When a neuron is stimulated by another neuron, an ionic gradient is created across the neuron's plasma membrane, which creates an electric current flow in the neuron. The current flow travels down the axon and causes neurotransmitters to be released at the terminals to stimulate downstream neurons.
Before a neuron is stimulated, it is in a resting state. The plasma membrane of the neuron is selectively permeable to some ions, such as sodium and potassium.
Upon stimulation by neurotransmitters or physical changes, some sodium ion channels in the neuron's plasma membrane open temporarily. Sodium ions enter the neuron and make the inside of the neuron more positive in a process known as depolarization. This creates an electric current that spreads to other parts of the neuron's membrane. The current is proportional to the size of the stimulation.
If the depolarization results in a membrane potential of -50 mV, it creates an action potential. This results in the opening of voltage-gated sodium ion channels, and sodium ions rush into the inside of the neuron. After the action potential current is conducted down the axons all the way to the terminal boutons, where neurotransmitters are released, the neuron returns to its resting state.