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Summary of ALL lectures Neuronal Networks and Behavior (AB_1051)
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This summary contains all lecture notes of the course NN&B from the Minor Biomolecular Sciences and Neuroscience
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NN&B summary & lecture notes 2022Neuronal Networks and Behavior Lecture Notes
Lecture 2: Vision 1
Photoreceptors are cones and rods
Optic discs are removed and processed by pigment epithelium of retina, takes about 12 days
for most inner discs to be processed.
Rods and cones difference: sensitivity to light, range of luminance, if no light=absolute
threshold where we start to see the light for rods, very sensitive to light, scotopic, no color
vision, rods are no sensitive to color
When more light, cones become sensitive, mesopic vision, rods and cones activated at same
time, we can see color here
Indoor lighting; rods get saturated and cones become the main processing factor of our
vision
Fovea: the place that processes most of visual information, distribution of cones and rods in
retina: the cones are low in peripheral retina but high in fovea, rods are mostly present in
peripheral parts
Cones and rods differ also in connectivity to bipolar cells, many rods can connect to 1 bipolar
cell, large parts of the visual field will be processed by 1 bipolar cell, so less detail, 1 cone
connects to 1 bipolar cell, so more detail and resolution
Rods and cones respond to light with graded hyperpolarization, the cells do not respond with
excitation. The higher the flesh of light the greater the hyperpolarization
Phototransduction: photoreceptors respond to light, the membranous discs of the
photoreceptor changes and a cellular cascade starts. But light is not always important in the
visual system, in the absence of light calcium channels in the outer membrane of the
photoreceptors are always open due to cGMP, sodium ions are then let inside the cells and
generate positive currents, so in absence of light there is depolarization. Potassium also
flows outside the cell. If light is present, it hits the photoreceptors the light is absorbed and
the receptor changes its conformation; a cascade starts: cGMP breaks down and the
channels close; hyperpolarization. cGMP is the messenger, it decreases in presence of light.
The channels close in the presence of light.
Why is this system so complicated? This process is very quick, there is no delay, the
advantage of this cascade is the amplification of the signal, one photon can cause closure of
200 channels in the membrane, so rods are very sensitive to light. The whole
phototransduction process holds place for the cones.
There are molecular ways to change the
sensitivity to light. If light is present calcium is
decreased, this plays a key role in adaption to
sensitivity of photoreceptors to light. Light
adaptation is thus a calcium dependent
mechanism.
Rod and cones differ in light response and
connectivity, the response of rods is longer and
stronger because they are more sensitive, cones
,NN&B summary & lecture notes 2022
are short in response. Rods respond with a long hyperpolarization and cones with a short
one. Cone have a one to one connection to bipolar cells and have sharp and short
responses, we view color via this ezebi
How do we have color vision? Light is electromagnetic radiation and it has wavelengths,
what happens in the retina is that we have cones that are sensitive to certain wavelengths of
light, different cones. 3 kinds: s cones, m cones and l cones. S cones stand for short, m for
medium and l for long wavelengths. The long cones are more sensitive to red light, m to
green and s to blue.
S cones are 5-10 precent of all cones and absent in fovea, the ratio of M to L cones differ
largely from individual to individual but this has no effect on color perception. Can cause
diseases, color blindness (partial); dichromacy, in which M or L cones are absent, no red-
green color distinction.
Achromatopsia: no cones, only rods to distinguish color, they see just shades of grey
Color perception starts in the retina but we need other mechanisms also to perceive color,
such as the role of context, if two objects with the same color are put in sunlight and in shade
you see 2 different colors.
Combined activation of different cones and higher-level processing in the brain allow us to
see multiple colors.
Retinal circuitry: the visual field can be split into a very small receptive field; one pixel that
can be processed by one cone. The receptive field can be split into 2 parts, center and
surround, different cells with same receptive field will respond differently to the center and
surround. How can the cell distinguish this? On center and off center ganglion cells detect
contrasts. Cells that respond only when light is in center of receptive field. Then on center
ganglion cell will fire an action potential. When there is light falling on the center the off
center ganglion cell will shut down, no response at all and vice versa. When light falls on
surround the on center gets active, when the light increases its activation decreases. The off
center ganglion cell fires when there is more light on surround, thus if the whole receptive
field is illuminated. This way we can detect contrast.
One cone connects to either on center bipolar cell or off center bipolar cell, and then it
connects to either on center ganglion or off center ganglion cell.
One light spot in center: center cones respond with hyperpolarization, on center bipolar cell
gets depolarized (change of sign), uses Mglur6, off center remains hyperpolarized, uses
AMPA kainate. On center bipolar cell is depolarized and fires action potential on on-center
ganglion cell, using glutamate, off center bipolar cells that are hyperpolarized do not fire any
action potentials.
