Anatomy · Cells & tissues

Neuron

The signalling cell of the nervous system

Nervous tissue · brain, spinal cord and nerves

The electrically excitable cell that carries messages through the nervous system. A typical neuron has a cell body, or soma, holding the nucleus; many short, branching dendrites that receive signals; and a single long axon that carries the signal away to the next cell across a synapse. Many axons are wrapped in a fatty myelin sheath that speeds conduction. The human brain contains on the order of 86 billion of them, supported by even more glial cells.

How to spot them

▸A cell body with a spray of branching dendrites and one long thread, the axon
▸Typical multipolar shape, like a star or a tree with a single long root
▸In stained sections, a large pale nucleus with a prominent nucleolus and dark Nissl granules in the cytoplasm
▸Axon wrapped in segments of myelin with small gaps, the nodes of Ranvier
▸Ends in swellings called axon terminals that meet the next cell at a synapse

What to know

A branching input tree, a cell body and one long output cable ending in a spray of terminals

Neuron — dendrites, soma and axon.

Insulated in segments by Schwann cells outside the brain or by oligodendrocytes within it, with gaps where the signal jumps

Neuron — myelin and the nodes of Ranvier give saltatory conduction.

The tapering start of the axon, where the electrical impulse is triggered

Neuron — the axon hillock and initial segment.

Its junction with the next cell was named by Charles Sherrington in 1897 (1897)

Neuron — the synapse.

A silver-stain method by Camillo Golgi let Ramón y Cajal show in the 1880s and 1890s that these were separate cells, not a continuous net (1906)

Neuron — Golgi and Cajal shared the 1906 Nobel Prize; the 'neuron doctrine'.

Some in the motor pathway to the foot run for more than a metre from the spinal cord

Neuron — a single motor neuron axon can exceed a metre in length.

About 86 billion of these are estimated in the human brain, a figure once quoted as 100 billion (2009)

Neuron — the count of Suzana Herculano-Houzel and colleagues.

Key ideas

A resting neuron holds a voltage across its membrane. When enough excitatory input arrives at the dendrites and soma to bring the trigger zone to threshold, voltage-gated channels fire an all-or-nothing action potential along the axon.
At the synapse the impulse triggers release of neurotransmitter, which crosses a tiny cleft and either excites or inhibits the next cell.

Quick-fire facts

•The word 'neuron' was popularised by Wilhelm von Waldeyer-Hartz in 1891 as part of the 'neuron doctrine' championed by Ramón y Cajal
•Camillo Golgi and Santiago Ramón y Cajal shared the 1906 Nobel Prize in Physiology or Medicine, despite disagreeing on whether neurons were separate cells
•Sherrington introduced the word 'synapse' in 1897
•Myelinated axons conduct at up to roughly 100 m/s; unmyelinated fibres conduct at around 1 m/s
•Neurons come as multipolar (most), bipolar (retina, olfactory) and unipolar or pseudo-unipolar (sensory neurons of the dorsal root ganglia)
•Most neurons cannot divide once mature, though new neurons are made in a few places such as the hippocampus
•Hodgkin and Huxley worked out the action potential in the giant axon of the squid and shared the 1963 Nobel Prize with Eccles
•Glial cells (astrocytes, oligodendrocytes, microglia and others) support them; the old '10 glia to 1 neuron' figure is now revised to roughly 1:1

Drill it — 8 cards

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Sources

OpenStax — Nervous Tissue (Anatomy and Physiology 2e, section 12.2)
NobelPrize.org — The Nobel Prize in Physiology or Medicine 1906