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NS 39 - Retinal Function + Visual Perception (i) (refractive power (cornea…
NS 39 - Retinal Function + Visual Perception (i)
2/3 of awake brain activity involves processing visual info
Visual system
sensory organ (eye) with photo Rs + optical elements (create sharp image)
corresponding neural system in brain
detects interprets photic stimuli
electromagnetic waves
wavelength = colour
visible between 400-750 nm
more=IR
less=UV
amplitude = brightness
Parts of eye
light goes through cornea (main refraction point) -> aqueous humour -> lens -> vitreous humour (produced by retinal cells) -> retina
choroid between pigmented retina + sclera (light absorbing pigment, nourishes retina)
ciliary body
muscle (controls lens convexity)
process (makes aqueous humour)
retina
near centre: macula lutea
centre of macula: fovea centralis
highest visual acuity
high cone conc
small receptive field
when looking directly at an object, image falls here
image formed on retina = inverted vertically + laterally (corrected by brain)
pigmented part + neural part
supplied by central retinal art + vein (run inside optic N)
130 000 000 photoRs on ext side (hence light must pass through all layers of retina)
6 000 000 bipolar cells (no axons)
1 000 000 ganglion cells (have axons, form optic N)
sclera: dense white connective tissue
iris: radial + longitudinal smooth muscle fibres
refractive power
ensures image is formed on retina
close objects - more divergent light - longer focal distance - more convex lens - ciliary muscle contracts - suspensory lig tension decreases (PARASYMP)
refraction occurs when light enters new medium @ any angle other than 90
convex surfaces converge light to focal point
cornea
convex
shape can't be changed
responsible for 2/3 of refraction
lens
biconvex
shape can be changed (accommodation)
responsible for 1/3 of refraction
far objects - less divergent light- shorter focal distance - less convex lens - ciliary muscle contracts - suspensory lig pulls
effects of pupil on vision
level of light
ensures optimal photoR stimulation
constriction decreases light level, dilation increases light level
depth of focus
contrition increases depth of focus (cuts out peripheral light + distant images)
pupil size controlled by ANS
symp - radial muscle contraction - dilation
parasymp - circular muscle contraction - constriction