The Reverend Dr. David CM Taylor

As a general rule, excitation comes from differences in

• concentration of ions (see diagram below, in mM)
• changes in the membrane permeability to those ions

The potential difference across the membrane (membrane potential) is the result of four major factors:

• the permeability of the membrane to potassium
• the impermeability of the membrane to the protein anions
• the permeability of the membrane to sodium, which is normally low, but increases when the membrane is excited
• the activity of the sodium pump, which is continuously active

The sodium channels are particularly interesting (!), in that the voltage activated sodium channels have three states, they can be

• open - in which case the sodium enters down its concentration gradient
• closed inactive - and stay closed for a period of time, after which they become
• closed active - and can be opened again by a change in membrane potential

The above diagram is, naturally an over simplification, as is the diagram below, which is a simplification of the Hodgkin-Huxley model. gNa refers to the sodium conductance, gK refers to the potassium conductance. A more detailed diagram, with the appropriate scales on the axes can be found in the original paper (Hodgkin & Huxley, 1952, Journal of Physiology. 117:500). This topic is covered well, in my opinion, in Berne & Levy (1992) Physiology (chapter 3), but many physiology text books tackle the topic.