What Is DNA? Chapter 17th, The Nobel Laureates In Physiology Or Medicine In 2013, What Is Neuron (2)
What Is DNA? Chapter 17th, The Nobel Laureates In Physiology Or Medicine In 2013, What Is Neuron (2)
[ ইংরেজীতে প্রকাশিত করা হল কিছু কিছু বাংলাদেশী বংশোদ্ভূত বিদেশে অবস্থানরত ছাত্র পাঠকেরা যারা বাংলা ভাষা পড়তে পারেনা, তাদের অনুরোধে, দুখিত ]
Figure source- http://www.nytimes.com/2013/10/08/health/3-win-joint-nobel-prize-in-medicine.html?_r=0
From left: Randy W. Schekman, Thomas C. Südhof and James E. Rothman.(NY TIMES)
1) Randy W. Schekman
Birth: December 30, 1948, at St. Paul, MN, USA
2) Thomas C. Südhof
Birth: December 22, 1955, at Goettingen, Germany
3) James E. Rothman
Birth: November 30, at Haverhill, MA, USA
(Dear readers, this chapter are the relevant part of the Chapter 16th. So you must have to go through the chapter 16th before you read this chapter.)
The Nobel Prize organization has declared Nobel award on Physiology or Medicine of the year 2013, on October 7th for this scientist trio. (1)
What did they discover?
It has been described somewhat in Chapter 16. How a cell transports its produced materials to the destination cell or from one organelle to another organelle inside the cell- was the subject of their discovery.
To make understood the discovery, it has been described what a simple animal cell is, in Chapter 16th.
But the discovery of James E. Rothman was upon brain Neuron.
For this, to understand his observation, it is necessary to know something about the structure and function of nerve cell or Neuron beforehand. So I want to describe a little about Neuron here earlier.
Let us see how the nerve cell or Neuron functions strangely!
What does the nervous system include and how does it function?
Nervous system includes-
1) Central Nervous System or CNS- It includes Brain that makes decision and transmits it up to Spinal Cord.
2) Peripheral Nervous System or PNS- That transmits the instructions received from the Brain to the working organ that will actually function, for example as muscle or secretory gland.
Now a days it has been a great challenge to the scientists to know how the brain can operate multipurpose functions. Yet, we could not know how much our brain holds capacity. We also could not know how much capacity we have been able to utilize thus far and how much is not yet.
What is Neuron?
Neuron is a very sensitive micro unit to maintain data in Brain including many other purposes.
Each Brain contains about 100 billion Neurons on an average, and each one is linked to about 1000 adjacent Neurons.
By this way, making connections of 100 billion neurons x 1000 neurons produce a gigantic network system in the Brain.
The head of a pin can hold about 30 thousand Neurons. Each Neuron receives electric signal from 1000 Neurons.
And if the electric signal is strong enough, the Neuron will start an electric impulse producing an Action Potential. This is also called the Firing of Neuron.
It is just like as if a gun will fire and strikes its bullets the target, if the pin of the trigger can produce enough strong hit upon the back of the cartridge when the trigger is pressured.
On the same way, if the electric charge received by the Dendrite becomes strong enough that can raise the charge as high as -55 m. volt; the Neuron will be able to produce an Action potential.
If this happens, Neuron will trigger and electric charge will pass through Axon as an Action Potential to the other Neuron or target cell. (6,7,8,9,)
How does look like the Neuron and what is its function?
Yes, it is invisible with our naked eye without microscope. See below a separate Neuron (Figure-5) and a pair connected in together (Figure-6).
Look it shows the different parts of a Neuron.
1) Dendrites- it receives electric signal. 2) Cell body or Soma- it decides to transmit the signal. 3) Axon Hillock- It is the joining place in-between cell body and Axon. This is the first place where an Action potential starts. 4) Axon- It transmits the electrical signal or Action Potential to the end of Axon called Axon Terminal. 5) Axon Terminal- It transmits the electrical signal through Synapse to the next Neuron or target cell.
You could also look how the Axon Terminal has been connected with the Dendrites of other Neuron without touching having a little gap in-between.
