Action potentials are one of my favourite topics of neuroscience. Once you get down to the small details, what seemed incredibly complex is infact relatively simple!
So what is an action potential?
An action potential is how one neuron communicates with another. As you may know, electricity is a flow of current, or a flow of charged particles. The term ‘charged particles’ doesn’t necessarily apply to electrons; rather it applies to anything with a charge. Within the brain, sodium and potassium constitute to this current.
When a neuron fires, this action potential is produced. You can think of your neurons like tiny batteries. Each has the ‘potential’ to release energy, in the form of moving charged particles.
This potential to release energy arises from the imbalance of charges in the neuron. A resting neuron is polarised. This means that there is an overall charge. The intracellular compartment of the neuron is full of potassium ions (K+), and the extracellular environment is full of sodium (Na+) ions. However, there are more sodium ions outside than there are potassium ions inside, which gives the neuron an overall negative charge. In fact, the resting potential of a neuron is -70mV.
So how does the neuron polarise?
Using the Sodium-Potassium pump. This is a protein present in the plasma membranes of neurons which transports sodium and potassium backwards and forwards within the neuron. For every 2 potassium it pumps in, 3 sodium ions are pumped out. This creates a concentration difference, called an electro-potential gradient. Since both sodium and potassium have the same charge, overall the neuron is negatively charged compared to the extracellular space. In this resting state, the neuron is polarised. The resting potential of neurons is usually around -70mv.
There are many different ion channels, each which function differently.
Some are voltage gated channels. This means they open and close in response to changes in the membrane potential, normally around -55mV.
Some are ligand-gated channels , which open when a neurotransmitter attaches, ie seratonin.
Finally there are mechanically gated channels, which open due to physical stretching of the membrane.
How do neurons fire?
Firstly, there is a stimulus. This causes sodium channels to open. For example, if the stimulus is pressure, this pressure may deform the cell membrane of neurons, causing the protein channel to change shape and thus open. Since there is a higher concentration of Na+ outside the cell, these sodium ions rush in. Due to this influx of positive ions, the membrane potential starts to increase (ie gets more positive). There is a phenomenon called the all or nothing principle which states that a neuron will only fire at a voltage of -55mV. If the membrane potential is any lower than this, it will not fire. Once the neuron reaches this Pd of -55mV, the voltage gated sodium channels rapidly open, causing more positive sodium ions to flood in. This further increases the membrane potential until it reaches a state of +40mv. At this point, the neuron is depolarised. This occurs in the space of milliseconds.
The definition of an action potential is a brief depolarisation caused by changes in currents. Thus, as a result of this change in ion concentration, an action potential is created.
Once the action potential is created in this neuron, it propagates down the axon. Since the local current would be quite strong, the neurons in close proximity will be affected in the same way (the voltage gated Na channels here too will open), resulting in depolarisation, which generates more action potentials so an electrical impulse is transmitted down neurons.
After the sodium channels have opened, they close again rapidly. Now the potassium channels open. This results in a net efflux of K+ ions in attempt to equalize the potassium concentration either side of the membrane. It is key to note that the potassium channels open much more slowly than the sodium channels. This allows time for the sodium to flush in and the depolarisation to be completed. If both channels were open simultaneously, the membrane potential would be constantly neutral resulting in no action potentials. As potassium floods out, the membrane potential begins to neutralise, heading towards -70mV.
However, since the potassium ions leave so rapidly, the membrane potential typically will overshoot the mark of -70mV, a stage called hyperpolarisation. The potential can reach values of around -90mV. Hyperpolarisation actually plays a key role in transmission; during hyperpolarisation the neuron cannot receive any other impulses, which prevents disruption of the initial action potential. This is because the new threshold value will understandably be larger than if the neuron was at its resting potential of -70mV (from -90 to -55, rather than -70 to -55). Furthermore, this hyperpolarisation prevents the action potential from travelling the wrong way down the axon. Without this crucial stage, another action potential could hypothetically be induced in the axon in the opposite direction. The period when another action potential cannot be produced is called the refractory period.
Now, the potassium ions are in the extracellular space, and the sodium ions are inside the cell. You may have realised that this is the wrong way round; Na+ should accumulate outside the cell. So,after this stage of hyperpolarisation the sodium potassium pump acts to bring the membrane potential back to its resting value of -70mV. It is important to point out that this Na/K pump acts continuously, but its role is crucial to restore the concentration of ions once an action potential has been produced. The pump requires ATP to work, as it actively transports the ions across the membrane.