1b Visual System Flashcards
What is the palpebral fissue?
The line of the eye where the eyelashes sit
What is the limbus?
The border between the cornea and the sclera
What is the caruncle?
The inner most fleshy corner part of the eye
What is the medial canthus?
Where the caruncle meets the sclera
What is the conjunctiva?
Thin, transparent tissue which covers the outer surface of the eye
What are the three layers of the eye?
Sclera - hard and opaque
Choroid - pigmented and vascular
Retina - Neurosensory tissue
What is the sclera?
The white of the eye - tough and opaque tissue which serves as the eyes protective outer coat
What is the water content of the sclera?
High
What is the cornea?
The transparent, dome-shaped window covering the front of the eye.
What is the water content of the cornea like?
Low water content
What is the function of the cornea?
Powerful refracting surface, providing 2/3 of the eye’s focusing power. Like the crystal on a watch, it gives us a clear window to look through
What happens if the cornea becomes hydrated?
becomes cloudy
What is the uvea?
Vascular coat of eyeball and lies between the sclera and retina.
What are the three parts of the uvea called?
Composed of three parts – iris, ciliary body and choroid.
What is the choroid?
Choroid - lies between the retina and sclera. It is composed of layers of blood vessels that nourish the back of the eye.
What is the function of the iris?
Controls the amount of light which is entering into the eye - has tiny muscles which dilate and constrict the eye
What is the structure of the lens?
It is composed of an outer acellular thin capsule,
encasing a core of regular elongated cell fibres.
What percentage of the eyes refractory power comes from the lens?
1/3 of the eye focusing power - higher refractive index than aqueous fluid and vitreous
What is a cataract?
The lens may loose its transparency with age,
resulting in an opaque lens, known as Cataract.
What is the retina?
Thin layer of tissue which lines the inner part of the eye
Responsible for capturing the light rays which enter into the eye - light impulses are then sent to the brain for processing via the optic nerve
What is the function of the optic nerve?
Transmits electrical impulses from the retina to the brain
What is the visible portion of the optic nerve called?
The optic disc
Where does the optic nerve connect to the back of the eye?
Near the macula
What is the blind spot?
Where the optic nerve meets the retina, there are no light sensitive cells
What is the macula?
A small and highly sensitive part of the retina responsible for detailed central vision - located roughly in the centre of the retina
what is the structure found at the center of the macula?
Fovea
Which part of the eye allows us to perform tasks which require central vision like reading?
Fovea
What is the concentration of cones and rods like in the fovea?
Cones are high, rods and low
Why is the fovea particularly important?
Only the fovea has the highest concentration of cones in order for us to perceive high detail
What is the anatomical land mark for the physiological blind spot?
Optic disc
What is central vision?
Detailed, day vision - only the fovea
what is peripheral vision for?
Shape, movement and night vision, as well as navigation
What is the result of extensive loss of visual field?
Unable to navigate in an unfamiliar environment,
What is found in the outer layer of the retina?
Photoreceptor cells - involved in the detection of light
What is found in the middle layer of the retina?
Bipolar cells (2nd order neurones)
What is the function of the bipolar cells?
Local signal processing to improve contrast sensitivity
What does the inner layer of the retina contain?
Retinal ganglion cells (3rd order neurones)
What is the function of the retinal ganglion cells
Transmission of signal from the eye to the brain
What is the structural differences between the cones and the rods?
Rods = Longer outer segment
Cones = Shorter outer segment
What is the difference between rods and cones?
Rods = More sensitive to light, but slower response
Cones = Less sensitive to light, but faster response
Which photoreceptor cell is responsible for day light fine vision and colour?
Cones
Which photoreceptor cells is responsible for night vision?
Rods
What is scotopic vision?
Peripheral and night vision
Where can one find the highest concentration of Rod photoreceptors in the retina?
20-40 degrees away from fovea
What colour are rods sensitive to?
Rods are used for night vision and spatial recognition and are not really sensitive to any particular colour
what is refraction?
Then light goes from one’s medium to another, it changes velocity
What is the index of refraction and how is it calculated?
Ratio of speed of light in a vacuum to speed of light in a medium
What can the index of refraction be used for?
Help identify a material
What happens to the path of light when it changes from one medium to another?
Path changes
What is the angle of incidence?
Equal to the angle of reflection, is the angle at which light is reflected off the new medium
What are the two types of lenses?
Concave and convex
What does a convex lens do to light?
Takes the light rays and brings to a single focal point
What does a concave lease do?
takes light rays and spreads them out
What is an example of an application of a converging lens?
A camera
What is emmetropia?
Perfect eye - adequate correlation between the axial length and refractive power
Parallel light rays fall on the retina without the need for accomodation
What is ametropia?
