S4) The Retina and Central Visual Pathways

Question 1

Identify the structures observed in the retina in a cross-section of the eyeball

Answer 1

Question 2

The eye has 3 layers.

What are they?

Answer 2

1. Outermost sclera
2. Uvea (pigmented vascular layer)
3. Retina (neural layer)

Question 3

Describe the outermost sclera.

Answer 3

tough and continuous with dural sheath of the optic nerve

Question 4

Describe the uvea

Answer 4

(pigmented vascular layer)

• Choroid sitting just deep to sclera
• Ciliary body and iris sitting anteriorly

Question 5

What cells can be seen in the retina (neural layer) from superficial to deep.

Answer 5

• Retinal pigment epithelium (prevents light from ‘bouncing around’ in the eyeball, causing glare)
• Photoreceptor cells
• Bipolar cells (first order neurones receiving input from photoreceptors).
• Bipolar cells are connected by horizontal cells which assist with enhancing edges through a process called lateral inhibition •
• Ganglion cell layer (receives input from bipolar cells. Axons of ganglion cells form the optic nerve)
• Nerve fibre layer
• Interestingly, our retina is the ‘wrong way around’, since light has to pass through the nerve fibre layer and other layers before getting to the photoreceptors. Cephalopods have a much better arrangement…!

Question 6

What are rod cells and what do they do?

Answer 6

Rod cells are photoreceptor cells in the retina which function in low intensity light and are responsible for black and white vision (night vision)

Question 7

What are cone cells and what do they do?

Answer 7

Cone cells are photoreceptor cells in the retina which function in high intensity light and are responsible for high acuity vision (detects colours)

Macula = highest acuity vision (central vision)

Question 8

Which part of the eye has the highest density of cones?

Answer 8

Fovea - found in the macula densa

Has high resolution vision

It is a pit - doesn’t have any ganglion cell axons .: less interruption for light to reach the cones

Question 9

What is the pigmented layer?

Answer 9

Contains melanin – prevents excessive reflections within the eyeball itself

They also maintain/ anchor the photoreceptor layer.

prevents light from ‘bouncing around’ in the eyeball, causing glare

clinical correlate: Albinism - don’t have melanin pigment present .: have to wear sunglasses if too much light reflections (everything seems to bright for them)

Question 10

What are bipolar cells and what do they do?

Answer 10

Bipolar cells are cells which exist between photoreceptors in the retina and act indirectly/directly to transmit signals from the photoreceptors to the ganglion cells

first order neurones receiving input from photoreceptors

Question 11

What are horizontal cells and what do they do?

Answer 11

- Horizontal cells are the laterally interconnecting neurons which help integrate and regulate the input from multiple photoreceptor cells

• They also allow the eyes to adjust to see well in both bright & dim light conditions

Bipolar cells are connected by horizontal cells which assist with enhancing edges through a process called lateral inhibition

Question 12

What are amacrine cells and what do they do?

Answer 12

Amacrine cells are inhibitory neurons and project their dendrites to the inner plexiform layer to interact with retinal ganglion cells and/or bipolar cells

Question 13

What are ganglion cells and what do they do?

Answer 13

A retinal ganglion cell is a type of neuron in the retina which receives visual information from photoreceptors via bipolar cells and amacrine cells

Receives input from bipolar cells. Axons of ganglion cells form the optic nerve

Question 14

What is a blind spot?

Answer 14

Region where there are no photoreceptors .: no detection of light or imaging

e.g. optic disc

Question 15

What is Amaurosis Fugax?

Answer 15

• Sudden transient loss of vision
• Caused by TIA - where small embolus has temporarily blocked the ophthalmic artery
• Ppl often describe it as a ‘curtain coming down over their vision’
• Vision then returns again after a couple of hours
• Often caused by hypoxia of the retina so photoreceptors and the neurones aren’t able to function
• Occlusion of the central retinal artery (a branch of the ophthalmic artery) causes sudden visual loss known as amaurosis fugax

Question 16

Identify the labels of the retina

Answer 16

Question 17

Examination of the retina by fundoscopy can detect signs of many diseases such as
hypertensive retinopathy, diabetic retinopathy and macular degeneration .

Identify the seven structures observed in a fundoscopy of the eye

Answer 17

The normal appearance of the fundus, with the macula (point of highest acuity)
sitting lateral to the optic disc (point of exit of ganglion cell axons). Branches of
central retinal artery and vein are visible on the macula.

Question 18

Why should we do fundoscopy?

Answer 18

Retinopathies
◦ e.g. hypertension, diabetes

Vascular occlusions
◦ Branch of central retinal artery or vein – ◦“Amaurosis fugax” (can be a symptom of stroke)

Macula (region of central vision)
◦ e.g. degeneration ◦ Optic disc
◦ e.g. papilloedema (optic disc swelling due to raised intracranial pressure)

Question 19

What specialist technique can be used to visualise the layers of the retina?

Answer 19

Optical coherence tomography (OCT)

Question 20

What can be seen through an ophthalmoscope?

