Visual system Flashcards
What are the three layers of the eye?
Sclera - hard, opaque
Choroid - pigmented, vascular
Retina - neurosensory tissue
What is the sclera?
Tough, opaque protective tissue (white of the eye), high water content
What is the uvea, and where is it located?
Vascular coat of eyeball,
lies between sclera and retina
What are the three parts of the uvea?
Iris, Ciliary Body, Choroid
Why can a disease in one part of the eye affect other portions?
Uvea is intimately connected so disease can travel between parts of the eye
What is the retina and what is its function?
Thin layer of tissue on the inner part of the eye, captures light rays and sends impulse to the brain via optic nerve
What is the visible part of the optic nerve called?
Optic disc
What is the optic nerve?
Transmits impulses from retina to the brain
connects to the back of the eye, near the macula
What is the macula?
Small, highly sensitive part of retina for detailed central vision
Where is the macula located?
Roughly in centre of retina, temporal (ie lateral) to the optic nerve
What is the very centre of the macula called?
Fovea, responsible for even finer detail
Blind spot in the eye
Where optic nerve meets retina (optic disc) - there are no light sensitive cells here
What is the difference between central vision and peripheral vision?
Central vision is directly in front of you, peripheral is what you can see in the edges without turning your head
Central vision
Detail day vision, colour, reading, facial recognition
How can you assess central vision?
Visual acuity assessment (reading letters eye test)
Peripheral vision
Shape, movement, night vision
aids with navigation
How can you assess peripheral vision?
Visual field assessment
What is the effect of losing visual field even with perfect visual acuity?
Unable to navigate, may even need white stick
What concentrations of cells does the fovea?
High concentration of cones, - colour receptors
low concentration of rods
What are the three layers of the retinal structure?
Outer, Middle, Inner Layer
What cells does the outer retinal layer contain and what is its function?
Photoreceptors (act as 1st order neurons - detection of light)
What cells does the middle retinal layer contain and what is its function?
Bipolar cells (act as second order neurons) - local signal processing to regulate sensitivity
What cells does the inner retinal layer contain and what is its function?
Retinal ganglion cells (act as third order neurons) - transmission of signal from eye to brain
axons converge to form the optic nerve
Retinal pigment epithelium
transports nutrients from the choroid to the retina and metabolites back to the choroid
What are the two main photoreceptors?
Rods and Cones
What is the difference between rod cells and cone cells?
Rod cells detect light, more sensitive to light than cnes but slow response
Cone cells detect colour, less sensitive to light but fast response
Why are rods slower to respond to light?
More photoreceptors, higher spatial and temporal summation of stimuli before response
What type of vision are rods responsible for?
Night vision - scotopic vision
What type of vision are cones responsible for?
Daylight fine vision and colour vision - photopic
What is refraction?
When light enters a new medium, changing its velocity and path
What are the two basic types of lenses?
Convex (CONVErging - brings rays together at focal point, + prescription),
Concave (diverging - spreads rays apart, - prescription)
What is Emmetropia
Adequate correlation btw refractive power of lens and cornea and axial length (size of the eye) to retina - parallel light rays fall on the retina
What is Ametropia
mismatch between refractive power and axial length, causes parallel lines miss the retina
e.g. myopia, hyperopia, presbyopia
What is myopia?
Parallel rays converge at a focal point anterior to the retina
Near sightedness
What are the two types of myopia?
Axial Myopia (more common - long globe, aka eye is long),
Refractive Myopia (excessive refractive power - lens is too strong)
What are the symptoms of myopia?
Blurred long distance vision,
squinting to improve visual acuity,
headache
What is hyperopia?
Parallel rays of light converge at a focal point posterior (behind) retina
Far sightedness
What are the two types of hyperopia?
Axial Hyperopia (more common - short globe, aka eye is short),
Refractive Hyperopia (insufficient refractive power - lens is too weak)
What are the symptoms of hyperopia?
Blurred short distance vision (can vary, worsens when person is tired or low light)
Asthenopic symptoms (aka eye strain symptoms - eye pain, headache, burning sensation of the eye)
How does the eye adapt for near vision?
Using the near response triad
What are the three parts of the near response triad?
Pupillary miosis (sphincter pupillae - pupil constricts to increase depth of field)
Convergence (medial recti from both eyes bring eyes towards near object)
Accommodation (circular ciliary muscle - contraction increases refractive power of lens)
What is presbyopia?
Natural loss of accommodation (focus on near objects) from 40 years+
Distant vision intact
How to correct presbyopia
Reading glasses (convex lenses) to increase the refractive power of the eye
What is the function of the visual pathway?
Transmits signals from eye to visual cortex
What are the landmarks of the visual pathway?
Eye
Optic Nerve (ganglion nerve fibres)
Optic chiasm, nerves from both eyes converge (53% decussate)
Optic tract
Lateral Geniculate Nucleus (ganglion nerves synapse here and relay with the thalamus)
Optic Radiation (4th order neuron relays the signal btw LGN + PVC)
Primary Visual Cortex or Striate Cortex (occipital lobe to the extra-striate cortex which has even higher visual processing)
What are the landmarks of the visual pathway in the retina?
