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Samenvatting - Integrative neuroscience (WBBY006-05)
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Summary of the lectures for the course Integrative Neuroscience for the bachelor Biology at the Rug.
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Integrative neuroscience SummaryLecture 1 The eye
Characteris cs of visible light. The electromagne c spectrum: only electromagne c radia on with
wavelengths of 400-700 nm is visible to the naked human eye. The part of the electromagne c
spectrum that captures the visual light is very small. Within this visible spectrum, different
wavelengths appear as different colours. The higher energy forms of life have a shorter wavelength,
blue light. The lower energy forms of life have a longer wavelength, red light. Colour percep on
doesn’t say anything about the warmth or energy of the wavelength. Colour is a percep on of the
brain. Colour percep on is subjec ve, so what someone sees as blue might look different for
someone else.
The eye consists of the pupil with around it the iris. The white part of the eye is the
sclera. The cornea is the transparent liquid layer
forming the front of the eye. Behind the pupil is the
lens, which has a round shape. There are muscles
that can alter the shape of the lens. Light travels
through the lens and hits the re na at the back of
the eye. Inside the re na are photoreceptors. The
light needs to be reflected from something in the
environment. The reflec on that enters the eye is
what is seen (perceived). The projec ons on the eye
need to be sharp.
The brain fills in informa on of things that are seen (or not). The op c disc is the place in the re na
where blood vessels enter and where the neurons from the ganglion cells leave the re na. The op c
disc doesn’t contain photoreceptor cells, so it is a blind spot. There is a trick to see the blind spot with
a plus and a black dot. Close the right eye, look at the plus, and move focus of the
le eye slowly towards the direc on of the spot (or further away). At a certain
moment, the black dot isn’t visible anymore.
The lens is important for ge ng a sharp image on the re na. The lens is a
converging type of lens. A bundle of light hits the eye. The rays of light enter the
eye in a parallel fashion. The medium of the lens is thicker than air, so the light
waves are bend inwards. They are bend further inwards to create a focal point
inside the eye. Light in a thicker medium will be refracted in the direc on of a line
perpendicular to the surface. The focal point is where all the refracted light waves
come together. Incident rays which travel parallel to the principal axis will refract
through the lens and converge to a point. Op mally the focal point is on the re na.
,An example of looking at a flower. Each ray of light from
the flower passes through the eye. The image is presented
upside down on the re na. The lens makes sure that all
the rays of the flower are sharp. When looking at
something far away, the lens doesn’t need to be as round.
When looking at something near, the lens needs to be
rounder as it needs to bend the light waves stronger than
when looking far away. When someone is far-sighted, the
lens does not get as round as it should. In this case the
focal point is behind the re na, which is called hyperopia.
When someone is near-sighted the eye is a li le bit longer
than it should be. In this case, the focal point is before the re na, which is called myopia. This can be
corrected by a lens that is the opposite.
When looking at a bright light the pupil constricts to limit the light entering the eye. A by light-reflex
is called the consensual reflex. The brainstem is involved in this as both eyes will constrict when one
eye is hit with a high intensity light. There can also be pupillary construc on to deepen the depth of
the field, aperture on camera. Each eye has a certain visual field. This can be determined with the
object-angle. The right eye has a different object-angle on the right side and on the le side, because
the nose is in between. The visual acuity or sharpness of the image is limited by the distance between
photoreceptor cells. The photoreceptor cells are stacked in the re na and they have a certain
dimension. The resolu on is dependent on how ghtly the photoreceptor cells are stacked on top of
each other. Example: the stars are bright, but some stars may be so small that they will fall in
between the photoreceptor cells. There is a limit of what can be seen based on the size and
how stacked the photoreceptor cells are.
Laminar organiza on of the re na by neuroanatomist Ramon Y Cajal. In 1880, parts of the re na
were already drawn. The re na with photoreceptor cells are located on the outside. Other
photoreceptor cells are located more towards the inner part of the re na. The most inner cells of the
re na are the ganglion cells. The photoreceptor cells are located in the vicinity of the epithelium. The
pigment of the re na allows the light to be reflected again. Light travels twice through the
photoreceptor layer, once from the outside and once when it is reflected back. On top of the
photoreceptor cells are the bipolar cells. These are intermediate between the photoreceptor cells
and the ganglion cells. The ganglion cells are the ones that travel the output from the photoreceptor
cells to the brain. There are also cells that can regulate the photoreceptor cells.