The joining place of the 2 Neurons is called Synapse and the gap in-between is called Synaptic cleft. (Figure- 6,
The main subjects of this chapter are-
1) After receiving the electric signal, how a Neuron transmits it to other Neuron.
2) How the first Neuron transmits the signal to the other Neuron crossing the synaptic cleft.
In fact, the discovery of James E. Rothman was upon Synaptic cleft. That is, on his observation, he showed how a Neuron transmits the electric signal through the Synaptic cleft to the other Neuron or target cell when needed at exact time without any delay.
To know only this discovery of James E. Rothman, we have to know what Neuron is and how it transmits the electric signal to the other Neuron and in particular how it transmits the signal through the gap of Synaptic cleft.
Let us see one-by-one earlier. Then it will be easier for you to understand their discovery in next chapter.
We will see-
1) How a Neuron, after receiving the electrical signal from a Neuron, transmits to other Neuron.
The Dendrites of a Neuron have innumerous branches. When it receives an electrical signal, transmits it to the cell body.
The cell body then takes decision and responds producing electrical signal through Action potential. Then this Action potential leaves the Neuron through Axon as an electrical signal.
If the Axon is short, it supplies only to the nearby Neuron in brain. And if the Axon is longer it supplies a distant cell either in brain or outside the brain.
The first Axon is covered with Myeline sheath built with Schwan cell. It acts as an insulator and also prompts transmission of the electric charge. If any way Myelin sheath is damaged, the person is attacked with a complicated disease called Multiple Sclerosis. The terminal axon is not covered with Myelin sheath, so the speed of transmission is slower there.(6,7)
(See figure- 5, 6 and videos, 1-4, 11,12)
What is Action potential?
Action potential is a very short time, sudden and temporary event of Neuron. In this event a sudden rise (Depolarization) followed by sudden fall (Repolarization) of electric membrane potential upon Neuron membrane happens. This very short time sudden rise and fall of electric membrane potential is called the Action potential. This is also called Spike. (2, 6,7,
How does the Neuron produce Action potential?
While cell remains at resting condition, it keeps at-70 m.volt charge on inside wall and positive charge on outside wall. This condition is called the ‘Polarized” condition of cell. At this time, there remains a lot of Sodium ion (Na+) outside the cell wall (See figure-2 for Na, figure-9 voltmeter)
To produce Action potential, it is very essential to hold -70 millivolt charge inside cell wall or it will be unable to produce an Action potential. This is called the resting condition of cell. The cell holds this resting condition through Na+/K+ ion pump mechanism.
At this time, there remains negative charged protein, Nucleic acid, and positive charged potassium ion (k+). (See figure-3, for K)
When Dendrite receives an enough strong signaling charge, it works as a stimulant causing a thrust upon cell. Then the thrust of stimulating action opens the Na ion channel at Axon Hillock. At this time, a lot of Na+ ion enter inside the cell wall and raise the -70 millivolt charge towards positive. This is called Depolarization.
If the signaling charge becomes so strong that it can raise the charge up to threshold level, (-55) then the Neuron fires and a lot of Na+ ion enter inside the cell wall raising the charge as high as up to +40 m.volt that is also called spike.
Now the cell controls and prevents the charge rising farther and fells lower.
Such a very short-time event of sudden rising and falling of change upon a Neuron membrane is called the Action potential. It takes 2 milliseconds (0.0002 sec.) to happen.
How does the cell prevent rising the electric potential above and will lower?
The cell performs it by 2 ways.
1) The cell now shuts down the Na+ channel so that more Na+ ion cannot enter inside and raise the charge further.
2) The cell opens the K+ ion channel and expels out K+ ion lowering the internal charge. By this way the charge will lower as low as up to -90 m. volt.
This condition of Neuron is called Refractory condition. In this condition, no charge can stimulate the cell causing an Action potential.
If the signaling charge cannot stimulate the charge up to threshold level, there cannot happen any partial action potential. The Action potential should either be complete or nothing. No partial Action potential can happen just like firing from a cartridge of a gun, where there will occur either a complete firing from the gun or nothing. No partial firing can occur. This phenomenon is called All-Or-None Phenomena.( See figure-7,9,10, also see Sodium in figure-2 and Potassium in figure-3)
Now the question is how and why does this Action potential proceed forward?
