The Visual Pathway Flashcards
Label the anatomical features of the eye
what is the neural layer of the retina?
what cells are found here?
A layer of the retina that contains neurones that transmit photons of light into electrochemical energy
contains photoreceptors, bipolar cells and ganglion cells
ganglion cell axons form the optic nerve
What is the optic disk?
The point at which the optic nerve leaves the retina
there are no photoreceptors in the optic disk so any image in this area cannot be detected
it is the blind spot
What is the macula and fovea?
The macula is along the visual axis and has a high density of photoreceptors
the fovea only contains cones and has the highest visual acuity
How can the retina be divided?
The retina is an extension from the diencephalon
it can be divided into a neuronal and non-neuronal layer
the non-neuronal layer consists of pigmented epithelium that is light absorbing
What are the 2 components of the outer layer of the eye?
What is the function of these areas?
Cornea:
- thick, transparent and avascular layer
- major area of refraction
sclera:
- “white of the eye” that covers most of the ocular surface and continuous with the cornea
- insertion point for muscles that move the eyeball
What are the 3 components of the middle (vascular) layer of the eye?
Choroid:
- highly vascular and nourishes the cornea and retina
iris:
- pigmented and vascular
- the muscles of the iris control the amount of light entering the eye by controlling the diameter of the pupil
ciliary body:
- controls the shape of the lens by pulling on the suspensory ligaments
What is the role of the lens?
It is a biconvex avascular structure through which light passes after passing through the pupil
What pigment is contained within the pigmented epithelium of the retina?
What is the role of this layer?
It contains melanin, which absorbs light
it provides nutrients to the photoreceptors
What are the 2 main neurones involved in the neuronal part of the retina?
1o bipolar cells:
- these link photoreceptors (rods and cones) to ganglion cells
2o ganglion cells:
- their axons exit the retina and fuse to form the optic nerve
What are the 2 different interneurones within the retina?
What are their roles?
They connect the rods and cones to the 1o and 2o neurones and modulate information
horizontal interneuron:
- modulates transmission
amacrine interneuron:
- modulates activity of ganglion cells
What does it mean that the optic nerve (CN II) is actually part of the CNS?
The optic nerve is an outgrowth from the diencephalon
when it exits via the optic canal, it takes all 3 layers of meninges with it
the optic nerve has meninges surrounding it to the eyeball
Label the components of the optic nerve
The sub arachnoid space contains CSF
How does a rise in CSF pressure affect the optic nerve?
A rise in CSF pressure leads to papilloedema
this is due to a thin layer of CSF surrounding the optic nerve
What is papilloedema?
What causes it?
A swelling of the optic disk
as the optic nerve is surrounded by meninges, increases in CSF pressure can swell the optic nerve
increase in pressure compresses the central retinal vein (and artery) preventing venous drainage from the eye
What are the symptoms of papilloedema?
- Headaches
- drowsiness
- blurred vision
- vomiting
Label the anatomy of the visual pathway
What structures are involved in the visual pathway from start to end?
- Both optic nerves converge to form the optic chiasma
- fibres from the optic chiasma enter the optic tract
- fibres radiate away from the optic tract and into the lateral geniculate nucleus of the thalamus
- this gives rise to optic radiations that lead to the primary visual cortex in the occipital lobe
What features are shown in the different colours?
A lesion at any point causes specific visual defects
What is shown here?
What areas surround this structure?
Calcarine sulcus
primary visual cortex:
- this is where visual information first gets perceived
visual association cortices:
- this is where information gets interpreted and given meaning
- e.g putting a name to a face and forming associations
What are the 3 neurones involved in the visual pathway from the photoreceptors in the retina to the primary visual cortex?
Within CNS:
- 1o neurones in the retina are bipolar cells
- these synapse with the 2o neurones, which are ganglion cells
- axons of the ganglion cells run over the retina to the optic disk to form the optic nerve
Within thalamus:
- the optic nerve enters the lateral geniculate nucleus (LGN)
- it synapses with the 3o thalamocortical neurones (optic radiations)
Cerebral cortex:
- optic radiations travel to the primary visual cortex
What is meant by the visual pathway being retinotopically organised?
- Left half of visual field goes to the right hemisphere
- Right half of visual field goes to the left hemisphere
- upper visual field goes to the lower bank of calcarine sulcus
- lower visual field goes to the upper bank of calcarine sulcus
- the centre of the visual axis (macula) goes to the occipital pole
What is the difference between images on the visual field and retinal field?
The images from the visual field are inverted onto the retinal fields
they are upside down and mirror reversed
How is the visual field and retinal field divided?
Each field is divided into hemi-fields by a vertical line
then divided into quadrants by a horizontal line
where they transect is the fixation point and this corresponds to the fovea
Which fibres cross at the optic chiasma?
What is the result of this?
Nasal fibres cross at the optic chiasma, but temporal fibres DO NOT
the left half of the visual field goes to the right hemisphere
the right half of the visual field goes to the left hemisphere
Where do fibres travel after leaving the LGN?