Answer 20

Question 21

The eye is a pinhole camera.

Illustrate what this implies.

Answer 21

This implies that light from a lateral visual field is detected by the medial retina (lateral retina = medial visual field) and that light from an upper visual field is detected by the inferior retina (lower visual field = superior retina)

Question 22

What 4 features make up the visual pathway?

Answer 22

1. The optic nerve (CNII)
2. The optic chiasm
3. The optic tracts
4. The optic radiations

Question 23

The optic nerve contains two types of fibres.

What are they?

Answer 23

The medial retina is referred to as nasal. Light from the temporal field is detected by
the medial retina

The lateral retina is referred to as temporal. Light from the temporal field is detected by the nasal retina

Question 24

In the optic chiasm, describe the path of the nasal and temporal retinal fibres.

Answer 24

Ganglion cells supplying the temporal retina project to the ipsilateral cerebral
hemisphere whereas ganglion cells from the nasal retina project to the contralateral
hemisphere via the optic chiasm (i.e. they decussate)
→ This implies that the left binocular visual field projects to the right hemisphere and vice versa

Question 25

In the optic tract, what does it connect and what does it contain?

Answer 25

Ganglion cell axons project to a part of the thalamus called the lateral geniculate nucleus via the optic tract

Question 26

Where are the optic radiations found?

Answer 26

From the **Lateral Geniculate nucleus to Primary visual cortex** (occipital lobe) The lateral geniculate nucleus projects to the visual cortex through the optic radiations

Question 27

We have two types of optic radiations. What are they and how do they differ?

Answer 27

* Ganglion cells from the **superior retina** (i.e. inferior field) project through the **superior optic radiation** running through the **parietal lobe** * Ganglion cells from the **inferior retina** (i.e. superior field) project through the **inferior optic radiation** running through the **temporal lobe**

Question 28

Illustrate the layout of the visual pathway

Answer 28

- **Temporal fibres** run **ipsilateral** - **Nasal fibres** **decussate** at the optic chiasm - **Optic tract** runs to the lateral geniculate nucleus - **Optic radiations** split into superior and inferior and runs to the primary visual cortex – Superior via parietal lobes and Inferior via temporal lobes

Question 29

What are visual fields?

Answer 29

These relate to **peripheral vision** Each eye has its own set of visual field: --These overlap to form our **binocular vision** --Good for depth and perception Visual fields also called **temporal or nasal**

Question 30

Briefly, describe the layout of the visual fields

Answer 30

- **Nasal fibres** are responsible for our **temporal field of vision** (sees peripheral vision) - **Temporal fibres** are responsible for our **nasal field of vision** (sees central fields) ***Lesions at any point in the pathway will correspond to a pattern of visual loss***

Question 31

What is a scotoma?

Answer 31

A **localised defect in the retina** can cause a **small patch of visual loss** called a scotoma

Question 32

How are visual field defects named?

Answer 32

These are named based on the **area of visual loss** rather than the site of the lesion **we always refer to the lost part of the field**, not the lost part of the retina

Question 33

What are some examples of visual field defects?

Answer 33

* Monocular blindness * Bitemporal hemianopia * Homonomous (affecting both eyes equally) hemianopia

Question 34

Describe the pattern of visual field loss at the following locations: - Before the optic chiasm - At/after the optic chiasm - After the optic chiasm

Answer 34

- **Before the optic chiasm:** signs are unilateral and ipsilateral - **At/after the optic chiasm:** signs are bilateral - **After the optic chiasm:** signs will be bilateral and contralateral

Question 35

What happens in a CNII lesion (on one side)?

Answer 35

Damage to the optic nerve can lead to **monocular blindness**

Question 36

What is the general cause of monocular blindness?

Answer 36

**Monocular blindness** is caused by a lesion of the optic nerve (right)

Question 37

Identify some of the underlying pathological causes of monocular blindness in children and adults

Answer 37

- Optic nerve glioma or retinoblastoma (children) - Optic sheath meningiomas (middle aged)

Question 38

What happens in a lesion at the optic chiasm?

Answer 38

Damage to the optic chiasm can cause **bitemporal hemianopia.**

Question 39

What is the general cause of bitemporal hemianopia?

Answer 39

**- Bitemporal hemianopia** is caused by a lesion at the optic chiasm - It affects both nasal fibres, and thus, both temporal fields lost

Question 40

What happens in an optic tract lesion?

Answer 40

Damage to the optic tract causes a **contralateral homonymous hemianopia**

Question 41

What is the general cause of left homonymous hemianopia?

Answer 41

- **Left homonymous** **hemianopia** is caused by a lesion of the right optic tract - It affects the right temporal and left nasal fibres

Question 42

What is the general cause of right homonymous hemianopia?

Answer 42

- **Right homonymous** **hemianopia** is caused by a lesion of the left optic tract - It affects the left temporal and right nasal fibres

Question 43

Identify some of the underlying pathological causes of right/left homonomous hemianopia

Answer 43

- Vascular causes (stroke – common) - Neoplasia - Trauma

Question 44

What does damage to the lateral geniculate nucleus result in?

