Membrane potential

Why this matters

Every dental injection is an intervention on this one mechanism: the drug you deposit at the mandibular foramen works by holding a sodium channel in a state it cannot open from. When it fails - and in a tooth with irreversible pulpitis it fails most of the time - the reason is membrane physiology, not technique. The same voltage decides whether a hypocalcaemic patient's perioral tingling is trivial or the first sign of tetany.

The big picture

A cell holds a voltage because a pump builds gradients and a leak lets one ion - potassium - dominate. Everything after that is the same equation with different permeability weights: swing sodium permeability up several hundred-fold and you get an action potential; hold sodium channels inactivated and you get anaesthesia. Excitability is not a level but a gap, between where the membrane rests and where it fires, and disease attacks either end of it.

The 15 cards in this lesson

1. 01Why a cell holds a voltage
2. 02The pump that builds the gradients
3. 03Nernst: one ion, one equilibrium
4. 04The gradients across the membrane
5. 05Goldman: near E_K, never at it
6. 06Sodium versus potassium
7. 07The action potential, phase by phase
8. 08Two gates, three states
9. 09Absolute versus relative refractory
10. 10How a signal travels
11. 11Jumping the internode
12. 12How local anaesthetic blocks the channel
13. 13Why the hot tooth resists
14. 14Hodgkin and Katz, 1949
15. 15Clinical alert — in the lesson

You have got it when

You can calculate an equilibrium potential from two concentrations and say why the resting potential sits near E_K but not at it. You can name which gate is shut during the absolute refractory period and which conductance is still raised during the relative one. Given a plasma potassium of 8 mmol/L you can predict which way the resting potential moves, and why the cell ends up unable to fire at all.