BiologyDifficulty 2-5

Neurons

Neurons are nerve cells specialised to carry signals. A neuron receives input on its dendrites, adds it up at the axon hillock and, if the input reaches threshold, fires an all-or-nothing electrical impulse — an action potential — along its axon. At most synapses the signal crosses to the next cell as a chemical neurotransmitter. The adult human brain holds about 86 billion neurons and a similar number of non-neuronal cells.

Signalling: integration, conduction and transmission

Dendrites receive synaptic input, and the axon hillock integrates it. When the summed depolarisation reaches threshold (about −55 mV), an all-or-nothing action potential starts. It travels along the axon and, in myelinated fibres, is regenerated at each node of Ranvier.

Chemical synapse: Ca²⁺ enters the terminal and makes vesicles fuse and release neurotransmitter. The transmitter diffuses across the cleft and binds ligand-gated channels, which depolarise or hyperpolarise the postsynaptic cell.
Electrical synapse: gap junctions join the two cells and pass current directly. These synapses are fewer, but every nervous system has them.
Glia: astrocytes supply nutrients, regulate the extracellular ion and chemical environment and support synapses structurally.

Common misconception: A synapse can be inhibitory as well as excitatory: neurotransmitter binding may hyperpolarise the postsynaptic cell, making it less likely to fire.

Full explanation — the complete reference version every reading depth is based on

What a neuron is

A neuron is a cell built for communication. Like other cells it has a nucleus, mitochondria and the usual organelles, but it also has extensions for sending and receiving signals. Neurons work alongside glial cells, which support them — astrocytes, for example, supply nutrients, control the chemical surroundings of neurons and give structural support to synapses.

Dendrites: branching extensions that receive messages from other neurons at synapses.
Cell body and axon hillock: the axon hillock integrates the incoming signals.
Axon: carries the signal away to the axon terminals.
Axon terminals: release chemicals that pass the signal to other neurons, muscles or organs.
Myelin: an insulating sheath on some axons that speeds conduction; the gaps in it are the nodes of Ranvier.

The action potential

At rest, the inside of a neuron is about 70 millivolts more negative than the outside (−70 mV, varying with neuron type and species). Excitatory input makes it less negative. If it reaches the threshold of about −55 mV, voltage-gated sodium channels open and positive sodium ions rush in, driving the membrane to about +40 mV. Then sodium channels close, potassium channels open and potassium leaves, returning the membrane to negative. During this refractory period the neuron cannot fire again.

\begin{aligned} \Delta V &= V_{\text{peak}} - V_{\text{rest}} \\ &\approx (+40\,\text{mV}) - (-70\,\text{mV}) \\ &= 110\,\text{mV} \end{aligned}

Worked example: the swing in membrane potential from rest to the peak of a typical action potential (our arithmetic from the cited values).

Worked example: from rest (−70 mV) the membrane must rise by 15 mV to reach threshold (−55 mV); once it does, it swings all the way to about +40 mV — a total change of about 110 mV. A stronger stimulus does not make a bigger action potential, because firing is all-or-nothing.

Crossing the synapse

When an action potential reaches an axon terminal, calcium ions flow in and make tiny synaptic vesicles release neurotransmitter into the synaptic cleft — the gap between the two cells. The neurotransmitter diffuses across and binds to receptors on the next cell, opening ion channels there. Fewer synapses are electrical: the two cells are joined by gap-junction channels and current passes directly.

Common misconception: A common misconception is that neurons are wired together like electric cables, passing current straight from one to the next. At most synapses the signal crosses a gap as a chemical neurotransmitter; direct electrical synapses exist but are fewer.

Common misconception: It is often said the brain has 100 billion neurons and ten times as many glial cells. Direct counts of adult male brains found about 86 billion neurons and roughly the same number of non-neuronal cells.

Where this connects

Neurons are a specialised kind of cell, and the signals they carry depend on ions — sodium, potassium and calcium — moving across the cell membrane through channel proteins: an electric current carried by ions, driven by a potential difference measured in millivolts. Artificial neural networks in AI were inspired by the brain, but they are engineered systems, not realistic models of real neurons.

How we know

Camillo Golgi and Santiago Ramón y Cajal shared the 1906 Nobel Prize for their work on the structure of the nervous system. In 1952 Alan Hodgkin and Andrew Huxley published a quantitative description of the membrane currents behind nerve conduction and excitation, and in 1963 they shared the Nobel Prize with John Eccles for discoveries about the ionic mechanisms of nerve cell membranes. In 2009 Azevedo and colleagues counted brain cells directly using a method called the isotropic fractionator.

Fruit fly: 139,255 neurons in one adult brain, counted neuron by neuron for a 2024 wiring diagram.
Mouse: about 75 million neurons (textbook figure).
Octopus: about 500 million neurons, roughly 350 million of them in the arms.
Adult human (male, average): about 86 billion neurons.

Assumptions and limits

−70 mV, −55 mV and +40 mV are representative textbook values; resting potential in particular varies with neuron type and species.
'86 billion neurons' is an average for adult male brains, with a spread of about ±8 billion, counted using one marker (NeuN) and one method.
Not every synapse excites: at inhibitory synapses the postsynaptic membrane is hyperpolarised, making firing less likely.
Neuron counts for other animals come from different methods and cover different parts of the nervous system, so compare them only roughly.

