Control Lecture 16: The Visual System

Question 1

What is the Macula?

Answer 1

Macula a region at the very back of the retina. It is medial to the optic disc. It falls right in the middle of the visual axis. Any light rays that travel directly in this region and fall here is interpreted with high visual acuity.

Question 2

What is the Fovea?

Answer 2

The area of the Fovea with the greater density of photoreceptors.

Question 3

What is the structure of the retina?

Answer 3

1. Non-neural layer - composed of pigmented epithelia cells. These contain melanin to absorb unreflected light ray. It also provides nutrients to the surrounding cells.
2. Photoreceptors - rods and cones

Between layer 2 and 3 are horizontal neurones. These integrate, modulate and adjust information from photoreceptors.

3. 1st order Bipolar cells - link photoreceptors to the second order neurones

Between layers 3 and 4 are Amacrine interneurones which modulate information.

4. Ganglion cells - combine to form the optic nerve

Question 4

What are differences between Rods and Cones?

Answer 4

Rods:

• Numerous
• Sensitive to light and therefore gives vision in dim light
• High level of convergence

Cones:

• Few
• Allows for colour vision
• High visual acuity
• Lower level of convergence
• At the macula, one cone to one ganglion cell

Question 5

What is the origin and therefore structure of the optic nerve?

Answer 5

The Optic Nerve is an extension of the diencepaholn. As a result the fibres are coated within a dural sheath with CSF in the subarachnoid space. The central artery and vein are also found in the centre of this dural sheath,

Question 6

Give causes of Papilloedema.

Answer 6

• Raised Intracranial Pressure
• Haemorrhage
• Head injury
• Inflammation - encephalitis or meningitis
• Intracranial HTN

Question 7

Where is the primary visual cortex located?

Answer 7

Either side of the Calcarine Sulcus (otherwise known as the Straite sulcus).

Question 8

Describe the visual pathway.

Answer 8

1. Photoreceptors pass information to the bipolar cells.
2. This is transmitted to the ganglion cells which merge to form the optic nerve.
3. The optic nerve projects its fibres to the lateral geniculate body of the thalamus. Nasal fibres crossover at the level of the optic charisma but temporal fibres do not.
4. Optic radiations (otherwise known as the genticulocalcarine tract) project to the primary visual cortex.

Question 9

How is the visual pathway retinotopically organised?

Answer 9

1. The left half of the visual field gets projected to the right hemisphere and vice versa.

This is as nasal fibres cross over but temporal fibres do not.

2. Images on the retina are inverted and laterally reversed.
3. Rays on the upper bank of the visual field is projected onto the lower bank of the primary cortex via the inferior trajectory (Meyer’s tract). This helps to correct the image. Rays projected on the lower bank of the visual field is projected onto the upper bank of the Visual Cortex via the superior trajectory.

Any information from the middle is carried and projected to the very back of the occipital lobe.

Question 10

What is a scotoma?

Answer 10

Localised patch of blindness

Question 11

What is a heteronymous hemianopia?

Answer 11

Loss of vision in half of the visual field in both eyes on opposite sides.

Question 12

What is the most likely visual defect in the following case:

A 55-year-old woman with a long history of menstrual irregularities consulted with her ophthalmologist, indicating that she was experiencing visual disturbances that seemed to have worsened during the past couple of months. Her ophthalmologist referred her to a neurologist. A CT scan of the patients head revealed the presence of a pituitary tumor impinging on the optic chiasma. What sensory defects is she likely to have?

Answer 12

Pituitary tumour impugning on the optic chaisma means loss of crossing over of nasal fibres.

Nasal fibres receive information from the temporal visual field (due to inversion of the image on the retina). This means there is a loss of temporal vision on both sides.

We would expect to see Bitemporal hemianopia (heteronymous hemianopia).

Question 13

What root do 10% of optic tract fibres take?

Answer 13

Approx. 10% of optic tract fibres take a medial root to the pre-tectal area (midbrain).

Question 14

Describe the reflex for the pupillary light response.

Answer 14

1. The afferent limb is the CN II and tract.
2. 10% of fibres these fibres travel to the midbrain to the pretecral area and the Edinger-Westphal nucleus.
3. CN II links here with parasympathetic fibres in CN III.
4. CN III is the efferent fibre that then is able to constrict the pupillary muscle. Some fibres cross over and so we get a direct and consensual pupillary response.

Question 15

What visual defect is seen in a CN III lesion?

Answer 15

Loss of consensual response. The information still reaches the eye.

Question 16

What visual defect is seen in a CN II lesion?

Answer 16

Loss of direct response and consensual response as the information is not reaching the eye at all.

Question 17

Describe the accommodation reflex pathway.

Answer 17

1. Afferent fibres carry visual input to the visual Cortex via CN II. These are then passed to the optic tract to the LGN and the visual cortex.
2. There is an interneuron the primary visual cortex to tell the association cortex the information is coming closer. This initiates the reflex. It sends projections to the midbrain to nuclei associated with CN III - Edinger-Westphal Nucleus and Motor Nucleus (found more lateral) of CN III. The motor nucleus of CN III, sends motor fibres to reach the eye to cause it to move medially by contracting medial rectus.
3. Efferent fibres all run within CN III - parasympathetic to the sphincter papillae and Pupillary muscles. Motor supply medial rectus.

This all leads to:

• Rounding (thickening) of the lends
• Eyes move medially
• Constriction of the Pupil

Question 18

Where is the macula represented in the primary visual cortex?

Answer 18

Most Posteriorly and the peripheral fields more anteriorly.

Question 19

What visual defects would we except to see in the following:

(a) Injury to the optic nerve
(b) Injury to the superior trajectory on the left

Answer 19

(a) Monocular blindness

(b) Inferior quadrantanopia in the right eye