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Nervous system: Resting and Action Potential

Learning about the resting and action potential of a neuron and the way electrical signals travel through our nervous system.




The most important concept of to understand in the process of electrical impulses of the nervous system is the difference in charge between the internal environment of the cell and external charge. The way this difference changes due to different stimuli determines how messages get carried across our nervous system.

Understanding Resting Potential


Every cell in our body has an internal environment which we refer to as cytoplasm of the cell. This is a thick liquid enclosed in the cell membrane supporting the organelles and comprised by mainly proteins, ions and water. These cells depending of which kind of tissue they are part of (connective, epithelial, muscular or nervous) exist in a extracellular matrix.


It is important to first understand how a neuron behaves at its resting state without any


In the nervous system the presence of the following ions in both the outside (extracellular matrix) and inside (the cytoplasm) of the neuron drive a lot of this action that we are seeing in these fascinating nerve cells. These ions are the following: Potassium (K+), Sodium (Na+), Calcium (Ca+) and a bunch of anions like Chloride (Cl-) and negatively charged protein molecules. The ions carry either a positive or a negative charge.


The concentration of Potassium is much higher outside of the cell and together with other positively charged ions (including smaller amounts of K) drive the extracellular fluid of the neuron to have be a POSITEVELY charged environment. On the other hand the intracellular fluid carries more negatively charged molecules which create a NEGATIVELY charged environment. These negatively charged proteins are manufactured by the cytoplasm.


These ions have two main forces acting upon them at all time: their concentration gradient and attractive forceopposite charges attract one another. This is how these two forces act on two of the most important ions involved in this process:

  1. Na+ is attracted to the inside of neurons at rest by two forces. First, the high concentration of Na+ outside the cell pushes it into the cell down the concentration gradient (I force). Second, the electrical gradient, due to the negative charge within the neuron, tends to pull the positively charged ion inside in the cell. The opposite charges attract one another (II force) but since there is resistance to the passage of Na+ across the neuronal membrane the Na/K pump is able to maintain the higher concentration of Na+ outside the neuron. Please note that the membrane doesn't just allow the passage of Na+ easily.


  1. K+ is found in a higher concentrations inside the neuron at rest. Since it moves freely across the neuronal membrane, there is a tendency for K+ to move out of the neuron down the concentration gradient more freely that Na+ is able to via so called leakage channels. This concentration gradient is partially offset by the electrostatic pressure induced by the increased negative charge inside the neuron. Therefore, the negative environment inside the neuron tends to attract the oppositely charged K+ ions. Nonetheless, the consternation gradient force is higher then the electrical one in this case, so K+ often leak outside of the cell.



Between the extracellular and intracellular fluid as we know from the anatomy of the cell we have a phospholipid bilayer cell membrane that controls the charge of these environments through its permeability to certain ions such as Na and K. 


So now that we understood that the inside and outside have different charges and all ions are trying to circulate either in or out of the nerve cell, let’s look at what allows these ions to travel through the membrane. 


Permeability


  • The negatively charged aions which are mainly negatively charged proteins are trapped on the inside of the membrane because of its high impermeability to protein which require channels to travel in and out of the cell

  • For other ions the membrane has leakage channels like the Na+/K+ pump which is of high importance for this process. This pump allows 3NA+ ions to leave the membrane and 2K+ to enter the cell. By moving more Na+ outside the cell, we are doing two things:  creating a more positively charged environment outside the cell and a more negative environment inside the cell (as there are less positively charged ions). This also results in having more K+ inside the cell.

  • We also have leakage channels for both Na+ and K+ that allow ions in and out. It’s worth notting that the membrane is more permeable to K+ then it is for Na+ .

  • There are also a few more leakage channels dealing with Calcium and Chloride that do the same: allow these ions to move in and out of the nerve cell to establish an equilibrium

The uneven distribution of ions between the inside and outside of the neuron due to the two forces acting on these ions and the different permeability of the membrane to them lead to a RESTING POTENTIAL  of -60 to -70mV.  Because there is a potential difference across the cell membrane, the membrane is said to be polarized.


  • If the membrane potential becomes more positive than it is at the resting potential, the membrane is said to be depolarized.

  • If the membrane potential becomes more negative than it is at the resting potential, the membrane is said to be hyperpolarized

Understanding Action Potential

Now that we understand what happens in the nerve cells when there is no stimuli involved, let's see what happens to the cell when there is certain inputs from the outside or inside of the body.


An action potential happens when the neuron will receive excitatory or inhibitory inputs mainly via the dendrites that cause changes to the -70mV resting potential we just went though.


The effect these inputs have on the membrane potential it depends on its potency. If the change in mini voltage is no bigger then > -55mV this is considered a graded potential. This doesn’t have the strength to push the potential through the axon so it remains in the soma of the neuron and decays with time and distance. 


If the potency of the input is greater and pushes the membrane potential threshold > -55mV, an action potential will be triggered in the Axon hillock


Depolarisation process


When reaching the Axon hillock point with a threshold of -50mM something amazing starts happening. The membrane of the axon has many voltage gated channels which most of them open at approximatevely -55mV threshold. These are mainly Na+ channels which start opening in a domino effect across the membrane allowing positively charged sodium ions to enter the cell shifting the charge inside the neuron to be positive reaching up to +50mV. This wave of positively charged ions travel along the axon. Action Potential doesn’t decay with distance as it can travel through up to 1m long axons and is very fast (especially in mylaineted axons which acts as an isolation layer).


In the case of mylenianated axons the nodes of Ranvier give a helping hand. These are gaps in the myelin rich in Sodium channels which allow another influx of Na+ to enter the axon. Positive sodium ions rejuvenates the action potential preventing it from dying out along the axon. The action potential slows down when reaching a node of ranvier because of the lack of myelin and speeds up again in a process called Saltatory conduction by leaps of depolarisation.


Repolarisation process


Following the influx of Na+ ions, the potential drops down even lower then the initial resting potential reaching -80mV due to intracellular environment trying to reach its ideal state opening up K channels which leave the membrane. In addition the Na+ channel close up. This goes up again to reach the -70mV resting potential shortly after in a refectory period. Please note that during this refectory period a nerve cannot generate another action potential due to the in-equilibrium of Na+ and K+ inside and outside of the nerve cell.

The electrical charge will travel down the axon into the Axon terminals where it will make its way to the next neuron in the nerve to reach its destination.


An example of how this happens will follow in another blog post.



Source:


1. College of Naturopathic Medicine lecture on the Nervous system https://www.naturopathy-uk.com/




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