What causes a nerve impulse move along the fiber?

An electrical signal called an action potential propagates along the nerve fiber, causing a nerve impulse to move. The movement is a result of the coordinated, sequential depolarization and repolarization of sections of the neuronal membrane.

Here's a step-by-step explanation of what causes a nerve impulse to move along a nerve fiber:

1. Resting Potential: Neurons maintain a resting membrane potential, which is a difference in electrical charge across their cell membrane. The inside of the neuron is negative compared to the outside. This potential is maintained through the selective permeability of the membrane to various ions.

2. Depolarization: When a stimulus reaches a sufficient threshold, it causes a change in the permeability of the membrane to ions. Sodium (Na+) channels open, allowing a rapid influx of sodium ions into the neuron, leading to a depolarization of the membrane.

3. Action Potential: If the depolarization reaches a threshold potential (usually around -55 to -50 millivolts), it triggers an action potential. During an action potential, the membrane potential rapidly becomes positive (around +40 millivolts) due to the continued influx of sodium ions and the inactivation of sodium channels.

4. Repolarization: Almost immediately after reaching its peak, the membrane potential begins to repolarize as sodium channels close, and potassium (K+) channels open. Potassium ions flow out of the neuron, restoring the negative resting membrane potential.

5. Hyperpolarization: Overshoot of the membrane potential beyond the resting potential may occur during hyperpolarization. This slight negative shift in the membrane potential is caused by the continued outward movement of potassium ions.

6. Refractory Periods: The neuron experiences refractory periods following an action potential. During the absolute refractory period, the neuron is completely unresponsive to any stimuli, while during the relative refractory period, a stronger-than-normal stimulus is necessary to generate another action potential.

7. Propagation: The depolarization and repolarization of the membrane at one point in the nerve fiber create a local current that depolarizes adjacent sections of the membrane, leading to the propagation of the action potential along the nerve fiber.

In summary, a nerve impulse is propagated along a nerve fiber due to the sequential depolarization and repolarization of membrane segments. This process involves the movement of sodium and potassium ions across the neuronal membrane, resulting in the propagation of an electrical signal called the action potential.