Introduction to Psychopharmacology - Part 4 (Action Potentials)

Action Potentials - How Neurons Communicate with each other. 

To learn more about psychopharmacology, please check out "Psychopharmacology: Drugs, the Brain, and Behavior," by Jerrold S. Meyer and Linda F. Quenzer.

Now that we have talked about neurons individually, we are going explain how they communicate between each other. We mentioned in the last post that they do it by action potentials. Today we are going into detail of what are these, how they work, and why they work. There are two important features in this process: electric charge and chemicals, specifically sodium, potassium, and calcium. 

What is it?

An action potential is the electricity traveling inside a neuron (remember that our brain works on electricity). It starts at the axon hillock, which is the part located before the axon and after the soma, and ends in the terminal button. When it reaches its end, neurotransmitters, which are chemicals that send messages across neurons (1), are released from the terminal button. After that they bind on the receptors of another neuron (receptors are usually on the dendrites, but the can be in other parts of the neuron). They either inhibit a neuron, and thus stop it from firing another action potential, or they can excite the neuron, making it fire another action potential. This is a basic description of how neurons are modulatory, but the real process is much more complicated.

Synapse

Before starting  adumbrating the process of an action potential it is important to describe what the synapse is. There are three parts that are important: the presynaptic cell, the post-synaptic cell, and the presynaptic cleft. The latter is the space between neurons. It is important to state that neurons never touch each other, there is always a small space between them. The presynaptic neuron is the neuron that is above or before the synaptic cleft, it is the terminal button pointing at the receptors of the other cell. The post-synaptic cell is the neuron that is after or below the synaptic cleft, it is the receptor side that receives the neurotransmitters (NTs). Thus, the presynaptic neuron releases NTs, they travel through the synaptic cleft and they bind to the receptors on the post-synaptic cell.

Chemicals

We already mentioned the three important chemicals for action potentials: potassium, sodium, and calcium. The former is concentrated more on the inside of a neuron; it is what gives the neuron a negative charge. When neurons are not firing, which is called the resting membrane potential, they have a negative charge, which is a critical component in order to fire action potentials (we will cover more of this later). Sodium is concentrated more on the outside of the cell and it gives it a positive charge. This means that the inside of a neuron is negatively charged and positively on its exterior. Calcium is needed so that neurotransmitters can be released.

Channels

There are two types of channels in the membrane of a neuron. They are ligand-gated and voltage-gated channels. The former opens up when a ligand, which can be a neurotransmitter or a drug, binds to the receptors of the channel. The latter opens up when a specific charge is detected.

Action Potentials

We already talked about how neurons have a negative charge. Their resting potential is -70 millivolts (mV). This means that neurons are polarized when they are resting (remember this is one of the reasons why neurons are specialized cells).

 Neurotransmitters can do two things when they bind: They can either excite the neuron or inhibiting it from firing another action potential. These are called excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs) (2). EPSPs start to depolarize the neuron. If they do not change the charge to -55 mV an action potential is not fired. These are called failed initiations. However, if it does reach the threshold of  -55 mV, depolarization starts to occur. This means that sodium channels open up and sodium enters the cell. It does this for two reasons. The first one has to do with the electric charge of a neuron. Remember that in chemistry positives attract. Therefore, because the inside of a neuron is negative, the sodium, which has a positive charge, tries to enter it once the sodium channels open up.

 This is why it is important that neurons have a negative charge in order to fire an action potential. Once the cell depolarizes an action potential is fired. When the neuron reaches a positive charge, the sodium channels close and the potassium one opens up, and by potassium leaving the cell the neuron starts to repolarize. Potassium channels are not efficient, in the sense that they know when to close up. For this reason, the cell hyperpolarizes. This means that it obtains a more negative charge than when it is at the resting potential. During this period, which is also called absolute refractory period, actions potentials cannot be fired. The potassium channels close at this point and a sodium and potassium pump return the cell to the resting membrane potential. As you saw the first two channels were voltage dependent, they needed a specific charge to be opened. This means that action potentials don't really require energy, however, the sodium and potassium pump does and it uses ATP as its source of energy. When the action potential reaches the terminal buttons, voltage-dependent calcium channels open up and help neurotransmitters release. They in turn either inhibit the next neuron from firing an action potential or they excite it and the process repeats itself over and over again.   

References

1. http://www.nimh.nih.gov/health/educational-resources/brain-basics/brain-basics.shtml

2. http://www.ncbi.nlm.nih.gov/books/NBK11117/