# Resting Potentials ## What is a Resting Potential? - Neurones transmit **electrical impulses** which travel rapidly along the neurone cell surface membrane from one end to the other - In a **resting axon** (one that is not transmitting impulses), the inside of the axon always has a **negative electrical potential** compared to the outside of the axon

The resting potential is the potential difference across the axon membrane when the neurone is at rest

- This potential difference (when there are no impulses) is usually about **-70mV** - The inside of the axon has an electrical potential about 70mV lower than the outside ## Establishing and Maintaining the Resting Potential - Two factors contribute to establishing and maintaining the resting potential: 1. The active transport of sodium ions and potassium ions 2. Differential membrane permeability ### Active Transport of Sodium and Potassium Ions - Carrier proteins called **sodium-potassium pumps** are present in the membranes of neurones - These pumps use **ATP** to actively transport **3 sodium ions out** of the axon for every **2 potassium ions** that they actively transport in - This means that there is a larger concentration of positive ions outside the axon than inside the axon - The movement of ions via the sodium-potassium pumps establishes an **electrochemical gradient**

The sodium-potassium pump works at a 3:2 ratio, removing more positive charges from inside the cell than it brings in

| Ion Movement | Direction | Number of ions | |--------------|-----------|----------------| | Sodium ions ($\text{Na}^+$) | Out of axon | 3 | | Potassium ions ($\text{K}^+$) | Into axon | 2 | | Net effect | - | 1 positive charge removed per cycle | ### Differential Membrane Permeability - The cell-surface membrane of neurones has **selective protein channels** that allow sodium and potassium ions to move across the membrane by facilitated diffusion - The protein channels are **less permeable to sodium ions** than potassium ions - This means that potassium ions can diffuse back down their concentration gradient, **out of the axon**, at a **faster rate** than sodium ions diffuse in

The membrane's selective permeability allows $\text{K}^+$ to leak out faster than $\text{Na}^+$ leaks in, maintaining the negative charge inside

## How the Resting Potential is Maintained - The resting potential is maintained by the **net movement of positively charged ions out of the axon** - This occurs through the combined effect of: - The sodium-potassium pump actively removing more positive charges than it brings in - Potassium ions diffusing out faster than sodium ions diffusing in ### Summary of Ion Distribution | Location | Sodium ions ($\text{Na}^+$) | Potassium ions ($\text{K}^+$) | |----------|---------------------------|------------------------------| | Outside axon | High concentration | Low concentration | | Inside axon | Low concentration | High concentration | - Inside of the axon: **negatively charged** relative to the outside - Membrane potential: approximately **-70mV** ## Energy Requirements

The resting potential requires ATP energy from respiration to power the sodium-potassium pumps that actively transport ions across the membrane

- The sodium-potassium pump is an **active transport** mechanism - It requires **ATP energy** from respiration in order to actively pump sodium and potassium ions across the membrane - Without ATP, the pump cannot function and the resting potential cannot be maintained - This is why neurones have high energy demands and contain many mitochondria ## Key Points About the Resting Potential - The resting potential is approximately **-70mV** (inside negative relative to outside) - It is established and maintained by: - **Active transport**: sodium-potassium pumps moving 3 $\text{Na}^+$ out and 2 $\text{K}^+$ in - **Differential permeability**: membrane more permeable to $\text{K}^+$ than $\text{Na}^+$ - The inside of the axon remains **negatively charged** relative to the outside - This creates the electrochemical gradient necessary for action potentials to occur - Maintenance requires continuous **ATP** supply from cellular respiration