Understanding Neurons: The Building Blocks of the Nervous System

Every thought you have, every movement you make, and every sensation you feel comes down to a single type of cell firing signals at lightning speed. The neuron is the basic working unit of the nervous system, and it is so fundamental that you cannot really understand psychology, behaviour, or mental processes without first understanding how these tiny cells operate. In this post, we will break down what neurons are, what they look like, how they are classified, and why they matter so much for everything we do.

What exactly is a neuron? 🔗

A neuron is a specialised cell that is excitable, meaning it can generate and transmit electrical signals called action potentials across a network of other neurons. These signals travel through the nervous system and help the body receive, process, and respond to information from both the outside world and within the body itself. According to a widely cited overview of neurons, neurons are the main components of nervous tissue in almost all animals, though they are absent in sponges, placozoans, plants, and fungi.

The human brain alone is believed to contain around 100 billion neurons at birth, and each neuron can form thousands of connections with its neighbours. This creates a communication network of staggering complexity, which is why neurons are often described as the “building blocks” of the nervous system. They work alongside supporting cells called glial cells, which provide scaffolding, insulation, and nourishment, but it is the neuron that does the actual signalling work.

The idea that neurons are discrete, individual cells (and not a continuous web of tissue) was established in the late 19th century. The Spanish anatomist Santiago Ramón y Cajal is credited with founding what became known as the neuron doctrine, which states that neurons are the basic structural and functional units of the nervous system. The term “neuron” itself was later popularised by the German anatomist Heinrich Wilhelm Waldeyer in 1891.

The three main parts of a neuron 🔗

While neurons come in different shapes and sizes depending on where they live and what they do, almost all of them share three core components. These are the dendrites, the cell body (also called the soma), and the axon. Together, these parts allow the neuron to receive, process, and pass on information.

Dendrites: the receiving end 🔗

Dendrites are the short, branch-like extensions that stick out from the cell body. The word itself comes from the Greek word for “tree,” and once you see a diagram of a neuron, the name fits perfectly. Dendrites are the neuron’s input sites, where signals are received from other neurons and transmitted electrically toward the cell body.

A single neuron can have dozens or even thousands of dendritic branches. The number and complexity of these branches often reflects the neuron’s job. For example, Purkinje cells in the cerebellum are famous for their incredibly elaborate dendritic trees, which let them gather signals from thousands of other neurons at once.

The cell body (soma): the control centre 🔗

The cell body, or soma, is the part of the neuron that houses the nucleus. Inside, you will find the usual cellular machinery – cytoplasm, mitochondria, endoplasmic reticulum, Golgi apparatus, and other organelles – just like in any other cell of the body. The nucleus contains the cell’s DNA and directs protein synthesis. These proteins are manufactured in the soma before being shipped out to the axon and dendrites, which is why a healthy soma is essential; without it, the whole neuron breaks down.

Signals arriving from the dendrites travel across the soma and, if strong enough, trigger an electrical impulse that fires down the axon. In that sense, the soma acts both as a power plant and a decision-making point for the neuron.

The axon: the transmitter 🔗

The axon is a long, tube-like fibre that carries the electrical impulse away from the cell body toward other neurons, muscles, or glands. Some axons are extremely short, while others are surprisingly long. The longest axon in a human motor neuron, for instance, can stretch more than a metre, all the way from the base of the spine down to the toes.

Many axons are covered in a fatty substance called the myelin sheath, which is produced by glial cells. Myelin acts as an insulator that lets electrical signals travel much faster along the axon. The sheath is not continuous; it has small gaps called the Nodes of Ranvier that help speed things up even more. When the myelin sheath is damaged, as happens in diseases like multiple sclerosis, signal transmission slows down and symptoms such as dizziness, fatigue, and loss of motor control can appear.

At the far end of the axon are the terminal buttons, which contain tiny sacs called synaptic vesicles. These vesicles hold chemical messengers known as neurotransmitters. When an electrical impulse reaches the terminal buttons, neurotransmitters are released into the tiny gap – the synapse – between one neuron and the next, where they bind to receptors on the receiving neuron’s dendrites in a lock-and-key fashion.

How neurons convert stimuli into electrical impulses 🔗

One of the most fascinating things about neurons is that they translate the world around us into a language the brain can understand: electricity. A stimulus – say, the warmth of a cup of chai, the pinch of a needle, or the sound of your name being called – is picked up by sensory receptors. These receptors convert the stimulus into an electrical signal that travels through the neuron.

