Resting Potential, Graded Potential, Action Potential

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MCalavera
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11 Jun 2013, 5:21 am

I have an exam coming up soon (Psychology), and I would really like to understand these concepts better. So to all those who have high knowledge regarding this topic, please read and inform me of any corrections or anything else I need to know. Note that I only need to understand what's necessary for a Psychology student to understand, so no need for overly technical terms and concepts, just the basics.

Here's how I understand it thus far:

A neuron is at a state of resting potential (potential for what?) when it isn't "firing" an impulse along its axon. At its resting potential, there tends to be more "negativity" inside the neuron than right outside of its membrane due to the presence of lots of positive ions (mainly Sodium ions) outside the membrane and the fact that negative ions inside are never allowed out. This is in spite of the presence of several positively charged Potassium (K+) ions that are inside. The measure of voltage across the membrane tends to be around -70 mV (I'd be grateful if someone could elaborate on this one). With this measure, it is said to be polarized (I also don't know what this means exactly).

That said, the neuron is not always at resting state and will quite often receive signals from neighboring cells that somehow stimulate the neuron's membrane to allow Sodium (Na+) ions to go through it into the cell. This causes a depolarization of the cell via reducing the negative difference in charge between the inside and the outside. Sometimes, however, the opposite occurs with the entrance of negative ions into the neuron or the exit of positive ions (mainly Potassium ions) to the outside, hyperpolarizing the cell via increasing the negative difference between the inside and outside. Whether it depolarizes or hyperpolarizes the neuron, the change in charge indicates a graded potential, and if enough depolarization occurs at various points of the neuron before the axon (and almost simultaneously), the tension to fire an impulse builds up and an action potential occurs (the threshold that needs to be crossed for this to happen is around -50 mV). Once it hits -50 mV, it will inevitably hit a positive difference of around +40 mV by the time it reaches the axon, and a firing occurs. This action potential is somehow interpreted as a firing (some elaboration here would be good as well).

Now in order to resolve the depolarization matter, what happens is that Potassium ions are forced out of the cell until the charge inside the neuron is back to -70 mV (it will actually go beyond -70 for a little while to compensate for the previous sudden increase in positivity but then restores itself to the default -70). And then the neuron is back to its resting state, but not before a message had already been sent to the next neurons or cells or whatever.

Any mistakes or incomplete information in what I said, let me know.



ruveyn
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11 Jun 2013, 6:55 am

The neuron has to build up a reserved of sodium and potassium ions so it can be ready to discharged when it receives a signal from a dendrite.

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MCalavera
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11 Jun 2013, 1:17 pm

What does electric flow really mean exactly? Negative charge moving towards positive charge?



NewDawn
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11 Jun 2013, 6:19 pm

You've got the basics correct, but I'm afraid that if you want to have a deeper understanding of the electrical circuits of the cell, understanding of physics (electromagnetism) and cell biology (ion-gated and voltage-gated protein channels) is unavoidable.

Quote:
A neuron is at a state of resting potential (potential for what?)


A potential is a difference in electrical charge between two points. It means that a current can flow. This can be either from positive to negative, or from negative to positive, but by convention, current is arbitrarily defined as going from positive to negative. In a cell, the current between the outer membrane and the inner membrane is restricted because of the impermeability of the cell membrane. The membrane is composed of phospholipids that are hydrophobic, which means they will repell a charged atom such as a sodium or potassium ion (or a polar molecule like water, for that matter). However, there is some diffussion of ions both into and out of the cell. When this rate of diffussion is at an equilibrium, it's called 'resting potential'. It is indeed around -70 mV for most neurons. Voltage is a measurement for electrical current (to be more accurate, but also more obscure if you have little knowledge of electromagnetism: the work done per unit charge to move the charge from one point to the other)

Quote:
With this measure, it is said to be polarized (I also don't know what this means exactly).


It means that there are opposite charges on the outside and the inside of the cell membrane.

There are a lot of resources to learn more about the electrical activity of a cell, but I don't know how far you want to go (at the deepest level it requires knowledge of calculus, electromagnetism and proteins).



MCalavera
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11 Jun 2013, 11:07 pm

Thanks for the extra information. I know I need to learn stuff about physics, but I'll just try to understand it now without worrying about all the advanced stuff.

So a membrane potential is the potential to move between the membrane?

