Sodium pump , polarised membrane , resting membrane potential , threshold stimulus

Answer

### Step-by-Step Solution

1. **Sodium Pump**:
- The sodium pump, also known as the sodium-potassium ATPase, is a vital membrane protein that actively transports sodium (Na+) out of the cell and potassium (K+) into the cell.
- This process is essential for maintaining osmotic equilibrium and the membrane potential of cells.
- The sodium pump works against the concentration gradient, meaning it moves sodium ions from an area of lower concentration inside the cell to an area of higher concentration outside the cell, and vice versa for potassium ions.

2. **Polarized Membrane**:
- A polarized membrane refers to the state of a cell membrane where there is a difference in electrical charge across the membrane.
- In a typical neuron, the inside of the cell has a negative charge relative to the outside. This charge difference is due to the unequal distribution of ions, primarily sodium and potassium, across the membrane.
- The polarized state is crucial for the function of neurons and other excitable cells, as it sets the stage for action potentials.

3. **Resting Membrane Potential**:
- The resting membrane potential is the voltage difference across the membrane of a neuron when it is not actively sending a signal.
- This potential is typically around -70 mV in neurons and is determined by the concentration gradients of ions (mainly Na+ and K+) and the permeability of the membrane to these ions.
- The resting potential is maintained by the sodium-potassium pump and the selective permeability of the membrane to different ions.

4. **Threshold Stimulus**:
- A threshold stimulus is the minimum level of stimulus required to trigger an action potential in a neuron.
- When a stimulus reaches this threshold, it causes the membrane potential to become less negative (depolarization) by opening sodium channels, allowing Na+ ions to flow into the cell.
- If the depolarization reaches a certain level, an action potential is generated, leading to the transmission of an electrical signal along the neuron.