Chapters 3- Neurophysiology basics 04. Neurotransmission

04. Neurotransmission

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The fundamental role of a neuron is to receive, propagate and transmit nerve signals. Its plasma membrane has special electrochemical properties that it can respond to a stimulus and spread its activities to the nerve ending.

The plasma membrane of neurons comprises channels and pumps capable of regulating the distribution of ions on either side of the membrane according to their electric charge and their concentration. We will see that this regulation plays a key role in the transmission of nerve impulses.

In contrast to an electric wire, it is not the flow of electrons which leads the signal [ 96 ], but this is an ion-exchange wave that occurs across the membrane. This spread is electrochemical in nature.

1. Basics:

Two concepts are very important to remember, the concentration gradient [ 39 ] and the electrical gradient [ 39 ].

Indeed, in biological molecules tend to diffuse environments of high concentration to low concentration of the media, then they say they follow their concentration gradient.

The charged molecules also follow a gradient power (potential gradient), and therefore positively charged molecules will diffuse to the negative charge of media and vice versa.

But often, these molecules are shared between electrical gradients and different concentration, sometimes even opposed. They will then diffuse in a balanced manner in the two gradients. They then follow an electrochemical gradient [ 100 ].

2. The rest potential [ 5 , 75 ]:

The diffusion of ions across the plasma membrane occurs at specific channels. Potassium channels are highly permeable [ 113 ], which is not the case for the sodium channel. Indeed, the membrane in the state of rest is slightly permeable to sodium, it considers that it is impermeable same thereto.

On the plasma membrane, there is a pump Na +-K +-ATPase that actively engages and each consumption of one molecule of ATP (the universal currency of cellular energy) three sodium ions out of the cell 2 against K + ions inside. The pump consumes so much energy that some have attributed to him 30% or 50% of all energy consumed by the brain.
Overall, the pump Na +-K +-ATPase fills the cell gap and potassium, sodium and each intervention mobilizes a record of a positive charge to the outside of the cell.

The molecules of intracellular K + gradient following chemical and out to the extracellular medium by providing more positive charges with them, the intracellular side of the membrane is thus negatively charged, which limits the diffusion of molecules of potassium.

So, without any transmission, the balance established with all these elements generates a potential difference between the extracellular medium and positively charged intracellularly negative charge. This is called transmembrane potential: the resting potential, it is often located between -50 and-75mV.

There are other molecules and others elements that would not detail here to simplify the phenomenon, otherwise it is much more complicated [ 39 , 41 , 100 , 133 ].

3. The action potential [ 39 , 41 ]:

There is a membrane on the voltage-gated sodium channel which opens only upon variation of the electric potential between the two sides of the membrane. When the membrane potential exceeds a threshold value, the voltage-gated sodium channels open and cause a massive influx of Na + ions within the cell (about 1 million / second [ 96 , 134 ]) until the polarity of the membrane is reversed (depolarization phase).

Potassium can then follow its concentration gradient is way outside of the cell, this will gradually bring the membrane potential to its resting state (repolarization phase). During this phase, the sodium channels inactivate and they can not be opened during a refractory period.

Sodium continues to be actively pumped out of the cell against the potassium molecules which join the inside of the cell.

The delay is potassium to enter the cell is responsible for hyperpolarization will gradually decline.

4. Signal propagation:

When there is a membrane depolarization for one reason or another, usually at the emergence of cone [ 113 ], where the concentration of voltage-gated sodium channels is greater, there is activation of proximity to other channels and so on.

This phenomenon of depolarization wave continues until the signals of membrane depolarization through the entire axonal length and ends at the terminal button.

The refractory period of voltage-gated sodium channels does not allow the signal to reverse [ 135 ], the signal propagates therefore always in one direction. We call this wave of action potentials: The nerve impulse. The spread of this influx obeys the law of all or nothing: Either the transmembrane potential exceeds a threshold value and results in an action potential or it is simply ignored.

On a single nerve fiber, the amplitude of the action potential does not change the encoding of the signal strength is by the frequency of action potentials, there is more action potentials, more signal is intense.

The transmission speed of nerve impulses varies from neuron to another. Indeed, the larger the diameter of the axon is big plus signal spreads quickly.
The speed of nerve impulses depends also on the axon myelination [ 100 ]: For myelinated fibers, the action potential jumps from node to node, it is called a transmission type (saltatory) which is very rapidly (up to 120 m / s [ 75 , 119 ]) opposite propagation (continuous) fibers amyélinisées of which is slower.

At the level of myelinated fibers, Na + channels are condensed at the nodes, the action potential recorded at this level is so important that it can quickly affect sodium channels that are in the next node and so on .