Fibres from the LGN travel to the upper and lower banks of the calcarine sulcus via different pathways
lower visual field fibres:
- travel in the superior trajectory to the upper bank of the calcarine sulcus
- this travels around the posterior horn of the lateral ventricle
upper visual field fibres:
- travel in the inferior trajectory / Meyer’s loop to the lower bank of the calcarine sulcus
- this travels around the inferior horn of the lateral ventricle
In the primary visual cortex, where is the macula represented?
The macula is represented most posteriorly - towards the tip of the occipital pole
the peripheral fields are represented more anteriorly
What is the visual pathway like from the left visual field?
From the left eye:
- temporal visual field passes to the nasal half of the retina
- fibres travel in the optic nerve
- nasal fibres cross the midline at the optic chiasma
From the right eye:
- nasal visual field passes to the temporal half of the retina
- fibres travel in the optic nerve
- temporal fibres DO NOT cross the midline at the optic chiasma and stay on the ipsilateral side
- fibres travel to the lateral geniculate nucleus
- 3o neurones travel to the primary visual cortex
What is the visual pathway like from the right visual field?
From the right eye:
- fibres from the right side of the visual field pass to the nasal hemiretina
- fibres travel in the optic nerve
- at the optic chiasma, nasal fibres cross the midline
from the left eye:
- fibres from the right side of the visual field pass to the temporal hemiretina
- fibres travel in the optic nerve
- at the optic chiasma, temporal fibres DO NOT cross the midline
How do fibres from the upper and lower visual fields travel to the primary visual cortex?
Lower visual field:
- fibres travel in the superior trajectory to the upper bank of the calcarine sulcus
upper visual field:
- fibres travel in Meyer’s loop to the lower bank of the calcarine sulcus
Why is it important to know the location of damage to the visual pathway?
Vision can be disrupted due to damage to the visual pathway as it passes from the retina to the cerebral cortex
knowing the location of the disruption can cause predictable deficits in the visual field
What is meant by scotoma?
A localised patch of blindness
What is anopia?
The loss of one or more quadrants of the visual field
What are the 2 different types of anopia?
Hemianopia:
- loss of half of the visual field
Quadrantanopia:
- loss of a quarter of the visual field
What is meant by homonymous and heteronymous?
Homonymous:
- visual field losses are similar for both sides
Heteronymous:
- visual field losses are on different sides
Where would a lesion be located in monocular blindness?
There would be a lesion of the optic nerve on the affected side
Where would a lesion be located in bitemporal hemianopia?
The lesion would pass through the centre of the optic chiasma
Where would a lesion be located in homonymous hemianopia (affecting the right side)?
There would be a lesion of the optic tract on the left (opposite) side
Where would a lesion be located in scotoma?
This is a lesion of the macula
it is often macular degeneration related to increasing age
What happens to the optic tract fibres that do NOT pass to the LGN?
Not all optic tract fibres pass to the LGN
approx 10% of optic tract fibres take a medial route to the pre-tectal area (midbrain)
What is meant by the pupillary light reflex?
What cranial nerves are involved?
The ability of both pupils to respond (constrict or dilate) dependent on the level of light that the retina receives
it uses two cranial nerves:
- Optic nerve
- Oculomotor nerve
What are the steps involved in the pupillary light reflex?
- 10% of optic tract fibres pass to the pretectal area
- they synapse with interneurones which connect with BOTH the contralateral and ipsilateral Edinger-Westphal nucleus (parasympathetic nucleus of CN III)
- Efferent fibres travel in BOTH oculomotor nerves which synapse in the ciliary ganglion
- this leads to constriction of sphincter pupillae muscles on BOTH SIDES
What is meant by the direct and consensual response in the pupillary light reflex?
Direct response:
- when light is shone in one eye, the SAME pupil constricts
Consensual response:
- when light is shone in one eye, the OTHER pupil constricts
What would be seen in a CN III and CN II lesion relating to the pupillary light reflex?
CN III lesion:
- loss of consensual pupillary light reflex
CN II lesion:
- loss of direct pupillary light reflex
What is meant by the accommodation reflex?
What are the three stages involved?
A series of changes that occur when the gaze is transferred from a distant to a near object (eyes turn medially)
1. Accommodation:
- the ciliary muscles contract and the lens becomes more rounded
2. Pupil constricts:
- caused by contraction of sphincter pupillae
3. Ocular convergence:
- this is adduction of the eyeballs acheived by the medial rectus muscle
What are the stages involved in the accommodation reflex?
- Afferent fibres carrying input to the primary visual cortex are:
- the optic nerve and tract to LGN
- then from the LGN to the visual cortex
- the fibres then pass to CN III nuclei in the midbrain
- descending information from the cortex is passed to Edinger-Westphal nucleus and motor nucleus of III
- Efferent fibres all run within CN III
- parasympathetic fibres from EW nucleus run to sphincter pupillae
- motor fibres from motor nucleus run to medial rectus
What are the 2 different types of receptors and how are they distributed?
Rods and cones
they are heterogeneously expressed across the retina
only cones are located at the macula
What are the differences between rods and cones?
Rods:
- 20 x more common
- sensitive to light
- vision in dim light
- high level of convergence
Cones:
- colour vision
- high visual acuity
- lower level of convergence
- at the macula, there is one cone to one ganglion cell
Label the anatomy of the visual pathway
What is shown in these images?
