Chapter 12 part 3

Membrane potential: the diffusion of ions down their

concentration gradients through the membrane

• selective permeability of the membrane, allowing some ions to pass more easily than others

An electrical potential is a difference in the concentration of

charged particles between one and another

It is a form of potential energy, under the right circumstances, can produce a current. An electrical current is a 

flow of charged particles from on point to another

The charge difference across the plasma membrane is called the

resting membrane potential (RMP)

We don’t have free electrons in the body as we do in an electrical circuit. Electrical currents in the body are

created, instead, by the flow of ions such as Na+ and K+ through gated channels in the plasma membrane

The resting plasma membrane is much less permeable to Na+ that to K+, but Na+ diffuse

down its concentration gradient into the cell, attracted by the negative charge in the ICF

The sodium leak is only a trickle, but it is enough to cancel some of the negative charge and reduce the 

voltage across the membrane

Sodium leaks into the cell and potassium leaks out, but the

sodium-potassium (Na+-K+) pump continually compensates for this leakage

It pumps 3 Na+ out of the cell for every

2 K+ it brings in, consuming 1 ATP for each exchange cycle

Ionic composition

• High Na+ outside 

• High K+ inside 

Potassium ions (K+) have the greatest influence on the RMP because the

plasma membrane is more permeable to K+ than to any other ion

Potassium ions diffuse freely through leak channels in the plasma membrane, down their 

concentration Gadient and out of the cell, leaving these cytoplasmic anions behind 

• as a result, the ICF grown more and more negatively charged 

Resting Membrane Potential

• K+ most important 

  • Differences in permeability 

    • 25% more K+

• Leak channels 

Na+/K+ pump compensates for the continual leakage of

Na+ and K+

• works continuously 

• 70% energy expenditure of the nervous system

Electrical currents in the body are created, instead, by the flow of ions such as

Na+ and K+ through gated channels in the plasma membrane

• gated channels can be opened and closed by various stimuli 

The resting membrane potential of -70 mV is the net effect of all these ion movements K+ diffusion

out of the cell, Na+ diffusion inward, and the Na+-K+ pump continually offsetting this ion leakage