L27. Visual Pathways

Question 1

Patient:

74 yo male presenting with losing vision.

• Visual acuity scale: RE 6/6 and LE 6/6
• Explain the findings and provide a hypothesis

Digital visual field test showed below…

Answer 1

The man has a bilateral temporal visual field hemianopia (he has lost his vision in the temporal fields of both eyes).

Based on the visual pathway of light, and based on the ‘golden eye rules’, he has some sort of lesion impacting the optic chiasm where the detectors of this visual field (nasal retinal fibres) cross over.

Question 2

What is the main neural output from the retina to the cortex?

Answer 2

The ganglion cells – their axons project from the retina into the optic pathway and together form the optic nerve (CN III)

Question 3

Where is the first level of information segregation occurring in the visual pathway? (ie. where is the parallel information set up?)

Answer 3

At the level of the ganglion cells because there are different types of ganglion cells encoding different information.

This segregation continues the whole way through the optic pathway.

Question 4

What are the three main types of ganglion cells?

Answer 4

1. M cells
2. P cells
3. Intrinsically photosensitive ganglion cells (ipGCs)

Question 5

Describe the M ganglion cell type in terms of:

• Morphology
• Functionality
• Target cell
• Relative proportion of total ganglion cells

Answer 5

• M cells (magnocellular cells) are large cells with long processes.
• They relay information about movement and where this movement is occurring in the visual field
• They target a specific place in the lateral geniculate nucleus (LGN)
• They make up only 10% of ganglion cells

Question 6

Describe the P ganglion cell type in terms of:

• Morphology
• Functionality
• Target cell
• Relative proportion of total ganglion cells

Answer 6

• P cells (parvo cells) have smaller nuclei and shorter processes
• They relay information about what you are seeing (acuity) and colour
• They also target the lateral geniculate nucleus (LGN)
• They make up 85% of the ganglion cell population

Question 7

The main ganglion cells are then subdivided within their functional types into 2 overlapping types. What are these and what is the significance of this division?

Answer 7

• On ganglion cells: depolarize in response to light
• Off ganglion cells: hyperpolarize in response to light

These set up another parallel relaying of information called the centre-surround organisation

Question 8

Where is the lateral geniculate nucleus (LGN) located?

Answer 8

In the thalamus, it acts as a major relay centre before heading off to the occipital lobe.

Question 9

Describe the pathway from the back of the retina to the LGN in the thalamus

Answer 9

The optic nerves from the back of the globes relay towards the optic chiasm, which is situated in the midline (in between the internal carotid arteries) at the base of the brain, anterior to the pituitary gland. From there they fan out again, moving posteriolaterally to the LGN of the thalamus.

Question 10

What is the difference between a visual field and a retinal field? What are the two types of fields? Draw these

Answer 10

A visual field is what the person is experiencing or seeing (from their point of view).

A retinal field is the point on the retina that receives information from a particular area in front of the person.

Question 11

Does the temporal visual field project onto the temporal retinal field? Why is this so/not so?

Answer 11

The temporal visual field projects onto the nasal retinal field because of the shape of the globe and the way in which the light enters the eye. Thus the nasal visual field projects/is sensed by the temporal retinal fibres.

Question 12

The optic fibres hemidecussate as they reach the optic chiasm, what is meant by this

Answer 12

Half of the fibres decussate while the other half remain ipsilaterally on the path through to the thalamus (the occipital cortex).

The nasal field fibres are the only ones to decussate in the optic chiasm.

Question 13

What is the general rule then about the encoding of information from the right and left visual hemifields?

Answer 13

Right visual cortex is encoding information from the left half of visual field in both eyes

The left visual cortex is reading information from the right half of the visual field in both eyes.

Question 14

What are the optic radiations?

Answer 14

Relatively large white matter tracts on either side of the brain that wrap around the ventricles travelling from the LGN to the occipital lobes

Question 15

Draw the visual field pathway including the:

• Visual fields
• Retina
• Optic chiasm
• Lateral geniculate nulcleus
• Optic radiations
• Visual cortex

Answer 15

Question 16

What are the three major ‘golden eye rules’ when dealing with the visual pathway in terms of pathology?

