Lectures 15 & 16 - Eyes & Ears Flashcards
Purpose of peripheral vision?
Detect movement
Are there more false positives or false negatives for our peripheral to detect movement?
More false positives
Where are the anterior and posterior chambers of the eye located?
Anterior: in front of iris
Posterior: behind iris and in front of lens
What is the eye filled with?
Vitreous humor
What is the sclera of the eye?
- Opaque connective tissue
- Posterior 5/6th of fibrous tunic
- Part of the eye commonly known as the “white”
What is the choroid of the eye?
Posterior majority of vascular tunic made of vascularized connective tissue between the sclera and the retina
What is the insertion point for the extraocular eye muscles?
Sclera
How many intraocular muscles in the iris? What are they? What is each innervated by? What do they do?
- Sphincter (constrictor) muscle - Parasympathetic NS (CN III) -> constricts pupil
- Dilator muscle -Sympathetic NS -> dilates pupil
==> Control pupil size by pulling on the lens
What is the pathway of light through the eye?
Light reflected from object => cornea => aqueous humor => pupil => lens => vitreous humor => retina photoreceptors and signal transduction => bipolar cells => ganglion cells
What are the 3 important structures of the retina? Describe each.
- Optic disc: hole in posterior retina = highly vascularized exit/entry point of optic nerve
- Fovea: depression of outer-most cellular neuronal layer of retina that has improved acuity
- Macula: contains the fovea
What constitutes the blind spot of the eye? What does it mean?
The optic disc of the retina
No photoreceptors in this spot
Position of optic nerve/blind spot with regards to the macula?
Medial
Position of blind spot image with regards to the macula?
Lateral
Can the blind spots of each eye be in the same spot at the same time?
NOPE
Describe the structure of the lens of the eye.
- Capsule: thick basement membrane containing collagen type IV and laminin
- Subcapsular epithelium: secretes the lens capsule
- Lens fibers (below the subcapsular epithelium): cells that have lost their nuclei and organelles, becoming thin, elongated, transparent structures
What nourishes the cells of the lens of the eye?
Fluid from the posterior to the anterior chamber of the eye
What can go wrong with the lens of the eye?
- Can become stiff
2. Can become cloudy
What are the 2 types of photoreceptor cells?
- Rods: black and white
2. Cones: color
In dim light are we primarily using rods or cones?
Rods
What are the only retinal cells capable of firing APs?
Ganglion cells
How many colors can you see? How many receptors/cone types do you have for each?
- Red: 1 or 2 (on X chromosome, so men only have 1)
- Blue: 2
- Green: 2
Do men or women see colors better? Why?
Women because they have 6 color receptors instead of 5 in men (see red colors better)
What is the dark current?
In the absence of light, photopigment proteins are inactive and therefore photoreceptors are depolarized and have current: cGMP opens cGMP gated cation channels (Na+ and Ca++ flow in)
Describe the phototransduction pathway in rods.
Light hits retinal portion of rhodopsin => rhodopsin is now activated and facilitates the exchange of GDP for GTP on transducin => transducin dissociates into Tα-GTP and Tβγ => Tα-GTP activates cGMP phosphodiesterase (PDE) by binding and removing its inhibitory subunit (I) => PDE converts cGMP to 5’-GMP and reduces [cGMP] to below the level needed to keep cation channels open => cGMP gated cation channels close preventing Na+/Ca++ influx => hyperpolarization of membrane photoreceptor cell + Ca++ efflux through Na+/Ca++ exchanger => IPSP => inhibition of glutamate release => signal transmitted to brain
How does light affect retinal on rhodopsin exactly? Explain.
It bleaches it: photo absorption breaks the double bond between C11 and C12 which permits rotation and conversion from 11-cis-retinal to all-trans-retinal
What is the major mechanism of dark adaptation?
Regeneration of unbleached retinal
Describe the mechanism of decrease in intracellular Ca++ during light adaptation/recovery?
Light => decreases Ca++ conductance => less Ca++ in cells to inhibit the GC => GC activated => more [cGMP] to dark levels => cGMP gated channels open => returning RMP to prestimulus level => rhodopsin kinase phosphorylates bleached rhodopsin (stimulated by low Ca++ and recoverin) => arrestin binds phosphorylated carboxyl terminal to inactivate rhodopsin => slowly arrestin dissociates, rhodopsin is dephosphorylated and all-trans-retinal is replaced with 11-cis-retinal => rhodhopsin ready for another phototransduction cycle
What kind of receptor is transducin?
Inhibitory GPCR
What causes glare?
Light scatters on the retina when it is very bright
Why doesn’t light exit back out of the pupil? What to note?
Because pigments in the epithelium of the retina absorbs it
This also prevents light scattering and glare
What is ocular cutaneous albinism?
Defect in pigment producing enzyme in the retina epithelium, causing:
- Pink eyes because the light reflects from the blood vessels in the back of the eye
- Poor vision due to glare that is not prevented by the pigment in the retina epithelium
Why does the fovea have improved acuity?
- Because the depression allows for the light to directly reach the photoreceptors
- Because it has a higher density of photoreceptor
What is the central visual pathway?
Retina => optic nerves => optic chiasm => optic tracts => synapse in lateral geniculate nuclei of thalamus (LGN) => optic radiations => visual cortex of occipital lobe
Result of optic chiasm lesion? 2 names
Bitemporal blindness = bitemporal hemianopia
Result of right optic radiations lesion?
Blindness in right nasal and left temporal visual fields
Purpose of optic chiasm?
Allows for 2 neurons receiving signals from the same part of the visual field but receiving signals from each the left or the right eye to be next to each other in the same cerebral hemisphere
Result of lesion of right optic nerve?
Right eye blindness
Result of left optic tract lesion?
Bilateral right hemianopia
What could cause a lesion at the optic chiasm?
Pituitary tumor
6 extrinsic muscles of the eye?
- Superior rectus
- Medial rectus
- Superior oblique
- Levator palpebrae superioris
- Lateral rectus
- Inferior oblique
What 3 cranial nerves control the extrinsic muscles of the eye?
Cranial nerve III: oculomotor => superior oblique
Cranial nerve IV: trochlear => lateral rectus
Cranial nerve VI: abducens => all other extrinsic muscles of the eye
Function of oblique muscles of the eye (superior and inferior)?
Prevents eye ball rotation and depression
What is strabismus? What does it cause?
Failure of eyes to converge on a same point => causes diplopia
What is diplopia?
Cross-eyed causing double vision