How a Nerve Signal Travels

In the picture

1. 1Dendrite — a branch of the cell body that receives signals from other neurons.
2. 2Cell body — adds up every incoming signal and fires only when the total crosses a threshold.
3. 3Myelin sheath — a fatty sleeve that insulates the axon so the charge stays inside it.
4. 4Node of Ranvier — the exposed gap where the pulse is rebuilt, so it leaps from gap to gap.
5. 5Axon terminal — the end bulb that releases neurotransmitters across the synapse to the next cell.

Examples

• Touching a hot pan: the signal runs up your arm at over 100 metres per second, so your hand pulls back almost instantly.
• The plastic coating on a charger cable does what myelin does — it keeps the current inside the wire.

Quiz: How a Nerve Signal Travels (Age 17)

1. 1

   A neuron is receiving several small signals through its dendrites at the same time. What decides whether it sends a pulse down its axon?

   • The signals are added together, and a pulse is sent only if the total crosses a set threshold.
   • A pulse is always sent, and it is stronger or weaker depending on the size of the incoming signals.
   • A pulse is sent only if one single incoming signal is strong enough by itself.
2. 2

   Nerve fibres are wrapped in a fatty insulating sleeve that is interrupted by small bare gaps. How does this arrangement speed the pulse up?

   • The fatty sleeve actively pushes the charge forward along each insulated stretch.
   • The bare gaps let the charge leak out into the surrounding fluid, which carries it onward.
   • The insulation keeps the charge inside, and the pulse is rebuilt only at the gaps, so it leaps from gap to gap.
3. 3

   The pulse reaches the end of the neuron, but a tiny gap separates it from the next cell. What happens next?

   • The electrical pulse sparks straight across the gap into the next cell.
   • The end of the neuron releases chemicals that cross the gap and pass the message on.
   • The neuron grows until it touches the next cell so the current can flow straight through.

The answers, each with its explanation, are in the app. This lesson has left the free week, so its quiz is part of the subscription.