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03. Neuron (a brain cell)
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Neuron (nerve cell) is core cell composing brain.
Brain is composed of neurons and glial cells.
Glial cells are 10 times in amount than neurons.
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Unlike other organs, cell as nerve is surrounded
by fluid (cerebrospinal fluid specifically) and membrane.
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Brain is almost similiar with kind of tool growing cells.
When brain generates cell (neuron),
it almost doesn't use any other nutritions but almost use oxygen and glucose as energy source.
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Characteristics of brain can be summarized into following 7 items.
1. Brain is cell generator.
2. Brain (each neuron specifically) generates electric pulse.
Each cell in our body plays unique functionality.
Liver cell detoxicates. Muscle cell creates movements.
Neurons composing brain is almost only cell that generates electric pulse.
3. Brain associates the sense.
Above characteristic is related to the fact that neuron generates electric pulse.
Visual sense, auditory sense, somatic sense can be associated in brain cortex and sensory cortex.
Above 3 stimulus is nothing but electric pulse reaction in terms of brain and neuron.
That's why senses like visual, auditor, somatic, smell, etc can be associated in brain cortext
because they're all nothing electric pulse.
4. Brain creates sense, perception, thinking.
5. Brain creates memory.
Memory is not one of various recognition actions
but memory can be considered as entire of recognition action,
because all recognition actions refer the memory.
6. Brain creates intentional movements.
All animals move. Human creates movements that have goal.
7. Brain creates language.
Human creates symbolic system using language.
If you only know the fact that brain is cell generator which is physical characteristic of brain,
then, you actually can infer any other characteristics of brain
aside from above fact that brain is cell generator.
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When brain create neurons, neurons move, propagate, divide.
Fetal, that is, at initial developmental stage, neuron cell moves, propagates, divides.
In neural tube, cells propagate, then neural tube becomes thicker,
resulting in forming brain.
Head and tail of backbone are sutured in the at last developmental stage.
But if they're not suture correctly, it can become abnormal.
In the line that neural tube is sutured, there exist raphe nucleus.
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What you can see outside is activity of human.
What creates activity of human is neural system.
If you look into neural system in detail,
it's composed as connected net.
If you look into neuron which consists of part neural system,
there are branches around neuron,
and there are spines on those branches.
We call location where 2 spines meet as synapse.
Mind activity of human happens on synapse.
One neuron can have thousands to 10 thousands of spines.
Chemical materials exchanges on synapse,
and chemical materials are converted into electric pulse.
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Synapse is divided into synapse fore membrane and synapse rear membrane.
In synapse fore membrane, there are vesicles which contain neurotransmitter.
Neurotransmitters released from synapse fore membrane is spread into synapse rear membrane.
Released chemicals like dopamine, serotonin, noradrenaline, acetylcholine, etc are attached
onto ionic channel which is kind of receptors in synapse rear membrane.
Then, structure of ionic channel changes,
then, another chemicals and ions like GABA, glutamate
which are located in synapse gap-around go into synapse rear membrane.
In the end, entered chemicals into synapse rear membrane create electric pulse.
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Ion channel and receptor are composed of protein.
Therefore, it means that body creates these structures
by extracting genetic information from gene.
mRNA is generated from DNA inside of cell nucleus,
then mRNA goes from nucleus of cell via nuclear pore into inside of cytoplasm
Inside of cytoplasm, amino acid ordering is created with mRNA and ribosome.
Ordering of amino acid is determined by base code of gene.
After amino acids are chained, it becomes protein.
So, in conclusion, we can say
that memory is also phenomenon of creating protein based on reaction of gene.
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Synapse happens in various location among neuron bodies, axons, somatic synapses, dendrites.
Various and many branches comes out of neuron.
Biggest branch is called "axon".
Many thin branches like tree branches are called dendrite.
Suppose there are spines from "neuron A" and "neuron B".
Parts which form synapses can be various.
Actually, spines can form synapse in all of parts and areas.
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When neuron gets stimulus from learning, spine divides.
After around 1 hour, spine divides into 2.
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Sensory cortext generates picture of the world by using visual, auditory, somatic information.
Important information out of senses is memorized via Papez circuit working in temporal lobe
To pass to long-term memory, synthesizing protein should be followed,
in other words, gene should come in.
To interpret passed sensory information,
brain performs perception (in other words, comparison)
for passed sensory information to existing memory using working memory mechanism,
resulting in judge, infer, predict.
Then, basal ganglia selects movements.
Basal ganglia composed of large chunk under cerebral cortex is composed
of large nucleus like putamen, caudate nucleus, globus pallidus.
Basal ganglia selects movement anyway.
Center which helps basal ganglia for selecting movement is frontal lobe.
