Visual Pathway Flashcards
Give the anatomical overview of the visual pathway
- Light information reaches the back of the eyes in the retina.
- The optic nerves (CN II) emerges from back of retina and travels back until it reaches the optic chiasma (where the 2 optic nerves converge).
- The optic tracts then pass towards the back of the midbrain until they hook upwards to connect with the lateral geniculate body of the thalamus (this includes the lateral geniculate nucleus).
- From the LGB, there are the optic radiations; the fibres that bring visual information from the thalamus to the primary visual cortex in the occipital lobe.
Where do the optic nerves emerge from?
The retina
After converging at the optic chiasma, where do the optic tracts then pass?
To the lateral geniculate body of the thalamus (this includes the lateral geniculate nucleus)
What is the lateral geniculate nucleus important for?
This is an important nucleus in the thalamus; important in relaying visual information.
From the lateral geniculate body of the thalamus, how is information brought to the primary visual cortex?
From the LGB there are the optic radiations
Where is the primary visual cortex?
Occipital lobe
There are 3 layers encircling the eye. What are they composed of?
- Outer layer; cornea and sclera (these are continuous)
- Middle (vascular) layer; choroid
- Inner layer; retina
Describe the cornea. Main function?
- Description:
- Transparent part of the eye that covers the front portion of the eye (pupil and iris)
- Thick, transparent, avascular.
- Function: major area of refraction
Describe the sclera. Main function?
- Description:
- Covers most of the ocular surface, “white of the eye”.
- Continuous with the cornea
- Tough, white, fibrous tissue
- Function: Insertion point for muscles that move the eyeball.
Describe the choroid. Function?
- Description:
- Vascular
- Continuous with the lens and ciliary muscle.
- Functinon: Nourishes the cornea and retina
Describe the iris. Function?
- Description: Pigmented, vascular.
- Function:
- Eye color is defined by that of the iris
- Muscles of the iris control the amount of light entering the eye by controlling the diameter of the pupil (the central opening of the iris).
Function of the ciliary body?
Controls the shape of the lens by pulling on the suspensory ligaments.
Describe the lens. Function?
- Description: Biconvex, avascular. Light passes through the lens after passing through the pupil.
- Function: changes shape (due to ciliary body)
- When rounded, can focus on objects which are near.
- When flattened, can focus on distant objects
When rounded, what can the lens focus on?
can focus on objects which are near
When flattened, what can the lens focus on?
distant objects
What is the retina an extension from?
Diencephalon
How many layers is the retina composed of?
2
What are the 2 layers of the retina?
-
Non-neuronal layer:
- Consists of pigmented epithelium.
- Sits against the choroid and is light absorbing
-
Neural layer:
- Nerve tissue; photoreceptors, bipolar cells, ganglion cells
- Ganglion cell axons form the optic nerve –> This allows transmission of light information (photons) into electrochemical information
What is the optic disk?
Where the optic nerve exits –> this is a blind spot
Why is the optic disk a blind spot?
we do not perceive any light information that falls in this region as there are no photoreceptors
What is the macula? Where is it located? Function?
- Region at very back of retina that falls right in middle of visual axis (peripheral vision not as clear)
- Found medial to optic disk
- Any light rays that travel directly in this region and fall upon the macula, this information is interpreted with high visual acuity –> as the macula contains a high density of photoreceptors
Why does the macula have a high visual acuity?
as the macula contains a high density of photoreceptors
Where is the fovea located? Function?
- Found in centre of macula
- Contains some specialised photoreceptors called cones –> highest point of visual acuity
What are ‘cones’? Where are they located?
Cone cells, or cones, are photoreceptor cells in the retinas (in the fovea) of vertebrate eyes
What is the yellow spot?
Optic disk; corresponds to exit of optic nerve (surrounded by arteries)
Why is the macula lutea region darker?
As contains higher density of photoreceptors
What is the function of the non-neuronal layer of the retina? What does it contain?
- Pigmented epithelium
- Contains melanin which absorbs light; provides nutrients to photoreceptors
- Function: maintains the metabolic activity of the photoreceptors, bipolar neurones and ganglion cells (found in neural layer)
What does the non-neuronal layer of the retina sit against?
The choroid of the eye
How does the non-neuronal layer of the retina protect the retina?
Absorbs excess light which is not passed on to the photoreceptors to reduce degeneration of the retina.
What does the neuronal layer of the retina contain?
