Vision and Hearing Flashcards
(44 cards)
What are the features of the eyes surface anatomy?
- Eyelids (palpebra)
- Sclera (white part)
- Sclerocorneal junction (where sclera meets the cornea)
- Pupil
- Iris
What are the muscles involved in the eye?
-Levator palpebrae superioris (lifts up the upper eyelid)
-Recti muscles:
Superior (above the eye)
Lateral (further away from the body
Medial (closer to the body)
Inferior (underneath)
-Obliques:
Superior (comes out the back and hooks through a bone hook above eye)
Inferior (Comes from underneath the eye)
What are saccades?
Intentional eye movements
How are each of the muscles innervated to cause eye movement?
3 Cranial nerves cause innervation
CN III (oculomotor nerve):
- Superior rectus
- Inferior rectus
- Medial rectus
- Inferior Oblique
- Levator Palpebrae superioris
CN IV (trochlear): -Superior oblique
CN VI (abducens): -Lateral rectus
What are the types of fixational eye movements (FEM’s)?
- Microsaccades (snap it back to the centre to keep image at highest part of sensitivity in the eye)
- Drifts (move image across the retina)
- Tremors (Ignore)
FEM’s prevent fading of the field of vision
What is the anatomy of the anterior chamber of the eye? (front of the eye)
- Cornea (transparent membrane allowing light to enter)
- Anterior chamber filled with aqueous humor to bring nutrients into the eye
- Iris (coloured opaque muscle that regulates light entry)
- Ciliary body (changes lens shape)
What is the anatomy of the posterior chamber? (the back of the eye from lens back)
- Posterior chamber is filled with vitreous humor
- Fovea (point at which there is the highest sensitivity of light due to highest concentration of photoreceptors)
- Retina (photoreceptor layer)
- Choroid layer (light-absobing pigment prevents glare)
- Sclera (protection)
- Optic disc (where the optic nerve meets the eye and there is a blind spot due to no photoreceptors)
- Optic nerve (axons projecting to visual cortex)
Where is the most concentrated area of cones in the eye?
Fovea (also where lowest concentration of rod cells are present except blind spot)
What is the macula?
Large area surrounding where the fovea is located
What is acommodation?
Where the shape of the lens changes in order to better focus on an object
How does the lens adjust when the object is far away? (greater than 6m)
- The ciliary muscles are relaxed causeing the suspensory ligaments to be tightened.
- This causes the lens shape to be flat so light refraction is small
-If an eye can’t do this then they are near-sighted (myopia) which can be corrected with a biconcave lens to move rays further apart
How does the lens adjust when the object is near the eye? (less than 6m away)
- The ciliary body is pulled forward and inward which causes the suspensory ligaments to relax allowing the lens to become more round which causes large light refraction
- If you are unable to do this you are far sighted (hyperopia) and it can be fixed with a boconvex lens to move the rays closer together
How do photoreceptors create an impulse?
When its dark the Na+ channels are open and the Na+/K+ ATP-ase pump is active. This balances out the molecules and keeps the cell at baseline depolarised (-40mV)
When light is present, the pigment changes shape and blocks the Na+ channels so they close. This causes a build up of positive ions in the synapse so the cell becomes hyperpolarised (more -ve). This prevents glutamate from being released
What are the photosensitive proteins called?
Opsins (rhodopsin/conopsin)
-Contain Vitamin A
How do rhodopsin cause a hyperpolarisation?
- Rhodopsin is sensitive to approx500nm wavelength (takes the lowest intensity of green light to be activated)
- The rhodopsin is turned into metarhodopsin which creates a shape change which closes the Na+ channel
How is hyperpolarisation acheived in cone cells?
three different cones have three opsins:
- S Cones (420nm so blue light)
- M Cones (534nm so green light)
- L Cones (564nm so red light)
What are the features of cone cells?
- 6 million per eye
- located mostly in Fovea
- Can see Colour
- Higher resolution due to 1-to-1 with ganglion cells
What are the features of rod cells?
- 120million per eye
- Periphery
- Monochrome
- Low resolution due to many to 1 with ganglion cells
What occurs to the image before reaching the optic nerve?
- Horizontal Cells that inhibit adjacent cells with GABA (lateral inhibtion)
- Bipolar cells form a modified 2nd image
- Amacrine cells form a modified third image
- Ganglion cells then produce action potentials that project along the optic nerve to the CNS
What are common problems that can occur in the eye?
Glaucoma - fluid outflow is blocked causing pressure to build which constricts the optic nerve and artery which causes the loss of the peripheral visual field
Macular degeneration - If the eye stops clearing debris properly, drusen can build up that can block of damage photoreceptors in the fovea causing loss of central visual field
What happens after the AP has left the eye through the optic nerve?
- The projections from each eye switch sides at the optic chiasm
- The optic tract projects to the LGN in the thalamus with some projections to the hypothalamus
- The optic radiations then project to the visual cortex at the back of the brain
What is retinotopy?
Mapping the visual field onto the cortex so by seeing if an area of the visual cortex is damaged it can be directly linked to an area on the eye that is damaged
How is the visual system adapted to light conditions?
When in the dark:
- cones adapt in less than 10minutes but not suited to dark so low sensitivity
- rods adapt in about 30minutes but will increase hugely in sensitivity by increasing the amount of rhodopsin produced
The reverse happens in light but its much quickers with both taking around 5mins to adapt
How is sound transmitted?
Disturbance in the air through compression and rarefraction
