The nervous system, one member of the team of body systems, carries fast signals. Its cells, the neurons, use the ion channels and pumps you studied in membranes to send electrical messages.
Key idea
A neuron receives signals at its dendrites, and sends an electrical impulse, the action potential, from the cell body along its axon. At the axon's end, the synapse, it releases chemical messengers called neurotransmitters, which cross a tiny gap and signal the next cell.
Why it works
A resting neuron is charged like a tiny battery: the sodium-potassium pump keeps more sodium outside and more potassium inside, so the inside sits at about −70 millivolts. When a signal is strong enough, sodium channels open and sodium rushes in, flipping the voltage; then potassium channels open and potassium flows out, resetting it. That flip triggers the next patch of membrane, so the impulse travels down the axon like a wave in a stadium.
Worked example: Signal speed
A nerve impulse travels 1.2 meters from the spinal cord to a toe in 0.02 seconds. How fast is it?
- Speed = distance ÷ time.
- 1.2 m ÷ 0.02 s = 60 m/s.
Answer: 60 meters per second.
Worked example: Direction
Why does a signal cross a synapse in only one direction?
- Only the axon ending releases neurotransmitters.
- Only the next cell has the receptors that bind them.
Answer: Release happens on one side and reception on the other.
Many axons are wrapped in myelin, a fatty layer that lets the impulse jump between gaps and travel much faster: from about 1 meter per second in thin, bare axons to more than 100 meters per second in thick, myelinated ones. A reflex, like pulling a hand from something hot, uses a short path through the spinal cord, so it happens before the brain even notices.
Common mistake
It is natural to picture the electrical signal jumping straight from one neuron to the next. At most synapses it does not. The electrical signal is turned into a chemical one: neurotransmitters cross the gap, and the next cell starts its own electrical signal.