Week 6: Peripheral Visual System Flashcards
What is the main function of accessory structures in the eye?
To focus, amplify, or limit light before it reaches the photoreceptors.
What are the three layers of the eye?
Outer fibrous (sclera/cornea), middle vascular (choroid/ciliary body/iris), and inner neural (retina).
What type of sensory system is the eye?
Type III – it uses receptor cells that synapse with intermediaries before afferent neurons.
What is the function of the cornea?
Primary site of light refraction due to its curved surface.
What structure changes curvature for accommodation?
Lens, via contraction or relaxation of ciliary muscles.
What is the pupillary light reflex?
Constriction of both pupils in response to light entering one eye.
What is spherical aberration?
Light rays from lens edges focus differently than central rays, causing image blur.
What structures form the near response?
Convergence, accommodation, and pupillary constriction.
What causes myopia?
Excessive corneal/lens power or a long eyeball; corrected with concave lenses.
What causes hyperopia?
Insufficient lens/cornea refraction or a short eyeball; corrected with convex lenses.
Where are cones concentrated in the retina?
In the fovea, providing high-acuity and color vision.
Where are rods most densely located?
In the parafoveal region (just outside the fovea).
Which photoreceptors are used in dim light?
Rods (scotopic vision).
Which photoreceptors are responsible for daytime color vision?
Cones (photopic vision).
What are the three types of cone opsins?
S-cone (blue), M-cone (green), and L-cone (red).
What is the photopigment in rods called?
Rhodopsin (opsin + 11-cis retinal).
What ion channels are open in darkness?
Na+ and Ca2+ channels (gated by cGMP), causing depolarization.
What triggers phototransduction in rods?
Light activates rhodopsin → transducin → PDE → cGMP breakdown → channel closure.
What happens to membrane potential in response to light?
Hyperpolarization due to channel closure.
What maintains low intracellular Ca2+ in light?
Continuous Ca2+ pumping out of the photoreceptor.
What is light adaptation?
Decrease in photoreceptor sensitivity via cGMP regeneration and Ca2+ removal.
What is dark adaptation?
Gradual increase in rod sensitivity over 20–30 min after exposure to darkness.
Why is cone adaptation faster?
Cones are less affected by bright light and recover more quickly.
What is the fovea’s functional specialization?
High-acuity, direct photoreceptor access with minimal overlying layers.
What determines wavelength sensitivity in photoreceptors?
Type of opsin bound to 11-cis retinal.
What is the role of bipolar cells?
Relay photoreceptor signals to retinal ganglion cells (RGCs).
What are the two types of bipolar cells?
ON bipolar cells and OFF bipolar cells.
How do ON bipolar cells respond to glutamate?
They are inhibited by glutamate (via metabotropic mGluR6 receptors).
What happens to ON bipolar cells in light?
Less glutamate → disinhibition → depolarization → increased firing.
How do OFF bipolar cells respond to glutamate?
Excited by glutamate (via ionotropic AMPA/kainate receptors).
What happens to OFF bipolar cells in light?
Less glutamate → hyperpolarization → decreased firing.
What are horizontal cells?
Interneurons that mediate lateral inhibition from photoreceptors to bipolar cells.
What are amacrine cells?
Interneurons that modulate signals between bipolar cells and ganglion cells.
What is a receptive field in the retina?
Area of retina providing input to a single ganglion cell.
What is centre-surround organization?
Direct pathway excites centre; indirect lateral inhibition forms antagonistic surround.
What is the functional significance of centre-surround organization?
Enhances contrast and edge detection.
What is the difference in receptive fields between fovea and periphery?
Foveal fields are small (low convergence); peripheral are large (high convergence).
What type of ganglion cell responds to light in the centre?
ON-centre cell.
What type of ganglion cell is inhibited by light in the centre?
OFF-centre cell.
How does lateral inhibition work via horizontal cells?
Light hyperpolarizes photoreceptors → less glutamate → hyperpolarized horizontal cells → less GABA → less inhibition on centre.
What neurotransmitter is used by photoreceptors?
Glutamate.
How does GABA influence centre-surround interactions?
GABA from horizontal cells inhibits centre cone response in dark; inhibition is lifted in light.
What are the red-green colour opponent receptive fields?
ON-centre red/green: excited by red centre, inhibited by green surround (or vice versa).
What are the blue-yellow colour opponent receptive fields?
ON-centre blue/yellow: excited by blue centre, inhibited by yellow surround (or vice versa).
Why are colour opponent fields useful?
Enable the brain to detect chromatic contrast and fine hue differences.
What determines if a cell is ON- or OFF-centre?
Its response to light in the receptive field centre (excitation vs. inhibition).
What is basal firing rate in ganglion cells?
Spontaneous firing when no light is present; modulated up/down by inputs.
How does convergence affect visual acuity?
More convergence → lower acuity (periphery); less convergence → higher acuity (fovea).
What are the three synaptic layers in the retina?
Photoreceptor → Bipolar → Ganglion.
What is the function of the retinal ganglion cell?
Conveys final retinal output via optic nerve to the brain.
What types of glutamate receptors are on bipolar cells?
Metabotropic (ON cells) and ionotropic (OFF cells).
What is antagonistic surround inhibition?
Light in the surround produces the opposite effect of light in the centre.
