Neurons: The Core Units of the Nervous System and Their Role in Processing Information

Every thought you’ve ever had, every movement you’ve made, and every sensation you’ve ever felt – all of it comes down to the activity of tiny cells called neurons. These nerve cells are the fundamental building blocks of your nervous system, and understanding how they’re built and how they work gives us a window into how the brain processes the world around us. With roughly 86 billion neurons in the human brain alone, it’s no exaggeration to say they are the machinery behind everything that makes us human.

What is a neuron? 🔗

Neurons, also known as nerve cells, are electrically excitable cells that generate and transmit signals throughout the nervous system. Unlike most other cells in the body, neurons are structurally and functionally unique – they are specifically designed to carry information rapidly across long distances, sometimes spanning over a meter in length. The nervous system itself is divided into two major parts: the central nervous system (CNS), which includes the brain and spinal cord, and the peripheral nervous system (PNS), which connects the CNS to the rest of the body. Neurons are the core working units of both.

The concept that neurons are the basic structural and functional units of the nervous system was first formally established as the neuron doctrine, put forward by Spanish anatomist Santiago Ramón y Cajal in the late 19th century. It was a revolutionary idea at the time – that the nervous system is composed of discrete, individual cells rather than a continuous meshwork. This foundational principle remains central to modern neuroscience.

The structure of a neuron 🔗

While neurons come in many shapes and sizes depending on their location and role, all neurons share three core structural components: dendrites, a cell body (soma), and an axon. These three parts work together to receive, integrate, and transmit signals.

Dendrites 🔗

Dendrites are the branching, tree-like extensions that project from the cell body. Their primary role is to receive incoming signals from other neurons and relay them toward the cell body. They receive and conduct electrical signals toward the cell body, acting as the neuron’s main input receivers. A single neuron can have many dendrites, each equipped with receptor sites that bind to chemical messengers called neurotransmitters. The more dendrites a neuron has, the more signals it can potentially receive and integrate at one time.

The cell body (soma) 🔗

The cell body, or soma, is the metabolic core of the neuron. It houses the nucleus, which contains the cell’s genetic material, along with organelles like mitochondria and the endoplasmic reticulum that keep the cell functioning. Beyond its housekeeping role, the cell body plays a critical decision-making function: it is where inputs from dendrites are processed and integrated to determine whether an electrical impulse will be generated. Think of it as the neuron’s command center – it collects all incoming signals and decides whether to pass them on.

The axon 🔗

The axon is the long, slender projection that carries electrical signals away from the cell body toward other neurons, muscles, or glands. In contrast to the shorter dendrites, the axon can extend for more than a meter – making it one of the longest structures in the human body. The axon begins at a region called the axon hillock, where the signal is first generated, and ends at the axon terminals, where the message is passed on to the next cell.

Many axons are wrapped in a fatty coating called the myelin sheath. This insulating layer, produced by glial cells, dramatically increases the speed at which electrical signals travel down the axon. The myelin sheath protects the axon and prevents the loss of electrical charge during transmission of action potentials, allowing signals to jump efficiently between small gaps in the sheath called nodes of Ranvier. Damage to myelin – as occurs in diseases like multiple sclerosis – disrupts this fast transmission and leads to a wide range of neurological symptoms.

The three functional types of neurons 🔗

Neurons are not all the same. While there are thousands of subtypes, neurons can be broadly classified into three functional groups: sensory neurons, motor neurons, and interneurons (also called relay or association neurons). Each type plays a distinct role in how information flows through the nervous system.

Sensory neurons 🔗

Sensory neurons (also called afferent neurons) carry signals from the outside world – or from within the body – toward the brain and spinal cord. They are activated by sensory input from the environment, such as pressure on the skin, changes in temperature, light entering the eye, or sound waves hitting the ear. Once activated, they transmit this sensory data to the CNS for processing. Most sensory neurons are pseudounipolar in structure, meaning they have a single axon that splits into two branches.

Motor neurons 🔗

Motor neurons (also called efferent neurons) do the opposite – they carry signals away from the CNS and out to the body’s muscles and glands. They release neurotransmitters to trigger responses leading to muscle movement, making them directly responsible for every voluntary and involuntary physical action. Motor neurons are typically multipolar, with one long axon and many short dendrites. There are two types: lower motor neurons that travel from the spinal cord to muscles, and upper motor neurons that travel between the brain and spinal cord.

Interneurons (association neurons) 🔗

Interneurons are found exclusively within the CNS and serve as the connectors between sensory and motor neurons. In addition to transferring signals between sensory and motor neurons, interneurons can communicate with each other, forming circuits of various complexity. They are the most numerous type of neuron in the human body, and they are central to reflexes, learning, decision-making, and all complex cognitive processes. When you recall a memory, process an emotion, or work through a problem, interneurons are at work.

