Introduction

What is a neuron? Think of neurons as the foundation of your entire existence. They are tiny cells that make up your brain and nervous system. Without neurons, your body wouldn’t know how to move, how to breathe, how to see, feel, or believe. Neurons, because of their vital importance, deserve to be studied.

Therefore, in this post, I will write so you may understand the basics of these cells, how they are structured, how they send messengers to other neurones, and how they are divided into subcellular zones. By the end of this reading, you should be able to picture and understand how neurons work.

What is a Neuron?

To begin, neurons are the living cells of the central nervous system. They are also called nerve cells. They exist in the CNS which includes the brain and spinal cord. The brain alone is composed of about 86 billion neurons.

Going back to the basics of biology. Cells are the fundamental unit of life. They are what all living things are made of from trees to people to my once-alive goldfish.

Cells group together to form tissues. These tissues work together to form organs. These organs are organised together to form organ systems that function collectively to create an organism.

In this case, the cell is each neuron, the tissue is each brain region (like the prefrontal cortex [PFC] & hippocampus), the organ is the brain itself, the organ system is the central nervous system, and the organism is you.

Neurons are far more fascinating than typical body cells because they are highly specialized. They are designed to perform specific tasks, execute them at the right time, and carry them out with the utmost efficiency.

As we will see in the next section, neurons have specific structures located in different regions that enable them to perform their duties with other neurons. Before reading the next section, however, it is important to understand the general process of how neurons communicate.

Neurons communicate with each other electrochemically. This means communication between neurons is possible via two processes: the conduction of action potential (electro-) down the axon and the transmission of chemical signals (neurotransmitters) across the synapse.

Neurons receive a stimulus, most commonly from other neurons, in the form of a neurotransmitter, which serves as a communication molecule. This neurotransmitter is released from the presynaptic (signal-sending) neuron into the synaptic cleft where it binds to receptors in the postsynaptic (signal-receiving) neuron.

This stimulus eventually triggers an action potential (AP) which acts as an electrical signal carrying the converted information down the neuron where it will cause the release of its transmitters restarting the entire process of neuronal communication. This communication is made possible by the seamless interplay between electrical signals (APs) and chemical signals.

For a deeper understanding of each process, read these 2 articles.

Zones & Structures

Understanding the answer to “What is a neuron?” involves breaking it down into its functional zones and corresponding structures.

This specialised cell can be divided into four zones. Each zone has congruent structures that play a role in communication.

The first zone is known as the input zone. The input zone is characterised by the reception of a stimulus. In other words, it is the zone where the postsynaptic (signal-receiving) neuron receives information in the form of a chemical signal from the presynaptic (signal-sending) neuron.

This signal exerts its effects at the synapses, Think of a synapse as a connection. This connection occurs when the presynaptic chemical signals bind to postsynaptic receptors. These synaptic effects result in an action potential that is initiated at the axon hillock. An AP is the rapid reversal of the neuron’s membrane potential which, from the hillock, travels down the axon to the axon terminals.

The most notable structures of the input zone are the dendrites which are branches. They contain dendritic spines which are finger-like projections that increase the neuron’s surface area to allow more synapses to occur. As mentioned before, synapses occur when another structure, a receptor, binds to a ligand. These effects travel across the soma (cell body containing the nucleus) activating the next zone.

The second zone is the integration zone. It is characterised by the initiation of the AP due to the effects of the input zone. The most notable structure of the integration zone is the axon hillock. It is directly responsible for adding up the effects of the input zone to initiate an AP.

The third zone is known as the conduction zone. This zone is characterised by a long fibre-like projection extending from the soma (cell body) to the final zone. It is known as the axon. This structure conducts or propagates the AP all the way down to the axon terminals.

The axon has special structures called myelin sheaths. Myelin is a fatty layer that wraps around the axon and helps it increase the AP conduction speed. It does this in two ways.

First, the axon has a property and consequence that occurs as the AP travels down it. This inherent property, known as capacitance, decreases the speed of the action potential because the axon lipid membrane stores and separates the electrical charge. However, when myelin is present, this fatty layer increases conduction. The insulation wraps around an axon but does not physically compress it. It is highly non-conductive and there are relatively few sodium channels at myelinated regions so propagation does not occur.

