The basic building block of the nervous system is the nervous cell or neuron. The special property of the neuron, the one which allows us to perceive and interact with our environment and makes it possible to talk about communication in the nervous system, is the generation and conduction of action potentials (APs). However, in order to understand how APs are generated and conducted, we need to talk about the structure of the neuron (because, as it usually happens in biology, structure dictates function).
A neuron (Figure 1) can typically be divided in three parts: dendrites, soma (cell body), and axon.
The typical structure of the soma is outlined in Figure 2. As every other cell, the neuronal soma has the following substructures:
- Golgi apparatus
- smooth endoplasmic reticulum
- rough endoplasmic reticulum
While the somatic structure is conserved across cells, the two types of projections mentioned above, the axon and the dendrites, are unique to the neuron and are particularly designed for conduction of electrical signals. They are thinner compared to the rest of the cell body and generally devoid of organelles (with a few exceptions, these stay mainly in the soma). In the following paragraphs we will take a closer look at each one of them.
Dendrites are tree-like filaments of the neuron and are specialized in receiving and integrating inputs from other neurons and passing these processed signals to the soma. As a dendrite extends from the cell body, it branches out extensively, becoming thinner with each branching. Typically, dendrites don’t extend for more than a few hundred micrometers away from the soma.
The axon is a single, long protrusion, which is specialized in conducting APs and transmitting them to other neurons or muscle cells. Each axon leaves the cell body at a swelling called the axon hillock. It can extend for great distances and give rise to numerous branches called axon collaterals. Unlike dendrites, the axon retains the same diameter across its length. Some axons can be covered by a myelin sheat – a fatty substance which acts as an insulator, i.e. it does not allow electric charge to pass through the membrane. However, this sheat is interrupted at certain points in the membrane (you can learn why here). These spaces are called the nodes of Ranvier. As we will see later, the myelin sheat leads to a tremendous increase in the propagation speed of APs.
The last structural elements in the composition of neurons which we have to understand before being able to delve into the details of AP generation and propagation are the ion pumps and the ion channels. Ion pumps are transmembrane proteins that move ions across the cell membrane against their concentration gradient. Therefore, ion pumps help build or maintain concentration gradients, which in turn serve as the electrochemical basis for APs. Ion channels, in turn, are pore-forming membrane proteins which allow the passage of ions in the direction of the concentration gradient.
As mentioned in the introduction, the function of the neurons is to generate and conduct APs. An AP can be described as a rapid rise and fall in the membrane potential. The typical membrane potential (so-called “resting-state” potential) is generated due to a difference in ion concentration between the two sides of the plasma membrane (Figure 3). Usually, there is a larger concentration of Na+ on the outside of the cell, and a larger concentration of K+ on the inside of the cell. Furthermore, large proteic anions (negatively charged proteins) are found inside the cell. While the Na+ and K+ ions are small enough to pass through the cell membrane at specific points (ion channels*), the large proteic anions are trapped inside. This makes the inside of the cell negatively charged with respect to the outside and gives rise to the aforementioned resting-state potential.
During the resting-state, the Na+ channels are closed and some of the K+ channels are open, causing an efflux of K+. If a stimulus causes the membrane voltage to become more positive, some of voltage-gated Na+ channels open, causing Na+ to flow inside the cell (according to its concentration gradient, i.e. from high concentration to low concentration), which in turn leads to more Na+ channels opening. At this point, two scenarios become possible: if not enough Na+ channels are opened, the membrane potential starts returning to its resting-state due to the ongoing K+ efflux; if, in turn, enough Na+ channels are opened, the Na+ influx becomes strong enough to counter the K+ efflux and the neuron depolarizes further (i.e. the membrane potential becomes more positive), until it reaches a positive value (peak of the AP). Once this peak is reached, the Na+ channels inactivate and the previously closed K+ channels also open. These two events lead to the membrane potential falling back towards the resting-state. Finally, due to the fact that many K+ channels are opened and they close much slower compared to the Na+ channels, the membrane potential overshoots the resting-state potential. This stage in the AP is called afterhyperpolarization. As most of the K+ channels start closing, the neuron reaches resting-state potential once more and the ion pumps reestablish the concentration gradient by actively pushing Na+ outside of the cell and bringing K+ inside.
The process described above repeats itself at nearby membrane patches until it reaches the synapse. The repetition is why myelinated axons have a much faster conduction speed: since the axon is electrically isolated by myelin, the AP can only be generated at the nodes of Ranvier, which means that it can jump from node to node (this process is also known as saltatory conduction). Importantly, naturally generated APs are usually initiated at the axon hillock, as this area has the largest concentration of Na+ channels (which means that lower intensity stimuli have higher chances of triggering an AP here compared to other parts of the neuron). Furthermore, these APs only travel in one direction, from the soma towards the synapse. This is ensured by the inactivation of Na+ channels, which leads to a refractory period*.
As the AP reaches the synapse, it causes neurotransmitter release, which in turn binds to another neuron, where it might trigger the whole process once again. While there are many more subtleties to this intricate mechanism, the action potential is the building block of neuronal communication and of our perception, thoughts, and emotions.
*Ion channels = large proteins which span the cell membrane and allow passage of ions under certain conditions; can be voltage-gated (only open when a certain voltage is reached), ligand-gated (only open when a specific molecule, e.g., the neurotransmitter acetylcholine is bound to them) or stretch-gated (open in response to mechanic deformation of the membrane)
*Refractory period = period of time during which, regardless of how strong a stimulus is, it cannot elicit a new action potential (absolute); or a much stronger stimulus than usual is needed in order to elicit a new action potential (relative)
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