Visual Physiology

Question 1

Photoreceptors:
What is the structure of a photoreceptor? How is the outer segment different for a cone and rod cell?

Which neurotransmitter is used?

Answer 1

• • Outer segment; Invaginations of membrane (Cone), or separate discs of phospholipids (Rod)
  • Inner segment; Nucleus, Mitochondria, etc.
  • Axon-like process and Synaptic terminal
• GLUTAMATE

Question 2

Signal transduction:
What is the RMP of a photoreceptor?

What occurs when there’s more light?

Where is the photopigment complex found? What is it formed of when INactivated?

What occurs to the photopigment when there’s more light?
→ How does this lead to the closure of Na+ channels?

How is this transduction signal terminated?

What occurs when there’s less light?

Answer 2

• -45mV; due to K+ and Na+ leak channels
• ↑Light closes Na channels = Hyperpolarisation, which stops the leakage of Glutamate at synaptic terminal
• On membrane discs; comprises OPSIN and 11-CIS RETINAL
• Light changes 11-cis retinal to 11-TRANS RETINAL, to Activate the photopigment
  → G proteins then activated = ↓cGMP, closure of Na+ channels
• Release of All-trans retinal from Opsin = opening of Na+ channels
• ↓Light opens more Na channels = Depolarisation, which causes more leakage of Glutamate at synaptic terminal

Question 3

Metabolic needs:
Why do photoreceptors need a rapid supply of oxygen and nutrients?

How is this rapid blood supply achieved? Where is it found?

What are the supportive roles of the retinal pigment epithelium?

What occurs with Age-related Macular Degeneration?

Answer 3

• Signal transduction occurs rapidly, higher metabolic rate
• By the CHOROID LAYER; highly specialised blood supply with very LEAKY walls (above retinal pigment epithelium)
• o Holds retina in place
  o Blood-Retina barrier
  o Conversion of All-trans retinal back into 11-cis retinal
  o Renews outer segment membranes discs
• YELLOW, FATTY PLAQUES between Retinal pigment epithelium and Choroid layer, causing the death of the photoreceptors below them

Question 4

What are the 4 types of ganglion cells? What does each of them do?

What do the retinal ganglion cells respond to? What is it due to?

How do we see colours if the response of a single cone cell is ambiguous?

Answer 4

• • “Off” cells; excited by decreased light
  • “On” cells; excited by increased light
  • Parvocellular cells; extracts fine detail and colour
  • Magnocellular; detects fast movements and broad outlines
• CONTRAST between the illumination of centre and surround; due to Inhibitory interneurons
• Brain compares the responses of different types of cone cells

Question 5

What does the Retina and Lateral Geniculate Nucleus encode?

What does the Primary Visual Cortex encode?

Higher Visual Cortex:
Which pathway processes shape and form?
→ What does a lesion in this pathway cause? What is it?

Which pathway processes location and movement?

Which pathway controls self-movement?

Answer 5

• Contrast (e.g. edges) and Wavelength
• Orientation of edges, Presence of corners, Direction of motion, Binocular Disparity

• Primary Visual Cortex → Inferior Temporal → Frontal Cortex
  → ASSOCIATIVE AGNOSIA; normal perception but with a loss of significance
• Primary Visual Cortex → Superior Temporal → Parietal Cortex
• Primary Visual Cortex → Superior Temporal → Parietal Cortex → Motor/Somatosensory Cortex

Question 6

Control of Conjugate eye movements:
What is involved in the Reflexive Saccade? What occurs?

What is involved in the Exploratory and Voluntary Saccade?

What is involved in Smooth Pursuit? What are its inputs and outputs?

Control of Disonjugate (Convergence) eye movements:
What’s the pathway occurring for the Convergence of the eyes?

What’s the pathway occurring for accommodation and pupil constriction?

Answer 6

• SUPERIOR COLLICULUS; activates respective nuclei (gaze centres) of the cranial nerves innervating the extraocular muscles
• Parietal cortex and Frontal lobe Eye Fields
• Cerebellum; receives input from Pontine Nuclei, and produces output to the Vestibular nuclei

• Input from Visual cortex to Pretectal Nucleus → both Oculomotor Nuclei → Leaves as Oculomotor nerve = Contraction of Medial Rectus muscles (eyes converge)
• Input from Visual cortex to Pretectal Nucleus → both Edinger-Westphal Nuclei → Leaves as Oculomotor nerve = Contraction of Sphincter Pupillae and Ciliary muscles (pupil constriction and lens fattening)