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...
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.
What is the main neural output from the retina to the cortex?
The ganglion cells – their axons project from the retina into the optic pathway and together form the optic nerve (CN III)
Where is the first level of information segregation occurring in the visual pathway? (ie. where is the parallel information set up?)
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.
What are the three main types of ganglion cells?
- M cells
- P cells
- Intrinsically photosensitive ganglion cells (ipGCs)
Describe the M ganglion cell type in terms of:
- Target cell
- Relative proportion of total ganglion cells
- 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
Describe the P ganglion cell type in terms of:
- Target cell
- Relative proportion of total ganglion cells
- 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
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?
- 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
Where is the lateral geniculate nucleus (LGN) located?
In the thalamus, it acts as a major relay centre before heading off to the occipital lobe.
Describe the pathway from the back of the retina to the LGN in the thalamus
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.
What is the difference between a visual field and a retinal field? What are the two types of fields? Draw these
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.
Does the temporal visual field project onto the temporal retinal field? Why is this so/not so?
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.
The optic fibres hemidecussate as they reach the optic chiasm, what is meant by this
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.
What is the general rule then about the encoding of information from the right and left visual hemifields?
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.
What are the optic radiations?
Relatively large white matter tracts on either side of the brain that wrap around the ventricles travelling from the LGN to the occipital lobes
Draw the visual field pathway including the:
- Visual fields
- Optic chiasm
- Lateral geniculate nulcleus
- Optic radiations
- Visual cortex
What are the three major ‘golden eye rules’ when dealing with the visual pathway in terms of pathology?
- Unilateral visual field defects must always be due to a lesion before the optic chiasm
- Bilateral visual field defects must always be due to a lesion in or beyond the optic chiasm
- Bilateral, same sided visual field defects must always be due to a lesion in beyond the optic chiasm
Describe the relationship between the pituitary gland and the optic chiasm in terms of pathology
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.
How does the lateral geniculate nuclei of the thalamus receive and project the visual information?
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.
Describe the organisation of the lateral geniculate nuclei of the thalamus
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
Where in the occipital lobe is the primary visual cortex (V1)? What is a major landmark used to describe it?
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)
What is meant by the retinotopic organisation of the primary visual cortex (V1)
Neighbouring cells within the retina project to neighbouring cells in the LGN and the visual cortex (the organisation is preserved throughout the pathway)
Describe where in the pathway the lesions would be to cause the following visual field defects:
The third type of ganglion cell is called the intrinsically photoreceptive Ganglion Cell (ipGC). What is special about this type of ganglion cell?
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.
Compare the response of ipGCs to light with the other major types of ganglion cells
ipGCs respond to light by depolarizing, M and P ganglion cells respond to the presence of light by hyperpolarising
What is the population of these ipGC cells in the retina?
Very small, there are only about 5000 cells per retina
What are the three main functions of these ipGC cells?
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)
- Regulate the circadian rhythm
- Pupillary responses
Describe the pupillary response, what are the two major types?
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.
What does the pupillary response depend on?
- Detection of light (CN II)
- Functioning muscles of the iris (ciliary muscles: dilator pupillae and sphincter pupillae)
- Functioning CNIII to innervate the parasympathetic response of constricting the pupil (by sphincter pupillae muscle)
Describe and draw the pathway of information for the pupillary reflex
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.
Describe how ipGCs are involved in controlling the circadian rhythm
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
How are ipGCs involved in photophobia?
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)