L43 - Vision Flashcards
Compare the innervation of retinal cells at the fovea vs peripheral retina.
Peripheral retina: many cone/rod cells > many bipolar cells > converge onto 1 retinal ganglion cell
Central retina: 1 cone/rod cell > 1 bipolar cell > 1 ganglion cell
= highest visual acuity
Compare the cones and rod cells in their function, receptive field and acuity?
Cones:
- photoreceptor for daytime, color vision
- Small receptive field
- High acuity
Rods:
- Photoreceptor for Night-light vision
- Large receptive field
- Low acuity
Describe the function of photopigments in day vs night vision?
Night: rhodopsin opens cGMP-gated Na+ channel»_space; Na+ entry/influx»_space; continuously depolarize photoreceptor»_space; tonic transmitter release to bipolar cell
Light: rhodopsin decomposed to opsin + retinal/retinol
-Bleaching of photopigment activates G protein-coupled biochemical processes»_space; Na+ channels close» hyperpolarize photoreceptor»_space; decrease/no transmitter release to bipolar cell
Classify retinal ganglion cells by functional responsiveness to light and dark?
On-centre ganglion cell
Off-centre ganglion cell
Describe the organization of bipolar cells in the retina?
concentric centre + surround receptive field (RF):
RFs of CENTER and SURROUND are antagonistic in nature
illumination of one RF causes depolarization while illumination of the other causes hyperpolarization
Describe the physiology of On- center retinal ganglion cells?
depolarize in response to illumination of the CENTER receptive field»_space; AP to brain
hyperpolarize in response to illumination of the antagonistic SURROUND RF via horizontal cells – lateral inhibition by GABA
» Silence ganglion cell
Describe the physiology of Off- center retinal ganglion cells?
depolarize in response to light off/ darkness (i.e. from light to dark) at CENTER RF
hyperpolarize with light to center RF or by dark stimulus to the surround
Function of On and Off- center retinal ganglion cells?
change in firing as edge of shadow sweeps across the retina
i.e. enhance the contrast of image during light to dark transitions, shadows
Classify retinal ganglion cells according to functional features?
i) ON-center cells vs OFF-center cells
ii) Magnocellular (M) cells
iii) Parvocellular (P) cells
Compare the function between Magnocellular cells and Parvocellular cells?
Magnocellular (M) cells
– detect stimulus movement / form of objects,
– insensitive to colour;
Parvocellular (P) cells
– sensitive to fine detail,
– colour discrimination.
Sequence of visual pathway?
i.e. Left nasal retina, right temporal retina
> > partial decussation at optic chiasma
> > right thalamus (lateral geniculate body)
> > right primary visual cortex processes left visual field
Describe the organization of the thalamus in processing visual information?
Lateral geniculate body/nucleus:
- Axons from each eye synapse on alternate cell layers
- Inputs from M cells, P cells are directed to separate layers of the LGB:
Layers 1-2: M cell
Layers 3-6: P cell
> > Concentric RF in the retina excited the same number of thalamic cells in the same concentric RF
Function of the Retinotopic organization of the retino-geniculo-cortical projection?
pattern recognition
feature extraction
Describe the transformation of receptive field throughout the visual pathway?
functional hierarchy:
Concentric center-surround RF in retina and in thalamus;
Inputs from thalamic cells with RF aligned along 1 axis converge onto 1 cell
> > Rectangular (parallel zones) RF in cortex
i.e. Many retinal cells excited in concentric RF
» Same number of thalamic cells excited in Concentric RF
» One cell** in primary visual cortex excited in parallel zone RF**
Describe the physiology of cortical processing of orientation from vision? (oritentation specificity)
Inputs from thalamic cells with RF aligned along 1 axis converge onto 1 cell in primary visual cortex [PARALLEL RECEPTIVE FIELD]
Simple Cells: can be turned on or off and each has a specific orientation (e.g. vertical bar of light) that cause depolarization
» Cortical cells with the optimal orientation fire maximum AP
» Signal converged into complex cells
» Hyper-complex cells and grandmother cells for central processing