First order (rod and cones),
Second order (retinal bipolar),
Third Order (retinal ganglion)
What fibres cross at the optic chiasma?
Crossed Fibres - from nasal retina (medial retina), responsible from temporal (lateral) visual field
What fibres do not cross at the optic chiasma?
Uncrossed fibres - from temporal (lateral retina), responsible from nasal (medial) visual field
What parts of the retina does left visual field stimulate?
Nasal retina of left eye, temporal retina of right eye (just vice versa for other one)
How many eyes do lesions anterior to the optic chiasma affect?
One eye only
How many eyes do lesions posterior to the optic chiasma affect?
Both eyes (due to crossover)
What does a lesion at the optic chiasma cause?
Damage to nasal retina from both eyes - leads to temporal field deficit in both eyes so
BITEMPORAL HEMIANOPIA
What does a lesion posterior to the optic chiasm cause?
Contralateral HOMONYMOUS HEMIANOPIA
(right sided lesion causes left homonymous hemianopia in both eyes)
What does a lesion to a unilateral optic nerve cause?
Monocular blindness
What does a lesion to the primary visual cortex cause?
Contralateral Homonymous Hemianopia with Macular Sparing
(right sided lesion causes left sided visual defect, but avoids macula region)
What is a typical cause of homonymous hemianopia?
stroke (cerebrovascular accident)
Why does macular sparing occur?
Macula receives blood supply from posterior cerebral arteries from BOTH sides - one damaged will not stop the other
What is the pupil response to light?
Constriction
How does the pupil constrict?
Circular muscles contract innervated by parasympathetic nerves within oculomotor nerve
What are the effects of pupil constriction?
Decrease glare
Increases depth of field (recall near response triad), Decrease bleaching of photopigments
What is the pupil response to dark?
Dilation
How does the pupil dilate?
Radial muscles contract innervated by sympathetic nerve
What is the effect of pupil dilation?
Increases light sensitivity in the dark by allowing more light to enter the eye
Describe the afferent pathway of the pupillary reflex
THINK SENSORY: rod and cone photoreceptors > bipolar cells > retinal ganglion, exits at posterior third of optic tract > enters lateral geniculate nucleus > brainstem pretectal nucleus > synapse at Edinger-Westphal Nuclei in both sides of the brainstem
(stimuli from one eye affects both due to synapse system)
Describe the efferent pathway of the pupillary reflex
THINK MOTOR: common for both eyes - oculomotor nerve efferents leave Edinger-Westphal nucleus, synapse at ciliary ganglion, follows short posterior ciliary nerve, to pupillary sphincter
What is the direct pupillary reflex
Constriction of the light stimulated eye
Consensual pupillary reflex
Constriction of the eye that is not light stimulated
What is the neurological explanation for the consensual pupillary reflex?
Afferent pathway on one side will stimulate common efferent pathway on both sides
What is an example of a right afferent defect?
Damage to right optic nerve
In a Right Afferent Defect, what will happen when the right eye is light-stimulated?
Neither pupil will constrict
In a Right Afferent Defect, what will happen when the left eye is light-stimulated?
Both pupils will constrict
Example of right efferent defect
Damage to the right 3rd nerve
In a Right Efferent Defect, what will happen when either eye is light-stimulated?
No right pupil constriction
Left eye constriction
How can you assess an Afferent Pupillary Defect?
Swinging Torch Test - alternating light stimulation of right and left eye
What are the 6 extraocular muscles?
Straight muscles: Superior Rectus, Inferior Rectus, Lateral Rectus, Medial Rectus,
Superior Oblique, Inferior Oblique
Where is the attachment of the superior rectus and what is its function?
Attaches at 12 o’clock, elevation
Where is the attachment of the inferior rectus and what is its function?
Attaches at 6 o’clock, depression
Where is the attachment of the lateral rectus and what is its function?
Attaches on temporal side of the eye, abduction (away from midline)
Where is the attachment of the medial rectus and what is its function?
Attaches on nasal side, adduction (towards midline)
What is the origin and attachment for the superior oblique?
Originates at the apex of the orbit, passes through the trochlea of the nasal bone, attaches on the superior temporal side of eye, underneath superior rectus
What is the function of the superior oblique?
Depression and Adduction
What is the origin and attachment for the inferior oblique?
Originates at the base of the orbit, passes through the trochlea of the nasal bone, attaches on the inferior nasal side of the eye, over the inferior rectus
What is the function of the inferior oblique?
Elevation and Adduction
What muscles does the oculomotor nerve innervate?
superior branch:
Superior Rectus, Levator Palpebrae Superioris (raises eyelid),
Inferior branch:
Inferior Rectus, Medial Rectus, Inferior Oblique, Circular Muscles of Iris
What muscles does the trochlear nerve innervate?
Superior Oblique
What muscle does the abducens innervate?
Lateral rectus
