NATS 1860 Note 13
Basics of vision:
- Synapses conclusion
- Photoreceptors (rods and cones)
- Colour vision and colour blindness
- Retina neurons
- Lateral inhibition and edges
- Projection to visual cortex
- Neurons in visual cortex
- Neurons and motion
Review of Neural Communication
- Spike (Action Potential) there is a threshold amount of depolarization that is
o Above polar levels causes Sodium to flow into the cell and depolarize
o Lower polar levels do nothing.
o Potassium then flows out and the cell hyperpolarizes
o Neurons signal using spike rate: up to 300spikes a second
o This is how cells signal
- If a spike travels down axon jumping from gap to gap in the myelin (it’s
absence causes multiple sclerosis) at up to 100m/sec
- It arrives at the synapses
- Calcium enters and triggers neurotransmitter release
- Neurotransmitter opens ion channels in dendrite
- Ions flow into pout of postsynaptic cell changing its potential.
- Glutamate (GLU) is a neurotransmitter
- Opens Sodium ion channels in post-synaptic neuron
- Sodium ions enter and depolarize new ion
- It’s called EPSP = Excitatory post synaptic potential THIS WILL BE ON THE
o NO SPIKES ARE generated at the synapse, they’re generated at the cell
- GABA = Gamma acid. It’s the major inhibitory transmitter
o It opens Potassium channels in the post-synaptic neuron
o Potassium opens the ion charges and allowing them to exit and
hyperpolarize the neuron
o They’re called Inhibitory Post-Synaptic Potential = IPSP
o If all cells fire at once, you may have an epilepsy
- There must be a balance between EPSP and IPSP EPSP and IPSP Interaction
- There is potentials spreading to soma and combining there.
- The two synapses that have IPSP and EPSP produce slowly move down to the
body and they combine. All the EPSPs (1000 synapses, 700 EPSP and 300
- EPSP depolarization add
- IPSPs hyperpolarization subtract from the EPSPs because they try to make
the cell more negative.
- If it’s not enough to depolarize, nothing happens, if it’ depolarizes to
threshold, there is a spike
o These must occur simultaneously for the interaction to take place
o The spike itself is actually triggered if the Axon
The dendrites combine all the information from thousands
nerve cells for stimulating and inhibiting. That combination
determines whether the cell fires the spike.
Drugs and the Brain
- In every case where a drug has effects on our thought processes, it is active at
- It releases at least 30 different neurotransmitters in the brain.
o GLU and GABA are the two most popular and important ones
o Each area in our brain uses Glutamate as primary source of excitation
and GABA as the primary source of inhibition, and several others.
- A drug that effects on only one neurotransmitters has a specific effect on one
part of the brain and not on the other
- All psychoactive drugs affect synapses
o In moderate amount, these drugs do not kill you but may alter your
- Cocaine: increase the effect of excitation in pleasure areas of the brain.
- Opium and Morphine are mistaken by the brain for a natural
neurotransmitter that blocks pain
o It has very similar characteristics to this natural neurotransmitter
Runner’s high feel euphoric because its due to the release of a
natural version of these chemicals in the brain
They’re called endorphins and encapholins
- Valium (anti-anxiety and anti-panic) increases GABA hyperpolarization
o It tends to lower the level of activity in the brain. The implication is
that when you’re feeling anxious your brain is over active, over
excited in areas that involve unhappiness and fear
By increasing inhibition you can damp that down.
Dopamine and Parkinsonism
- Dopamine is a neurotramsitter that is produced in the center of the skull
where ther are specialized neurons that produce and transport it all over the
brain - If an individual produces too little dopamine they develop Parkinson’s where
they can’t move, shuffle, and have cognitive consequences
- The treatment for this is drugs to help brain produce more dopamine
(chemical precursors of dopamine that the brain uses to make more)
o These can alleviate the symptoms of Parkinson’s
- You can have electrodes implanted in the dopamine locations in the brain to
produce more dopamine.
o Stem cells can also be put into the areas that produce dopamine (it’s
- If someone takes too much of these drugs that produce more dopamine, it
creates schizophrenia, especially in paranoid schizophrenia.
o Since dopamine effects pleasure, too much pleasure would disrupt it
- One of the goals of drugs research of the brain is to find treatments that can
be limited to only one area
o The current problem is the instability of specific areas that the drug
Basic Neural Network in Brain
- Excitation and Inhibition everywhere in brain
o It’s how these cells are hooked up as well as how they get stimulated
that determines what that part of the brain is doing
o Excitatory and Inhibitory cells are all interconnected
One cells re-excites another cells
An E cell can excite an I cell, which in term is exciting it.
If the inhibition is strong, one will shut the other off. They
It fosters both competition and co-operation within the brain.
Eye and Retina
- An imagine is focused by the combination of a cornea, and the lens inside of
o It’s focused onto the retina at the back of the eye
o For evolutionary reasons, our retina is created backwards in the sense
that the photoreceptors are at the very back of the eye, and in front of
them are two more layers of cells (ganglion)
The axons of the ganglion cells go off into the brain and form
the optic nerve.
If the axons that are going into the optic nerve that are
in the front of the eye, they have to cut back to the
retina at some time, causing a blind spot (because this is
where the optic nerve
Photoreceptors: Rods and Codes
- Photoreceptors: they are specialized neurons
o How do neurons normally get stimulated?
By other neurons in synapses, or environmental energy Light. Photoreceptors are specialized neurons is that
proteins in their membranes (photopigments) evolved
to capture photons of light of the wavelengths that we
When Light falls on these proteins in either rods or
cones, it opens ion channels and excites the cells.
o Afterwards, they transmit signals to the retina
and along axons to the brain
- Rods: specialized for night vision (low level) and there’s a majority of them.
They’re very sensitive to light but do not code for colour. They send
information even for 1 photon of light (smallest measure)
o In a very dim illumination you can see shades of gray, but you cannot
o If you want to determine whether only your rods are operating you
have to be aware that if you can’t see colours in a dark hallway, your
rods are only operating
- Cones: you are using them right now because they’re specialized for daylight-
bright intensity vision to light.
o They code light colour using three different photopigments
Red (50% cones) in most of our retinas
Green (45% of cones)
Blue (5% of cones)
This is because in mammals, only monkeys, great apes, and
humans have the red photo-pigments.
A hypothesis is that it evolved as a way of det