When signals received at the dendrites reach a certain threshold at the cell body, the neuron “fires” an action potential. This electrical wave rushes down the axon, triggers the release of neurotransmitters at the synapse, and the message is passed to the next cell. This is how touching something hot can lead, within a fraction of a second, to you yanking your hand away.

Classifying neurons by function 🔗

Neurons are not all alike. Based on what they do, they fall into three functional categories: sensory neurons, motor neurons, and association neurons (also called interneurons).

Sensory neurons 🔗

Sensory neurons, sometimes called afferent neurons, carry information from sensory receptors in the skin, eyes, ears, nose, tongue, and internal organs toward the central nervous system. They are the reason you can feel the warmth of sunlight, taste your morning tea, or hear a doorbell. These neurons convert signals from the external environment into corresponding internal stimuli that the brain and spinal cord can interpret.

Motor neurons 🔗

Motor neurons, or efferent neurons, do the opposite. They carry commands from the central nervous system out to the muscles and glands, telling them what to do. When you decide to pick up a glass of water or blink your eyes, motor neurons are the ones delivering the instructions. According to clinical neuroanatomy resources on motor neurons, damage to these cells can lead to conditions like amyotrophic lateral sclerosis (ALS), which causes progressive muscle weakness and paralysis.

Association neurons (interneurons) 🔗

Association neurons, more commonly called interneurons, sit between sensory and motor neurons and act as the connecting link. They are found exclusively within the central nervous system and are by far the most abundant type of neuron in the human body. Interneurons process incoming sensory information, integrate it with what the brain already knows, and help decide what response to generate.

A classic example of interneurons at work is the knee-jerk reflex arc. When a doctor taps the patellar tendon, sensory neurons pick up the stretch and send a signal to the spinal cord, where interneurons quickly coordinate with motor neurons to contract the quadriceps muscle. The entire reflex arc can occur without any input from the brain, which is why your leg jerks before you even register the tap.

Classifying neurons by structure 🔗

Neurons can also be grouped based on how their processes extend from the cell body. The three main structural categories are unipolar, bipolar, and multipolar neurons.

Unipolar neurons 🔗

A unipolar neuron has only one process, or neurite, extending from the cell body. This single process then branches out to form dendritic and axonal endings. True unipolar neurons are mostly found in the central nervous systems of invertebrates like insects, while humans usually have what are called pseudounipolar neurons – a related form in which the single process splits into two branches shortly after leaving the cell body. Most sensory neurons in the human peripheral nervous system are pseudounipolar.

Bipolar neurons 🔗

Bipolar neurons have two processes extending from the cell body: one axon and one dendrite, each sticking out from opposite ends. They are uncommon and occur only in a few specific areas, such as the olfactory epithelium (which plays a role in smell) and the retina of the eye. They are specialised for transmitting signals related to special senses.

Multipolar neurons 🔗

Multipolar neurons are the most common type in the human body. They have one axon and many dendrites, which lets them receive and integrate a huge amount of information. Multipolar neurons make up the vast majority of neurons in the brain and spinal cord, and all motor neurons that control skeletal muscles fall into this category.

Why understanding neurons matters for psychology 🔗

Since every psychological process – from learning a new language to feeling anxious before an exam – ultimately depends on neurons firing and communicating, understanding their structure and function gives us a biological foundation for understanding behaviour. Disorders like depression, schizophrenia, Parkinson’s disease, and Alzheimer’s all involve problems at the level of neurons and their neurotransmitters. That is why psychopharmacology, a field that develops medications to treat mental health conditions, focuses so heavily on how drugs affect neural communication.

What do you think? If every thought, feeling, and action ultimately comes down to neurons firing, does that change the way you view free will or personal experience? And given how central these cells are to everything we do, which aspect of neural function do you find most surprising or worth exploring further?

References

1. https://en.wikipedia.org/wiki/Neuron
2. https://openstax.org/books/psychology-2e/pages/3-2-cells-of-the-nervous-system
3. https://courses.lumenlearning.com/waymaker-psychology/chapter/cells-of-the-nervous-system/
4. https://byjus.com/biology/neurons/
5. https://www.ncbi.nlm.nih.gov/books/NBK554616/
6. https://en.wikipedia.org/wiki/Reflex_arc
7. https://www.britannica.com/science/reflex-arc
8. https://en.wikipedia.org/wiki/Unipolar_neuron
9. https://www.medicalnewstoday.com/articles/unipolar-vs-bipolar-vs-multipolar-neurons