Prefrontal lobe is in charge of instinct coming from intestine,
and limbic system is in charge of emotion and feeling,
then we become to know desire of ourselves.
Prefrontal lobe,
especially dorsolateral prefrontal lobe where working memory runs
can merge data passed from outside and instinct and desire.
By being helped by prefrontal lobe, brain selects movement in striatum
Cerebellum outputs movement for selected movement to be appropriate for situation, time, timing.
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The job that a brain does is that it takes inputs and it outputs 5 kinds of movement.
The movements of the intestine system is intervened by the autonomous nervous system.
The movements of the body is intervened by the motor nerves which are passed via the spinal cord.
There are various forms in the movements;
automatic movement, rhythmic movement, balacing movement with the attention,
fine motor movement like the movements of fingers.
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When you do aerobics exercise, blood vessels around frontal lobe are generated.
With more blood vessels, supplying oxygen and glucose to brain will be much better,
then, in the end, activeness of cells will increase.
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Neuron can lengthen up to 1 meter.
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p1.
Neuron (nerve cell) generally has one long axon.
Suppose above neuron is alpha motor neuron.
Then, axon of this neuron creates synapse with skeletal muscle at the end.
Once synapses are created with skeletal muscle,
muscle reaction happens by myosin and actin chain.
The muscles contract after releasing acetylcholine (Ach) there.
The dendrites are formed randomly in shapes.
There are over 10000 spines on one Purkinje cell of the cerebellum.
There are nucleus in which the DNA is in a cell body.
The insulator is wrapped around axons.
In the case of alpha motor neuron, it can have schwann cells.
The schwann cell is also a insulating cell for axons,
and is also another form of the glial cell which has hundreds of variants.
Importantly note that each cell which wraps axon is the individual cell which has each nucleus.
In the above illustration, oligodendrocyte (one form of the glial cell) wraps one axon.
Only a vertebrate has the oligodendrocyte.
The invertebrate animals in the marine don't have the oligodendrocyte and the schwann cell.
They have only the unmyelinated nerve system.
To make faster speed without the myelin, the axon should be thicker and thicker.
So, a squid has almost 500 micron axon.
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p2.
Pattern of wrapping axon is as follow.
Let's express axon as stick.
Schwann cell flattens by itself.
So, flattened schwann cell can wrap axon almost 100 times.
As athlet performs more and more practices,
his axon is more and more wrapped,
resulting in fast conduction speed.
There happens patches onto flattened schwann cell,
which are parts affected by air.
Wrapping axon is called "myelinate".
Myelin is insulating wire which prevents axon from leaking electricity.
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p1.
All electicity is gathered in the cell body of the neuron.
When the value which can be obtained
by summing all analog signals exceeds the action potential value (-50mV),
those analog signals become digital signal in the form of spikes.
The spikes are passed with leaping the myelin.
At the end of a region, acetylcholine (Ach) is released, resulting it the movements.
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p1.
In the top-right illustration,
you can see the neuron B which has branches.
The regions where branches of B and A meet are called the synapse.
Let's zoom out the synapses (top-right circle which has 7 dots).
The presynaptic membranes release chemicals,
then a electrical pulse happens in the postsynaptic membrane.
Seeing, listening, all are nothing but the electrical pulses.
Since the part of a brain (which is in charge of the functionality of visual)
of visually impaired people can't be given the visual electrical pulses,
other form of electrical pulses
like auditory electrical pulses can enter the the visual function part of the brain.
The theory which claims that regions can be changed for other functions is neuroplasticity.
p1 e.
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p3.
Let's think of synapse of 1,2,3,4
Electric potential value inside of a neuron when it has no stimulus is -70mV.
That's why the default electric potential value is -70mV.
As the negative value decreases up to -50mV,
a electrical pulse becomes spikes as a digital pulse.
This spike is called also as the action potential.
Let's understand the reason why -70mV can be.
When the voltage happens around the ionic membrane,
ions should move with being against the voltage.
A electrical energy which should used in the above step is $$$W_{e}=ZFV$$$.
F: Fermi constant value
Z: ionization tendency of each molecule
V: Voltage
Besides, since above phenomenon is a diffusion phenomenon,
you need to consider the following.
When molecules of the gas is diffused,
the diffusion energy
which is used for rearranging molecules in the density1 $$$[X]_{1}$$$ to density2 $$$[X]_{2}$$$
can be calculated by using $$$W_{c}=RT\ln{\frac{[X]_{2}}{[X]_{1}}}$$$
If you set "$$$W_{e}$$$ = $$$W_{c}$$$", you can get a voltage pulse.
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Sodium and Potassium are most important for step of creating electric pulse.
