Lecture 6– The retina and the visual pathway

Question 1

purpose of the pigmented layer of the retina

Answer 1

• Acts as the site of absorption of light- to modulate amount of light being received by the photoreceptors
  
  • People with albinism (lack of pigment- melanin) have photophobia
• ​Anchors photoreceptor

Question 2

function of Rods and cones

Answer 2

• Rods- black and white vision
• Cones – colour vision- high acuity vision
  • Found in the fovea (macula densa)

Question 3

function of bipolar cells

Answer 3

• Act to connect photoreceptor cells to the ganglion cells
• Ganglion cells axons come together to form the optic nerve
• Optic nerve exits via the optic disc
  
  • Blind spot- not photoreceptor cells

Question 4

function of Horizontal cells

Answer 4

Lateral inhibition- detects area where image is coming from and detects the photoreceptor that is most in line to pick up that signal and inhibits photoreceptors next to it prevents too many neural impulses

Question 5

fundoscopies can be used to look at the….

Answer 5

• Retinopathies
  
  • E.g. hypertension, DM
• Vascular occlusion
  
  • E.g. branch of central artery or vein
  • ‘amaurosis fugax’- like a curtain coming down
• Macula
  
  • E.g. degeneration
• Optic disc
  
  • E.g. papilledema

Question 6

Optical coherence tomography (OCT)

Answer 6

Question 7

label the retina

Answer 7

thin layer of tissue that lines the back of the eye on the inside

Question 9

the central visual pathway is comprised of:

Answer 9

1. The optic nerve (CN II)
2. The optic chiasm
3. The optic tracts
4. Optic radiations

Question 10

the optic nerve can be split into…… divisions

Answer 10

4 different fibres of the retina

• Temporal (lateral)- orange
• Nasal (medial)- green
• Also have up and down

Question 11

nasal fibres

Answer 11

medial

Question 12

temporal fibres

Answer 12

lateral

Question 13

The optic chiasm​

Answer 13

• Nasal fibres decussate
• Temporal fibres remain ipsilateral

Question 14

The optic tracts runs from the

Answer 14

From optic chiasm to lateral geniculate nucleus (LGN

Question 15

what do optic tracts contain

Answer 15

Contains temporal fibres from the ipislateral side

Contains nasal fibres from tbe contralateral side

Question 16

Optic radiations run from the

Answer 16

From LGN to primary visual cortex (x2 ) in the occipital lobe (x2 lobes)

Question 17

the optic radiations have

Answer 17

2 routes to the occipital lobe

1. Superior route via the parietal (superior optic radiations)
   
   • Continuation of superior quadrant fibres (temporal and nasal)
   • ‘Baums loop’
2. Inferior route via the temporal (inferior optic radiations)
   
   • Continuation of inferior quadrant fibres (temporal and nasal)
   • “meyes loop’

Question 18

summary of the visual pathway

Answer 18

Question 19

Visual fields

Answer 19

Visual fields relate to peripheral vision (also called temporal and nasal). Each eye has its own set of visual filed

• These overlap to form binocular vision
• Good for depth perception

Question 20

nasal fibres are responsible fore …….. visual fields

Answer 20

temporal

Question 21

temporal fibres are responsible for

Answer 21

nasal visual fields

Question 22

If we want to detect something in the temporal visual field, light will travel through the pupil straight to

Answer 22

the nasal retinal fibres (temporal visual field detected by nasal retinal fibres)

Question 23

• If we want to detect something in the nasal visual field, light will travel through the pupil straight to the

Answer 23

temporal retinal fibres (nasal visual field detected by temporal retinal fibres)

Question 24

Visual field defects

Answer 24

Named based on the area of visual loss rather than site of lesion

• monocular blindness
• bitemporal hemianopia
• homonomous hemianopia

Question 25

**CN II lesion: Monocular blindness**

Answer 25

* **Temporal and nasal fibres** on the ipsilateral side affected * Therefore **the nasal and temporal visual field** are lost on the **ipsilateral** side

Question 26

**Bitemporal hemianopia** ​

Answer 26

* lesion at the chiasm where the nasal fibres decussate * **Nasal fibres** on **both** sides affected * **Temporal** visual field loss on **both sides** * **‘tunnel vision’** * **Cause e.g. pituitary adenoma**

Question 27

**Homonomous hemianopia**

Answer 27

* Lesion of the **optic tract** on the right hand side * Left nasal retinal fibres (contralateral) and right temporal nasal fibres (ipsilateral) affected * Left temporal (contralateral) visual field lost and right nasal (ipsilateral) field loss * ‘left homonomous hemianopia’- even though lesion is on the right- due to decussation * Name the visual defect on visual loss not lesion i.e. homo- loss of both left or loss of both right

Question 28

**Optic radiation lesion**

Answer 28

* Superior visual fields are detected by inferior retinal fibres * Inferior visual fields are detected by superior retinal fibres

Question 29

**Quadrantanopia’s**

Answer 29

Affects **left/right** superior quadrant fibres and inferior quadrant fibres

Question 30

**Homonomous inferior quadrantanopia (left)** x

Answer 30

Lesion on right superior optic radiation (parietal lobe) * Superior ipsilateral temporal fibre affected * Loss of inferior nasal visual field * Superior contralateral nasal fibre affected * Loss of inferior temporal visual fields

Question 31

**Homonomous superior quadrantanopia (left)**

Answer 31

* Inferior temporal fibres on ipsilateral side affected * Loss of superior nasal visual field * Inferior nasal fibre on contralateral side is affected * Loss of superior temporal visual field

Question 32

**What if superior and inferior radiations are affected? E.g. stroke**

Answer 32

* Superior and inferior ipsilateral temporal fibres affected * Superior and inferior nasal fibres contralateral affected * Homonomous hemianopia

Question 33

macula sparing strokes

Answer 33

Occipital lobe has dual blood supply * Middle Cerebral Artery (occipital pole) * Posterior Cerebral Artery In stroke affecting the posterior cerebral artery… * Most occipital lobe will be lost * However middle cerebral supplies the occipital pole (represents the macula) * Therefore macular function (central vision) will be saved

Question 34

**The light reflex** (i.e. light shone in left eye)

Answer 34

1. Sensory afferent from left retina (optic nerve) 2. Some branches leave the optic nerve and enter the midbrain and synapse in **pre-tectal nucleus** 3. Synapse with EDW nuclei (left and right) 4. Pre-ganglionic parasympathetic fibres from EDW leave brainstem with oculomotor (left and right) 5. Pre-ganglionic synapse in the ciliary ganglion, becoming post-ganglionic 6. Reach sphincter pupillae muscle of iris 7. Direct and consensual light reflex

Question 35

accomodation reflex required for

Answer 35

near vision

Question 36

accomodation reflex requires 3 C's

Answer 36

* **Convergence** (medial rectus) * Pupillary **constriction** (constrictor pupillae) * **Convexity (becomes thicker)** of the lens to increase refractive power (ciliary muscle)

Question 37

why must the cerebral cortex be involved in the accomodation reflex

Answer 37

* Cerebral cortex must be involved because its relating to image analysis * Therefore the reflex follows the visual pathway via the **lateral geniculate nucleus to the visual cortex** * Midbrain receives info from visual cortex * Sends off information via cranial nerve 3 nuclei (MR) and the Edwinger Westphal nuclei (EWN) (pupillary constriction)