AT REST

A charged membrane

Resting potential The inside of a resting neuron is about 70 millivolts negative compared with the outside

Outside Sodium (Na⁺) is concentrated outside the cell

Inside Potassium (K⁺) is concentrated inside the cell

The pump The sodium-potassium pump uses energy to keep these gradients in place

A resting neuron is like a charged battery, ready to fire.

THE SPIKE

Voltage over a millisecond or two

The whole spike lasts about a millisecond or two.

A spike is a quick swing from negative to positive and back.

THE STEPS

Ion channels do the work

Threshold If input pushes the potential to about −55 mV, voltage-gated sodium channels open

Rising Na⁺ rushes in and the inside swings positive, to about +40 mV

Falling Sodium channels close and potassium channels open, so K⁺ flows out

Reset The pump and leak channels restore the starting balance

A spike is a chain reaction of ion channels opening in sequence.

ALL OR NONE

Strength is a count, not a size

🎯

All or none

A neuron fires a full spike or none at all

🔢

Rate coding

A stronger stimulus makes more spikes per second, not bigger spikes

⏳

Refractory period

Right after a spike the neuron briefly cannot fire again, so the signal moves one way

Information is carried in the timing and rate of identical spikes.

SPEED

Myelin makes it jump

Myelin insulates the axon, and the spike is regenerated only at small gaps called nodes of Ranvier, so it appears to jump from node to node. This lets signals travel up to about 100 meters per second. Diseases such as multiple sclerosis damage myelin and slow the signal.

Insulation turns a slow crawl into a sprint.

UNIT 2 STUDY COMPLETE

⚡

Ready for the Fab Challenge?

You've followed a spike from rest to threshold to reset, and seen how myelin speeds it up.