Neural pathways of vision Flashcards
What is its function?
Ganglion nerve fibres synapse here (in lateral geniculate nucleus) and go on in tract as 4th order neurons in optic radiation
What are the first, 2nd and 3rd order neurons in the optic pathway?
- st= photoreceptors
- nd bipolar cells
- rd= ganglionic cells
Explain the role and pathway of the ganglionic cells in the visual system
Come from retina, 53% cross at optic chiasm, continue further as optic tract to lateral geniculate nucleus
What is the receptive field of a photoreceptor?
It is the area of the retina where light infall causes fiering of the cell
What does convergence mean?
Convergence describes the number of low order neurons that synapse on to a high order neuron
Explain the convergence of cones and rods in the visual system
Cones have a low convergence (few photoreceptors are connected to higher order neuron) whereas rods have a higher convergence
Explain the convergence in the central part of the retina vs the peripheral part of the retina
In central part= low convergence (at macula almost a 1:1 matching of neurons) whereas there is a high convergence at peripheral retina
Explain the characteristics of low and high convergence within the visual system
Low convergence
- small receptive field
- fine vision
- low light sensitivity
High convergence
- larger receptive field
- coarse visual activity
- high light sensitivity
Explain the concept of on-centre and off centre ganglions
What is their function?
On centre
- when light falls on centre of ganglion–> ganlion fires
- when light falls off centre of ganglion–> inhibition of fiering
Off centre
- when light falls on centre of ganglion–> inhibiton of fiering
- when light falls off centre of gaglion–> ganglion fires
–> important in contrast vision
Which fibres cross at the optic chiasm?
Which part of the visual field to they percieve?
Crossing fibres= located nasally, responsible for temporal vision
Ipsilateral fibres=located temporaly, responsible for nasal vision
What does a lesion anterior to the optic chiasm lead to?
It leads to loss of one eye
Unilateral field loss
What does a lesion at the optic chiasm lead to?
Bitemporal hemianopia –> loss of temporal vision at both sides
What does a lesion posterior to the optic chiasm lead to?
What is the most common reason for it?
Homonymos hemianopia –> loss of vision on either right or left side on both eyes –> normally due to Stroke
When does a right nasal hemianopia occur
With a lesion in the right perichiasmal area
Where would a lesion be in a patient presenting with Quadrant anopia?
A lesion within the optic radiations would be the cause
Where would a lesion be in a patient presenting with a macula sparing visual field defect?
Why might the macula not be affected by visual field defects?
Normally in the visual cortex
–> Area representing the macula gets blood supply from different part of the brain: still supplied
Visual cortex organisation
Explain the location of the
- Macula
- Superior/inferior visual fields
- Right /left visual fields within the visual cortey
- Macula: has a disproportiaonal big representation lateral in the visual cortex
- Superior visual field information is located inferior to calcarine fissure
- Inferior visual field information is located superior to calcarine fissure
- Right visual fields are located in left visual cortex
- Left visual fields are located in left right cortex
What is the dorsal pathway in the visual system?
What is its function?
Dorsal pathway= pathway from primaryvisual cortex to posterior parietal cortex
Involved in:
- motion detection
- visually guided action
- damage results in motion blindness
What is the ventral pathway in the visual system?
Pathway from primary visual cortex to inferiotemporal cortex
Involved in
- object representation
- face recognition
- detailed fine central + colour vision
- damage may result in cerebral achromatopsia (cerebral colour blindness)
Explain the route of infomation from light infall to pupil constriction in the pupillary reflex
Afferent: Optic nerves form optic tracts after optic chiasm
Information goes to brainstem (pectetal nucleus –> edinger-westphal nucleus in midbrain) each optic tracts synapses on both occulumotor nuclei (at each side)–> thats why it is biocular
Occulumotor afferent to each eye
When there is a lesion to the right occulumotor nerve- how would the pupillary reflex look like?
When shine into the right eye: left eye constricts
When shint into left eye: also only left eye constricts
–> no constriction in right eye possible
What is the swiniging torch test?
When is it used?
Relative Afferent Pupillary Defect
- Swing torch from right to left eye
- done when there is still aPartial pupillary response when the damaged eye is stimulated
Observations:
- Both Pupils constrict when light swings to left undamaged side
- Both Pupils paradoxically dilate when light swings to the right damaged side –> looks like dilation because it is only partial constriction in comparison to full constiction
What are the four strait muscles of the eye?
Which movements are they involved in?
Superior, inferior, lateral and medial rectus muscles
Superior= moves eye up
infeiro= moves eye down
Lateral/external = moves eye outwards (towards temple)
Medial= moves eye inwards towards nose
What is the function of the superior oblique muscle of the eye? Explain its route
- Attached high on the Nose temporal side of the eye.
- Passes under the Superior Rectus.
- Moves the eye in a diagonal pattern – down and in.
- Travels through the trochlea
What is the function of the inferior oblique muscle of the eye?
Explain its route
Attached low on the nasal side of the eye.
- • Passes over the Inferior Rectus.
- • Moves the eye in a diagonal pattern – up and out.
Which extraocular muscles are innervated by the third cranial nerve?
- Superior branch
- superior rectus (eye elevation)
- eye lid elevation
- Inferior branch
- Inferior Rectus – depresses eye
- Medial Rectus – adducts eye
- Inferior Oblique – elevates eye
- Parasympathetic Nerve – constricts pupil
Which extraoccular muscles re innervatred by the fourth cranial nerve?
Which movement does it make?
Fourth Cranial Nerve
– Superior Oblique–-> depresses eye inwards
Which extraocular muscle is innervated by the adbucens nerve? (CN VI)
Which movement does it controll?
Lateral rectus muscle .–> abducts eye
Which symptoms does someone present with a third cranial nerve palsy?
Affected eye down and out
- Droopy eyelid
- Unopposed superior oblique innervated by fourth nerve causes: (down)
- Unopposed lateral rectus action innervated by sixth nerve causes :(out)
How does osmeone with a 6th nerve paralysis present?
Affected eye unable to abduct and deviates inwards
How does someone with a fourth cranial nerve paralysis present?
Pupil muscules
Radial (SNS) and circular muscle (PNS)
