Lesson 8: The Eye Flashcards
The Eye
Vision is the most dominant
human sense
Over 70% of all the receptors in the body are the photosensitive cells of the eyes.
About one third of all the fibers that carry impulses to the CNS come from the eye.
Parts of the Eye
Sclera – the fibrous “white of the eye”
- gives the eye shape
Cornea – the transparent layer at the front of the eye.
- helps focus light on the retina
Iris – the coloured part of the eye
- muscle tissue that controls the amount of light that enters the eye by opening and closing the pupil.
Pupil – the opening of the eye
-region where light enters the eye
Ciliary Body – region that contains ciliary muscles that controls eye movement
Choroid – contains the large capillary network that supplies blood to the retina
Lens – primary structure used to focus light on the retina.
- can change shape based on the distance of the object being viewed
Anterior Chamber – the space between the Iris and Cornea
- Contains fluid called the Aqueous Humor which provides support for the
Iris and Cornea.
Posterior Chamber – the space between the Lens, Ciliary body and the
Iris.
- contains Aqueous Humor which supports the Lens and the Iris
Vitreous Chamber – large gelatinous mass filled with a fluid called Vitreous Humor
- maintains the shape of the eye and supports the retina
Retina
Region of the eye where the image is formed
The retina converts the light image into an action potential that can be interpreted by the brain.
The retina contains 2 types of
photoreceptors that detect light.
Rods
Sensitive to all colours of
light but cannot distinguish
between colours
Active in dim light
Cones
Less sensitive to light
Provide colour vision
Require more light to
function
Fovea – the focal point of the eye
- contains cones only
Optic Disk – the area where the network of retinal cells converge
- known as the “blind spot” since it contains no photoreceptors
- connects the sensory neurons of the eye to the Optic Nerve
Optic Nerve – thick sensory nerve that carries nerve impulses from the eye to
the brain.
Eye Function
The iris controls the amount of light entering the eye.
When there is low light the
pupil will be opened to allow
more light in.
When there is too much light
the pupil will be constricted
to protect the retina.
The image is focused on the retina,
which is composed of three layers:
The ganglion cell layer
The bipolar cell layer
The rod and cone cell layer
The bipolar cells synapse with the rod and cone cells and transmit impulses to the ganglion cells
The ganglion cells join together and form the optic nerve as they exit the eye.
Rods and Cones
The retina is composed of approximately 150 million rods cells and 6 billion
cone cells.
Rods and cones function through the use of a purple pigment called
rhodopsin.
When light strikes rhodopsin, it breaks down into two proteins – retinal
and opsin. Opsin releases the energy that is required to stimulate the
bipolar cells.
Rods
Rods are sensitive to light and are therefore ideal for night vision, however,
they cannot distinguish colours, resulting in images that are made up of
shades of grey.
Many rods can be attached to a single bipolar cell which is why night vision is
blurry and indistinct.
Cones
Cones exist in three forms characterized by changes in the structure of opsin.
The three kinds of cones are sensitive to red, green or blue wavelengths of
light.
Each cone is connected to a separate bipolar cell, which allows the brain to
form a very detailed image from the information it receives.
Cones are concentrated in an area called the fovea centralis, which is located
directly behind the centre of the lens.
When doing something that requires fine detail, we move the object directly
in front of our eyes to focus the image onto the fovea.
Accommodation
The shape of the lens changes in response to your distance from the object being viewed.
If you are looking at something far away, the ciliary muscles relax which causes the suspensory
ligaments to tighten and the lens to flatten.
If you are looking at something up close, the ciliary muscles contract, releasing the tension on the suspensory ligaments and the
lens becomes more rounded.
Common Vision Disorders
Myopia – commonly known as
“nearsighted”
- have trouble seeing distant objects
- caused by the image being focused
in front of the retina. - is corrected by concave lenses
Hyperopia – commonly known as
“farsighted”
- have trouble seeing close objects
- caused by the image being focused
behind the retina - corrected by convex lenses.
Astigmatism
- have blurred vision
- caused by irregular lens or irregular
cornea which lead to an inability to
focus objects clearly on the retina. - some astigmatism can be corrected
by irregular lenses or surgery.