Answer 44

Damage to the lateral geniculate causes a **contralateral homonymous hemianopia**

Question 45

What is the relationship between the optic radiations and their role in terms of field of vision?

Answer 45

Question 46

What is a quadrantanopia?

Answer 46

A **quadrantanopia** is an anopia affecting a quarter of the field of vision, associated with a lesion of an optic radiation

Question 47

What happens in a superior optic radiation lesion?

Answer 47

Damage to the superior optic radiations (in the parietal lobe) causes **contralateral homonymous inferior quadrantanopia**

Question 48

What happens in an inferior optic radiation lesion?

Answer 48

Damage to the inferior optic radiations (in the temporal lobe) causes **contralateral homonymous superior quadrantanopia**

Question 49

What happens if both the superior and inferior radiations are affected e.g. in a stroke?

Answer 49

Damage to both optic radiations causes a **contralateral homonymous hemianopia**

Question 50

Localise a quadrantopia observed in the following visual fields: - Left field - Right field - Superior field - Inferior field

Answer 50

- **Left field** → Right hemisphere - **Right field** → Left hemisphere - **Superior field** → Inferior radiations - **Inferior field** → Superior radiations

Question 51

The occipital lobe has dual blood supply. Identify these arteries

Answer 51

- Posterior cerebral artery - Middle cerebral artery (occipital pole)

Question 52

In four steps, explain the concept of macular sparing after a stroke

Answer 52

⇒ Stroke can affect the **posterior cerebral artery** ⇒ Most of **occipital lobe** will be lost ⇒ But, **Middle cerebral artery** supplies the occipital pole (represents the macula) ⇒ **Macular function** (central vision) will be spared

Question 53

If there is non vascular damage to the occipital lobe, what can happen?

Answer 53

Can cause a **contralateral homonymous hemianopia without macular sparing**

Question 54

If there is an occlusion of the posterior cerebral artery, what can happen?

Answer 54

Causes a **contralateral homonymous hemianopia with macular sparing** This is due to the fact that the area of visual cortex that supplies the macula receives blood from the deep branch of the middle cerebral artery

Question 55

In four steps, describe the nervous pathways involved in the light reflex

Answer 55

⇒ Light stimulates the **afferent nerve** in the pathway (CN II) ⇒ Afferent nerve synapses in **pretectal area** ⇒ Gives rise to neurones supplying **Edinger Westphal nuclei** bilaterally ⇒ **Both CN III** (oculomotor nerve) are stimulated to cause **direct** and **consensual pupillary constriction** (parasympathetic fibres)

Question 56

What is the purpose of the accommodation reflex?

Answer 56

Required for **near vision** Needed for focussing on something very close to you - eyes need to change to accommodate this i.e. lens need to thicken, refraction occurs more

Question 57

Describe the three aspects of the accommodation reflex as well as the structures involved

Answer 57

- **C**onvergence (**medial rectus**) (pulls eye medially/adducted) - Pupillary **c**onstriction (**constrictor pupillae**) (constricts pupil to help focus light) - **C**onvexity of the lens to increase refractive power (**ciliary muscle**) (thickens the lens)

Question 58

Briefly, explain how the cerebral cortex is involved in the accommodation reflex (image analysis)

Answer 58

**Cerebral cortex must be involved because it is relating to image analysis** The reflex follows the visual pathway via the **lateral geniculate nucleus** to the **visual cortex**

Question 59

Difference between Light reflex and Accommodation reflex

Answer 59

In accommodation reflex → oculomotor nucleus → mediates convergence response by acting on medial rectus → adduction of eye .: convergence edinger westphal nucleus → stimulates ciliary ganglion → stimulates sphincter pupillae → pupils constrict. Also contracts ciliary muscle → lens thicken

Question 60

Illustrate how the medial longitudinal fasciculus and its relationship with how we get co-ordination of eye movements e.g. when looking to the left: we use the LEFT lateral rectus and the RIGHT medial rectus.

Answer 60

Question 61

If we get a problem/ disorder of the medial longitudinal fasciculus pathway, what can arise?

Answer 61

**Internuclear ophthalmoplegia** → results in un-coordinated eye movement! - can result in diplopia

Question 62

What is sensory ataxia?

Answer 62

* caused by involvement of somatosensory nerve → leads to interruption of sensory feedback signals and body incoordination is caused * loss of co-ordination of due to loss of sensory input for motor output

Question 63

LABEL THE MAJOR RETINAL LAYERS IN THE OCT IMAGE.

Answer 63

Question 64

If we get a problem/ disorder of the medial longitudinal fasciculus pathway, what can arise?

Answer 64

Internuclear ophthalmoplegia →

Question 65

If we get a problem/ disorder of the medial longitudinal fasciculus pathway, what can arise?

Answer 65

**Internuclear ophthalmoplegia** →

Question 66

look over session 3 groupwork questions