When the Action potential occurs initially at Axon Hillock, the thrust of the stimulating charge cause to open the next Na+ ion channel station first and K+ ion channel station next, causing exactly another Action potential over there, just like, as occurred at the previous station. And the thrust of this Action potential opens the next N+ and K+ ion channel causing another Action potential over the next station. By this way the Action potential heads forward.
This Action potential passes as electric signal and arrives up to Axon terminal using Axon fiber as electric wire.
The speed of its flow varies from 10-100 meter according to its site.(See the videos of Action potential)
Why does the Action potential travel only forward not backward?
We have already known, to occur an Action potential, Neurons’ resting condition (-70 m.volt) is required over there or Action potential cannot happen. When Action potential occurs at an Ion channel station, the cell, within 1 m.second lowers the charge at previous channel station below -70 m volt as low as up to -90 m.volt through Sodium/Potassium ion pump. Then the cell shuts down Sodium channel. As a result, there remains -90 m volt charge at previous station.
This condition of a cell is called the Refractory Period, mentioned before.
If the Action potential is to go back, it has to produce an Action potential at the back station where the charge has already fallen up to -90 m.volt just 1 m.second ago, that is in fact impossible.
For this, the Action potential does have no way except advancing forward. The gate of the back door is shut down by lowering the charge up to -90 m.volt that is called Refractory Period of the cell.
But at more backward station i.e. at the station of 2 m.seconds time before, the charge becomes - 70 m.volt or resting condition by Sodium/Potassium ion exchange that is suitable for producing Action potential. But the Action potential does not have so much strength that it can stimulate so much back back crossing one station. It has ability to stimulate and thrust only its adjacent station, not beyond that.
So it has no way but to proceed forward.
Could you see how the Neuron has applied a great technology to assure the advancement of Action potential only towards forward?
It is just like, asking someone to move forward and when he drives forward, his backway door is shut down with lock and key so that he has the only way to go forward until reaches the destination.
Could see what type of a strange technology of Neuron is!!
Why does the Neuron take such high technology measure to assure only forwarding the electric signal?
Yes, it is essential for operating the body. When an Action potential starts from brain carrying an electric signal towards a target organ, if it delays or faces any obstacle on the way the nervous system will fail keeping our body active or alive leaving our body a non -living object. That means we will no loger remain as a living animal but a non-living material, since none of our cell or organ can work without the instruction of brain.
By this way our nervous system keeps all the biological works and capabilities of our body with its so high sophisticated technological manner.
For this reason, some writers called our brain “GOD”.
This is why, as the electric wire of our house is covered with plastic or rubber sheath as insulator, Neuron has also all over covered its wire, the Axon, with Myelin Sheath as insulator where necessary, so that the electric signal can transmit promptly without any obstacle or delay on the way.
Did you see how the Neuron has taken a precaution to avoid any possible accidents, hazards or risks?
It takes 2 milliseconds to occur an Action potential. (0.002 sec)
(2,,8,9,15 video- 5, figure- 7,9,10)
Thus far, by this way, the electric charge arrived at the Axon terminal. If the target cell or organ is too far, the signal will have to cross another Neuron to reach the target.
But we will have to remember, unlike the electric wire, our Neurons are not in-touch directly with one another at the Synapse or junction.
The electrical signal will face a junction at Axon terminal with the dendrites of another Neuron. This junction is called Synapse. (See the figure- 6,
2) How does the first Neuron transmit the electrical signal through minute gap or Synaptic cleft of Synapse?
What is this Synaptic cleft? How does the electric signal will hand over its responsibility i.e. electrical signal to the Dendrites of other Neuron crossing over this minute gap of Synaptic cleft?
Although the synapse is junction of 2 Neurons but they don’t touch each other.
They are separated with a minute gap in between. Then who will cross the electric charge this minute gap and will reach to the other Neuron?
Now see how the Neuron crosses the electric signal this gap.
The Neuron which carries the signal is called Presynaptic Neuron and the Neuron will receive the charge is called the Post synaptic Neuron. And the gap in- between is called Synaptic cleft.
It is just like a break on a bridge making 2 parts. Then how does the electric charge will pass through this break or gap?
