What is the external and internal change that causes a nerve impulse to be created?

A nerve impulse is generated when a neuron is stimulated by external factors, such as a chemical signal (neurotransmitter) or physical stimulus (like pressure or temperature). This stimulation leads to a change in the neuron's membrane potential, causing sodium channels to open and sodium ions to rush into the cell, resulting in depolarization. If the depolarization reaches a certain threshold, an action potential is triggered, allowing the nerve impulse to propagate along the axon. Internally, this change in ion concentrations and electrical charge is crucial for transmitting signals within the nervous system.