00:00
The next sense is vision.
00:03
Vision uses visible light which is
part of the electromagnetic spectrum
with wavelengths from
about 400 to 700 nanometers.
00:14
This is referred to as the visible light spectrum
and corresponds to the colors of the rainbow
ROYGBIV or red, orange, yellow,
green, blue, indigo, and violet.
00:28
Taking a step back, wavelength
is defined as the distance between
two consecutive peaks of
an electromagnetic wave.
00:37
The shorter the wavelength, the
higher the electromagnetic energy
therefore violet light which has the shortest of
the wavelengths of our visible light spectrum,
admits more energy than red light
which has the longer wavelength.
00:56
So how does light actually pass through
the eye and send messages to our brain?
Light passes through the cornea, the anterior
chamber, the pupil, then to the posterior chamber,
then to the lens, then to the vitreous
humor and then is projected onto the retina.
01:21
In order to determine how
the light produces an image,
we must first understand how
light travels through different media.
01:30
Light refracts or bends when it
passes through a transparent substance
such as the different
humors found in the eye.
01:40
When it passes through a substance with one density
and to a second substance with a different density,
bending occurs at the junction
of these two substances.
01:54
Due to the refraction
of light through the eyes,
images focused on the retina are
inverted and then right to left reverse.
02:04
The brain is then responsible
for correcting the image.
02:09
Also, the lens must accomodate
to properly focus the objects.
02:15
Accommodation involves the changing of the shape
of the lens by the ciliary muscles attached to it.
02:24
An image is projected onto the fovea centralis,
the site where our vision is the sharpest.
02:33
In a normal emmetropic eye, light will refract
correctly and focus a clear image onto the retina.
02:43
However in some instances, the
shape of the eye leads to imperfect vision
for example, in myopia or nearsightedness,
the eyeball is shaped longer than it should be.
02:58
This causes an image to converge on the
front of the retina instead of onto the retina.
03:06
This allows for close
objects to be seen sharply
but as you move away,
distant objects become blurry.
03:15
We correct this with concave lenses which elongate
the pathway so that it reaches to the retina.
03:25
The opposite of myopia is hyperopia.
03:29
In hyperopia or farsightedness, the
eyeball is shaped shorter than it should be.
03:35
This causes an image to
converge behind the retina.
03:40
This allows for distant objects to be seen more
sharply than close objects which appear blurry.
03:48
We correct this with convex lenses which shorten
the pathway of light so that it hits the retina.
03:58
Next we have astigmatism.
04:00
Astigmatism is a condition
where either the cornea of the eye
or the lens of the eye or sometimes
both have an irregular curve.
04:10
An astigmatism is going to
cause blurred or distorted vision.
04:18
So now let's take a look at
the process of light transduction.
04:23
Recall that the retina is the
structure onto which light is projected.
04:29
The retina contain sensors
known as photoreceptors
and these photoreceptors
are known as rods and cones.
04:39
The rods are necessary for us to see in dim light
and our cones are necessary to produce color vision.
04:49
Once light hits the rods or cones, information
is going to flow through the outer synaptic layer
to the bipolar cells through the
intersynaptic layer to the ganglion cells.
05:02
From there, axons of the ganglion cells bundle together
and exit out of the back of the eye as the optic nerve.
05:11
Notice that the nerve impulse goes in the
opposite direction of the direction of light.
05:22
Rods and cones, the photoreceptor of the retina are
going to convert light energy into neural impulses
Rods and cones get their name because
of their appearance of their outer segments.
05:38
Rods and cones contains photopigments
which are necessary for the absorption of light
and that it will initiate events that leads
to the production of a receptor potential.
05:50
Rods contain only the
photo pigment rhodopsin.
05:55
Cones contain three different photopigments,
one for each of the three types of cones.
06:02
We have red cones, green cones and blue
cones and each has a different photopigment.
06:10
The photopigments are going to
respond to light in a cyclical process.
06:16
So let's take a look at
the process in our rods.
06:20
First, a photon of light is going
to bind to the rhodopsin in the rod.
06:27
This causes the conversion of the molecules
cis-retinal to be transformed to trans-retinal.
06:34
This is referred to as
the isomerization step.
06:38
Next, the trans-retinal will detach from the
the opsin molecule and lead to photo bleaching.
06:47
During this time, the enzyme
retinoisomerase in the retina
is going to convert the dissociated
trans-retinal back into cis-retinal.
06:59
From here, the cis-retinal will
attach to the bleached opsin
in a regeneration step and
the cycle can start all over again.
07:12
So with our vision, we have to sometimes
adapt to changes in the amount of light.
07:18
Light adaptation takes place when an
individual moves from a dark surrounding
to light ones such as
exiting out of a movie theater.
07:28
This process occurs in
seconds as sensitivity decreases.
07:35
Dark adaptation is the opposite.
07:38
In this process, we're going to be
moving from a lighted area into a darker one
such as driving along the
highway and then entering a tunnel.
07:48
The process of dark adaptation
takes minutes to complete
as it takes a moment to
increase the sensitivity to light.
07:57
This is because rhodopsin in rods generates much
slower than the photo pigments found in our cones.
08:06
So how does photo transduction
occur at the photoreceptor?
This is best explained by starting with
the events that happened in the darkness.
08:17
In darkness, cis-retinal is
associated with the photopigment.
08:23
Also, cyclic GMP, which is a
molecule very similar to cyclic AMP
but contains guanine instead of
adenine, is present in high concentrations.
08:35
Cyclic-GMP binds to and opens cation channels
allowing sodium to flow into these cells.
08:45
Sodium flowing in causes the
depolarization and this depolarization
causes the opening of
voltage-gated calcium channels.
08:56
This then leads to the release
of the neurotransmitter glutamate.
09:01
Glutamate inhibits the bipolar cells from
transmitting signals to the ganglion cells
which provide output
from the retina to the brain.
09:13
So in the darkness, this
signal is being blocked.
09:19
In the light, this inhibitory effect is
blocked when the isomerization of cis-retinal
also causes the breakdown of cyclic GMP.
09:31
This now leads to a hyperpolarization that stops the
release of the inhibitory neurotransmitter glutamate
and now allows for the bipolar cells to
be activated and form an action potential.
09:46
From there, it sends signals to the ganglion
cells which then sends signals to the brain.
09:54
The neural pathway for vision begins
when the rods and cones convert light energy
into neural signals that are
diorected to the optic nerves.
10:07
From the optic nerve, exiting out of the optic
disc of the eyeball at the back of the retina,
we find the optic chiasm
which is like an x-shape.
10:19
From there, the pathway or the stimulus
is going to travel down the optic tract
to the lateral geniculate
nuclei of the thalamus.
10:30
Here, optic radiations are going to allow information
to arrive at the primary visual cortex area
of the occipital lobe of the cerebral
cortex for perception of what you're viewing.
10:46
Because of where our eyes are located
which is on the interior surface of our bodies
or in the front of our bodies, our visual
fields are going to overlap with each other.
10:58
This gives us binocular vision.
11:01
To give you some contexts, horses'
eyes are on the lateral portions of their body.
11:08
Therefore, what they see out of one
eye can not be seen out of the other eye.
11:15
The two visual fields of each eyes are the nasal
or medial field, and the temporal or lateral fields.
11:24
Visual information from the right half of each visual
field will travel to the left side of the brain.
11:33
Visual information from the left half of each visual
field will travel to the right side of the brain.