At Axon terminal there is a lot of vesicle or sac containing some types of chemical compounds called Neurotransmitter. See chapter 16th to learn what is vesicle. These Neurotransmitters are of various types according to their functions e.g. Serotonin, Acetylcholine, Norepinephrine, Dopamine and Gamma-Amino Butyric Acid (GABA) etc.
These Neurotransmitters cross over the Synaptic cleft the electric signal arrived at Axon terminal. Since this electric signal is associated with chemicals, it is also called Electrochemical signal.
How does it cross over?
When the Action potential arrives at Axon terminal, its thrust opens the door of Calcium ion channel over there (See figure -4 for Calcium ion)
Then a lot of calcium ions enter inside, and through a complex strategy, these Calcium ions cause the vesicles to pour down the Neurotransmitters at synaptic cleft without any delay and with assurity.
What is that complex strategy?
That is the subject of discovery of scientist James E. Rothman that you will know in next chapter.
Thus far it is known to us that a neuron may have more than 1 type of Neurotransmitter. (5)
Each Neurotransmitter has different types of function.
Then this Neurotransmitter sits on its specific receptor upon the wall of Dendrites of post synaptic Neuron. This Neurotransmitter now acts as a stimulator on this neuron to produce an Action potential over there. And the thrust of this Action potential produces another Action potential at the next station of Sodium/Potassium ion channel just as occurred in presynaptic Neuron. This Action potential as an electric signal travels forward until it reaches at Axon terminal and then to another neuron or target cell e.g. a muscle or a secretory gland etc.
It is to remember these electric signals are not only excitatory. They have also inhibitory function. Both the excitatory and inhibitory functions are coordinated to operate the body properly.
For example, an excitatory signal causes our leg to kick the football and the inhibitory signal causes our leg to stop kicking the football while we play football to perform the game correctly.
How an excitatory or an inhibitory impulse occur-
It depends upon Neurotransmitters. When the Neurotransmitter opens Sodium (Na+) channel, depolarizes the membrane causing excitatory impulse. And when the Neurotransmitter opens Potassium (K+) channel, hyperpolarizes the membrane causing inhibitory impulse. (2)
Did you see another great technology of Neuron!!
The vesicles which are once used, the cell enters it inside and throws them in its dustbin, Lysozyme, if not usable further, and saves those which are usable to use for next time.
There are some chemicals that can loss sensation temporarily shutting down the door of ion channels e.g. local anesthesia, venom of some animals and excessive cold etc.
(2.8.14,and see the video-6,7)
From where does the Axon terminal receive the vesicles containing these Neurotransmitters?
Yes, like the other cells, the Soma or cell body of Neuron has Nucleus, Ribosome, ER, Golgi etc. Ribosome produces Neurotransmitters and fills it in vesicles with Golgi. Then the cell, like food grains in a plastic bag, transports them at Axon terminal and stores sufficient quantity there for utility while required there immediately and urgently.
This is also another big talent activity of Neuron!!!
Is there any difference between the electric current used in our daily life and the electric signal passes through the Axon of Neuron?
Yes, surely there is.
1) Our domestic current means flow of electrons from a negative pole to a positive pole at a speed rate of 1,86,000 miles/sec.
2) Unlike the domestic current, to pass electric signal through Axon, electron does not flow from one negatively charged end to another positively charged end of Axon. Such arrangement is not in Neuron. If it would occur the purpose also would not serve.
What happens in Neuron? Here, the Action potential passes from the Axon hillock causing sudden rise and fall of electric charge creating a thrust upon Sodium/Potassium ion channel station up to the Axon terminal. (See figure-7). For this, its speed-rate is also too much slower (10-100 meter/second) than the domestic electric current. It crosses over the synaptic cleft using the chemicals called Neurotransmitter.
No flow of current passes across Neuron. Here an electric charge is produced initially through a sudden rise and fall of Sodium/Potassium ion exchange through the ion channel, inside cell wall. This sudden up and down of charge is called the Action potential that travels stimulating each ion channel station along the wall of Axon and reaches at the Axon terminal where it makes another thrust and starts working there that described above.
