Week 6: Peripheral Visual System

Question 1

What is the main function of accessory structures in the eye?

Answer 1

To focus, amplify, or limit light before it reaches the photoreceptors.

Question 2

What are the three layers of the eye?

Answer 2

Outer fibrous (sclera/cornea), middle vascular (choroid/ciliary body/iris), and inner neural (retina).

Question 3

What type of sensory system is the eye?

Answer 3

Type III – it uses receptor cells that synapse with intermediaries before afferent neurons.

Question 4

What is the function of the cornea?

Answer 4

Primary site of light refraction due to its curved surface.

Question 5

What structure changes curvature for accommodation?

Answer 5

Lens, via contraction or relaxation of ciliary muscles.

Question 6

What is the pupillary light reflex?

Answer 6

Constriction of both pupils in response to light entering one eye.

Question 7

What is spherical aberration?

Answer 7

Light rays from lens edges focus differently than central rays, causing image blur.

Question 8

What structures form the near response?

Answer 8

Convergence, accommodation, and pupillary constriction.

Question 9

What causes myopia?

Answer 9

Excessive corneal/lens power or a long eyeball; corrected with concave lenses.

Question 10

What causes hyperopia?

Answer 10

Insufficient lens/cornea refraction or a short eyeball; corrected with convex lenses.

Question 11

Where are cones concentrated in the retina?

Answer 11

In the fovea, providing high-acuity and color vision.

Question 12

Where are rods most densely located?

Answer 12

In the parafoveal region (just outside the fovea).

Question 13

Which photoreceptors are used in dim light?

Answer 13

Rods (scotopic vision).

Question 14

Which photoreceptors are responsible for daytime color vision?

Answer 14

Cones (photopic vision).

Question 15

What are the three types of cone opsins?

Answer 15

S-cone (blue), M-cone (green), and L-cone (red).

Question 16

What is the photopigment in rods called?

Answer 16

Rhodopsin (opsin + 11-cis retinal).

Question 17

What ion channels are open in darkness?

Answer 17

Na+ and Ca2+ channels (gated by cGMP), causing depolarization.

Question 18

What triggers phototransduction in rods?

Answer 18

Light activates rhodopsin → transducin → PDE → cGMP breakdown → channel closure.

Question 19

What happens to membrane potential in response to light?

Answer 19

Hyperpolarization due to channel closure.

Question 20

What maintains low intracellular Ca2+ in light?

Answer 20

Continuous Ca2+ pumping out of the photoreceptor.

Question 21

What is light adaptation?

Answer 21

Decrease in photoreceptor sensitivity via cGMP regeneration and Ca2+ removal.

Question 22

What is dark adaptation?

Answer 22

Gradual increase in rod sensitivity over 20–30 min after exposure to darkness.

Question 23

Why is cone adaptation faster?

Answer 23

Cones are less affected by bright light and recover more quickly.

Question 24

What is the fovea’s functional specialization?

Answer 24

High-acuity, direct photoreceptor access with minimal overlying layers.

Question 25

What determines wavelength sensitivity in photoreceptors?

Answer 25

Type of opsin bound to 11-cis retinal.

Question 27

What is the role of bipolar cells?

Answer 27

Relay photoreceptor signals to retinal ganglion cells (RGCs).

Question 28

What are the two types of bipolar cells?

Answer 28

ON bipolar cells and OFF bipolar cells.

Question 29

How do ON bipolar cells respond to glutamate?

Answer 29

They are inhibited by glutamate (via metabotropic mGluR6 receptors).

Question 30

What happens to ON bipolar cells in light?

Answer 30

Less glutamate → disinhibition → depolarization → increased firing.

Question 31

How do OFF bipolar cells respond to glutamate?

Answer 31

Excited by glutamate (via ionotropic AMPA/kainate receptors).

Question 32

What happens to OFF bipolar cells in light?

Answer 32

Less glutamate → hyperpolarization → decreased firing.

Question 33

What are horizontal cells?

Answer 33

Interneurons that mediate lateral inhibition from photoreceptors to bipolar cells.

Question 34

What are amacrine cells?

Answer 34

Interneurons that modulate signals between bipolar cells and ganglion cells.

Question 35

What is a receptive field in the retina?

Answer 35

Area of retina providing input to a single ganglion cell.

Question 36

What is centre-surround organization?

Answer 36

Direct pathway excites centre; indirect lateral inhibition forms antagonistic surround.

Question 37

What is the functional significance of centre-surround organization?

Answer 37

Enhances contrast and edge detection.

Question 38

What is the difference in receptive fields between fovea and periphery?

Answer 38

Foveal fields are small (low convergence); peripheral are large (high convergence).

Question 39

What type of ganglion cell responds to light in the centre?

Answer 39

ON-centre cell.

Question 40

What type of ganglion cell is inhibited by light in the centre?

Answer 40

OFF-centre cell.

Question 41

How does lateral inhibition work via horizontal cells?

Answer 41

Light hyperpolarizes photoreceptors → less glutamate → hyperpolarized horizontal cells → less GABA → less inhibition on centre.

Question 42

What neurotransmitter is used by photoreceptors?

Answer 42

Glutamate.

Question 43

How does GABA influence centre-surround interactions?

Answer 43

GABA from horizontal cells inhibits centre cone response in dark; inhibition is lifted in light.

Question 44

What are the red-green colour opponent receptive fields?

Answer 44

ON-centre red/green: excited by red centre, inhibited by green surround (or vice versa).

Question 45

What are the blue-yellow colour opponent receptive fields?

Answer 45

ON-centre blue/yellow: excited by blue centre, inhibited by yellow surround (or vice versa).

Question 46

Why are colour opponent fields useful?

Answer 46

Enable the brain to detect chromatic contrast and fine hue differences.

Question 47

What determines if a cell is ON- or OFF-centre?

Answer 47

Its response to light in the receptive field centre (excitation vs. inhibition).

Question 48

What is basal firing rate in ganglion cells?

Answer 48

Spontaneous firing when no light is present; modulated up/down by inputs.

Question 49

How does convergence affect visual acuity?

Answer 49

More convergence → lower acuity (periphery); less convergence → higher acuity (fovea).

Question 50

What are the three synaptic layers in the retina?

Answer 50

Photoreceptor → Bipolar → Ganglion.

Question 51

What is the function of the retinal ganglion cell?

Answer 51

Conveys final retinal output via optic nerve to the brain.

Question 52

What types of glutamate receptors are on bipolar cells?

Answer 52

Metabotropic (ON cells) and ionotropic (OFF cells).

Question 53

What is antagonistic surround inhibition?

Answer 53

Light in the surround produces the opposite effect of light in the centre.