Neuron Structure: How a Nerve Cell Sends and Receives Signals

A neuron is a cell built for one job: moving information. Its shape matches that job. It has a wide receiving surface, a control center, and a long cable for carrying what it decides. Once you know those parts, the nervous system stops looking like magic. It starts looking like engineering.

This guide covers the parts of a neuron. It explains how an electrical signal travels along one. It then follows the signal across the gap to the next cell. Finally it describes the supporting cells, which take up about half the brain by volume.

The Four Parts of a Neuron

Neurons are excitable cells. They fire electrical signals called action potentials. They link up into networks called neural circuits. They pass information to each other at junctions called synapses. At most synapses the signal crosses as a chemical. Nerves first appeared in worm-like animals about 550 to 600 million years ago. The ability to fire a fast electrical signal probably came earlier, between 700 and 800 million years ago.

The four functional regions of a neuron: dendrites, cell body, axon and axon terminals. Image: Curtis Neveu, CC BY-SA 4.0, via Wikimedia Commons

Dendrites: The Receiving Surface

The dendrites are the branches that reach out from the cell body. Their job is to collect signals. Branching gives one neuron a very large surface. The membrane of each branch carries receptors. Those receptors bind chemicals released by other cells. A pyramidal neuron in the cortex can carry tens of thousands of dendritic spines. Each spine is a separate receiving site.

The Cell Body: Where Signals Are Added Up

The cell body, or soma, holds the nucleus and the organelles the neuron needs to stay alive. It is also the decision point. Every incoming signal causes a small voltage change. The cell body adds those changes together. If the total crosses a threshold, the neuron fires. If it does not, the signal fades and nothing is sent.

The Axon: The Cable

The axon is a long, slender extension. It carries the action potential away from the cell body. Its length matches its job. A motor neuron running from the spinal cord to a muscle in the hand needs an axon about a meter long. A signal bound for a fingertip cannot afford to travel slowly. Interneurons inside the cortex may have axons a fraction of a millimeter long.

Axon Terminals: The Output

At the far end the axon splits into branches called axon terminals. These do not touch the next cell. They stop a tiny gap away. That gap is where the electrical signal becomes a chemical one.

Sending the Signal: From Rest to Spike

Every neuron starts at a baseline voltage. The resting membrane potential sits near −70 millivolts. That means the inside is negative compared with the outside. Two things set it up. A pump moves sodium out of the cell and potassium in. The membrane is also far more permeable to potassium than to sodium. Potassium therefore leaks outward. It carries positive charge away with it. That leaves the inside negative. The voltage settles near −70 mV. The pump maintains the ion gradients behind this, rather than creating the voltage directly.

Uneven ion distribution, plus a membrane far more permeable to potassium than to sodium, holds the resting potential near −70 mV. Image: explorebiology, CC BY 4.0, via Wikimedia Commons

When the summed input crosses threshold, the neuron fires. Sodium channels open. Sodium rushes in, and the inside swings positive. This reversal is called depolarization. It triggers the same change in the membrane just next to it. So the signal travels down the axon as a wave. Potassium channels then open. Positive charge leaves, and the membrane returns to its resting state.

An action potential is all-or-nothing: the voltage spikes, then returns to rest. Image: explorebiology, CC BY 4.0, via Wikimedia Commons

Two features make this signal reliable. First, it is all-or-nothing. There is no half-strength spike, so a signal arriving at a terminal is always a full one. Second, it is followed by a refractory period. During that short window, more stimulation has no effect, because the membrane has not reset. The refractory period sets the fastest firing rate of the neuron. It also stops a signal from bouncing back and forth, so it travels one way along the axon.

Myelin and Saltatory Conduction

A bare axon conducts, but slowly. Most axons in the body are wrapped in myelin. Myelin is a lipid-rich insulator. Think of the insulation on an electrical wire, except that it does not cover the whole length in one piece. It covers defined stretches called internodal segments. Between them are short bare gaps called the nodes of Ranvier. Each node is about one micrometer long.

The signal is regenerated at each node, so it appears to jump from gap to gap. Image: Mgcaptainzanko, CC BY 4.0, via Wikimedia Commons

Those gaps are the point. Myelin greatly limits ion movement along the internode. So the action potential can only be rebuilt at the nodes. It therefore appears to leap from one node to the next. This is called saltatory conduction, from the Latin saltus, meaning leap. It is much faster than pushing the signal along bare membrane. It is also far more efficient. The neuron pumps ions only at the nodes, not along the whole axon.

Crossing the Gap: The Synapse

A synapse is a junction between a neuron and a cell next to it. At a chemical synapse, the sending membrane is called the presynaptic membrane. It sits very close to the receiving cell. When the action potential arrives, vesicles in the terminal release neurotransmitters. These chemicals diffuse across the gap and bind to receptors on the target. The target may be another neuron, a muscle cell, or a gland.

The chemical step: vesicles empty into the cleft, and receptors on the far side decide what happens next. Image: Scientific Animations, CC BY-SA 4.0, via Wikimedia Commons

Nobody knows exactly how many neurotransmitters humans use. More than a hundred have been identified. The common ones include glutamate, GABA, acetylcholine, glycine, dopamine and norepinephrine. What the chemical does to the target depends less on the molecule than on the receptor it lands on. The same transmitter can excite one cell and inhibit another. That happens purely because the two cells carry different receptors.