Dark spot in center: center cone respond with depolarization, on center bipolar cell
hyperpolarizes and off center depolarizes, on center ganglion cells hyperpolarize and do not
fire, they get silenced, off center ganglion cells get depolarized
Horizontal cells: responsible for center-surround antagonism. Cells between bipolar cells and
cones: amacrine cells, they connect the layers with each other, these cells help to detect the
contrast, these cells are GABAergic cells, when light falls on center the horizontal cells
release inhibitory transmitter that maintain the hyperpolarization. If there is more light, more
horizontal cells are activated and more hyperpolarized, they release less GABA and center
cones are not hyperpolarized but depolarized. Off center cones influence signal in on center
cones through inhibition by horizontal cells.
,NN&B summary & lecture notes 2022
For the visual system:
Receptors are sensitive to photons, light is the stimulus, photons bind to receptors, receptors
changes conformation, signal cascade starts (hyper of depo etc)= modality
Where the light comes from is important to retina, the light where it falls (on and off center)=
location
Intensity is also important, if light increases hyperpolarization increases= intensity
Lecture 3: Vision 2
Visual system; neurons in retina will send axons through optic nerve, goes to hypothalamus
and then info go through optic radiation to visual cortex
Central visual pathway; parallel processing; multiple pathways from retina to brain
1. Main one: info that we see as images; info to lateral geniculate nucleus and then to visual
cortex
2. Pathway to hypothalamus; hypothalamus is regulating hormones, one of such regulation
of circadian rhythm, day and night rhythm, visual info goes to hypothalamus and this
regulates circadian rhythm based on that visual information
You need exposure to bright light into a blue spectrum, this light acts on M ganglion cells in
retina containing melanopsin (most sensitive to blue light), these are light sensitive
photoreceptors, when they are activated they project to hypothalamus, the
pathway/projection arrives in the SCN, this nucleus regulates the body clock
Disturbance of the circadian clock is that we expose ourselves to blue light at night due to
screens and phones, melatonin is then decreased, blood sugar level increase, leptin goes
down; more prone to feel hungry
3 & 4. 2 more pathway that process visual information; pathway going to pretectum
regulating reflex that controls pupil constriction and dilation. During bright light pupil becomes
smaller, in dark pupil dilates and more light enters, regulated by pretectum, controls the
muscles that cause constriction or dilation of the pupil. Last pathway: pathway going to
superior colliculus; regulates the movement of head and eye and
coordination of those
The lens of the eye project an inverted and left-right reversed image on
the retina, the image is upside down, upper field from both eyes is in
lower part of retina, nasal part of visual field is projected to temporal
part of retina, so right-left reversed
Visual field is projected like the film of a camera, left visual field falls on
nasal retina and right visual field falls on temporal retina. The fibers
cross over; all information that arrives in left part of the body will be
projected in the right part of the brain; happens due to optic chiasm,
right visual field will cross over in right eye but in left eye not, partial
crossing over in this way you have that everything is right visual field
projected to left hemisphere and vice versa = contralateral
How is information represented in the cortex? Info coming from upper field goes to lower part
of visual cortex, info from lower visual field goes to upper part of visual cortex. Here also
, NN&B summary & lecture notes 2022
upside down. The visual cortex is divided by calcarine sulcus, everything above is lower
visual field and vice versa
Main features retinotopic representation visual field:
The information then goes from the visual field to the LGN.
The LGN is the first principal subcortical site for processing visual info; info from left and right
eyes are left separate in different layers, also in visual cortex it is kept separate, it arrives in
different columns of the cortex. Why?: its important for seeing depth
The LGN has 6 layers, cells of these layers have monocular input from on eye, the layers
alternate the input from each of the eyes, the top 4 layers (3456) are called parvocellular
(info on shape and size of object) and the bottom 2 (1,2) are magnocellular (info on
movement and motion).
The 6 layers are there for 3 layers of information.
The retina has M cells (for magnocellular layers), P cells (for parvocellular layers) and K cells
(for koniocellular layers, in between layers, specialized for short-wavelength light (blue)).
These are ganglion cells. Collect info from cones and rods. P cells from cones. M cells from
rods. K cells from blue-sensitive cones. These layers project to different sublayers in the
cortex. Again kept separate.
M cells receptive fields are very big, collect a lot of info and cover big receptive field. P cells
have smaller receptive field, specialized in processing color so they are sensitive to specific
light, not all light. If different light is shined on the cells than it what they are sensitive for the
cells shut off.
Primary visual cortex= V1
Visual cortex has retinotopic maps of the visual space
Cortical magnification
All cortical areas have 6 layers with different cells, the visual cortex, input comes mainly in
layer 4, layer 5 and 6 are the output layers, project to other areas
What kind of cells process the info in visual cortex? simple cells, in visual cortex simple
cells get combined and form a receptive field
Information in retina and LGN is very simple and simple receptive field, in visual cortex the
receptive field is more complex
How is color processed? Blobs, sensitive for color and process color
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