Excitable cells/ action potential

Types of excitable cells

striated skeletal muscle cells,
smooth muscle cells,
cardiac muscle cells,
Secretory cells (pituitary cells , insulin producing cells of pancreas / islets of laangerhans)

electrochemical gradient

K+ (potassium)
150 millimoles /L inside cell
5 millimoles/L outside cell
Na+ (sodium)
15 millimoles/L inside cell
150 millimoles/L outside cell

How is the electrochemical gradient established?

By the sodium/ potassium pump actively transporting ions against their concentration gradient.
3 Na+ ions are pumped OUT
2 K+ ions are pumped IN

How is the electrochemical gradient maintained ?

Sodium/ potassium pump & "leak channels

leak channels

channels that are always open and allow ions permeability into & out of the cell (passive transport)

Why is potassium more permeable than Sodium ?

Potassium has more leak channels (30x more than sodium)
-so any change in potassium outside the cell will affect the cell

Which direction do ions move thru leak channels?

Potassium leaks out
Sodium leaks in

resting membrane potential

An electrical potential established across the plasma membrane of all cells by the Na+/K+ ATPase and the K+ leak channels. IN most cells, the resting membrane potential is approximately -70 mV with respect to the outside of the cell.

Which side of the membrane is more positive/ relatively negative?

The outside has more + ions = more positive charge
The inside has less + ions = less positive charge (relatively negative)
[due to Na/K ATP pump ,
-> 3 Na+ ions outside cell
<- 2 K+ ions inside cell
& more K+ leak channels -> outside cell]