This is also where the neat idea of a digital signal breaks down. The change caused in the receiving membrane is a postsynaptic potential. Postsynaptic potentials are graded, not all-or-nothing. A weak input produces a small change. A strong input produces a large one. Hundreds of these graded signals are added up in the receiving cell. Only the total decides whether that cell fires its own action potential. Learning and memory work partly by strengthening or weakening single synapses. That is why this graded stage matters so much.

Not all synapses are chemical. At an electrical synapse, two cells are joined directly by gap junctions. Signals pass both ways at once, with no chemical step. These synapses help large groups of cells act together. Their direction is often not well defined, and they can also produce chaotic activity.

The Supporting Cast: Glial Cells

Neurons get all the attention, but they are not the whole story. Glial cells do not fire action potentials. Yet they make up about half the volume of the human brain. They hold the chemical environment steady, build myelin, and give physical and metabolic support. A neuron cannot work without them. (In cell numbers the glia-to-neuron ratio is actually less than one to one; the half is a measure of volume, not of count.)

A single oligodendrocyte can myelinate several axons at once. Image: Holly Fischer, CC BY 3.0, via Wikimedia Commons

Oligodendrocytes and Schwann Cells: The Myelin Makers

In the central nervous system, myelin is built by oligodendrocytes. Each one sends out extensions that wrap around several axons. Depending on the region, one oligodendrocyte can myelinate 20 to 60 axons. In the peripheral nervous system the job falls to Schwann cells, named after the German physiologist Theodor Schwann. Schwann cells come in myelinating and non-myelinating forms. They also support nerve growth and repair. Which cell does the insulating is not a detail. It is why a demyelinating disease behaves differently depending on where in the body it strikes.

Astrocytes: The Environment Department

Astrocytes are the star-shaped cells of the brain and spinal cord. Their name literally means star cells. They make up roughly 20 to 40 percent of the glial population. They control the cells that form the blood-brain barrier. They supply nutrients to nervous tissue, keep extracellular ions balanced, and regulate cerebral blood flow. They also help repair and scar the brain after injury. Also, they are the main source of cholesterol in the central nervous system. They carry it to neurons on a transport protein called apolipoprotein E.

Astrocytes branch out to contact blood vessels, synapses and other glia. Image: Bruno Pascal, CC BY-SA 3.0, via Wikimedia Commons

Microglia: The Immune System of the Brain

Microglia are the resident immune cells of the nervous system and its first line of defense. They are about a tenth of the glial population. Unlike other glia, they do not come from nervous tissue. They start life in the yolk sac. Microglia constantly search for plaques, damaged or surplus neurons and synapses, and infectious agents. They are also unusually sensitive. Part of the reason is that they carry potassium channels that respond to very small changes around them. That lets them notice trouble early.

Ependymal Cells: The Lining of the Fluid Spaces

Ependymal cells form the thin, ciliated lining of the brain’s ventricles and the central canal of the spinal cord. Their cilia help move cerebrospinal fluid through those spaces. The fluid itself is made mainly by a different tissue, the choroid plexus.

Glial type	Where it is found	Main job
Oligodendrocyte	Central nervous system	Builds myelin; one can cover 20 to 60 axons
Schwann cell	Peripheral nervous system	Builds myelin; supports nerve growth and repair
Astrocyte	Brain and spinal cord	Blood-brain barrier, nutrients, ion balance, blood flow, repair
Microglia	Brain and spinal cord	Immune defense; clears debris and monitors synapses
Ependymal cell	Brain ventricles, spinal canal	Lines the fluid spaces and helps move the fluid
Satellite cell	Peripheral nervous system	Supports neuron cell bodies in ganglia

When the Structure Fails

A neuron depends on its shape and its covering. So structural damage shows up as a loss of signal. Doctors can measure the speed of conduction directly. This is called nerve conduction velocity. A drop in that number is one of the clearest signs that myelin has been damaged.

The best-known example is multiple sclerosis, or MS. MS is an autoimmune disease of the central nervous system. The immune system attacks myelin. As the covering is stripped away, the axon behaves more like bare membrane. The signal slows down and may fail altogether. Damage can also reach the axon itself. Symptoms depend on which nerves are affected. They include double vision, vision loss, eye pain, muscle weakness and stiffness, tingling, numbness, balance problems, fatigue, and trouble with memory and thinking. Diagnosis uses symptoms, MRI scans, and sometimes a lumbar puncture to test the spinal fluid. There is no cure. But disease-modifying treatments can cut the number of relapses. Physical therapy and other care help manage the symptoms.

Loss of myelin in an MS lesion: the dark myelin staining is broken up and patchy. Image: Marvin 101, CC BY-SA 3.0, via Wikimedia Commons

Demyelinating diseases fall into two broad groups. In myelinoclastic diseases, healthy myelin is destroyed by something external: a toxin, a virus, or an autoimmune attack. In leukodystrophic diseases, the myelin was never built correctly in the first place. Suspected causes range from genes and viral infection to environmental exposures. Even a vitamin B12 deficiency can cause faulty myelin formation, rather than damage to good myelin.

