Physiology: Vision Flashcards
Requirements to visualise an object (4)
- Pattern of the object must fall on vision receptors (rods and cones in retina)
- Amount of light entering the eye must be regulated
- Energy from the waves of photons must be transduced into electrical signals
- Brain must receive and interpret signals
Retinotopy
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.
Where is visual field mapped?
Mapped in the
- retina
- LGN
- Superior colliculus
- Cortex
Central. visual field is overrepresented
Discrete point of light
Acitvates many cells in target structure due to overlapping receptive fields
Perception of light
Based on the brains. interpretation of distributed patterns of activity
Primary visual cortex
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
How can visual perception be shaped by early experience?
Congenital cataracts
Ambylopia
Congenital Cataracts
Opaque covering of lens
Impaired vision from birth
Difficulty perceiving shape and form
Ambylopia
One eye has better vision
Can be caused by strabismus (wandering eye) if not corrected in infancy
Cellular structure of retina
Direct (vertical) pathway for signal transmission
Lateral connections influence signal processing
Retina- Direct pathway for signal transmission
Ganglion Cells
Bipolar Cells
Photoreceptors
Retina- lateral connections
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
Photoreceptors
Converts electromagnetic radiation to neural signals
->transduction
Photoreceptors main regions
Outer segment
Inner segment + Cell body
Synpatic terminal
Type of photoreceptors
Rods and Cones
- Outer segments: membranous disks containing photopigment
Phototransduction
- Vertebrate photoreceptors have depolarised rmp (Vm). Compared to other neurons, resting Vm is more positive (~ -20mV)
- Light exposure hyperpolarizes Vm
Dark Current and Vm
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
modulation of dark current in the dark
In the dark
PNa = Pk (Na channels in outer segment)
Vm between Ena and Ek.
Modulation of dark current in response to light
Pna is reduced (Pk >Pna)
Vm –> Ek , hyperpolarizes
Change is local and graded
Visual pigment molecules
Rhodopsin (for rods)
Retinal + Opsin
Present in membrane folds
Light converts 11-cis-retinal to. all-trans-retinal 9activated form)
Molecular Mechanism
- all-trans-retinal activates transducer
- molecular cascade
- decreases cGMP
- closure of cGMP-gated Na+ channel
- Lowered Na entry= hyperpolarization
Dark Current channel
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
Facilitation of High Acuity
Photoreceptor spacing facilities high acuity
Visual Acuity
Ability to distinguish between 2 nearby points
Determined by
- photoreceptor spacing
- refractive power
Rods
Seeing in dim light
more convergence. in rod system, increasing sensitivity with decreasing acuity
Cones
Seeing in normal daylight.
Colour vision
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
Duplex Theory on Vision: Rods
Achromatic peripheral retina high convergence high sensitivity low acuity.
Duplex theory on vision: cones
Chromatic Central retina - fovea Low convergence Low sensitivity High acuity
