mnsr 31 Flashcards
eyes have developed —- at least — times in —-
independently, 3, metoza as arthropods, mollelus, and vertebrates
image formation requires a
lens - the eye
the independent development of the eye is an example of
convergent evolution
the eye converts
light energy into electrical signals using photoreceptors cells
- eye placement , binocular vision
- wider field of view
- distance and depth perception
for horizontal movements we use
medial and lateral rectus muscles
for vertical movements we use
posterior and anterior rectus muscles and superior and inferior oblique
eye movement can be — when we —- or can be —- by maintaining focus while moving our head aka vestibular ocular reflex
conscious, focus, unconscious
the tunic layers of the eyes
1- fibrous layer: scalera and cornea
2- vascular layer:iris, choriod, ciliary body
3- retina layer: pigmentent layer, natural layer
the outermost layer of the eyes composed of dense connective avascular tissue
fibrous layer
the function and properties of the scalera
- white, opaque layer
- provides strength and protection
- has an attachment point for extra ocular muscles
the function of the cornea
- transparent layer
- 2/3 of the focusing power allowing the light to enter the inner eye
- the corner can be reshaped by laser or transplanted from donor patient,
the vascular/ uvea layer consist of
- lens
- iris
- choroid
- cillary body and ligaments
the function of the choroid
- supplies nutrients and 02 to the retina
- layer of blood vessels
- contains melanin , reduces reflection of light
the function of the ciliary body
- ring shaped structure
- consist of: radial ciliary muscles and cirucllary mucles
- suspensory ligaments and zonule filaments
the lens has
- transparent and consist of:
- inner epithelial layer
-lens fiber cells - lens corspule - basement membrane
- it separates the aqueous humour from the front and the vitreous humour behind
the function of the iris
- circular ring structure
- has radial pupillary dilators muscles
- circular pupillary sphincters
in bright light, the pupil constricts by
cirucallry muscle contract in the parasynpthatic
in dim light, the pupil dilates by
radial muscles constricts in sympathetic
nervous/sensory tunic layer - retina layer consist of
- inner epithelial pigmented layer w/ melanin
- natural layer w/ inner light sensitive consist of photoreceptors( modified nerve cells to detect light), bipolar cells and retinal ganglion cells
—- consist of large concentration of the cones and the largest density at —-
macula , fovea
for high resolution, color and central vision
damage in the macula is
macular degeneration
optic disk consist of
optic nerve and blood vessels
— is an area w no photoreceptors
blindspot
rods and cones are in contact w —— that provides —-
pigmented epithelium, retinal for rhodopsin
both rods and cones contain numerous —- of the plasma membrane and they contain —-
discs/ foldings, contains photopigment and the discs increase the light absorbing surface area
— is back to front in relation to light entering the eye
retina
- the photorecptors face away from the incoming light
the retina has five main types of neurons
- ganglion
- amacrine
- horizontal
- bipolar
- photoreceptors ( rods and cons )
rods, cones, bipolar and ganglion release —-
horizontal and amacrine release —-
glutamate ( excitatory )
GABA ( inhibitory )
the fucntions of amacrine and horizontal cells
both are lateral interneurons and both help in the process of visual input in the retina
- HCs: connection between photoreceptors and bipolar cells
- ACs: connection between bipolar and ganglion cells
ganglion cells receive impulse for
- brightness, motion, color
- they are sensitive to light
- don’t play a role in vision BUT it can control the pupil size and synchornizng the circadian rhythms
are repsonsible for the photooptic vision
cones cells
activated in bright light, good visual details, is color sensitive
cones cells
nocturnal animals contain mainly
rods
responsible for high sensitivity but poor discrimination - scotopic vision
rods
rods work best in — illuminatuin and is not functional in bright light bc - photo-sensitive pigment is degraded by bright light.
low illumination
rods need time to adapt aka the dark adaptation which takes
30-60 minutes
The visual pigment of the rods is rhodopsin
Consists of:
Opsin protein
Cofactor 11-cis-retinal
11-cis-retinal is derived from
11-trans-retinol and vitamin A
11-cis-retinal is maximally sensitive to
blue-green in the 500nm wavelengths and does NOT react to red light
Cis-retinal rests within a pocket in the
opsin protein
A deficiency of vitamin A leads to —– and eventual deterioration of the outer segments of the rods
night blindness
explain the visual cycle
Light isomerises the 11-cis-retinal to 11-trans-retinal
Induces conformational change in the protein structure to generate activated rhodopsin.
Activated rhodopsin activates other signalling proteins, resulting in generation of a signal.
Activated rhodopsin quickly breaks down into the protein opsin (scotopsin in rods), releasing 11-trans-retinal
Later the 11-trans-retinal is reconverted to 11-cis-retinal by an enzyme
Rhodopsin very sensitive - a single photon of light energy can excite a single rod cell
Approx. 5 rods need to be excited before a flash of light is seen
There are about 125 million rods in each retina mostly away from the fovea.
Dark adapted, indirect gaze in low light can discern objects in shades of grey or silver. Light level below the threshold of colour sensitivity
the summation effect is the
In the fovea the ratio of cones to ganglion cells is almost 1 : 1
High visual acuity
Peripheral areas of the retina may have some 300 rods / optic fibre
This explains the greater sensitivity in these areas to weak light because of the summation effect
no. of cones and rods
6 million, 120 million
- we have 900,000 optic nerve axons
- 140 rods per 1 ganglion cells
- 6 cones per 1 ganglion cell
The three types of cones
-Blue sensitive (max 430nm),
-Green sensitive (max 530nm)
-Red sensitive (max 562nm)
-Each contains a slightly different opsin - maximally sensitive to different wavelengths of light.
Visible spectrum in humans is between
380nm and 760 nm
most primates are — most mammals expect primates are — vertebrates as birds/reptiles/fish/amphilia have —
- trichromatic
- dichromatic
- 4 classes of cones
color blindess genes encoding the —- and is linked to —-
green and red sensitive (commonest type ), X chromosomes
- 1:12 males
-1:200 females
the opening of the iris ( the coloured part of the eye) is called
the pupil