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Neural networks — Related to
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Cell — Special case of

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Sources and methodology

Dendrites receive messages from other neurons at junctions called synapses, the axon hillock integrates the incoming signals, and the axon carries the integrated signal to axon terminals that synapse on other neurons, muscles or target organs. (awaiting scientific review)
- Biology 2e, 7.3.2: Neurons and Glial Cells (OpenStax section 35.1) — Textbook
Myelin acts as an insulator that increases the speed of action-potential conduction along an axon, and the nodes of Ranvier — gaps in the myelin about one micrometre long — contain voltage-gated channels that regenerate the action potential, so it effectively jumps from node to node. (awaiting scientific review)
- Biology 2e, 7.3.3: How Neurons Communicate (OpenStax section 35.2) — Textbook
A neuron at rest has a membrane potential of about −70 millivolts, the inside being negative relative to the outside, although the value varies with neuron type and species. (awaiting scientific review)
- Biology 2e, 7.3.3: How Neurons Communicate (OpenStax section 35.2) — Textbook
When a stimulus depolarises a neuron to its threshold potential of about −55 mV, voltage-gated sodium channels open and the membrane depolarises to about +40 mV; action potentials are all-or-nothing events. (awaiting scientific review)
- Biology 2e, 7.3.3: How Neurons Communicate (OpenStax section 35.2) — Textbook
After the peak of an action potential, sodium channels close and voltage-gated potassium channels open, repolarising the membrane; during the refractory period the neuron cannot produce another action potential. (awaiting scientific review)
- Biology 2e, 7.3.3: How Neurons Communicate (OpenStax section 35.2) — Textbook
At a chemical synapse, calcium ions entering the axon terminal cause synaptic vesicles to release neurotransmitter into the synaptic cleft, and the neurotransmitter diffuses across and binds to receptors on the postsynaptic membrane. (awaiting scientific review)
- Biology 2e, 7.3.3: How Neurons Communicate (OpenStax section 35.2) — Textbook
Electrical synapses, in which the two cells are physically connected by gap-junction channel proteins, are fewer in number than chemical synapses but are found in all nervous systems. (awaiting scientific review)
- Biology 2e, 7.3.3: How Neurons Communicate (OpenStax section 35.2) — Textbook
The adult male human brain contains on average about 86 billion neurons (86.1 ± 8.1 billion) and about 85 billion non-neuronal cells (84.6 ± 9.8 billion). (awaiting scientific review)
- Equal numbers of neuronal and nonneuronal cells make the human brain an isometrically scaled-up primate brain (Azevedo et al.) — Peer-reviewed paper
Direct counts found roughly equal numbers of neuronal and non-neuronal cells in the human brain, contradicting the widely quoted figures of 100 billion neurons and ten times as many glial cells. (awaiting scientific review)
- Equal numbers of neuronal and nonneuronal cells make the human brain an isometrically scaled-up primate brain (Azevedo et al.) — Peer-reviewed paper
A complete neuronal wiring diagram of an adult female fruit fly (Drosophila melanogaster) brain, published in 2024, contains 139,255 neurons connected by about 5 × 10⁷ chemical synapses. (awaiting scientific review)
- Neuronal wiring diagram of an adult brain (Dorkenwald et al., FlyWire Consortium) — Peer-reviewed paper
The mouse nervous system has about 75 million neurons. (awaiting scientific review)
- Biology 2e, 7.3.2: Neurons and Glial Cells (OpenStax section 35.1) — Textbook
An octopus has about 500 million neurons, the largest nervous system among invertebrates, and about 350 million of them are in the arm nervous system. (awaiting scientific review)
- Where Is It Like to Be an Octopus? (Carls-Diamante S.) — Peer-reviewed paper
Astrocytes, a type of glial cell, provide nutrients and other substances to neurons, regulate the concentrations of ions and chemicals in the extracellular fluid, and provide structural support for synapses. (awaiting scientific review)
- Biology 2e, 7.3.2: Neurons and Glial Cells (OpenStax section 35.1) — Textbook
Neurotransmitters can have excitatory or inhibitory effects: at an inhibitory synapse the postsynaptic membrane is hyperpolarised, making the neuron less likely to fire an action potential. (awaiting scientific review)
- Biology 2e, 7.3.3: How Neurons Communicate (OpenStax section 35.2) — Textbook
In 1952 Alan Hodgkin and Andrew Huxley published a quantitative description of the membrane currents of nerve and its application to conduction and excitation. (awaiting scientific review)
- A quantitative description of membrane current and its application to conduction and excitation in nerve (Hodgkin & Huxley) — Peer-reviewed paper
The 1963 Nobel Prize in Physiology or Medicine was awarded jointly to John Eccles, Alan Hodgkin and Andrew Huxley for their discoveries concerning the ionic mechanisms involved in excitation and inhibition in the peripheral and central portions of the nerve cell membrane. (awaiting scientific review)
- The Nobel Prize in Physiology or Medicine 1963 (Eccles, Hodgkin, Huxley) — Other (unclassified)
Camillo Golgi and Santiago Ramón y Cajal shared the 1906 Nobel Prize in Physiology or Medicine in recognition of their work on the structure of the nervous system. (awaiting scientific review)
- The Nobel Prize in Physiology or Medicine 1906 (Golgi, Ramón y Cajal) — Other (unclassified)

Claims marked “awaiting scientific review” cite the sources listed but have not yet been signed off by a scientific reviewer.

Content status: published 1 October 2026.

Scientific review: this version has not yet been signed off by a scientific reviewer.
The Advanced explanation has not yet been reviewed for age suitability.