The neuron's battery: rough map
Na+ sodium
K+ potassium
1 · the bag
a fat skin wraps salty water
charged salt can't cross fat
→ the skin is a dam
2 · the pump
a protein in the skin burns
fuel (ATP) to shove salt:
3 sodium OUT, 2 potassium IN
→ it charges the battery
−−
−
++
+
3 · the leak
a few doors fit only K+
K+ trickles out of the bag
inside settles at −70 mV
(− inside, + outside)
+
4 · the spark
a signal opens sodium doors
Na+ floods IN for ~1 ms
inside flips to about +30 mV
→ that flip is the spike
K+ is positive and it's leaving. So why does it STOP at −70 mV
instead of draining the whole bag? What's pulling it back?