🧠Neurology🧠 - Visual system Flashcards
Name these eye structures
Name these eye structures
What is the antero-posterior diameter of the eye?
24mm in adults
What are the layers of the coat of the eye?
3 layers
Sclera - hard an opaque
Choroid - pigmented and vascular
Retina - neurosensory tissue
Name these structures?
Give more detail about the sclera
Commonly known as the “white of the eye”
Tough, opaque tissue - serves as the eye’s protective outer coat
High water content
Sits superficially to the choroid and retina
What is the uvea?
Vascular coat of the eyeball
Lies between the sclera and retina
Composed of the iris, ciliary body and choroid
Intimately connected - a disease of one part also affects the other portions (though not necessarily to the same degree)
Give more detail about the retina
Very thin layer of tissue lining the inner part of the eye
Responsible for capturing light rays
Light impulses then sent to the brain via the optic nerve
Outline the optic nerve
Transmits electrical impulses from the retina to the brain
Connects to the back of the eye near the macula
Portion visible on the retina is called the optic disk
What is the macula and what purpose does it serve?
Located roughly in the centre of the retina
Small and highly sensitive part of the retina - responsible for detailed central vision
Fovea is the very centre of the macula - allows the appreciation of detail in central vision, crucial for tasks such as reading
How does the optic nerve form a blind spot?
Where the optic nerve meets the retina there are no light sensitive cells - it is a blind spot
Which anatomical landmark corresponds to the physiological blind spot?
The optic disk
What is the purpose of central vision?
Detail day vision, colour vision, reading, facial recognition
Fovea has the highest concentration of cone photoreceptors
Assessed by visual acuity assessment
Loss of foveal vision – Poor visual acuity
What is the purpose of peripheral vision?
Shape, movement, night vision
Navigation vision
Assessed by visual field assessment
Extensive loss of visual field – unable to navigate in environment, patient may need white stick even with perfect visual acuity
What are the 3 layers of the retina, from superficial to deep?
Outer layer
Middle layer
Inner layer
What does the outer layer of the retina contain?
Photoreceptors (rods/cones - 1st order neurons)
Responsible for the detection of light
What does the middle layer of the retina contain?
Bipolar cells (2nd order neuron)
Local signal processing - improves contrast sensitivity
What does the inner layer of the retina contain?
Retinal ganglion cells (3rd order neuron)
Transmission of signal from the eye to the brain
What is meant by the order of a neuron?
The sequence or hierarchy of neurons in a neural pathway
i.e. 1st order neurons create impulses from a stimulus (e.g. photoreceptors), 2nd order neurons receive impulses from 1st order neurons and so on…
Outline the cells responsible for visual processing
Two main classes - rods & cones
Rods - 100x more sensitive to light than cones
Slow response to light
Responsible for night vision
Cones - Less sensitive to light, but faster response
Responsible for daylight fine vision and colour
120million rods vs 6million cones
Outline the 2 types of lenses
What is emmetropia?
What is ametropia?
Mismatch between axial length and refractive power
Parallel light rays don’t fall on the retina
Near-sightedness - myopia
Far-sightedness - hyperopia
Presbyopia
Outline myopia
Presents with blurred distance vision
Squinting when looking into the distance
Headache
Outline hyperopia
Patient’s visual acuity at near will blur relatively early
Nature of the blur can vary - tends to be more noticeable when tired, printing is weak or light is inadequate
Asthenopic symptoms: eye pain, headache in frontal region, burning sensation in eyes
What is the near response triad?
Triad of mechanisms that help the eye adapt for near vision
Pupillary miosis (sphincter pupillae)
Convergence of eyes (performed by medial recti muscles)
Accommodation (circular ciliary muscle)
What is the role of the sphincter pupillae in the near response triad?
Causes pupillary miosis - increases depth of field
What is the purpose of convergence in the near response triad?
Performed by the medial recti from both eyes
Aligns both eyes towards a nearby object
What is the purpose of the circular ciliary muscle in the near response triad?
Accommodation
Increases the refractive power of the lens for near vision
What muscle is responsible for pupillary miosis, and what does it mean?
Sphincter pupillae
Increase the depth of field
What muscle is responsible for convergence of the eyes?
Medial recti in both eyes
What muscle is responsible for accommodation, and what does it mean?
Circular ciliary muscle
Increase refractive power of lens for near vision
What is presbyopia?
Naturally occurring loss of accommodation (focus for near objects)
Onset from age 40 years
Distant vision intact
Corrected by reading glasses - effectively increase refractive power of the eye
Outline the visual pathway and its landmarks
Visual Pathway transmits signal from eye to the visual cortex
Eye
Optic Nerve – Ganglion Nerve Fibres
Optic Chiasm – Optic nerves from both eyes converge at the optic chiasm, 53% decussate to contralateral optic tract
Optic Tract – Ganglion nerve fibres continuation
Lateral Geniculate Nucleus (relay centre within thalamus) – Ganglion nerve fibres synapse
Optic Radiation – forms 4th order neuron, relay signal from the Lateral Geniculate Ganglion, to the Primary Visual Cortex
Primary Visual Cortex or Striate Cortex – within the Occipital Lobe, relays to extra-striate cortex (higher visual processing)
Briefly outline the pathway from first order photoreceptors to the thalamus
First Order Neurons – Rod and Cone Retinal Photoreceptors
Second Order Neurons – Retinal Bipolar Cells
Third Order Neurons –Retinal Ganglion Cells
-Optic Nerve (CN II)
-Partial Decussation at Optic Chiasm – 53% of ganglion fibres cross the midline
-Optic Tract
-Destinations
–Lateral Geniculate Nucleus (LGN) in thalamus – synapse with optic radiations (Fourth Order Neurons) – relay visual information to visual cortex
What is the optic chiasm
Important landmark in visual pathway
53% of ganglion fibres cross at optic chiasm
Crossed Fibres – originating from nasal retina, responsible for temporal visual field
Uncrossed Fibres – originating from temporal retina, responsible for nasal visual field
How is it obvious whether a lesion has occurred anterior or posterior to the optic chiasm?