There are different cell types in the eye. The ganglion cells are the only
ones providing output of the re na. Amacrine cells are mul polar re nal
neurons branching within the inner plexiform layer of the re na to collect
and decode bipolar cell signals. Bipolar cells provide the main pathways
from photoreceptors to ganglion cells. Horizontal cells are the laterally
interconnec ng neurons having cell bodies in the inner nuclear layer of the
,re na of vertebrate eyes. They help to integrate and regulate the input from mul ple
photoreceptor cells. Light-sensi ve cells in the re na are photoreceptor cells.
There are two different types of photoreceptor cells: rods and cones. The cone photoreceptor
cells can have three types of pigment. Cone cells are ac ve under “photopic” or light condi ons.
The rod photoreceptor is 20 mes more sensi ve than the cone photoreceptor (in the human
re na). Rod cells only have one type of pigment. Rods are 1000 mes more sensi ve than cones.
The rods contain far more stacked layers of pigment, so they are way more sensi ve to the light
than cones. Rod cells are ac ve under “scotopic” or dark/dim light condi ons, cones are then
not working as there is too li le light. This is why people don’t see colour at night. The re na is a
“duplex”.
There is a place on the re na that allows to really focus on something and see in great detail.
This is some sort of sweet spot of the re na and is called the fovea. The ganglion cell layer and the
inner nuclear layer are displaced laterally to allow light to strike the foveal photoreceptors directly. At
the fovea, light doesn’t have to pass through cell layers. This part of the re na does only contain
cones, so it is used under daylight condi ons. When it gets dimmer, this part of the re na is not used
much anymore. Under dim light condi ons a person’s sharpness (or acuity) also decreases.
The fovea is the place in the eye where the most cones are. The number of cones declines very
quickly away from this spot, and the number of rods increases. Photoreceptor cells are almost
coupled one to one to ganglion cells in the central re na. Towards the peripheral side, mul ple
photoreceptor cells are coupled to a ganglion cell. Therefore, the resolu on decreases at the
peripheral re na.
How photorecep on works. Under “res ng” condi ons neurons have a membrane poten al of -65
mV and rods have a membrane poten al -30 mV. The neurotransmi er hits a receptor on the cell
membrane, which causes a conforma onal change. This can alter a G protein. This change alters a
second messenger system, which can open ion channels. Then there this a depolariza on.
Depolariza on in rods is caused by influx of sodium (Na+). This is how a normal neuron works. In the
case of light it is actually the opposite. Opsin has a molecule called re nol in its core. This can change
conforma on when hit by light. Normally re nol has a bend tail, but when hit by light it has a straight
tail. This change closes the ion channel, so instead of membrane depolariza on, there is
hyperpolariza on. Now, the ac on poten al will stop. “Dark current” is the inward flow of sodium
, ions into the photoreceptor while an individual is in darkness.
The hyperpolariza on of photoreceptors in
response to light. Photoreceptors are
con nuously depolarized in the dark because of
an inward sodium current, the dark current.
Under dark condi ons glutamate is being
released. Under light condi ons this stops. Sodium enters the photoreceptor through a cGMP-gated
channel. cGMP keeps the channel open, so sodium can enter the cell under dark condi ons. Light
leads to the ac va on of an enzyme that destroys cGMP, thereby shu ng off the Na1 current and
hyperpolarizing the cell. But under light condi ons cGMP is converted to GMP, so the ion channel is
closed. Opsin consists of seven alpha helices that can expand the membrane. The conforma on of
re nal changes when light bleaches rhodopsin. Transducin is the G protein that is coupled to the
receptor.
The light-ac vated biochemical cascade in a photoreceptor. Under dark condi ons, cGMP is bound to
the ion channel so sodium can enter. In the dark, cGMP gates a sodium channel, causing an inward
Na1 current and depolariza on of the cell. Under light condi ons, cGMP is converted to GMP, so the
levels of the cGMP decrease and the sodium channel is closed. The ac va on of rhodopsin by light
energy causes the G-protein (transducin) to exchange GDP to GTP, which in turn ac vates the enzyme
phosphodiesterase (PDE). Already one photon hi ng the re nol can lead to this cascade under light
condi ons. This is because there is signal amplifica on. PDE breaks down cGMP and shuts off the
dark current.
Structure of cGMP. The intracellular messenger of phototransduc on is 5’-cyclic guanosine
monophosphate (cGMP). Upon illumina on the ac vated phosphodiesterase (PDE) hydrilyzes cGMP
to GMP by cleaving the cyclic phosphate bond. Guanylyl cyclase does the opposite of PDE.
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