• What is action potential and how are the signals transmitted?
The nerve cells, which are called the neurons, unlike other cells,
have axons and dendrites. The dendrites consist of many short
extensions and look like the roots of the cell. These extensions
receive signals from other neurons and the recipient cells and
convey it to the body of the cell. Axons, on the other hand, are
thin structures that arise from the cell body. One axon arises from
the cell body and they help carry the messages to the brain. These
neurons make a huge network of communication, all together.
Every cell membrane has an electrical charge and every neuron is
like a tiny biologic battery ready to discharge itself. And each
neuron is like a mini, biological battery ready to discharge its
energy. The ions are electric-charged molecules inside and outside
the nerve cells. This creates a difference in the voltage
throughout the cell membrane. In order for the the neurons to send
signals, minus 50 milivolt (one millivolt is one in one thousandth of
a volt) is needed at an average. 1 The signal is transmitted
through the axon. After every signal, potassium ions are exchanged
through the cell membrane. The neuron has to be recharged after
every signal. To do that, the neuron gets back the ions to reach
its potential value.
One neuron takes onethousandth of a second to send a signal. This
makes it possible to send a maximum of 1000 signals a second,
however generally only 300-400 signals are sent in one second. The
nerve cells have varying lengths in people 2 and the speed of
transmission is around 100-300 meters a second. 3
Prof. Peter Suckling, a neuroscientist at the Downstate Medicine
Center, explains his awe at the cell membrane:
"This thin cell memberane preserves the electric voltage more
efficiently than any other insulating material. This is a very high
insulating power. It has to be strong and at the same time, it is
very thin." 4
The fact that the nerve cells can signal each other through the
electricity produced in the cell membrane, the fact that they can
send messages and ensure that the body functions continue, is
something to be pondered upon. Furthermore, these electrical
signals find the right address and make sense to the recipient cell.
Every cell knows what the signal means and acts accordingly. This
is a miraculous incident that has to be thought about very
comprehensively. If it wasn't for this flawless system between the
cells, no living thing could survive. Then how did this system that
displays an amazing intelligence and engineering come about?
Surely, it is impossible to say that unconscious piles of atoms and
molecules came together to build the cells and led to this
amazingly engineered system between the cells. The existence of
such a conscious system proves us the existence of creation in the
living things. This incredible micro structure that left scientists
amazed, belongs to our Lord, the Creator of Everything:
"Is He who creates like him who does not create? So will you not
pay heed?" (An-Nahl, 17)
Neurons at rest
A nerve cell is at rest, when it is not communicating a signal.
However, this doesn't mean that the neuron is completely immobile.
It has to be ready to transmit signals that could be received from
the neighbors at any moment. A neuron that is at rest has to
polarized. This means that the liquid inside has to be negatively
charged compared to one outside. One nerve cell has an electric
potential of around 70 milivolts across the cell membrane. This is
called the membrane potential or the resting potential. Although
this might seem like a small amount, it means that it produces a
voltage that is equal to 1/20 of the energy in the battery of a
small flashlight and creates a potential for the electricity
production throughout the axon membrane.
Outside the axon, there are sodium and (Na+) chloride (Cl-) ions,
while inside, there are charged proteins and potassium (K+) ions.
The electrical imbalance between outside and inside of the cell,
creates the resting potential throughout the membrane. This
imbalance created by the charged ions, is obtained through the cell
membrane being semipermeable for different ions. Even if the
sodium, potassium and chlorine ions pass through the cell membrane,
the entrance of big molecules creating an electric potential, is
restricted.
However, semipermeability is not the only solution, as the
potassium ions inside the cell (K+) are always higher in number than
the sodium ions (Na+), while the sodium ions outside the cell
membrane (Na+) are always more than the potassium ions (K+). In
order to ensure the necessary ion balance, the intensity levels in
the nerve cell, has to be reverted.
The cell does this using a kind of ion pump. The sodium-potassium
pump is a big protein molecule that builds a channel in the cell
membrane. This pump gets its energy from the ATP (Adenosine-
5'-triphosphate: the cellular energy molecule that living things
directly use) and after sending the sodium (Na+) ions out, accepts
the potassium (K+) ions inside. Thus, it preserves the right ion
ratio inside and outside the cell. In every square micrometer of the
cell membrane, there are some 100–200 sodium-potassium pumps and
each of them sends out 200 sodium ions while accepting 130 potassium
ions inside.
Action potential and transmission of the signal
When a neuron is prompted by another neuron or the conditions, the
signal starts and right after this, the signal moves along the axon
and makes the cell membrane potential to be reverted. As there are
thousands of protein channels or gates on the neuron membrane
allowing the passage of the ions. These gates are usually closed.
When there is a signal, the sodium channels are opened and the
positive-charged sodium ions flow inside. This way, the inside of the
cell membrane has more positive charge and the resting potential is
reverted, which increases the cell membrane potential up to +50
milivolt. The reverting of these charges is called the 'action
potential'. During the action potential, the potassium gates are
opened and the positive charged potassium ions flow outside. This
re-balances the resting potential and the inside of the neuron
becomes negative charged while outside becomes positive charged.
A single electric signal triggers all this process. Therefore, we
can say that the transmission of the signals triggers a domino
effect. As each domino falls, the next one falls. After the signal
is sent, the dominos stand back and get ready for the next action
potential.
The molecular traffic between the nerve cells continues on a
constant basis. It is the ions and some proteins that tell this
traffic to move or to stop. It is surely impossible for unconscious
moelecules to build this amazing nervous system in our bodies and
then organize it perfectly. They have come together to serve a
single purpose within a system. This fascinating system in our
bodies is one of the manifestations of Allah's beautiful creation
and infinite control over everything.
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