Flashcards in Eye Physiology Deck (18):
What spectrum of light is visible?
- visible light: 400-750 nm
What components make up the three layers of the orbital wall?
- outer fibrous layer: cornea, conjunctivae, sclera
- middle vascular layer: iris, choroid (and ciliary body)
- inner neural layer: retina (contains the photoreceptors)
What is the blind spot? What about the macula? The fovea?
- the retina covers the entire posterior eye except for the optic disc (the head of the optic nerve); this is called the blind spot
- the macula is the center of the retina; it is the area with the highest visual acuity
- light is focused onto a small depression in the macula called the fovea
What are the two fluids present in the eye and where is each found?
- 1) aqueous humor: fills the (smaller) anterior chamber
- 2) vitreous humor: fills the (larger) vitreous chamber
- (anterior chamber: between cornea and iris)
- (vitreous chamber: between lens and retina)
- (posterior chamber is a very small space between the iris and the lens)
What are the two types of photoreceptors? What are the major differences between them?
- rods and cones
- rods: low threshold, sensitive to low intensity light (function well in darkness); low acuity, no color
- cones: higher threshold, works best in daylight; high acuity, color vision
What are the eight layers of the retina?
- 1) pigment cell layer: (lies just inside the choroid) made up of pigment epithelium (absorbs stray light and reduces scatter)
- 2) photoreceptor layer: rods and cones
- 3) outer nuclear layer: contains the nuclei of the photoreceptors
- 4) outer plexiform layer: contains the synapses between photoreceptors and interneurons
- 5) inner nuclear layer: contains the nuclei of the interneurons
- 6) inner plexiform layer: contains the synapses between interneurons and ganglion cells
- 7) ganglion cell layer: contains ganglion cells (these are the output cells of the retina)
- 8) optic nerve layer: the axons of the ganglion cells pass through the retina and become the optic nerve after passing through the optic disc
- *this means light actually passes through the entire retina before hitting the photoreceptors in the outer layer, and then the resulting neural activity flows back towards the inner layer*
Why do rods have low acuity and cones have high acuity?
- rods have lower acuity because many rods synapse onto each interneuron
- cones have higher acuity because only a few cones synapse onto each interneuron
What is rhodopsin and where is it found? What is it composed of?
- rhodopsin is the light sensitive pigment found in the outer segments of photoreceptors
- it is present in much more quantity in rods than cones, making rods way more sensitive (one photon will excite a rod, several hundred are needed to excite a cone)
- rhodopsin is made up of opsin and retinol (an aldehyde of vitamin A)
What major event triggers photoreception? What happens next (hint: it's weird!)? What is "dark current"? What role does vitamin A play?
- the triggering event of photoreception is photoisomerization: the conversion of 11-cis rhodopsin into all-trans rhodopsin (light catalyzes this reaction)
- this sparks a series of conformational changes in opsin (a component of rhodopsin), eventually forming metarhodopsin II
- metarhodopsin II activates transducin (a GPCR, Gt), which activates a phosphodiesterase! (this will result in DECREASED cGMP levels)
- the decrease in cGMP causes HYPERPOLARIZATION of the photoreceptor, which causes a DECREASE in the release of inhibitory neurotransmitters
- (in the dark, "dark current" dominates, and cGMP levels are high and the cell is depolarized, triggering release of inhibitory NTs)
- (vitamin A is needed to return the trans rhodopsin back into its cis form in order for another transduction to occur)
What is the basic pathway of photoreception?
- 1) photoreceptors
- 2) interneurons (bipolar and horizontal cells... amacrine cells indirectly)
- 3) ganglion cells
- 4) optic nerve
- 5) lateral geniculate nucleus of thalamus
- 6) geniculocalcarine tract
- 7) visual cortex
Outline the pathway for each eye's nasal and temporal fields. Information from which fields does each optic tract carry? What does this mean for the information projecting to each cortex?
- first off, temporal visual fields project onto the nasal half of the retina and nasal visual fields project onto the temporal half of the retina
- the nerve fibers from each nasal retina (which contain the temporal fields) cross at the optic chiasm and ascend contralaterally
- the nerve fibers from each temporal retina (which contain the nasal fields) do not cross and ascend ipsilaterally
- therefore each optic tract carries info from the ipsilateral nasal field (temporal retina) and the contralateral temporal field (nasal retina)
- this means the right cortex is getting each eye's left visual field (so the right eye's nasal field and the left eye's temporal field) and the left cortex is getting each eye's right visual field (so the right eye's temporal field and the left eye's nasal field)
What does cutting an optic nerve result in? Cutting the optic chiasm? How about cutting an optic tract? Cutting a geniculocalcarine tract?
- (note that the optic nerve becomes the optic tract after passing through the optic chiasm)
- optic nerve: ipsilateral anopsia
- optic chiasm: bitemporal hemianopsia
- optic tract: homonymous contralateral hemianopsia (loss of ipsilateral nasal field and contralateral temporal field AKA loss of the contralateral visual field hemispheres in each eye; so if you cut the right optic tract, you will lose the left visual field in each eye)
- geniculocalacarine tract: same as above but with macular sparing
What is the uvea (or uveal tract)?
- the uvea consists of the choroid, the ciliary body, and the iris (these make up the middle layer of the eye)
- uveitis is therefore inflammation of any of these structures
- (anterior uvea: iris and ciliary body)
- (posterior uvea: choroid)
Which arteries supply the eye? From which major arteries do they branch?
- the ophthalmic artery supplies the entire eye; it is derived from the internal carotid artery and has two major groups of branches:
- the central retinal artery branch supplies the inner layers of the retina
- the ciliary artery branches supply the outer layers of the retina, the choroid, and the sclera
What are the extrinsic/extra-ocular muscles? What does each do and what nerve controls each?
- LR: abduction (towards ear)
- MR: adduction (towards nose)
- SR: elevation of abducted eye
- IR: depression of abducted eye
- SO: DEPRESSION of adducted eye
- IO: ELEVATION of adducted eye
- CN III (oculomotor) supplies MR, SR, IR, and IO
- CN IV (trochlear) supplies SO
- CN VI (abducens) supplies LR
What results from a CN III palsy?
- (CN III is oculomotor nerve)
- the ipsilateral eye is turned down and out (because of unopposed activity of LR and SO)
- the ipsilateral eye shows ptosis (because CN III normally supplies levator palpebrae superioris muscle) and mydriasis (pupil dilation because CN III normally supplies parasympathetic activity)
What results from a CN IV palsy?
- (CN IV is trochlear nerve)
- when asked to adduct the ipsilateral eye and look down, the head tilts to compensate for the inability to depress the adducted eye (the normal role of the SO)