Neurons: The Building Blocks of the Nervous System

Your brain is running a communication network more complex than anything humans have ever engineered. At the core of that network is a single type of cell: the neuron. The human brain contains roughly 86 billion neurons, each one constantly sending and receiving signals that shape your thoughts, emotions, movements, and mental health. Understanding how neurons work – structurally and functionally – is foundational to understanding why we think, feel, and behave the way we do.

Structure of a neuron
The cell body (soma)
Dendrites
The axon
Neural transmission: how the signal travels
The resting state and threshold
Depolarization and repolarization
Propagation along the axon
Synaptic communication: when neurons talk to each other
The synaptic cleft
Neurotransmitters: the chemical messengers
Reuptake, degradation, and clearance
Why neurotransmitter balance matters for mental health

Structure of a neuron

Every neuron, regardless of where it sits in the nervous system, shares three core components: a cell body (soma), dendrites, and an axon. Each part has a distinct role, and the three work together as a seamlessly integrated unit.

The cell body (soma)

The soma is the neuron’s control center. According to neuroscience research on NCBI, it contains the nucleus, where the cell’s DNA is stored, along with mitochondria, endoplasmic reticulum, and other organelles that keep the cell alive and functioning. Proteins needed throughout the neuron are manufactured here before being shipped out to the axon and dendrites. Without a healthy soma, the entire neuron breaks down.

Dendrites

Dendrites are the neuron’s input structures – branching extensions that receive incoming signals from other neurons and relay them toward the cell body. The word itself comes from the Greek for “tree,” and the branching structure lives up to that name. Neurons can have more than one set of dendrites, known as dendritic trees, and the number and complexity of these branches often reflects the neuron’s functional role. For example, Purkinje cells in the cerebellum have highly elaborate dendritic trees that allow them to receive signals from thousands of other neurons simultaneously. Dendrites are also covered in tiny protrusions called dendritic spines, which serve as the postsynaptic contact points for incoming signals.

The axon

While dendrites handle input, the axon handles output. The axon extends from the cell body and, after branching, ends at nerve terminals that pass signals to the next cell. Axons can be remarkably long – in humans, some stretch up to a meter in length. Most axons are wrapped in a fatty coating called the myelin sheath, produced by specialized support cells called glia. Myelin acts as an insulating layer that allows nerve impulses to travel more rapidly along the axon, much like insulation around an electrical wire. At the very end of the axon are the axon terminals – the sites where chemical messengers are stored and eventually released to communicate with neighboring cells.

Neural transmission: how the signal travels

Knowing the structure of a neuron is only half the story. What makes neurons remarkable is their ability to generate and transmit electrical signals rapidly across the body. This process is called neural transmission, and it hinges on a phenomenon known as the action potential.

The resting state and threshold

When a neuron is not actively firing, it exists in a resting membrane potential. The inside of a resting neuron is electrically negative relative to the outside – around -70 mV. This electrical imbalance is maintained by the unequal distribution of ions (mainly sodium and potassium) across the cell membrane. The membrane potential isn’t fixed; it fluctuates constantly depending on the incoming signals the neuron receives. An action potential is only generated when a stimulus reduces this negativity enough to reach the threshold potential, typically around -50 to -55 mV. Stimuli that fall below this threshold produce no response – this is known as the all-or-none principle.

Depolarization and repolarization

Once the threshold is crossed, a rapid and predictable sequence of events unfolds. Voltage-gated sodium channels open, allowing a flood of positively charged sodium ions into the cell – this is depolarization, where the inside of the cell briefly becomes electrically positive. At the peak of this change, the sodium channels close, and potassium channels open. Potassium ions rush out of the cell, restoring the negative charge inside – this is repolarization. The cell briefly overshoots into a hyperpolarized state before returning to its resting potential, ready to fire again.

Propagation along the axon

The action potential doesn’t just stay put – it travels. An action potential is generated near the cell body portion of the axon and then conducts down the axon toward the axon terminals. The myelin sheath plays a critical role here: instead of the signal traveling continuously, it jumps from one gap in the myelin (called a node of Ranvier) to the next, a process called saltatory conduction. This dramatically increases the speed of transmission. Diseases like multiple sclerosis, which destroy myelin, slow this conduction significantly, with real consequences for movement, sensation, and cognition.

