Eyelid movements are mediated by what three muscles?
How are they innervated?
orbicularis oculi and levator palpebrae superioris (ACh acts on nicotinic receptros to cause contraction)
superior tarsal muscle (sympathetic innervation via a1 receptors)
focusing power =
fatter lens = more accomodation
relaxation of ciliary muscle
via activation of ß2 receptors
contraction of ciliary muslces
Axial length < focal length
caused by lens that is too short or lens that is too weak
correct with convex lens to increase refractive power
correct with concave lens
axial length > focal length
less flexible lens
decrease in accomodation
uneveness in lens
part of visual field will be out of focus
opaquness in lens
physical damage, radiation, high [glucose]
remove lens, replace
pupillary light reflex
causes miosis (parasympathetic stimulation of sphincter pupillae via muscarinic receptors)
mydriasis (sympathetic stimulation (a1 receptors) that activates the dilator pupillae muscles)
light in one eye --> constriction in another... used to test brain injury.
increased intraocular pressure can cause permanent vision loss
closed angle glaucoma (medical emergency) -- caused by blockage of fluid outflow
open angle glaucoma (slowly developes) -- caused by increase in aqueous humour production or a blockage of outflow
increased interocular pressure!!
1. cornea -- halos, blurring; 2. photoreceptor pressure; 3. optic nerve pressure; 4. arterial supply
aqueous humor volumn and intraocular pressure
inflow -- sympathetics --> ß2 (increases flow) and alpha1 (decreases flow) of H2O via carbonic anhydrase
outflow -- parasympathetics --> canal of Schlemm contract sphincter pupillae
intraocular pressure = 20 mmHg (>30 glaucoma)
(Pressure = flow x resistance (V = IR))
Cells and circritry of the retina
1. Generate receptor potential-- transduction
2. Bipolar cells -- graded potential
3. Ganglion cells -- action potential generated (axons form optic nerve)
Rods and Cones
Rods are responsible for scotopic vision (black and white; monochromatic vision)--high sensitivity! less precise!
Cones; 3 types-- blue, green, and red; or short, medium, and long (respectively).
Cones have better spatial and temporal resolution than rods; photopic vision
spacial acuity: where is something in space (snellen eye chart to determine). Cones: Best in fovea.
Temporal acuity: ability to distinguish 2 events as separate.
Spectral acuity: ability to distinguish differences in wavelength (color) -- how well cone system is functioning
1. activation of rhodopsin
2. rhodopsin joins with transducin
3. GTP --> GDP; activated phosphodiesterase, which breaks down cGMP
4. cGMP holds open Na+ channel, which then releases.