Neuroscience: The Action Potential

Action potential is something I find a lot of people who study psychology or neuroscience struggle with. For me, I just could never remember where it was sodium or potassium on the inside or outside of the cell membrane. Luckily, action potential is not as difficult to understand when you break it down into its major subsections.

Introduction

Action potentials are generated by neurons and are always consistent in size and duration; their strength does not diminish as it propagates down the axon. The strength of the action potential is determined by the frequency it propagates down the axon; the frequency and pattern of the action potentials generate a code used by neurons to transfer information appropriately from one location to another. Neuroscientists use an oscilloscope to study action potential as it sensitive to changes in voltage over time. The action potential works its way down the axon until it reaches the axon terminal. At the axon terminal, a synaptic transmission is initiated.

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Generating the Action Potential and All-or-None 

The perception of any sensation such as warmth or pain as well as any movement is the direct result of action potential. Depolarization of the neuron is what is necessary for an action potential to be generated, and it can arise in a variety of ways depending on the kind of neuron. The most common way to depolarize a neuron is by entry of sodium ions. The important thing is that the stimulus is strong enough to depolarize the neuronal membrane. Action potential works on what is known as all-or-none response, meaning it works on a threshold basis. As it is consistent in size, it relies on the stimulus exceeding a threshold called generator potential in order to fire, rather than the strength of the stimulus itself. This may seem confusing because yes, the strength of the stimulus must exceed the threshold , so in a way strength of the stimulus matters. However, what is important to remember is that once the threshold has been exceeded, the strength of the stimulus has no importance or effect on the strength of the action potential whatsoever.

Properties of Action Potential 

The membrane of a neuron at rest is around -65mV or -65 milli-volts with the regard to the outside. Volts is a good indicator that we are dealing with electrical current. During action potential, however, the membrane potential becomes briefly positive. This change from a negative to positive membrane potential is depolarization. Depolarization, a very rapid process, will continue until the membrane potential reaches 40 mV. To give you an idea how rapid the entire process of action potential including reaching back to -65mV it takes 2 milliseconds.

An action potential will only ever travel in one direction because of the refractory period. When the action potential travels away from the soma or cell body towards the axon terminals it is called orthodromic conduction. When the action potential travels towards the soma, the action potential is called antidromic conduction. Regardless of the direction it is traveling, action potential conductance rate varies, but the typical rate is 10m/s. The average length of an axon is about 2cm.

The Process 

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1. Resting Potential: As stated before, the resting membrane potential is around -65 mV but it ranges depending on the neuron. Different textbooks will tell you different resting membrane potentials, but it’s not something you need to worry too much about. It is also important to note than Vm or the membrane action potential is equal to 0. Vm=0.

Threshold: The membrane potential at which there stimulus is strong enough to open the sodium channels. The opening of the voltage-gated sodium channels changes the ionic permeability of the membrane in favour of sodium.

2. Depolarisation

3. Rising Phase: As the membrane potential is negative at the resting phase, the opening of the sodium channels creates a great pull. This is because the sodium ions (Na+) are positive. The result is sodium ions rushing into the cell causing rapid depolarisation. This changes the membrane action potential towards the equilibrium value of sodium or ENa.

Overshoot

4. Falling Phase: The voltage-gated sodium channels become inactivated, and in turn the voltage-gated potassium channels open. This time around, there is a great pull on the potassium ions (K+, also positive) outside the cell as the sodium ions have made the membrane potential very positive. Potassium ions rush outside the cell, resulting in a negative membrane potential once more, close to  Eor the equilibrium value of potassium.

Undershoot: The membrane potential is more negative than -65 mV.

Absolute Refractory Period: The cell cannot fire during this period as there is a gradual restoration of the membrane potential back to -65mV and the Vm to 0. During this time, even if the stimulus is strong enough to exceed the threshold, the membrane will not fire. It lasts about 1ms.

Relative Refractory Period: The membrane potential stays hyper-polarised or at the undershoot until the potassium channels close. It can be difficult to initiate another action potential even after the absolute refractory period, longer than 1 ms.

Factors Influencing the Conduction Velocity 

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The rate of the action potential depends on the strength of the depolarising current; specifically, it is reflected in firing frequency of the action potential. The maximum frequency, however, is 1000 Hz. Generally, axon potential conduction velocity increases diameter of the axon as well as the size of the axon itself and the number of ion channels. Smaller axons tend to require a much greater stimulus to reach the threshold necessary for depolarisation, as such as they are more sensitive to local anaesthetics.