Resting Potential

When a neurone is not transmitting an impulse, there is a potential difference of about −70 mV across its membrane, with the inside negative relative to the outside. This is the resting potential, and the membrane is said to be polarised.Three factors produce the resting potential.1. The sodium-potassium pump. This carrier protein uses ATP to actively transport 3 Na⁺ out of the axon for every 2 K⁺ in. This sets up concentration gradients: Na⁺ is more concentrated outside and K⁺ more concentrated inside. Because more positive charge leaves than enters, the pump also makes the inside slightly more negative.2. Different permeability. At rest the membrane is much more permeable to K⁺ than to Na⁺. Some K⁺ channels are open, so K⁺ diffuses out of the axon down its concentration gradient. Most Na⁺ channels are closed, so few Na⁺ ions diffuse back in. The loss of positive ions leaves the inside negative.3. Negative ions inside. The cytoplasm contains large negatively charged molecules, such as proteins, that cannot leave the axon.As K⁺ leaves, the inside becomes more negative, and this negative charge attracts K⁺ back in. The resting potential is reached when the electrical gradient pulling K⁺ in balances the concentration gradient pushing it out. The combination of the two is the electrochemical gradient.The ion gradients must be maintained all the time, which is one reason neurones use a lot of ATP and contain many mitochondria.