Also, in a current, what exactly is flowing within the context of the inside of a neuron in the form of a nerve impulse? The textbook really sucks at expounding on this, and the Internet resources always seem to miss stating the answers I'm looking for.

I know about the Sodium-Potassium Pump, but I don't want to hurt my head too much while preparing for the exam (it's not examinable), so I've intentionally put this one aside to avoid confusion. Later after the exam, I'll aim to understand this one even better.



NewDawn
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12 Jun 2013, 3:22 am

This is where we get to the ion-gated and voltage-gated channels. I'll try not to be too technical, but that means I'm also oversimplifying

What travels along the axon is the action potential. When a neurotransmitter binds to a receptor, the ligand-gated sodium ion channels are opened. (These are not the sodium-potassium pumps. The function of the s-p pumps is to maintain the resting potential). This causes sodium ions to flow into the cell, thereby causing a current and a lowering of the membrane potential. When the potential reaches -50mV (threshold), another transporter protein channel.that is sensitive to voltage change (voltage-gated ion channel) opens, letting even more sodium ions in. We now have an 'action potential'. The neuron 'fires'. Because voltage-gated ion channels are sensitive to voltage change, they will 'infect' neigbouring voltage-gated channels. In this way, the action potential is propagated along the axon.



NewDawn
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12 Jun 2013, 2:34 pm

This an explanation that isn´t too technical and yet very accurate to what actually happens. It also has pictures that make it easier to understand.

http://www.ncbi.nlm.nih.gov/books/NBK26910/#A2041



nebrets
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12 Jun 2013, 4:48 pm

The inside of a neuron axon is more negative than the outside, not just because there are negative ions inside but also because there are more positive ions outside.

The measure of voltage is the difference between the charge on the outside compared with the charge on the inside. There is a formula that describes this but you probably do not need to know it. Just know that the voltage is the difference in charge between the outside and inside.

Polarized is when this resting voltage has been built up. To do this the cell has to expend energy to bring in and maintain certain ions against their concentration gradient.

When a neuron receives an incoming signal, it received a neuro transmitter at a dendrite and that causes a local change in the charge across the membrane. If this is big enough to reach the axon, or if enough dendrites receive the signal so there is an additive effect there is an action potential. This happens when the local potential, or the sum of local potentials is enough to change the charge across the cell membrane to reach the threshold value.

The reaching of the threshold value causes the protein gates in the membrane to open allowing sodium ions to enter the cell. This increases the positive charge inside the cell so that the full depolarized charge is reached. This increase in charge causes the area of the membrane next to it to open the sodium ion channels allowing more sodium to move across the membrane, increasing the charge, and this effects the next section of the neuron so and so forth all the way down the axon (propagation).

This change in value and opening the ion channels down the axon is an action potential. This occurs very fast even though it is one part opening, allowing a change in charge and that allows the next section to open, allowing a change in charge, allowing the next section to open allowing a change in charge. This occurs very fast in part because the axon is very narrow, so the surface area to volume ratio is high, so it only takes a relatively small number of ions to effect a change once in the axon.

An action potential is also called a cell firing because when the neuron receives the signal it is like a person pulling a trigger on a gun, the movement of the action potential down the axon is like a bullet moving down a barrel of a gun (again, very fast), and the when the action potential gets to the synapse a neurotransmitter is released, like the bullet exiting a gun.

Hyperpolarization increases the negative charge difference across the cell membrane, thus making it harder for the cell to reach threshold. This makes it harder for the cell to fire.


Message me if this is unclear or if you have further questions.


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MCalavera
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13 Jun 2013, 12:52 am

Thanks to you both for taking the time to provide some needed information. I'll check the link as well.



ruveyn
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03 Jul 2013, 9:44 am

MCalavera wrote:

A neuron is at a state of resting potential (potential for what?)


Storing up electrochemical energy for later causing motion or chemical reaction. Storage batteries for example are electro chemical capacitors in which energy is store for generating electric and magnetic fields along wires.

BTW, what is going through a circuit is energy described by the Poynting vector caused by the vector product of the elctric and magnetic fields at the surface of wires. When electrons move a magnetic field is created around the wire on whose surface the electrons are moving under the influence of the electric field created by the battery or generator.

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OddDuckNash99
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04 Jul 2013, 7:02 am

All you need to know about action potentials: Na+ influx, K+ efflux. :lol: :lol: :lol:


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