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HNNS > HNS43 Vision > Flashcards

HNS43 Vision Flashcards

1
Q

Pathways for pupil constriction + dilatation

A 
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)
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2
Q

***Visual pathway overview

A

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

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3
Q

Functions of visual system

A
  1. Visual acuity
  2. Form recognition (Spatial orientation)
  3. Motion perception
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4
Q

Clinical relevance - diseases of visual system

A
  1. 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
  1. Lens disease
    —> Cataract (clouding of lens, protein aggregates)
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5
Q

Cataract

A

Clouding of lens (Protein aggregates)

Symptoms:

  • Blurred vision
  • Poor night vision
  • Double vision (1 eye)
  • Colour fading

Causes:

  • Aging
  • Family history
  • Diabetes
  • Medication
  • Trauma
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6
Q

Lens accommodation

A

Changing of optical power

Innervation: ***Ciliary ganglion

Far accommodation: Ciliary muscle relaxed —> thinner lens
Near accommodation: Ciliary muscle contracted —> thicker lens

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7
Q

***Neural structures in retina

A

Receptor cells: base of retina

  1. Rod —> Night-time vision (Rhodopsin)
  2. 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)

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8
Q

Sensory transduction

A 
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

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9
Q

***Light and Dark environment

A

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
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10
Q

5 Retina cell types

A
  1. Rod / Cone cells (receptor cell) —> at base of retina (最入)
  2. Bipolar cell (linking cell)
  3. Retinal ganglion cells (output cell) —> outermost layer (最出)
  4. Horizontal cell (lateral pathway)
  5. Amacrine cell
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11
Q

Direction of information processing

A

Rods / Cones
—> Bipolar cell
—> Retinal ganglion cells (On-centre + Off-centre —> Different responsiveness to Light/Dark) (Concentric receptive field: RF centre + RF surround)
—> CN2

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12
Q

Bipolar cells

A

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
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13
Q

***Retinal ganglion cell

A
  1. 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
  1. 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
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14
Q

***Function of off-centre RG cells

A

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

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15
Q

Retinal processing

A

Retinal ganglion cells with ***distinct receptive field (dependent on classification):

  1. On-centre vs Off-centre
  2. Red-green vs Blue-yellow
  3. M cells vs P cells
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16
Q

Magnocellular cells vs Parvocellular cells (NOT examined)

A

Magnocellular cells:

  • Large RF
  • Movement / form of object
  • ***Contrast
  • ***Insensitive to colour

Parvocellular cells:

  • Small RF
  • ***Fine detail
  • ***Colour discrimination
17
Q

***Visual pathway in CNS

A

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

  1. **Binocular vision —> **Depth perception
  2. **Retinotopic projection —> **Pattern recognition + ***Feature extraction
18
Q

Lateral Geniculate Nucleus (Thalamus)

A

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
19
Q

***Modular organisation of Primary Visual Cortex (Striate cortex V1)

A
  1. 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)
  1. Blobs (cylindrical column for colour vision)
  2. 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

20
Q

**Functional hierarchy - **Transformation of Receptive field along visual pathway in CNS

A

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)

21
Q

Orientation specificity of cells in visual cortex

A
  1. Orientation columns
  2. Simple cells vs Complex cells —> each cell still retains “ON” zone and “OFF” zone
  3. Directional selectivity to moving stimulus
  4. Hyper-complex cells

—> Allow **Pattern recognition + **Feature extraction

22
Q

Higher visual cortex

A

Function relatively unknown

  1. Hyper-complex cell
  2. 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):
  1. Ventral pathway (***form/colour recognition, spatial tasks, visual memory) —> Temporal association cortex
  2. Dorsal pathway (***motion perception) —> Parietal association cortex

HNNS (58 decks)

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