dendrites
cell body
axon hillock
axon
axon terminals
−70 mV
inside, compared with outside
Na⁺, crowded outside
K⁺, crowded inside
Na⁺ channel
In: there is far less Na⁺ inside, and the negative inside pulls it in too.
Na⁺ rushes in
K⁺ flows out
undershoot
K⁺ flows out
Na⁺ channel
K⁺ channel
As Na⁺ channels shut, K⁺ carries + charge out. Slow-closing K⁺ channels cause the dip.
No myelin
With myelin
myelin, with gaps between
Myelin insulates the axon, so the spike jumps from gap to gap, many times faster.
A neuron at rest
A resting neuron holds its inside about 70 mV below the outside: its resting potential.
Which way for Na⁺?
A pump keeps Na⁺ outside and K⁺ inside. If a Na⁺ channel opens, where does Na⁺ go?
Past the threshold
Give the cell a push. A small one fades away. Find the smallest push that fires it.
Past about −55 mV it fires. A harder push gives a spike barely any taller: all or none.
Inside the spike
Two kinds of channel open in turn. Move through the spike and watch them.
Coming back down
After the peak, the voltage falls back below rest. What brings it down?
Down the axon
Each patch fires the next. The one behind can't fire again yet, so the spike only goes forward.
Start to finish
Past −55 mV at the hillock, Na⁺ rushes in, then K⁺ flows out, and the spike runs to the terminals.
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