Synaptic communication: when neurons talk to each other

An action potential traveling down one neuron is just the beginning. For information to move through the nervous system, neurons must communicate with each other – and they do so at specialized junctions called synapses.

The synaptic cleft

Neurons don’t physically touch each other. Chemical synaptic transmission occurs primarily through the release of neurotransmitters from presynaptic neural cells to postsynaptic receptors, across a tiny fluid-filled gap called the synaptic cleft. This gap between the presynaptic axon terminal and the postsynaptic dendrite is only 20-40 nanometers wide – impossibly small, yet absolutely essential to how brain communication works.

Neurotransmitters: the chemical messengers

When an action potential reaches the axon terminal, it triggers the release of neurotransmitters – chemical messengers stored in small sacs called synaptic vesicles. Each vesicle can contain thousands of neurotransmitter molecules, and when the electrical signal arrives, the vesicles fuse with the cell membrane and release their contents into the synaptic cleft. These molecules then drift across the gap and bind to specific receptors on the postsynaptic neuron.

The effect of this binding depends entirely on the type of neurotransmitter involved. Neurotransmitters are categorized as excitatory, inhibitory, or modulatory depending on how they affect brain activity. Excitatory neurotransmitters like glutamate increase the likelihood that the receiving neuron will fire its own action potential. Inhibitory ones like GABA do the opposite – they calm neural activity. Neurotransmitters influence numerous functions, including emotions, thoughts, memories, learning, and movements.

Reuptake, degradation, and clearance

After a neurotransmitter delivers its message, it doesn’t linger indefinitely in the synaptic cleft. Neurotransmitter molecules are cleared from the synaptic cleft through one of three processes: reuptake (reabsorption by the releasing neuron for reuse), degradation by enzymes within the synapse, or diffusion away from the cleft. This clearance mechanism keeps signaling precise and prevents overstimulation. Many psychiatric medications directly target this process – for example, selective serotonin reuptake inhibitors (SSRIs) block the reabsorption of serotonin, increasing its availability in the synapse and helping to regulate mood in people with depression.

Why neurotransmitter balance matters for mental health

The efficiency of synaptic communication has direct consequences for mental health. Significant imbalances or disruptions in neurotransmitter systems are associated with a range of mental health and neurological conditions, including depression, anxiety, ADHD, Parkinson’s disease, and addiction. Serotonin deficiency is closely linked to depression and anxiety; dopamine dysregulation underlies conditions ranging from schizophrenia to addiction; and GABA deficits can make the nervous system overactive, contributing to seizure disorders and anxiety. Research has shown that patients with endogenous depression have low plasma levels of tryptophan, a precursor of serotonin, which has directly informed the development of antidepressant medications.

It’s important to note that neurotransmitter function is not a simple “more is better” equation. The nervous system relies on a finely tuned balance between excitation and inhibition. The effect of a neurotransmitter depends on its type, the receptors it binds to, and where in the nervous system the signal is released – meaning the same chemical can have very different effects in different contexts. This complexity is why mental health conditions are rarely explained by a single neurotransmitter and why treatments continue to evolve.

What do you think? Given that neurotransmitter imbalances can influence mood, cognition, and behavior at a biological level, how does this change the way you think about mental health conditions – are they still purely psychological, or does the biology fundamentally shift the picture? And if drugs like SSRIs work by modifying how neurons communicate, what does that suggest about the relationship between brain chemistry and personal experience?

References

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Table of Contents

Structure of a neuron 🔗

The cell body (soma) 🔗

Dendrites 🔗

The axon 🔗

Neural transmission: how the signal travels 🔗

The resting state and threshold 🔗

Depolarization and repolarization 🔗

Propagation along the axon 🔗

Synaptic communication: when neurons talk to each other 🔗

The synaptic cleft 🔗

Neurotransmitters: the chemical messengers 🔗

Reuptake, degradation, and clearance 🔗

Why neurotransmitter balance matters for mental health 🔗