Why the target organ or cell is thrusted or jabbed sending an Action potential?
Since the target organ or cell normally remains in rest, sleep or inactive. It cannot start working until instructed by brain through stimulation or awakening. So this Action potential acts as an electric thrust that stimulates them and starts working. It is just like as you start working your idly sitting worker hitting a jab or thrust upon him /her, when you want working or stopping.
Otherwise why a cell or organ will be working? Like us, they also like to keep in rest naturally until receicives any instruction from head office, the brain. Our body has chosen to activate them to stimulate by sending electric jab or thrust upon them through Action potential.
Thus far, this process has been chosen better to our body to maintain all our body function.
To understand clearly this chapter, some scientific terms are defined below. You can take help with them.
STIMULATION- The act of arousing a cell or organ to action e. g. an Action potential produces another Action potential by arousing the adjacent ion channel and advances forward.
POLARIZED- Here it means the resting condition of cell at – 70 millivolt, when the cell is able to produce an Action potential.
DEPOLARIZATION- Here it means the Neuron raises the charge from -70 (resting condition) upwards. Thus it raises up to +40 millivolt (see the Voltammeter at figure-9)
REPOLARIZATION- Here it means the cell again polarized or the charge falls down again
POTENTIAL- Here the quantity determining the energy of charge in an electric field e.g. by voltage. (see the Voltammeter at figure-9)
CHANNEL- Here, it is a very narrow pore on cell wall, through which ions and other body materials can pass in and out. Neuron causes Action potential by passing Sodium/Potassium ions in and out through it and transmits it up to Axon terminal,
Each ion has different channel. One cannot enter through others’ path. The cell has control over it. Cells open or close it as required just as we can open or shut down a gate of a water dam.
It is also controlled by ionic concentration.
When a particle losses or gains electron, it turns into a positively or negatively charged particle accordingly. This charged particle is called Ion. If it losses electron, called Positive Ion and if it gains electron, called negative Ion e.g. Na+ ion or Cl- ion.
What is positive Sodium ion (Na+)?
(See figure-2, Sodium) (17)
At the center of a Sodium atom, there are 11 atoms (positively charged), 12 Neutrons (No charge) and 11 electrons on its 3 shells whirling around.
See the photo of Sodium atom below.
At this normal condition there is no charge in the atom or neutral, since there are equal numbers of electrons and protons in the atom.
Its outer shell contains 1 electron that has weakly attraction to the central proton. So this outer most electron may add to any other particle. Then this Sodium atom will have 11 protons and 10 electrons with one proton number more.
Then the atom will be a positively charged ion. So, it is called positive Sodium ion or Na+.
If it loses 2 or 3 electrons, it is expressed as Na2 +or Na 3+
And when electrons add at its outer most shell, it forms a negative ion that is expressed as Na-, Na2 - or Na3- according to the number of elections added. The electric charge of a particular place depends upon these positive or negative Ions in that place e.g. if the outside of the cell wall contains more positive or negative Ions it will be positively or negatively charged accordingly. The same condition will also be inside or the cell wall.
The difference between outer and inner cell wall is called electric potential that is measured by Voltammeter.
See the example of a Sodium Ion.
Figure source- http://www.chemicalelements.com/elements/na.html
Sodium (Na) atom. It contains 11 Protons, 12 Neutrons and 11 Electrons.(17)
It converts into a Sodium positive ion (Na+) when it losses its outer shell- electron. Then it will supply positive electric charge to Neuron wall.
What is positive potassium (K+) ion?
(See the figure-3, Potassium atom)
A Potassium atom.
Figure source- http://www.chemicalelements.com/elements/k.html
Figure-3, a potassium atom. It contains 19 protons, 20 Neutrons and 19 Electrons. It als0 converts into a positive ion when it losses the Electron of it’s 4th shell.(17)
What is Calcium (CA2+) Ion?
(See figure-4, a Calcium atom)
A Calcium atom.
Figure source- http://www.chemicalelements.com/elements/ca.html
Figure-4, a Calcium atom. It contains 20 Protons, 20 Neutrons and 20 Electrons.(17)
Look, it converts into a positive ion losing it’s 2 Electrons from it’s outer shell. It works at Axon terminal expelling Neurotransmitters out of the cell wall.