The neuronal layer contains photoreceptors, primary bipolar neurons, secondary ganglion neurons and interneurons
What are the 2 photoreceptors of the retina?
Rods and cones
After light information is passed to the neuronal layer of the retina, describe the order of the visual pathway in this layer
- After the non-neuronal layer, light information is then passed onto our photoreceptors (rods and cones) which transmit this information to the neurons that make up the visual pathway:
- 1st order neurons (bipolar cells) link photoreceptors (rods and cones) to 2nd order neurons
- 2nd order neurons (ganglion cells) axons of ganglion cells combine to form optic nerve
(optic nerve then leaves retina to travel to thalamus)
What are the 1st order neurons of the visual pathway?
Bipolar cells; these link photoreceptors (rods and cones) to 2nd order neurons
What are the 2nd order neurons of the visual pathway?
Ganglion cells
What forms the optic nerve?
Axons of ganglion cells combine to form optic nerve
Interneurons are also present in the neuronal layer of the retina. What is their function? What are the 2 types of interneurons found?
Purpose is to modulate transmission of information:
- Horizontal interneurons
- Amacrine interneurons
Where are the horizontal interneurons found?
between bipolar cells and rods and cones (photoreceptors)
Where are the amacrine interneurons found?
Between bipolar cells and ganglion cells
Describe the organisation of rods and cones across the retina.
They are heterogeneously expressed across the retina with only cones being located at the macula. Rods are 20x more common than cones.
Individual advantages of rods and cones?
Rods:
- Sensitive to light; allow vision in dim light
- High level of convergence
Cones:
- Colour vision
- High visual acuity –> at the macula, one cone to one ganglion cell
- Lower level of convergence
What is the black spot?
Optic disk is black spot; absence of rods and cones
Identify this
Calcarine sulcus (can only appreciate in medial view of brain)
When the optic nerve exits through the optic canal, what does it take with it? Why?
- Exits through optic canal, taking layers of meninges with it.
- CN II is an outgrow from the diencephalon (is part of the CNS)
Which fissure transmits the optic nerve?
Optic canal (the optic canal opens into the skull base at the optic foramen)
As the optic nerve has meninges surrounding it to the eyeball, this means there is CSF present in the subarachnoid space (just like the spinal cord and brain).
What can happen if there is a rise in intracranial pressure (e.g. rise in volume of CSF)?
Rise in volume of CSF can be transmitted to the area of the eyeball via the subarachnoid space surrounding the optic nerve –> papilloedema can occur
What is a pailloedema? What are the symptoms? What can contribute to these symptoms?
- Swelling of the optic disc (due to increase in CSF pressure swelling the optic nerve) - swelling is usually bilateral
- Increase in pressure compresses the central retinal vein (and artery) preventing venous drainage from the eye
- Symptoms: headaches, drowsiness, blurred vision and vomiting
What causes a papilloedema?
caused by increased intracranial pressure due to any cause
Anatomy of visual pathway
What is found in the grey areas above and below the calcarine sulcus?
The primary visual cortex (also referred to as the striate cortex) –> responsible for perceiving visual information.
Horizontal section through midbrain at level of colliculi (look for mickey mouse shape).
- Photons received by the photoreceptors
- Signal transduced to the ganglion cells; axons form the optic nerve
- Crossing over occurs at the optic chiasma
- Signals continue via the optic tract
- Synapses at the LGB with 3rd order neurons
- 3rd order neurons have optic radiations which fan out to the cortex
- Visual cortex found along the banks of the calcarine sulcus in the occipital lobe (area surrounding the primary visual cortex is the association visual cortex, where information is interpreted).
What arteries are found lateral to the optic chiasma?
2 internal carotid arteries
What visual structure can an aneurysm of the ICA damage?
the optic chiasma
What visual structure can a pituitary tumour affect?
Pituitary gland also is in the midline, so a tumour may cause compression of the optic chiasma
What is the most common problem with vision to result from a pituitary tumour?
bitemporal hemianopia (tunnel vision) due to compression of optic chiasma
What is the area surrounding the primary visual cortex?
