the visual system Flashcards
the visual system
~ 50% of the cortex is devoted to visual perception
- has the most complex circuitry of all sensory systems
- over 1 million fibers
anatomy of the human eye
the surface of the retina
- has numerous vessels fan out over the surface
- vessels arise from othalamic artery and vein
where does the opthalmic artery and vein enter the eye through?
optic disk
retinal axons leave the eye through
optic disk
the optic disk
- has no photo receptors
- blind spot
macula lutea
- located near the center of the retina
- contains yellow pigment
- high visual acuity
where is acuity the greatest?
center of macula (fovea)
physical properties of light
- electromagnetic radiation visible to eye
- visible light consists of wavelengths 400-700nm
cornea
- the site of most refractive power of the eye
- responsible for the majority of the refractive power of the eye (40D)
refraction of the cornea
- has refractive power of about 40 diopters
focal distance
distance from the refractive surface of the point where parallel light rays converge
greatest amount of refraction is at
the anterior surface of the cornea
corneal refractive power is
constant
lens
- 20 D of refractive power
- changes with accommodation
accommodation by the lens
- lens adds refractive power by changing the shape of the lens
- contraction of ciliary muscles releases tension in zonula
- lens becomes rounded
- greater curvature provides greater refraction
ciliary muscle is innervated by
parasympathetic fibers in CN III
amplitude of accommodation
- the range of accommodation decreases with age
- crystalline lens and ciliary muscles become less elastic
refractive errors
- ametropia
- myopia
- hyperopia
- presbyopia
ametropia
a refractive error is present
emmetropia
normal vision
myopia
near sightedness
hyperopia
far sightedness
correction of myopia and hyperopia
presbyopia
loss of accommodative ability of the lens resulting in difficulties with near tasks
where do the photoreceptor cells lie?
in the back of the retina
- light rays must pass through retina before reaching photopigment molecules to excite
the laminar organization of the retina
(furthest back toward front)
- pigment epithelium
- rods
- cones
- outer plexiform layer
- horizontal cells
- bipolar cells
- amacrine cells
- inner plexiform layer
- ganglion cells
- nerve fiber layer
retinal pigment epithelium
- plays critical roles in maintenance of photoreceptors and photopigments
- has melanin
- absorbs any light not captured bu the photoreceptors
- thus minimizes reflection of light that could blur image
retina: neural circuitry
- light hits photoreceptors
- sends signals to the bipolar cells
- bipolar cells send signal to ganglion cells
- ganglion cells send signal to the brain
photoreceptors include
rods and cones !!
rods
- 100 million
- light sensitive
- found in periphery of retina
- low activation threshold
- achromatic
cones
- 6 million
- are color sensitive
- found mostly in fovea
- high acuity
the outer segments of a rod or cone contain
different photopigments
cones work under _____ light.
rods work under _____ light.
bright, dim
photopigments of rods are
rhodopsin
rhodopsin…
absorbs light in a very narrow range
photopigments of cones are
cone opsins (iodopsins)
3 isoforms of cones include
- short WL
- medium WL
- long WL