lecture 25 - nervous system 5: vision and the eye Flashcards
what is the eye made up of
cornea iris pupil lens retina
what does the cornea do?
responsible for refracting light to bring about focus on the retina
responsable for most of the refractive power of the eye
what does the iris do?
pigment determines eye colour
muscles controlled by ANS
• control pupil diameter according to light and emotional signals
what does the pupil do?
opening in the centre of the iris that allows light entry
2-8mm range
what does the lens do?
responsible for accommodation - adjusting the refractive properties of the eye
what does the retina do?
layer at the back of the eye
photoreceptors, horizontal cells, bipolar cells, amacrine cells, ganglion cells and nerve fibres
part of the brain
where does laser treatment act?
on the cornea
what do photoreceptors on the retina do?
convert light energy into electrical signals which go to the brain
what is the anterior chamber of the eye filled with?
aqueous humour
what is the vitreous chamber of the eye and what is it filled with?
maintains shape of the eye
filled with vitreous humour
what is the optic disk?
where the nerve fibres and optic nerves leave the eye
what is the visual field of each eye?
covers 150 degrees
fields of view overlap
image is laterally inverted in the eye
limited by the nose
what is accommodation?
the ability of the eye to adjust its focal length for different object distances
lens brings light rays into focus at the focal length at a particular position on the retina
accommodation: what happens if the object is close?
lens isn’t powerful enough to bring object into focus
image will form behind the retina so will be out of focus
you need a stronger lens
• rounder lens gives a stronger refractive power
• brings light into focus on the retina
how does the lens accommodate?
it is in a capsule which is attached to the ciliary muscles by the zonulas
how do the ciliary muscles allow accommodation?
contraction of the ciliary muscles allows zonulas of zinn to slacken, lens expands and becomes more rounded
when relaxed the zonulas are pulled tight and the lens is pulled into a flat configuration
what is the retina made of?
fovea
blind spot or optic disk
blood vessels
what is the fovea?
pit in the centre of the macula in the retina
area of maximum acuity (highest image resolution) - highest density of cone receptors
what is the blind spot / optic disk?
region where the nerve fibres and blood vessels leave the eye
blind in this region
no photoreceptors here
brain fills in the missing area
what do the blood vessels in the retina do?
deliver oxygen and nutrients to the retina
organisation of the retina
pigment epithelium
2 types of photoreceptors
other cells connecting vertically and horizontally
what is the pigment epithelium?
cells at the very back of the retina
photoreceptors embedded in this layer
cells contain melanin black pigment
absorbs light to prevent scattering stray light that would affect the image
what are the 2 types of photoreceptors?
rods
• scotopic vision
• low light levels
cons
• photopic vision
• high light levels
more rods than cones
what are the other cells connecting vertically and horizontally?
bipolar cells
horizontal cells
amacrine cells
ganglion cells
cones and rods attached to bipolar cells
what does light have to pass through to reach photoreceptors?
blood vessels and nerve fibres
except at the fovea for maximum acuity
what happens at the fovea?
fovea cells in upper layers of the retina are pushed aside to allow light direct access to photoreceptors
features of rods
- scotopic vision
- most sensitive to light
- sensitive to brightness
- only detect brightness
- function at low light levels
- bleached at high light levels
features of cones
- photopic vision
- 3 types sensitive to long, medium and short wavelengths
- enable colour vision
- responsible for high acuity (sharpness)
- work only at high light levels
what is dark adaptation?
at high light levels rods bleached so rely on cone vision
when switched from bright to low light levels its initially hard to see - cones not sensitive and rods still bleached from light before
gradually visual sensitivity increases ass rods recover - pigment in the rods gradually builds up
this is called dark adaptation
distribution of rods and cones
cones
• low density in retina
• high density in fovea
• not at the blind spot
rods
• high density in retina
• not in the fovea
• not at the blind spot
where there aren’t cones there are rods
what are the 3 cone types?
short wavelength
medium wavelength
long wavelength
each cone is sensitive to a range of wavelengths
what is the trichromatic theory of colour vision?
colour is represented by the unique ratio of outputs from the 3 different cones
what happens when theres an absence of a cone type?
colour confusion or colour blindness
light sensitive photopigments in rods and cones
rods - rhodopsin
cones - 3 different pigments (but wavelength sensitive)
what is rhodopsin?
the light sensitive photopigment in rods
made of 2 chemicals
1) opsin (protein)
2) 11-CIS-retinal (derived from vitamin A)
in the dark opsin and retinal are bound together
on light exposure, retinal molecule changes shape and splits from opsin
this process is called bleaching
where are retinal and opsonin found?
in the outer segment of photoreceptors
rhodopsin are embedded in the membranes of the membraneous disks
rods in the dark vs in the light
Rods contain channels for Na+ and K+
In dark:
• cyclic GMP levels high, keeps Na+ channel open
• Na+ current into rod > than K+ current out
• rod depolarised to – 40mV & transmitter released
• constant release of neurotransmitter in dark
Light bleaching of rhodopsin activates transducin (G protein):
• reduces level of cGMP so Na+ channels close
• less inward current (fewer positive ions flow in)
• K+ continues to flow out
• rod hyperpolarises to -70 mV
• less transmitter release
light hyperpolarises the cell and reduces transmitter release (not what you might expect)
