What causes an electrical impulse to move down a neuron?

The movement of electrical impulses down a neuron is known as an action potential. It occurs due to changes in the electrical potential across the neuron's cell membrane. Here's a step-by-step explanation of what causes an electrical impulse to move down a neuron:

1. Resting Potential: Neurons maintain a resting potential, where the inside of the neuron is negative relative to the outside. This difference in electrical potential is maintained by the uneven distribution of ions (charged particles) across the cell membrane.

2. Depolarization: When a neuron receives a stimulus that exceeds a certain threshold, it causes the cell membrane to become more permeable to sodium (Na+) ions. Sodium channels open, allowing an influx of Na+ ions into the neuron, making the inside less negative. This initial phase is called depolarization.

3. Action Potential: If the depolarization reaches a critical level called the threshold potential, it triggers an action potential. During an action potential, the membrane potential rapidly reverses, and the inside of the neuron becomes positive relative to the outside. This is caused by the opening of voltage-gated sodium channels, leading to a massive influx of Na+ ions.

4. Repolarization: Almost immediately after the peak of the action potential, voltage-gated potassium (K+) channels open. K+ ions flow out of the neuron, causing the membrane potential to return to its resting negative value. This process is called repolarization.

5. Hyperpolarization: In some cases, the K+ channels remain open for a brief period longer than necessary to restore the resting potential, resulting in a slight overshoot of negativity inside the neuron. This phase is known as hyperpolarization.

6. Refractory Periods: After an action potential, the neuron undergoes refractory periods. The absolute refractory period is a short phase during which the neuron cannot generate another action potential, even with strong stimuli. The relative refractory period follows, where the neuron can generate an action potential, but it requires a stronger stimulus.

The sequence of depolarization, action potential, repolarization, and refractory periods propagates the electrical impulse down the length of the neuron, carrying the signal to its target.