Key ideas

• The nervous system is made up of billions of neurons, plus supporting cells, that communicate in enormous networks.
• Information flows through a neuron from the dendrites, to the cell body, to the axon.
• A neuron starts at its resting potential. It fires only when incoming signals bring it to threshold. If the signals are too weak, it doesn't fire. Once threshold is reached, the neuron generates an action potential.
• Under the all-or-none principle a neuron can't fire halfway, so a stronger stimulus makes neurons fire more often, not harder.
• During an action potential, each section of the axon depolarizes as the signal travels, and afterward the neuron enters a refractory period before returning to resting potential.

Key terms

neuron

A specialized cell of the nervous system that receives and sends signals to other neurons.

dendrites

Branching structures that receive incoming signals from other neurons.

cell body

The part of the neuron that incoming signals move toward from the dendrites and from which the axon extends.

axon

The long fiber extending from the cell body that carries the electrical signal when the neuron fires.

resting potential

The stable electrical state of a neuron when it is not firing.

threshold

The level incoming signals must reach for a neuron to fire.

all-or-none principle

A neuron either fires a complete signal or doesn't fire at all (a light switch, not a dimmer).

action potential

The rapid electrical change that travels down the axon when a neuron fires.

depolarization

The electrical change in which the inside of a section of the axon briefly becomes more positive.

refractory period

A brief reset after an action potential when the neuron can't fire again.

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