre cell
***Retinal ganglion cell
- On-centre vs Off-centre cell
On-centre cell:
- Light directed to RF centre —> Excitation —> action potential from retinal ganglion cell
- Light directed to RF surround —> Inhibition (via horizontal cells: Lateral inhibition) —> no action potential from retinal ganglion cell
Off-centre cell:
- Darkness (Light off) directed to RF centre —> Excitation —> action potential from retinal ganglion cell
- Darkness (Light off) directed to RF surround / Light directed to RF centre—> Inhibition —> no action potential from retinal ganglion cell
- M cells vs P cells
—> Distinct ***parallel output pathways (i.e. project to different areas / layers of higher centres)
Magnocellular cells:
- Large RF
- Movement / form of object
- Contrast
- Insensitive to colour
Parvocellular cells:
- Small RF
- Fine detail
- Colour discrimination
***Function of off-centre RG cells
Able to ***enhance contrast of image at light-dark borders
—> Perceive darkness / movement of shadows (e.g. register edge of shadow sweeping across RF)
Initial: Light at centre —> No firing
Then: Darkness cover whole centre + some surround (some light still shine at RF surround) —> Intense firing
Lastly: Darkness cover whole RF —> Some firing
—> Difference in firing pattern when edge of shadow sweeping across RF
Retinal processing
Retinal ganglion cells with ***distinct receptive field (dependent on classification):
- On-centre vs Off-centre
- Red-green vs Blue-yellow
- M cells vs P cells
Magnocellular cells vs Parvocellular cells (NOT examined)
Magnocellular cells:
- Large RF
- Movement / form of object
- ***Contrast
- ***Insensitive to colour
Parvocellular cells:
- Small RF
- ***Fine detail
- ***Colour discrimination
***Visual pathway in CNS
Retina-geniculo-cortical pathway
Left visual field
—> Medial left eye (Nasal) + Lateral right eye (Temporal)
—> Cross at **Optic chiasm to right side + Stay at right side (Partial decussation)
—> Right **Lateral Geniculate nucleus (Right Thalamus)
—> Right visual cortex
Images from 2 eyes converge in brain
- **Binocular vision —> **Depth perception
- **Retinotopic projection —> **Pattern recognition + ***Feature extraction
Lateral Geniculate Nucleus (Thalamus)
6 layers of cells:
- **Contralateral nasal visual field information: 1, 4, 6
- **Ipsilateral temporal visual field information: 2, 3, 5
M cells (from retina) register at: 1, 2 P cell (from retina): 3, 4, 5, 6
***Modular organisation of Primary Visual Cortex (Striate cortex V1)
- Ocular dominance columns (alternating columns representing ipsilateral temporal / contralateral nasal visual field) —> ***Depth perception
(- Right visual cortex
—> Right temporal visual field
—> Left nasal visual field
- Left visual cortex
—> Left temporal visual field
—> Right nasal visual field)
- Blobs (cylindrical column for colour vision)
- Orientation columns (perpendicular to visual field columns) —> ***Spatial orientation (特定orientation trigger firing from 特定cortical cells)
—> ***Interaction between cortical modules (R/L, Colour vision, Orientation)
—> Visual perception
**Functional hierarchy - **Transformation of Receptive field along visual pathway in CNS
Neighbouring Retinal cells (a patch of retina) excite same number of Thalamic cell
—> Receptive field remains **Concentric centre-surround RF (dots-like) from Retina to Thalamus
—> multiple Thalamic cells converge
—> excite ONLY one cell in Primary Visual Cortex
—> RF becomes **Rectangular Parallel zones RF (Bar-like)
Convergent inputs (dots) from Thalamic cells
—> allow Cortical cells to have ***optimal orientations
—> each Cortical cell respond to a particular orientation (i.e. a particular RF)
Orientation specificity of cells in visual cortex
- Orientation columns
- Simple cells vs Complex cells —> each cell still retains “ON” zone and “OFF” zone
- Directional selectivity to moving stimulus
- Hyper-complex cells
—> Allow **Pattern recognition + **Feature extraction
Higher visual cortex
Function relatively unknown
- Hyper-complex cell
- Grandmother cell
Parallel pathways:
- Distinct M and P pathways (from Retina —> Visual cortical regions)
- Segregated pathways beyond Primary visual cortex (i.e. to High visual cortex):
- Ventral pathway (***form/colour recognition, spatial tasks, visual memory) —> Temporal association cortex
- Dorsal pathway (***motion perception) —> Parietal association cortex