Instead, the action potential “jumps” from one node of Ranvier to the next in a process called saltatory conduction, where the signal regenerates only at these spots. These nodes of Ranvier are gaps of the exposed axon that sit in between myelinated regions of the axon.

This presence of myelin may seem counter-intuitive at first, however, the conduction speed increases due to it reducing capacitance and increasing resistance. Increased resistance is a result of myelin reducing the leakage and loss of ions.

Myelin forces the action potential to propagate only at the nodes of Ranvier. This process restricts sodium channels to places of the nodes which dramatically increases the speed at which electrical signals travel along the axon.

Think of saltatory conduction as skipping a rock (the AP) on water (the axon). When you throw the rock, the longer it hits the water the slower and shorter the distance it travels. However, if you skip the rock it jumps from distance to distance increasing its overall distance across the lake. This jumping technique is what makes rocks and action potentials travel further and faster.

Finally, the action potential conducts to the final zone known as the output zone. This zone is characterised by the transmission of a chemical signal across the synapse. This is the location where the presynaptic (signal-sending) neuron releases neurotransmitters into the cleft where they eventually form synapses.

The most notable structures in this stage are the axon collaterals, but more so, the axon terminals. The axon collaterals are just branches that give rise to the terminals. These terminals store and release neurotransmitters in the cleft which is the gap that exists between neurons. The output zone also contains receptors but I will explain that in a separate post.

One thing to note is that after the presynaptic transmitter binds to postsynaptic receptors it is released back into the cleft where it is broken down or reabsorbed by the presynaptic neuron for later use at the output zone.

Neuronal Potentials

The electrochemical communication and functionality of neurons depend on the changes that occur with their membrane potentials.

If you have taken a chemistry class, you may know that potential energy is a type of energy an object has because of its position and/or mass. For example, if a 12-lb bowling ball is about to be dropped from the top of a building, it has more potential energy than that of a whiffle ball at ground level.

Also, if someone (like a kid who plays chess at the level of a grandmaster by the age of 10) has potential, they are understood to have some capacity to become someone great in the future (like a world chess champion).

Neurons exhibit different types of potentials. In this case, a potential is just a stored electrical charge. Neuronal potentials are measured in millivolts (mV). The first potential to be aware of is that of a neuron when it is not producing or conducting any activity.

In this state, we can say a neuron is at rest and therefore has a resting potential or stored electrical charge when at rest. This potential serves as its baseline or Goldilocks zone otherwise referred to as homeostasis.

Neurons, because of their cellular mechanisms, have a resting membrane potential (inside the cell) that is more negative than the outside of the cell. A neuron’s resting potential is somewhere around -70 mV.

Membrane potential just refers to the neuron’s potential. The membrane is the fatty lipid bilayer acting as a wall separating the inside of the nerve cell (intracellular) from the outside of the nerve cell (extracellular).

So, when I mention, membrane potential I am referring to the difference in stored electrical charge between the inside and outside of the neuron.

At rest, the negatively charged neuron (relative to the positive outside) is maintained by the sodium-potassium pump which actively moves 3 Na⁺ ions out and 2 K⁺ ions in. This creates a negative resting membrane potential usually around -70 mV (milliVolts).

Ion movement across the membrane is influenced by two pressures: the concentration gradient, which drives ions from high to low concentration, and the electrical gradient, which attracts positive ions to negative areas.

Together, these gradients generate the electrical potential needed for the initiation of an action potential.

Conclusion

Neurons are the foundation of everything in our mind, from our thoughts to our ability to experience the world. By understanding their structures, mechanisms, and roles within the nervous system, we gain a deeper appreciation of how these cells participate in communicative processes within the nervous system.

Hopefully, you are now equipped with the appropriate knowledge to continue your neuroscience journey. However, if you still cannot answer “What is a neuron?” and would like more practice and something to help you consolidate the information you read about, see the mastery sheet below.

References

Erulkar, S. D. and Lentz, . Thomas L. (2024, December 6). Nervous system. Encyclopedia Britannica. https://www.britannica.com/science/nervous-system

Morell, P., & Quarles, R. H. (1999). The Myelin Sheath. Nih.gov; Lippincott-Raven. https://www.ncbi.nlm.nih.gov/books/NBK27954/

Watson, N. V., & S Marc Breedlove. (2021). The mind’s machine: foundations of brain and behavior. Sinauer Associates/Oxford University Press.

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