Ratio is as follow.
If you suppose $$$K^{+}$$$ is 100 inside of cell, $$$K^{+}$$$ outside of cell is 5,
so ratio of $$$K^{+}$$$ between inside and outside is 20 times.
Since cell has evolved in sea, $$$Na^{+}$$$ is much outside of cell.
If you supposee $$$Na^{+}$$$ is 15 inside of cell, $$$Na^{+}$$$ is 150 outside of cell,
so the ratio of $$$Na^{+}$$$ between inside and outside is 10 times.
Unit is mM/l when you say this.
Intensity of passing is different.
Passing $$$K^{+}$$$ is 40 times easier than $$$Na^{+}$$$.
Let's apply above 2 concepts into following.
$$$V_{membrane}=61.5\log\frac{P_{K} \times [K^{+}]_{outcell}+P_{Na} \times [Na^{+}]_{outcell}}{P_{K} \times [K^{+}]_{incell}+P_{Na} \times [Na^{+}]_{incell}}$$$
$$$V_{membrane}$$$: Voltage of membrane
$$$P_{K}$$$: probability of K ion passing
$$$[K^{+}]_{incell}$$$: density of K ion inside of cell
$$$P_{Na}$$$: probability of Na ion passing
$$$[Na^{+}]_{incell}$$$: density of Na ion inside of cell
$$$[K^{+}]_{outcell}$$$: density of K ion outside of cell
$$$[Na^{+}]_{outcell}$$$: density of Na ion outside of cell
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Let's apply following "ratio" prior knowledge.
$$$K^{+}$$$ 100 5
$$$Na^{+}$$$ 15 150
$$$V_{membrane}=61.5\log\frac{P_{K} \times [K^{+}]_{outcell}+P_{Na} \times [Na^{+}]_{outcell}}{P_{K} \times [K^{+}]_{incell}+P_{Na} \times [Na^{+}]_{incell}}$$$
$$$V_{membrane}=61.5\log\frac{40 \times 5+1\times 150}{40 \times 100+1 \times 15}$$$
$$$V_{membrane}=-65mV$$$ 가 나온다.
If you include Cl and etc strictly, it becomes -70mV.
When the neurons have no stimulus,
the stable energy level of inside of the neurons is less
as much as -70mV than energy level of outside of the neurons
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p4
There are synapses which inhibits.
They're mostly located around 세포체막.
And they use neurotransmitter called GABA.
When synapses do active reaction,
they use glutamate.
Since when synapses use GABA, synapses inhibits reaction,
occuring action potential hardly happens.
Let's call inhibitation as I
call activation as $$$E_{1}, E_{2}$$$
You will be able to understand the perspective of creating electric pulse,
once you grasp the relationship among above 3 ones.
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As said before, when a neuron has no stimulus, a starting point is -70mV.
When a single neuron $$$E_{1}$$$ becomes stimulated,
the electrical pulse increases,
then, it decreases to the -70mV if a continuous stimulus is not given.
When the two of neurons become stimulated,
and if the electrical pulse reaches to -50mV, the action potential happens.
The action potential value can increase up to +30mV maximal.
If the continuous stimulus is not given,
the electrical value decreases up to slightly under -70mV,
then the electrical value goes to the default value (-70mV).
If only "I" works, the electrical value decreases to the down.
When single "inhibitation" and single "promotion" work at the same time,
the electrical value increases up to a half of height of case
that only single "promotion" works.
When $$$E_{1}+E_{2}+I$$$ works, the electrical value can't reach to -50mV,
which means that electrical value can't be converted into the action potential.
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p5
This wave is created when two signals are merged.
This wave is created when Na ion comes in.
Green color wave is create when K ion comes out.
But since direction is towards outside,
you should specify direction opposed.
That is, opposed direction green wave lets wave
which is formed in high to be lower.
Left increasing is wave which is formed when Na ion comes in.
Right decreasing is wave which is formed when K ion comes out.
These are 2 elements which create "action potential"
The width of highly peaked wave is 1ms.
It takes almost 3ms for that peak to go below -70 and to go up to 70.
It is almost 1ms that the shortest time of electrical stimulus in brain.
You can consider increasing wave as flooding salty sea water into cell.
You can think electric pulse occurs after salty sea water comes into cell.
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Neural reaction is to create electric pulse.
In middle of creating electric pulse,
receptors which are composed of protein are intervened,
so that creating electric pulse is connected up to gene.
In the middle of creating "action potential",
Ions of sodium comes in, ions of potassium comes out.
Neural cell (neuron) consumes much energy for this Na-K pump.
Seeing ions of Na and K and Na-K pump,
you also can think of lifes of initial Earth and seas.
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