In essence, the language of brain or Neuron is the language of “electric charge or signal” either stimulatory or inhibitory. When it receives any information receives in the language of electric charge and when it replies, it replies also using the language of “electric charge” . When it transmits any instruction, transmits through the language of “electric charge” . it corresponds all over the body world through “electric charge”. It has no other way to operate its function without correspondence through electric charge or electric signal, either stimulatory or inhibitory.
It is also confirmed that our Nervous system or brain operates all the important function of body through electric power and the raw materials needed to generate this electricity are the Ions e.g. positively charged Sodium (Na+), Potassium (k+) and Calcium (Ca2+) ions etc.
No doubt, we should be very grateful and also thankful to these particles.
So, our brain including the whole nervous system keeps us all the time alive and active operating all the system, organs glands etc. of our body during day and night for 24 hours.
Now let us see some relevant photos-
Figure source- https://simple.wikipedia.org/wiki/Neuron
Figure-5, a Neuron
Look, it has some extra parts rather than a simple cell.
1) Dendrites (left)
2) Soma (cell body)
3) Axon (right)- It is covered and insulated with Schwan cell (Yellow color) extended from cell body like fiber.
Look below at figure-6 how Dendrites of a Neuron is connected with an Axon of another Neuron.
Figure source- http://www.usdbiology.com/cliff/Courses/General%20Biology/7%20neuronXIII-XIV.html
Here it has been shown how a Neuron connects its Axon to the Dendrites of another Neuron and advances by this way. In this way it reaches the target organ or cell. The last point is called the Motor End Plate.
Figure source- https://en.wikipedia.org/wiki/ATPase
This is the Sodium/Potassium channel of cell wall. These are the minute pores made of specialized protein through which Sodium and potassium enter or exit the cell increasing or decreasing the cell wall charge. Cell controls its charge by pumping Sodium/potassium Ions.
Figure source- http://cephalove.blogspot.com/2010_05_01_archive.html
Synaptic cleft (15) or the gap at the junction of 2 Neurons. The thrust of electrical signal of presynaptic Neuron helps in releasing Neurotransmitters at Synaptic cleft, that stimulates and produces Action potential at the postsynaptic Neuron wall and then advances forwards on the same way
Figure source- http://faculty.washington.edu/chudler/ap.html
See the up and down of electric charge on cell wall during Action potential with the Voltammeter.
Figure source- https://en.wikipedia.org/wiki/Action_potential
Figure-10, see the level of charges upon cell wall during Action potential.
[Approximate plot of a typical action potential shows its various phases as the action potential passes a point on a cell membrane. The membrane potential starts out at -70 mV at time zero. A stimulus is applied at time = 1 ms, which raises the membrane potential above -55 mV (the threshold potential). After the stimulus is applied, the membrane potential rapidly rises to a peak potential of +40 mV at time = 2 ms. Just as quickly, the potential then drops and overshoots to -90 mV at time = 3 ms, and finally the resting potential of -70 mV is reestablished at time = 5 ms.]
See the animated figure of Action potential on link below.
The description of Neuron or nerve cell finishes here.
You will see the discovery of the scientists trio in next chapter.
See the other chapters here- https://dnaandthemystryofhumanbody.wordpress.com/
Keep in touch to know the mystery of human body.
VIDEOS FOR NEURON-
5).ACTION POTENTIAL VIDEOS
8)ACTION POTENTIAL PROPAGATION VIDEO ----good
9) SYNAPSE WORKS (ACETYLCHOLINE)-----good
10) Neurology - Neuron
11) Neurology - Glial Cells, White Matter and Gray Matter
References of chapter 17
NERVE IMPULSE/ACTION POTENTIAL/POLARIZATION
6) ACTION POTENTIAL/CHANNEL/BRISTOL
7) ACTION POTENTIAL/WIKI
9) SYNAPTIC TRANSMISSION
10) DIAGRAM OF THE OF THE DISCOVERY
12) /DIAGRAM/DISCOVERY DETAILED/ cen.acs.org
14) For kids
15) SYNAPSE FOR KIDS
17) CHEMICAL ELEMENTS