How neurons transmit information: the action potential 🔗

The signal that travels within a neuron is an electrical event called an action potential. Action potentials are the fundamental units of communication between neurons and occur when the sum total of excitatory and inhibitory inputs causes the neuron’s membrane potential to reach a threshold of around -50 mV. When this threshold is crossed, the neuron “fires” – generating a rapid electrical signal that travels down the entire length of the axon.

This follows the all-or-nothing principle: if the neural threshold is not reached, the neuron will not fire; if it is reached or exceeded, the amplitude of the action potential is the same regardless of the level of stimulation. The neuron either fires fully or not at all – there is no in-between.

How neurons communicate: synaptic transmission 🔗

Once an action potential travels down the axon and reaches the axon terminal, it faces a physical barrier: a tiny gap between the sending neuron and the receiving one. This gap is called the synapse (or synaptic cleft). Neurons do not make direct contact with each other – instead, electrical signals within a neuron are converted into chemical messages that cross this gap using chemicals called neurotransmitters.

Here is how the process unfolds step by step:

When an action potential arrives at the axon terminal, it triggers the opening of calcium channels. Calcium floods into the terminal, causing synaptic vesicles (small pouches containing neurotransmitters) to fuse with the cell membrane and release their contents into the synaptic cleft. After travelling across the synaptic cleft, the neurotransmitter attaches to receptors on the postsynaptic side, causing either positive or negative ions to flow through channels in the membrane. This either excites the receiving neuron – making it more likely to fire its own action potential – or inhibits it, making firing less likely.

Common neurotransmitters include dopamine, which plays a key role in movement and reward; serotonin, involved in mood regulation; glutamate, the brain’s main excitatory signal; and GABA, the main inhibitory messenger. Understanding synaptic transmission is crucial in psychopathology and treatment – for instance, drugs like SSRIs work by preventing the reuptake of serotonin in the synapse, prolonging its mood-regulating effects. Similarly, the dopamine hypothesis for schizophrenia implicates excessive dopamine activity in certain brain regions as a driver of symptoms.

From stimulus to response: neurons in action 🔗

To see how all of this comes together, consider a simple example: you accidentally touch a hot pan. Sensory neurons detect the heat at the skin and fire signals to the CNS. Interneurons in the spinal cord process that signal and relay it both upward to the brain (for conscious awareness of pain) and directly to motor neurons (triggering a reflex withdrawal before you even consciously decide to pull away). Motor neurons then instruct your arm muscles to contract, pulling your hand back. This entire loop – from sensation to reaction – involves all three types of neurons, orchestrated through action potentials and synaptic transmission.

This kind of reflex arc is one of the clearest demonstrations of how neurons are organized into functional circuits. More complex behaviors – learning, memory, language, emotion – rely on vastly larger networks of neurons forming and strengthening connections over time, a process called synaptic plasticity.

When neurons are damaged 🔗

Because most neurons are amitotic (they lose the ability to divide and replicate), damage to them is often permanent and difficult to reverse. This is why serious brain or spinal cord injuries can result in lasting impairment. Structural degeneration of dendrites in hippocampal neurons is linked to the early pathology of Alzheimer’s disease, while demyelination of axons underlies conditions like multiple sclerosis and Guillain-Barré syndrome. Understanding neuron biology, therefore, is not just academically interesting – it is central to understanding and treating some of the most challenging conditions in medicine and mental health.

Research into neurogenesis – the creation of new nerve cells – continues to advance, with studies suggesting it occurs throughout life, particularly in areas like the hippocampus. This holds exciting promise for conditions involving neuronal loss, including Alzheimer’s and Parkinson’s disease.

What do you think? Given that most neurons cannot regenerate once lost, how does this change the way we should think about protecting brain health across a lifetime? And if synaptic transmission underlies both our thoughts and our emotions, what does it mean for the relationship between brain chemistry and personal experience?

References

1. https://www.healthline.com/health/neurons
2. https://en.wikipedia.org/wiki/Neuron
3. https://byjus.com/biology/neurons/
4. https://www.kenhub.com/en/library/physiology/parts-of-a-neuron
5. https://content.byui.edu/file/a236934c-3c60-4fe9-90aa-d343b3e3a640/1/module6/readings/neuron_structure.html
6. https://www.simplypsychology.org/neuron.html
7. https://qbi.uq.edu.au/brain/brain-anatomy/types-neurons
8. https://qbi.uq.edu.au/brain-basics/brain/brain-physiology/action-potentials-and-synapses
9. https://www.cliffsnotes.com/study-guides/psychology/psychology/psychology-biological-bases-of-behavior/neural-transmission
10. https://www.simplypsychology.org/synaptic-transmission-a-level-psychology.html