Answer 16

1. Unilateral visual field defects must always be due to a lesion before the optic chiasm
2. Bilateral visual field defects must always be due to a lesion in or beyond the optic chiasm
3. Bilateral, same sided visual field defects must always be due to a lesion in beyond the optic chiasm

Question 17

Describe the relationship between the pituitary gland and the optic chiasm in terms of pathology

Answer 17

The optic chiasm is located directly anterior to the pituitary gland and tumours of the pituitary gland often impinge on the chiasm causing visual defects.

Question 18

How does the lateral geniculate nuclei of the thalamus receive and project the visual information?

Answer 18

In a very organised and segregated manner. There is no mixing of information from each eye in the LGN - maintaining segregation set up by the ganglion cells and pathway.

Question 19

Describe the organisation of the lateral geniculate nuclei of the thalamus

Answer 19

The LGN is organised into six layers, each receiving information only from one eye per layer and one type of cell (M cell or P cell) per layer.

• M cell = layers 1 and 2
• P cell = layers 3 and 6
• Right eye = layers 2,3,5
• Left eye = layers 1,4,6

Question 20

Where in the occipital lobe is the primary visual cortex (V1)? What is a major landmark used to describe it?

Answer 20

Broddmans Area 17

At the very posterior aspect of the posterior lobe around the calcarine fissure (fissure runs through middle of the primary visual cortex)

Question 21

What is meant by the retinotopic organisation of the primary visual cortex (V1)

Answer 21

Neighbouring cells within the retina project to neighbouring cells in the LGN and the visual cortex (the organisation is preserved throughout the pathway)

Question 22

Describe where in the pathway the lesions would be to cause the following visual field defects:

Answer 22

Question 23

The third type of ganglion cell is called the intrinsically photoreceptive Ganglion Cell (ipGC). What is special about this type of ganglion cell?

Answer 23

These ganglion cells have a visual pigment protein in them called melanopsin that is very similar to rhodopsin and coneopsin.

This makes them intriniscially sensitive to light (like photoreceptors) and able to behave like photoreceptors by sending input to other regions of the brain.

Question 24

Compare the response of ipGCs to light with the other major types of ganglion cells

Answer 24

ipGCs respond to light by depolarizing, M and P ganglion cells respond to the presence of light by hyperpolarising

Question 25

What is the population of these ipGC cells in the retina?

Answer 25

Very small, there are only about 5000 cells per retina

Question 26

What are the three main functions of these ipGC cells?

Answer 26

They give non-vision related information on the presence of light and not the pattern of light. (ie level of light in the environment information only)

1. Regulate the circadian rhythm
2. Pupillary responses
3. Photophobia

Question 27

Describe the pupillary response, what are the two major types?

Answer 27

Shining a light into an eye will cause the pupil to constrict in the direct response.

Shining a light into an eye will cause the opposite pupil to constrict in the consensual response.

Question 28

What does the pupillary response depend on?

Answer 28

1. Detection of light (CN II)
2. Functioning muscles of the iris (ciliary muscles: dilator pupillae and sphincter pupillae)
3. Functioning CNIII to innervate the parasympathetic response of constricting the pupil (by sphincter pupillae muscle)

Question 29

Describe and draw the pathway of information for the pupillary reflex

Answer 29

Direct response pathway:

• ipGC cells receive light information and relay this through the optic nerve.
• these do not synapse at the LGN, but instead enters the optical pretectal nucleus in the midbrain.
• From here it travels to the Edinger-Westphal nucleus and synapses onto an efferent fibre of CN III.
• These send parasympathetic fibres back to through the ciliary ganglia to the eye.

The consensual response occurs because of a local synapse in the pretectal nucleus with another tertiary neuron that decussates and provides identical information to the opposite sided EW nuclei.

Question 30

Describe how ipGCs are involved in controlling the circadian rhythm

Answer 30

The circadian rhythm is highly dependent on the detection of light to control sleeping patterns.

Some ipGCs receive information and project it ipsilaterally and directly to the suprachiasmic nucleus of the hypothalamus.

This SCN region is very important for driving the circadian rhythm

Question 31

How are ipGCs involved in photophobia?

Answer 31

Migraine pain pathway: dura>trigeminal nerve>brainstem>thalamus>cortex

It is believed that these melanopsin ganglion cells target one of the pain parts of the thalamus: posterior nucleus of the thalamus making them fire more and the person perceive this as worsened pain (hyperalgesia)