What starts and stops excitation?

Excitation in biological systems, particularly in neurons, is primarily initiated by the influx of ions, such as sodium (Na+), through ion channels in response to a stimulus, leading to depolarization of the membrane. This process is stopped when potassium ions (K+) exit the cell, repolarizing the membrane and restoring its resting potential. Additionally, neurotransmitters can modulate excitation by either promoting or inhibiting neuronal firing, thus influencing the overall activity of the nervous system.