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The Action Potential Scheme

This topic pops up a lot when talking about the nervous system. It is this action potential that allows us to have any sort of feelings in our senses! No kiddin’. So, I hope this will help it stick in your mind!

Our neurons work by an all or nothing method. This means, the action potential is either relaying info to and from your central to your peripheral nervous system and vice versa. In other words, you touch a hot stove, the hotness stimulates your peripheral nervous system which sends a signal to your brain (central nervous system), which gets your neurons to send out a signal to other nearby neurons to eventually get to the part of the brain that tells you “yes, it is hot!”

The function of the action potential is simply causing the release of a neurotransmitter into the synaptic cleft. The synaptic cleft is this guy right over there. It is the spacing between an axon and a dendrite.

There exists a potential difference between the inside of the neuron and its outside or the extracellular space. This difference is -70 mV. The neuron has a semipermeable plasma membrane that has similar characteristics of all other plasma membrane. The interesting thing is, the inside of the neuron is more negative than the outside of the neuron. Hence, the potential difference cannot be 0 mV. Neurons maintain this more negative inside by selective permeability of ions via the sodium potassium ATPase channel.

The insides of the Neuron:

  • high POTASSIUM (K+)
  • low SODIUM (Na+)
  • The opposite is true for the outside of the neuron.

Sodium Potassium ATPase

This channel are present in other parts of the body as well such as in muscles. Know that this channel’s purpose is to pump out 3 Sodium out of the cell for ever 2 Potassium that enters with the usage of 1 ATP. ATP is needed because the sodium and potassium are going against their gradients.

How does an Action Potential start?

The action potential is triggered when the stimulus causes a depolarization. The axon end at the synapse will get an excitation by the stimulus sent from the other neuron firing from its dendrite. When the axon is excited, it becomes more positive and eventually will reach a threshold of around -55 or -40 mV. (Recall the resting potential is -70 mV)

So:

  • depolarization –> excitatory –> more positive–>reach threshold of -50mV = action potential
  • hyperpolarization –> inhibitory–> more negative –> goes back to resting potential and beyond

How does the action potential continue?

This is where some can be confused. The movement of the sodium and potassium in and out of the neuron happens in an electrochemical gradient. What this means is, instead of looking of the concentration of the ion, we’re talking about the overall charge of the environment inside and outside of the neuron.

So this is the sequence of events that happens after the threshold has been reached!

1. Sodium channel opens up. Since the outside of the cell is more positive than the inside of the cell, the sodium ions will move into the cell (or neuron) because sodium is positive. Thus, the sodium is moving with the electrochemical gradient.

Note: The influx of sodium into the cell will bring the potential in the cell to about 35-40mV (see picture). The inside of the cell is now positive instead of the -70mV (resting) or -50mV (threshold).

2. Sodium channel closes.

3. Potassium channel opens. Now that the inside of the cell is at positive (35-40mV), the positively charged potassium will move out of the cell. Again, the potassium is following its electrochemical gradient.

Note: The efflux of potassium will repolarize the cell. Often times, the potassium will efflux enough to cause a hyperpolarization (meaning that the inside of the cell is more negative than what it’s resting potential was). Based on the image, say its about -90mV.

Hyperpolarization is important because it gives the potassium channel time to close.

Can multiple action potential happen at the same time?

No. The action potential once started needs to be completed before it can accept another stimulus to cause another action potential. Once an initiation in an action potential occurs, a refractory period is observed. There are two types:

1. Absolute refractory period means there is absolutely no stimuli, no matter how strong, will cause another action potential. This is reasoned by the fact that the action potential initiation is dependent on the sodium channel opening. Also, the neuron or cell needs to, at least be at its resting potential or in hyperpolarization in order for it to go about its second stimuli.

2. Relative refractory period means the duration of an action potential that may be able to be depolarized again because of another stimuli. Usually, this will be seen after hyperpolarization.

Take home

Remember that while the potassium and sodium are moving along its electrochemical gradient, it is going against its ion concentration gradient thus needs ATP to allow the channels to open.

 

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