Anterior lesions will affect the visual field in only one eye
Posterior lesions will affect the visual field in both eyes
How does a lesions at the optic chiasm present?
Damages crossed ganglion fibres from nasal retina in both eyes
Temporal field deficit in both eyes – bitemporal hemianopia
How does a lesion posterior to the optic chiasm present?
Right sided lesion – left homonymous hemianopia in both eyes
Left sided lesion – right homonymous hemianopia in both eyes
What would a right sided lesion anterior to the optic chiasm cause?
What would a lesion at the optic chiasm cause?
What would a lesion at 4 cause?
What would a right sided lesion posterior to optic chiasm, but anterior to the lateral geniculate nucleus?
What would a lesion at the point of optic radiation cause?
Only quadrant as lesion here is unlikely to affect all fibres, as this is the point where they are spreading out
What would a lesion in the right sided visual cortex cause?
Homonymous hemianopia with macular sparing
Why is there macular sparing in a unilateral lesion of the primary visual cortex?
The area responsible for the macula receives dual blood supply - both posterior cerebral arteries, so a problem with one still leaves macula area with a blood supply
What are the pupillary reflexes?
In light - pupil constriction
In dark - pupil dilation
Which nervous system is responsible for pupil dilation, using which muscles?
Sympathetic stimulation causes radial muscles to contract
Which nervous system is responsible for pupil constriction, using which muscles?
Parasympathetic stimulation causes circular muscles to contract
What is the pupillary reflex pathway?
Reflex where the amount of light reaching the eye effects the diameter of the pupil
A small sub-section of retinal ganglion cells participate in the Pupillary Reflex Pathway
Outline the pupillary reflex pathway
Pupil-specific ganglion cells exits at posterior third of optic tract before entering the lateral geniculate nucleus
Synapses at brainstem pretectal nucleus
Afferent (incoming) pathway from each eye synapses on Edinger-Westphal Nuclei on both sides in the brainstem
Edinger-Westphal nucleus -> oculomotor nerve efferent
Synapses at ciliary ganglion
Short posterior ciliary nerve -> pupillary sphincter
What is the significance of the pathway from each eye synapsing on Edinger-Westphal Nuclei on both sides in the brainstem
Pupillary reflex occurs in both eyes - not just light-stimulated eye
Explain the difference between the direct and consensual pupillary reflex
Direct light reflex – constriction of pupil of the light-stimulated eye
Consensual light reflex – constriction of pupil of the other (fellow) eye
What is the neurological basis of the consensual pupillary reflex?
Afferent pathway on either side alone will stimulate efferent pathway on both sides
Explain how the afferent vs efferent defect can help pinpoint the location of a defect
What does the swinging torch test help identify?
Relative Afferent Pupillary Defect
Partial pupillary response still present when the damaged eye is stimulated
Elicited by the swinging torch test – alternating stimulation of right and left eye with light
Both Pupils constrict when light swings to left undamaged side (as an example)
Both Pupils paradoxically dilate when light swings to the right damaged side
How many extraocular muscles are there, and which cranial nerves innervate them?
Six extraocular muscles
Three cranial nerves (II, IV, VI)
Which 2 functional categories can the eye muscles be sorted into?
Straight and rotary movement muscles
Which extraocular muscles are responsible for straight movement?
Superior rectus
Inferior rectus
Lateral rectus
Medial rectus
Which extraocular muscles and responsible for rotary movement?
Superior oblique
Inferior oblique
Outline the superior rectus
Attached to the eye at 12 o’clock
Moves the eye up
Outline the inferior rectus
Attached to the eye at 6 o’clock
Moves the eye down
Outline the lateral rectus
Also called the external rectus
Attaches on the temporal side of the eye
Moves the eye toward the outside of the head (toward the temple)
Outline the medial rectus
Also called the internal rectus
Attached on the nasal side of the eye
Moves the eye toward the middle of the head (toward the nose)
Outline the superior oblique
Attached high on the temporal side of the eye
Passes under the Superior Rectus
Moves the eye in a diagonal pattern down and out
Travels through the trochlea
Outline the inferior oblique
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 3 cranial nerves innervate the extraocular muscles?
Oculomotor
Trochlear
Abducens
Outline the innervations of the oculomotor nerve
Superior branch:
-Superior rectus - elevates eye
-Levator palpebrae superioris - raises eyelid
Inferior branch:
-Inferior Rectus – depresses eye
-Medial Rectus – adducts eye
-Inferior Oblique – elevates eye
-Parasympathetic Nerve – constricts pupil
Outline the innervation of the trochlear nerve
Superior oblique - depresses eye
Outline the innervation of the abducens nerve
Lateral rectus - abducts eye