The association visual cortex, where information is interpreted
Describe the 3 neuron chain from photoreceptor to cortex (i.e. the entire visual pathway)
- 1st order neuron in retina (bipolar cells)
- 2nd order neuron in retina (ganglion cells)
- Axons of ganglion cells run over retina to the optic disc to form the optic nerve
- Optic nerve travels to optic chiasma (cross over)
- Optic tracts travel to LGN
- 3rd order neuron (thalamocortical neurons) travel in optic radiation to visual cortex
What are the 1st and 2nd order neurons of the visual pathway outgrowths of?
the diencephalon
What are the 3rd order neurons of the visual pathway called?
thalamocortical neurons
What is retinotopy?
the mapping of visual input from the retina to the neurons
Which hemisphere does the left half of the visual go?
right hemisphere
Which hemisphere does the right half of the visual go?
left hemisphere
Which bank of the calcarine sulcus is the upper visual field projected to?
the lower bank of calcarine sulcus
Which bank of the calcarine sulcus is the lower visual field projected to?
the upper bank of the calcarine sulcus
Where is the centre of visual axis (macula) transmitted to?
occipital pole
How are the images changed from the visual field to the retinal field?
Upside down and mirror reversed:
- Whatever you’re looking at on left hand side of visual field is projected to right had side of retina and vice versa –> mirror reversed
- Information from lower visual field is projected to upper half of retina and vice versa –> upside down
Light rays then travel in straight line to reach back of retina (except little bit of bending at level of lens).
What is each field divided into?
- Each field divided into hemi-fields by a vertical line
- Then into quadrants by a horizontal line
What is the fixation point?
Where the horizontal and vertical lines transect –> this corresponds to the fovea
The retinal field can be divided into; temporal and nasal.
- Where is the temporal field?
- Where is the visual field?
What does each field look at?
- Temporal: lateral part of eye BUT looks at medial half of visual field (as the right and left halves of each visual field are reversed in their projection onto the retina)
- Nasal: medial part of eye BUT looks at lateral half of visual field
(look at diagram)
Information from temporal and nasal fields is carried by CN II fibres to the optic chiasma. Which fibres cross at the optic chiasma? Why?
Only the nasal fibres cross but temporal fibres remain ipsilateral. Nasal fibres from each eye cross because light travels in a straight line.
At the level of the optic tract, what half of the visual field is the information carried from?
- Left optic tract –> information from right half of visual field
- Right optic tract –> information from left half of visual field
This is due to nasal fibres crossing over but temporal fibres remaining ipsilateral.
Fibres from the LGN pass to the upper and lower banks of the calcarine sulcus via different pathways (due to upper/lower visual field). What are these 2 pathways?
- Superior trajectory
- Inferior trajectory/Meyer’s loop
What does the superior trajectory pathway carry?
Carries lower visual field to the upper bank of the calcarine sulcus
What does the inferior trajectory/Meyer’s loop pathway carry?
carries the upper visual field to the lower bank of the calcarine sulcus
In the primary visual cortex, where are the macula and fovea represented?
most posteriorly (towards the tip of the occipital pole)
In the primary visual cortex, where are the peripheral field represented?
More anteriorly
View of optic radiation pathways
Define ‘scotoma’
localised patch of blindness
Define ‘anopia’
loss of one or more quadrants of the visual field
Define ‘hemianopia’
half of the visual field is lost
Define ‘Quadrantanopia’
quarter of the visual field is lost
If vision loss is ‘homonymous’, what does this mean?
visual field losses are similar for both sides
If vision loss is ‘heteronymous’, what does this mean?
visual field losses are on different sides
What defect would a lesion of optic nerve on the left cause?
monocular blindness of the left (as no information of the left or right hand side of the visual field can reach the brain)
What do lesions affecting the whole optic nerve result in?
Monocular blindness –> Lesions involving the whole optic nerve cause complete blindness on the affected side, that means damage at the right optic nerve causes complete loss of vision in the right eye
What visual defect would a tumour compressing the centre of the optic chiasma cause?
Bitemporal hemianopia:
- ‘Tunnel vision’
- This is heteronymous as loss of left visual field in one eye and right visual field in other eye
Is bitemporal hemianopia Homonymous or Heteronymous?
Heteronymous; as loss of left visual field in one eye and right visual field in other eye
What visual defect would any central lesion of optic chiasma lead to?
bitemporal hemianopia
What type of lesion would cause binasal hemianopia?
Lateral chiasmal lesions
What visual defect would a lesion of the optic tract lead to?
Homonymous hemianopia:
- Loss of same visual field on both eyes
What would a lesion of the left optic tract lead to?
right-sided homonymous hemianopsia (i.e. right visual field of both eyes affected)
What would a lesion of the macula cause?
Scotoma (localised patch of blindness)
Clinical 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 patient’s head revealed the presence of a pituitary tumor impinging on the optic chiasma. Which one of the following visual deficits is the patient likely to have?
