A neuron has three main parts: dendrites that receive signals, a cell body that processes them, and an axon that sends them forward. These structures work together as a communication unit, passing electrical and chemical messages through the nervous system. Supporting structures like the myelin sheath and axon terminals add speed and precision to the process.
Dendrites: The Receiving End
Dendrites are the branching, tree-like extensions that spread out from the cell body. Their job is to pick up incoming signals from other neurons. The more branches a dendrite has, the more connections it can form, and some neurons have thousands of dendritic branches.
Along those branches sit tiny protrusions called dendritic spines, each roughly 1 to 3 micrometers long. These spines are where most of the actual signal reception happens. The rounded head of each spine makes contact with a neighboring neuron, and proteins embedded in the spine’s membrane detect chemical messages crossing the gap between cells. Spines aren’t fixed structures. They grow, shrink, and change shape in response to activity. When a connection between two neurons strengthens (a process tied to learning and memory), spines tend to get larger and more numerous. When a connection weakens, spines retract. This constant remodeling is one of the physical mechanisms behind how your brain adapts to new information.
The Cell Body (Soma)
The cell body, or soma, is the neuron’s control center. It contains the nucleus, which holds the cell’s DNA and directs gene activity. Surrounding the nucleus are the organelles that keep the neuron alive and functioning: energy-producing structures, waste-processing systems, and protein-building machinery.
One feature worth noting is the clusters of protein-making equipment called Nissl bodies, found throughout the soma. These are dense collections of ribosomes and internal membranes that produce the proteins a neuron needs to maintain its structure, grow new connections, and repair itself. Neurons are unusually active protein producers because they have such long extensions to maintain. A motor neuron running from your spinal cord to your foot, for example, needs to supply materials across a distance of up to a meter, all manufactured largely in the cell body.
The Axon Hillock: Where Signals Fire
Where the cell body tapers into the axon, there’s a small region called the axon hillock. This is the decision point. Dendrites feed incoming signals into the cell body, and those signals, some excitatory and some inhibitory, get summed together. If the combined signal is strong enough when it reaches the axon hillock and the initial segment just beyond it, the neuron fires an electrical impulse called an action potential.
This is an all-or-nothing event. If the threshold is met, the signal fires at full strength. If not, nothing happens. Research using simultaneous recordings from different parts of the neuron has shown that the action potential actually initiates in the axon just past the initial segment, then travels back to the cell body while also racing forward down the axon. The axon hillock region acts more as a gatekeeper, electrically isolating the firing zone from the noisy environment of the cell body.
The Axon: The Transmission Line
The axon is a long, slender cable that carries the action potential away from the cell body toward other neurons, muscles, or glands. Axons vary enormously in length. Some in the brain are less than a millimeter. Others, like those running from the base of your spine to your toes, can stretch over a meter.
Inside the axon, a network of tiny tubes called microtubules serves as a transport highway. Motor proteins act like molecular delivery trucks, hauling cargo in two directions: forward (toward the axon tip) to deliver fresh proteins and organelles, and backward (toward the cell body) to return used materials for recycling. These transport proteins move at roughly 1 micrometer per second. This internal supply chain is essential because the axon can’t manufacture its own proteins. Everything it needs comes from the cell body.
The Myelin Sheath and Nodes of Ranvier
Many axons are wrapped in myelin, a fatty insulating layer produced by specialized support cells. In the brain and spinal cord, cells called oligodendrocytes form the myelin. In the rest of the body, Schwann cells do the job. The myelin doesn’t cover the axon continuously. It wraps around in segments, leaving small gaps called nodes of Ranvier between each section.
This arrangement dramatically speeds up signal transmission. Instead of the electrical impulse crawling along every point of the axon membrane, it effectively jumps from one node to the next, a process called saltatory conduction. The myelin lowers the electrical capacitance of the covered sections, meaning the signal can skip ahead without losing strength. The result is striking: unmyelinated axons conduct signals at roughly 0.5 to 10 meters per second, while myelinated axons can reach speeds up to 150 meters per second. That’s the difference between a signal taking a full second to travel from your spine to your hand and one arriving almost instantly.
Axon Terminals and the Synapse
At the end of the axon, the cable splits into small branches tipped with button-like swellings called axon terminals (sometimes called synaptic boutons or terminal buttons). These terminals sit extremely close to the next cell but don’t actually touch it. The gap between them, called the synaptic cleft, is only about 12 to 50 nanometers wide, far too small to see with a standard microscope.
Inside each terminal, tiny sacs called synaptic vesicles sit loaded with chemical messengers (neurotransmitters), waiting near the membrane. When an action potential arrives at the terminal, it triggers voltage-sensitive calcium channels to open. Calcium floods in, and within less than a millisecond, it binds to sensor proteins on the vesicles. This causes the vesicles to fuse with the terminal membrane and dump their neurotransmitters into the synaptic cleft. The whole sequence, from action potential arrival to neurotransmitter release, takes under a thousandth of a second.
Those neurotransmitters then drift across the cleft and bind to receptors on the dendrites or cell body of the next neuron, starting the cycle over again. The speed and precision of this process depend on four things working in concert: a ready pool of loaded vesicles, calcium channels that open and close rapidly, sensor proteins that respond almost instantly to calcium, and tight physical proximity between the calcium channels and the sensors.
Neuron Shape Varies by Function
Not all neurons look the same. The layout of dendrites, cell body, and axon shifts depending on what a neuron does and where it sits in the body.
- Multipolar neurons have many dendrites branching from the cell body and a single axon. These are the most common type in the brain and spinal cord, including motor neurons that control muscles.
- Bipolar neurons have one dendrite on one side of the cell body and one axon on the other. They’re found in sensory pathways like the retina of the eye and the olfactory system.
- Unipolar (pseudounipolar) neurons have a single extension that splits into two branches, one heading toward the body’s periphery and one toward the spinal cord. Sensory neurons that detect touch, temperature, and pain use this design. The signal can travel from your fingertip to the spinal cord without passing through the cell body first, which saves time.
Despite these structural differences, every neuron relies on the same basic toolkit: dendrites to collect input, a cell body to maintain operations, and an axon to send the signal forward.