Mismatch between axial length and refractive power so parallel light rays do not fall onto the retina
What is myopia?
Near sightedness
What is hyperopia?
Far sightedness
Where do the light rays converge in myopia?
Parallel rays converge at a focal point which is anterior to the retina - in front of it
What are the causes of myopia?
Excessive long globe and excessive refractive power
What are the symptoms of myopia?
Blurred distant vision
Squinting to attempt to improve visual activity
Headaches
What are the treatments for myopia?
Diverting lens in glasses
Contact lenses
Refractive lens surgery
Where do light rays converge in hyperopia?
Parallel rays converge at a focal point posterior to the retina
Etiology : not clear, inherited Causes
What are the causes of hyperopia?
•excessive short globe (axial hyperopia) : more common
•insufficient refractive power (refractive hyperopia)
What are the symptoms of hyperopia?
visual acuity at near tends to blur relatively early
* nature of blur is vary from inability to read fine print to near vision is clear but suddenly and intermittently
blur
* blurred vision is more noticeable if person is tired , printing is weak or light inadequate
* asthenopic symptoms : eyepain, headache in frontal region, burning sensation in the eyes, blepharoconjunctivitis
What are the asthenopic symptoms of hyperopia?
Eye pain
Headache
Burning sensation in eyes
Blepharoconjuctivitis
What type of lens is used in glasses to treat hyperopia?
Converging lens
What is the near response triad?
Three actions of the eye in order for adapt for near vision
Pupillary Miosis- Increase depth of field
Convergence- Align both eyes towards a near object
Accommodation- Increase refractive power of lens for near vision
What is pupillary miosis ?
Pupil constricts which increases the depth of field of the eye
Which muscle is responsible for the contraction of the pupil?
Sphincter pupillae
What is the benefit of a larger depth of optic field?
Eye is able to maintain clear forms over a certain range of viewing distances which relying on accomodation
What is convergence?
When the eyes adduct medially at the same time to align the eyes towards a near object
How does convergence occur?
Simultaneous contraction of the medial rectus extra ocular muscles from both eyes
Which muscles allow for accomodation of the eye?
Circular ciliary muscles
What does accomodation do?
Increases the refractive power of lens for near vision
What is presbyopia?
Naturally occuring loss of accommodation onset from age 40
Distant vision is intact
How is presbyopia corrected?
Corrected by reading glasses - increases refractive power of the eye
Why does the lens lose its ability to accommodate in presbyopia?
Natural lens loses elasticity
What is the treatment of presbyopia?
Convex lenses in near vision - reading glasses or bifocal
What is the visual pathway?
The visual pathway is the neurological pathway,
where by vision is converted to neurological impulses, to be transmitted from the eye to the visual cortex, the posterior part of the brain.
Where do the cell bodies of the optic nerve originate?
The retina
Where do the optic nerve from each eye, converge?
Optic chiasm
What happens to the ganglion nerve fibres at the optic chiasm?
About half of the ganglion nerve fibres cross at the optic chiasma, and exit along the contra-lateral Optic Tract, while the remaining ganglion nerve fibres exit along the Optic Tract on the same side.
Where do the ganglion nerve fibres synapse?
Lateral Geniculate nucleus
What is the fourth order neurone in the visual pathway?
Optic radiation - takes information from the lateral geniculate ganglion to the primary visual cortex in the occipital lobe
What cortex is involved in higher visual processing?
Extra-striate Cortex (in Occipital cortex)
What are the first second and third order neurones of the retinal visual pathway?
First Order Neurons – Rod and Cone Retinal Photoreceptors
Second order Neurons – Retinal Bipolar Cells
Third Order Neurons –Retinal Ganglion Cells
what happens to the retinal ganglion fibres after entering into the optic nerve?
become myelinated to improve signal transmission
What type of visual defect do you get when you have a lesion which is anterior to the optic chiasm?
Lesions anterior to Optic Chiasma affect visual field in one eye only
What type of visual defect do you get when you have a lesion which is posterior to the optic chiasm?
Lesion occurring posterior to the Optic Chiasma will affect visual field simultaneously in both eyes,
because of the fibre crossing at the chiasma.
Where do the crossed fibres predominantly originate from?
predominantly originate from the nasal retina,
responsible for the temporal half of the visual field in each eye.
Where do the uncrossed fibres predominantly originate from?
The uncrossed fibres predominantly originate from the temporal retina,
responsible for the nasal half of the visual field in each eye.
Lesion at optic chiasm?
Bitemporal hemanopia - temporal field deficit in both eyes
Lesions posterior to optic chiasm?
Right sided lesion – Left Homonymous Hemianopia in Both Eyes
Left sided lesion – Right Homonymous Hemianopia in Both Eyes
What is the most common cause of bitemporal hemaniopia?
