Resting potential

The intracellular space is hyperpolarized, we have potential stored energy

The three basic ingredients

1. Cell membrane
2. Concentration gradients (thanks to ionic pumps)
3. Selective ionic channels

The most proeminent pump is the sodium-potasssium pump ($Na^+$/$K^+$) moving sodium from the inside to the outside, and potassium from the outside to the inside. $\rightarrow$ This process requires energy and sets up the concentration gradients.

As $K^+$ is present at high concentrations on the inside it will diffuse to the outside. As a result the outside of the membrane accumulates a net positive charge (because of the slight excess of $K^+$) and the inside accumulates a net negative charge. Because opposite charges attract, the excess charges will align locally to each side of the membrane.

The cell membrane

1. Separates in vs. out
2. Electrical insulator
3. Electrical capacitor (stores charge)

Selective ionic channels

They have a large conductance $g_{channel}=10^4 g_{membrane}$ and are selective for one type of ion.
Computations in neurons
result from:

• Channel selectivity
• Time dependance of $g_{ch}$ due to:
  • Neurotransmitters
  • Voltage
  • Secondary messengers ($Ca^{2+}$)

Ionic flux

The Nernst Equation

Goldman-Hodgkin-Katz equation

More realistic but more complex.

The reversal potential

(or also called the equilibrium potential for 1 ion)
It is the potential at which the flux is in equilibrium and will determine what will happen to the flux if we change the potential.

Example with $K^+$:

I-V curves

We can next graph an I-V (using ohm's law) curve for more insight into the system.

Two channel-types

With two ions and respective channels, we obtain a new equilibrium potential. But the respective currents are not 0 and they cancel out. Therefore, the respective concentrations of the ions are not fixed requiring pumps to be maintained.

Change my mind!