Study topic · about 3 minutes

Neurons and nerve signals

A neuron is a nerve cell built to send signals. It collects messages through its branching dendrites, adds them up in its cell body, and, if they are strong enough, fires an electrical pulse, the action potential, down its axon, which passes the message to the next cell at a synapse.

Key points

The human brain has about 86 billion neurons, and roughly as many glial cells that support them.
Signals travel one way: dendrites, cell body, axon, axon terminals.
An action potential is all-or-none: once a neuron reaches its threshold, it fires at full strength.
Myelin, a fatty wrapping made by glial cells, makes signals travel much faster.
At the synapse, the electrical signal becomes a chemical one: neurotransmitters cross a tiny gap to the next cell.

What are the parts of a neuron?

Part	What it does
Dendrites	Branches that receive signals from other neurons.
Cell body (soma)	Holds the nucleus, keeps the cell alive and adds up the incoming signals.
Axon hillock	Where the cell body meets the axon. The action potential starts here.
Axon	A long cable that carries the signal away from the cell body, in some nerves about a meter long.
Myelin sheath	Fatty insulation around many axons that speeds the signal up.
Nodes of Ranvier	Small gaps in the myelin where the signal is renewed as it jumps along.
Axon terminals	Endings that release neurotransmitters onto the next cell.

Types of neurons

Sensory neurons carry signals from the body's senses toward the brain and spinal cord.
Motor neurons carry commands from the brain and spinal cord out to muscles and glands.
Interneurons (also called relay neurons) connect neurons to each other inside the brain and spinal cord. Most neurons are interneurons.

In a reflex arc, all three work together in the spinal cord. Touch a hot stove, and a sensory neuron signals an interneuron, which fires a motor neuron that pulls your hand away, before the pain signal has even reached your brain.

Glial cells: the support team

Glial cells don't fire action potentials, but the brain could not work without them. Astrocytes feed neurons and control the chemicals around them. Oligodendrocytes wrap myelin around axons in the brain and spinal cord, and Schwann cells do the same in the nerves of the body. Microglia clean up damage and fight infection.

How does a neuron fire?

Resting potential. At rest, the inside of a neuron is negative compared with the outside, about -70 millivolts, because charged particles called ions are unevenly spread across its membrane.
Threshold. Signals from other neurons nudge the voltage up. If it rises past a threshold, about -55 millivolts, the neuron fires.
Depolarization. Sodium channels open, positive sodium ions rush in, and the inside briefly turns positive.
Repolarization. Potassium channels open, potassium ions flow out, and the voltage drops back.
Refractory period. For a moment the neuron cannot fire again, so the signal can only move forward along the axon.

The action potential is all-or-none: a stronger stimulus does not make a bigger action potential. It makes the neuron fire more often.

Why myelin matters

In a myelinated axon, the signal jumps from one node of Ranvier to the next instead of creeping along every bit of membrane. This is called saltatory conduction, and it lets the fastest axons carry signals at over 100 meters per second.

When myelin is damaged, signals slow down or fail. In multiple sclerosis, the immune system attacks myelin in the brain and spinal cord, which can cause weakness, numbness and vision problems.

How signals cross a synapse

Neurons don't touch. When an action potential reaches an axon terminal, the terminal releases neurotransmitters into a gap about 20 nanometers wide, the synaptic cleft. They attach to receptors on the next neuron and make it more likely to fire (excitatory) or less likely (inhibitory). Then they are cleared away, often by being pumped back into the neuron that released them, which is called reuptake.

A 3D synapse releasing neurotransmitter onto the next cell — A synapse in the 3D app: vesicles in the axon terminal release neurotransmitter across the cleft to receptors on the next cell.

In myasthenia gravis, the immune system blocks the receptors that muscles use to receive acetylcholine, so the muscles tire and weaken. Read more in neurotransmitters.

Zoom into a neuron and its synapse in 3D

Quick quiz

5 questions. Your answers are saved and come back in your daily review.

Which part of a neuron receives signals from other neurons?
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Dendrites. Dendrites branch out to collect incoming signals; the axon carries the signal away.
What does myelin do?
Show the answer
Speeds signals along the axon. Myelin insulates the axon, so the signal jumps from node to node much faster.
An action potential is "all-or-none". What does that mean?
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It fires at full strength or not at all. Past the threshold, the neuron always fires fully. A stronger stimulus makes it fire more often, not harder.
In a reflex arc, which cell connects the sensory neuron to the motor neuron in the spinal cord?
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An interneuron. Interneurons link neurons inside the brain and spinal cord.
Which disease attacks the myelin in the brain and spinal cord?
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Multiple sclerosis. In multiple sclerosis the immune system damages myelin, so signals slow down or fail.

In these study guides

MCATNeurons, the nervous system and hormones
AP Psychology1.3 The neuron and neural firing
Intro psychologyNeurons and the nervous system (sections 3.2 and 3.3)
Anatomy and physiologyNervous tissue (chapter 12)
A-level PsychologyNeurons and synaptic transmission
GCSE BiologyThe nervous system and reflexes (4.5.2.1)

Sources

Anatomy and Physiology 2e, 12.2 Nervous Tissue, OpenStax
Anatomy and Physiology 2e, 12.4 The Action Potential, OpenStax
Anatomy and Physiology 2e, 12.5 Communication Between Neurons, OpenStax

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