- Left (contralateral) homonymous hemianopia
- Superior (right) homonymous quadrantanopia
- Inferior left homonymous quadrantanopia with macular sparing
- Non-homonymous bitemporal hemianopia
- Complete blindness (left eye)
Non-homonymous bitemporal hemianopia –> presence of pituitary tumour impinging on the optic chiasma impairs the travelling of the nasal retinal fibres, corresponding to the temporal visual field.
What do the nasal retinal fibres correspond to?
the temporal visual field
What do the temporal retinal fibres correspond to?
nasal retinal field
Not all information reaches the visual cortex via the optic radiations. Where do 10% of the optic tract fibres go instead?
10% of the optic tract fibres instead take a medial root to the pre-tectal area of the midbrain
- Some optic tracts synapse at the LGN
- SOME of the optic tracts do not synapse here, and instead take a medial root to the pre-tectal area of the midbrain.
What is the pupillary light reflex?
Ability for both pupils to respond (constrict or dilate) dependent on the level of light the retina reaches
Which 2 cranial nerves does the pupillary light reflex utilise?
CN II and CN III
Describe the pathway of the pupillary light reflex
This pathway is part of the 10% of optic fibres that go to the midbrain instead!
- Optic nerve and tract (CN II) sends afferent (sensory) information to the pre-tectal area of midbrain
- Interneurons then link information with Edinger-Westphal nucleus of oculomotor nerve
- Oculomotor nerve (CN III) then sends pre-ganglionic efferent parasympathetic motor fibres to the ciliary ganglion
- Post-ganglionic fibres then cause constriction of sphincter pupillae muscle
What is the Edinger-Westphal nucleus?
Edinger-Westphal nucleus of CN III is the parasympathetic pre-ganglionic nucleus that innervates the iris sphincter muscle and ciliary muscle
What parasympathetic ganglion of the head is involved in the pupillary light reflex?
Ciliary ganglion
What is a direct pupil response?
same pupil constricts
What is a consensual pupil response?
light in one eye, other pupil constricts
What is the normal response in the pupillary light reflex?
both pupils constrict when light shone in one eye
What would a loss of the consensual pupillary light reflex on one side indicate (i.e. only one eye doesn’t constrict)?
A CN III lesion –> CN III fibres are working on one side but not the other so pupil cannot constrict
What would a loss of the direct pupillary light reflex indicate?
CN II lesion on one side –> as light information has not reached the CNS
(N.B. there would also be a loss of the consensual response as well so make sure to check other eye)
Then shine light on other eye and see if it constricts (if it does then it is a lesion of CN II on one side only)
What is the accomodation reflex?
Is a series of changes that occur when the gaze is transferred from a distant to near object or when an object is travelling from a distance to closer.
What 3 things happen in the accommodation reflex?
- Accommodation –> ciliary muscle contract so lens become more rounded in order to allow more refraction to reach back of retina
- Pupil constriction –> via sphincter pupillae
- Ocular convergence –> medial rectus muscle contracts, adducting eyeballs (eyeballs converge medially to focus gaze on central object)
Describe the pathway during the accommodation reflex
Fibres pass to the visual cortex AND the pre-tectal area in the midbrain:
- Afferent fibres carrying visual input to the visual cortex via normal pathway (nerve, chiasma, tract, LGN, cortex)
- Information sent to association cortex; understand that object is coming closer
- Neurons then sent to pre-tectal area in midbrain to synapse with the nuclei of the oculomotor nerve;
- Edinger-Westphal nucleus AND motor nucleus
- Then efferent fibres sent to cause;
- Accommodation
- Pupil constriction
- Ocular convergence
Which CN III nuclei are involved in the accommodation reflex? Why?
- Edinger-Westphal nucleus; need parasympathetic input to sphincter pupillae & ciliary muscles to cause pupil constriction and rounding of lens
- Motor nucleus; need motor input to move medial rectus (move eyeball)
Describe the shape of the lens when light rays arrive from a distant source. Why?
Parallel light rays arrive from a distant source and so a flat lens is good so that less refraction occurs
Describe the shape of the lens when light rays arrive from a near source. Why?
Near sources lead to bent light rays and so a rounded lens is needed to refract the light more.
Function of the lateral geniculate body?
The LGN (also called the lateral geniculate body) is a relay center in the thalamus for the visual pathway; where the thalamus connects with the optic nerve.