Typically caused by enlargement of Pituitary Gland Tumour
What is the most common cause of homonymous hemianopia?
On the other hand, homonymous hemianopia is typically caused by stroke,
or cerebrovascular accidents in the brain.
What does damage of the primary visual cortex lead to?
Homonymous Hemianopia with Macular Sparing
Why is the macula spared in damage to the primary visual cortex?
Area representing the Macula receives dual blood supply from Posterior Cerebral Arteries from both sides
What happens to the pupil in the light?
Constriction
What nerve mediates pupillary constriction?
Parasympathetic nerve - within CNIII
What causes the circular muscles to contract and what does this lead to?
parasympathetic stimulation - pupil constriction
What causes the radial muscles to contract and what does this lead to?
Sympathetic stimulation - pupil dilation
When does the pupil dilate?
In dark environments
Why does the pupil constrict in light?
decreases spherical aberrations and glare
increases depth of field
reduces bleaching of photo-pigments
Describe the efferent pathway of the pupillary reflex?
Edinger-Westphal Nucleus -> Oculomotor Nerve Efferent ->
Synapses at Ciliary ganglion ->
Short Posterior Ciliary Nerve -> Pupillary Sphincter
Where do the afferent fibres of the pupillary reflex become the efferent?
Edinger-Westphal Nuclei
What is the direct and consensual 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 direct and consensual reflex?
Afferent pathway on either side alone will stimulate efferent (outgoing) pathway on both sides
What is the observed difference in afferent and efferent defects
In short, afferent defect produces different responses between eyes, depending on which eye is stimulated.
Efferent defect produces the same unequal responses between left and right eye, no matter whether left or right eye is stimulated.
What is the swinging torch test?
Shining light between right and left rapidly
What is a relative afferent pupillary defect?
When the damage to the afferent pathway is incomplete
What is the best way to test the relative afferent pupillary defect?
Swinging torch test
Describe a positive swinging torch test for a relative afferent pupillary defect?
Both pupils constrict,
when light swings to the left eye with intact afferent pathway.
Both pupils will paradoxically dilate,
when the light swings to the right eye with damaged afferent pathway, as a result of relatively reduced drive for pupillary constriction in both eyes.
What facilitates the movement of the eye?
It is facilitated by the six extraocular muscles in the orbit,
innervated by the three cranial nerves.
What are the straight muscles of the eye called?
Superior rectus
Inferior rectus
Lateral rectus
Medial rectus
What are the names of the two muscles of the eye which are not straight
Superior oblique
Inferior Oblique
what does the superior rectus do to the eye?
Moves the eye up
What does the inferior rectus do to the eye?
moves the eye down
what does the lateral rectus do?
Moves the eye towards the outside of the head - towards temple
What does the medial rectus do?
Moves the eye towards the midline
Describe the action of the superior oblique?
Passes under the Superior Rectus.
Moves the eye in a diagonal pattern down and out
Describe the action of the inferior oblique?
Moves the eye in a diagonal pattern - up and out.
Which eye muscles are innervated by the superior branch of the third cranial nerve - occulomotor?
Superior Rectus – elevates eye
levator palpebrae superioris - raises eyelid (not shown)
Which eye muscles are innervated by the inferior branch of the third cranial nerve - oculomotor?
Inferior Rectus – depresses eye
Medial Rectus – adducts eye
Inferior Oblique – elevates eye
Parasympathetic Nerve – constricts pupil
Which eye muscles are innervated by the fourth cranial nerve?
Superior oblique - depresses eyes
Which eye muscles are innervated by the 6th cranial nerve?
Lateral Rectus – abducts eye
What position of the eye best tests the lateral rectus?
Abduction
What position of the eye best tests the medial rectus?
Adduction
What position of the eye best tests the superior rectus?
Abducted and Elevated
What position of the eye best tests the inferior rectus?
Depressed and abducted
What position of the eye best tests the inferior oblique?
Elevated and adducted
What position of the eye best tests the superior oblique?
Depressed and adducted
What does the eye look like in third nerve palsy?
affected eye is down and out
How is the eyelid effected in third nerve palsy?
Droopy eyelid - due to loss of levator palepbrae superioris
Why does third nerve palsy result in a down and out movement of the eye?
Unopposed superior oblique innervated by fourth nerve (down)
Unopposed lateral rectus action innervated by sixth nerve (out)
What does 6th nerve palsy result in?
patients are deficit in abduction in the affected eye
Affected eye will deviate inwards
What is asthenopia?
Eye strain condition which manifests through non-specific symptoms such as fatigue, pain in and around the eyes, blurred vision and occasional double vision
