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Ophthalmology > Physiology: Vision > Flashcards

Physiology: Vision Flashcards

1
Q

Requirements to visualise an object (4)

A
  1. Pattern of the object must fall on vision receptors (rods and cones in retina)
  2. Amount of light entering the eye must be regulated
  3. Energy from the waves of photons must be transduced into electrical signals
  4. Brain must receive and interpret signals
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2
Q

Retinotopy

A

Nerve fibres from the nasal half of each retina cross over at the optic chiasm
Resulting 2 optic tracts allow right and left visual fields to reach separately the left and right hemispheres.

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3
Q

Where is visual field mapped?

A

Mapped in the

  • retina
  • LGN
  • Superior colliculus
  • Cortex

Central. visual field is overrepresented

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4
Q

Discrete point of light

A

Acitvates many cells in target structure due to overlapping receptive fields

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5
Q

Perception of light

A

Based on the brains. interpretation of distributed patterns of activity

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6
Q

Primary visual cortex

A

Eye specific inputs are segregated in layer 4.
Both eyes project to each visual cortex.

At the primary visual area they remain largely segregated into ocular dominance columns

Cells outside of layer 4 receive input from both eyes

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7
Q

How can visual perception be shaped by early experience?

A

Congenital cataracts

Ambylopia

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8
Q

Congenital Cataracts

A

Opaque covering of lens
Impaired vision from birth
Difficulty perceiving shape and form

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9
Q

Ambylopia

A

One eye has better vision

Can be caused by strabismus (wandering eye) if not corrected in infancy

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10
Q

Cellular structure of retina

A

Direct (vertical) pathway for signal transmission

Lateral connections influence signal processing

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11
Q

Retina- Direct pathway for signal transmission

A

Ganglion Cells
Bipolar Cells
Photoreceptors

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12
Q

Retina- lateral connections

A

Horizontal cells receive input from photoreceptors and project to other photoreceptors and bipolar cells

Amacrine cells receive input from bipolar cells and project to ganglion, bipolar and other amacrine cells

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13
Q

Photoreceptors

A

Converts electromagnetic radiation to neural signals

->transduction

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14
Q

Photoreceptors main regions

A

Outer segment
Inner segment + Cell body
Synpatic terminal

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15
Q

Type of photoreceptors

A

Rods and Cones

- Outer segments: membranous disks containing photopigment

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16
Q

Phototransduction

A
  1. Vertebrate photoreceptors have depolarised rmp (Vm). Compared to other neurons, resting Vm is more positive (~ -20mV)
  2. Light exposure hyperpolarizes Vm
17
Q

Dark Current and Vm

A

Positive Vm is because of ‘dark current’

cGMP-gated Na+ channel that is open in dark and closes in light

Change in Na+ with light is the the signal that enables the brain to perceive objects in visual field.

Modulation of dark current is the basis of photo transduction

18
Q

modulation of dark current in the dark

A

In the dark

PNa = Pk (Na channels in outer segment)

Vm between Ena and Ek.

19
Q

Modulation of dark current in response to light

A

Pna is reduced (Pk >Pna)

Vm –> Ek , hyperpolarizes

Change is local and graded

20
Q

Visual pigment molecules

A

Rhodopsin (for rods)

Retinal + Opsin
Present in membrane folds

Light converts 11-cis-retinal to. all-trans-retinal 9activated form)

21
Q

Molecular Mechanism

A
  1. all-trans-retinal activates transducer
  2. molecular cascade
  3. decreases cGMP
  4. closure of cGMP-gated Na+ channel
  5. Lowered Na entry= hyperpolarization
22
Q

Dark Current channel

A

Opens in dark
Closes in response to light
nucleotide gated channel (opened by cGMP)
Permeable to Na+
Keeps photoreceptor Vm more positive than other neurons

–> Steady release of neurotransmitter

23
Q

Facilitation of High Acuity

A

Photoreceptor spacing facilities high acuity

24
Q

Visual Acuity

A

Ability to distinguish between 2 nearby points

Determined by

  • photoreceptor spacing
  • refractive power
25
Q

Rods

A

Seeing in dim light

more convergence. in rod system, increasing sensitivity with decreasing acuity

26
Q

Cones

A

Seeing in normal daylight.

27
Q

Colour vision

A

Light comprises discrete wavelengths

photoreceptors are only activated by a small portion of the spectrum of electromagnetic waves

Different opsin for discrete wavelengths

4 photoreceptor types in human retina

28
Q

Duplex Theory on Vision: Rods

A 
Achromatic 
peripheral retina 
high convergence 
high sensitivity 
low acuity.
29
Q

Duplex theory on vision: cones

A 
Chromatic 
Central retina - fovea 
Low convergence 
Low sensitivity 
High acuity

Ophthalmology (8 decks)

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