Neuron

A neuron, or nerve cell, is a cell that sends and receives electrical and chemical signals. The human brain has about 86 billion of them, linked by trillions of connections, and together they produce thought, movement and sensation.

A typical neuron has a cell body that contains the nucleus, branching dendrites that receive signals, and a single long axon that carries signals away, sometimes for a meter or more. Many axons are wrapped in myelin, a fatty sheath that speeds up transmission. Axons depend on motor proteins such as kinesin and dynein to move materials along microtubules between the cell body and the far end.

At rest, the inside of a neuron is about 70 millivolts more negative than the outside, a voltage that results from ion gradients across the cell membrane, as explained by the Nernst equation. If an input pushes the voltage past a threshold, sodium channels open and a brief electrical spike, the action potential, travels along the axon. In 1952 Alan Hodgkin and Andrew Huxley described it using the giant axon of the giant squid. At the synapse, the signal is passed to the next cell by chemicals called neurotransmitters.

Glial cells supply nutrients, insulate axons and clear waste. The blood-brain barrier controls what reaches the brain. Santiago Ramón y Cajal showed that the nervous system is made of separate cells, work recognized in the 1906 Nobel Prize shared with Camillo Golgi. The study of neurons is the core of the work of the neuroscientist.

• Blood-Brain Barrier — the protective layer around the brain
• Nernst Equation — the physics of its resting voltage
• Kinesin — a motor that moves cargo along axons