The lesson is that structure is function. An axon cannot be rewired by good intentions. A dendrite cannot be widened. A myelin sheath cannot simply be reassembled once it has been stripped. What makes the nervous system remarkable is not that its cells are complicated. It is that each one is shaped so precisely for the single task it performs.

Key Terms in Plain English

These are the words in this article that would send you to a dictionary. Each one is given here the way it is actually used above.

Term	What it means here
Neuron	A cell that fires electrical signals to move information.
Dendrite	The branched surface of a neuron that collects incoming signals.
Cell body	The part of the neuron holding the nucleus; it adds signals up.
Axon	The long fiber that carries a signal away from the cell body.
Action potential	The fast electrical spike a neuron fires.
Resting potential	The baseline voltage of a quiet neuron, near minus 70 mV.
Depolarization	The moment the inside of the membrane swings positive.
Refractory period	A short pause after a spike when the neuron cannot fire again.
Myelin	A fatty insulator wrapped around an axon to speed the signal up.
Node of Ranvier	A bare gap in the myelin where the signal is rebuilt.
Saltatory conduction	The signal jumping from node to node along a myelinated axon.
Synapse	The junction where a neuron passes a signal to another cell.
Neurotransmitter	The chemical that carries a signal across a synapse.
Postsynaptic potential	The graded voltage change a signal makes in the receiving cell.
Glial cell	A supporting cell of the nervous system that does not fire spikes.
Oligodendrocyte	The glial cell that builds myelin in the central nervous system.
Schwann cell	The glial cell that builds myelin in the peripheral nervous system.
Astrocyte	A star-shaped glial cell that steadies the neuron environment.
Microglia	The immune cells of the nervous system.

The words in this article that would otherwise send you to a dictionary.

Key Takeaways

• A neuron has four parts: dendrites that receive, a cell body that adds signals up, an axon that carries the spike, and terminals that pass it on.
• The resting membrane potential sits near −70 mV. It comes from uneven ions and a membrane far more permeable to potassium than to sodium.
• An action potential is all-or-nothing. The refractory period after it limits firing rate and keeps the signal travelling one way.
• Myelin insulates the axon in segments. The signal jumps between the nodes of Ranvier in saltatory conduction, which is faster and cheaper.
• At a chemical synapse, over 100 known neurotransmitters cross a narrow gap. The receptor on the target cell decides what happens next.
• Postsynaptic potentials are graded, not all-or-nothing. Adding them up is where learning and memory begin.
• Glia take up about half the brain by volume. Oligodendrocytes, Schwann cells, astrocytes, microglia and ependymal cells each have a clear maintenance job.

Frequently Asked Questions

Why does an axon need myelin instead of being one bare wire?

Mainly for speed. Pushing an action potential along bare membrane is slow, because the voltage has to be rebuilt continuously. Myelin lets the signal leap from node to node instead, which is far faster. The second reason is cost. Ion exchange is what burns energy in a neuron, so doing it only at the nodes saves a great deal. Multiple sclerosis shows why this matters. When myelin is lost, conduction becomes measurably slower and can stop altogether.

Are glial cells just support staff, or do they do real work?

They are support staff, and that is the important job. Glia take up about half the brain by volume, but they do not fire spikes. Firing would interfere with the signaling they exist to allow. Instead they control the conditions neurons need. Astrocytes maintain the blood-brain barrier and the ion balance. Oligodendrocytes and Schwann cells build the insulation. Microglia patrol for damage. Ependymal cells manage the fluid spaces. A neuron cut off from all of this cannot hold a signal.

Can neurons divide or repair themselves after injury?

Mature neurons are very poor at dividing. That is a big reason why brain and spinal cord damage tends to last. The adult nervous system keeps a small ability to make new neurons from resident precursors. But it is limited, and in most cases it does not replace cells lost to injury. The peripheral nervous system fares better. Schwann cells and the tubes they help form give a severed axon a path to grow along. That difference is why the same injury can have very different outcomes depending on where it happens.

How is a chemical synapse different from an electrical one?

A chemical synapse has a gap, and the signal crosses it as a molecule. That makes it one-way, slightly slower, and adjustable. The strength of such a synapse can be raised or lowered, and that is the cellular basis of learning. An electrical synapse has no gap. The two cells are joined by gap junctions and share a continuous interior. Signals cross both ways at once, with no delay. Electrical synapses are good at making a group of cells act as one. But their direction is often not well defined.

Sources: resting potential and action potentials follow NCBI Bookshelf (Physiology, Resting Potential) and standard physiology reviews. Glial cell types and proportions follow Verkhratsky et al., The Concept of Neuroglia, and von Bartheld et al. on neuron and glial numbers. Microglial origin follows Ginhoux and Prinz. Ependymal cells follow Deng et al. The multiple sclerosis section follows NINDS and the National Multiple Sclerosis Society. Figures are from Wikimedia Commons, with the author and license named in each caption.