The nerve impulse starts when the neuron receives a signal from another cell, normally this stimulus is a chemical stimulus. When the neuron receives this stimulus a reversal of charge, called action potential (AP) occurs across the membrane.
The direction of the action potential is anterograde, it starts at the body of the cell and ends at the axonic terminal.
When the cell receives the stimulus, the sodium channels open, causing the entry of the cation inside the cell. Sodium causes the inside of the cell to be more positive than the outside of the cell. This change of charge of the cell membrane is called depolarization and travels down the axon rapidly as an electric current.
The far end of the axon is called the axonic terminal, this structure is very close to the dendrites of the next cell. These structures form the synapse, this is the place where the electric nerve impulse is transmitted from one cell to another, in this case, from one neuron to the next one. The axonic terminal corresponds to the presynaptic terminal, the dendrite of the next cell corresponds to the postsynaptic terminal, and the space between them is called the synaptic cleft.
In the presynaptic terminal, there are vesicles that contain substances that are called neurotransmitters. The neurotransmitters are substances that carry the signal from one cell to the next one.
In the membrane of the postsynaptic terminal, you will find receptors that are specific for each type of neurotransmitter and protein channels that will allow ions to flow inside or outside the cell as needed.
When the action potential reaches the axon terminal it activates the influx of calcium ions to the presynaptic terminal, which helps the vesicles carrying the neurotransmitters to fuse with the membrane of the presynaptic terminal and release the neurotransmitters to the synaptic cleft. The neurotransmitters travel through the cleft towards the postsynaptic terminal where they bind with the receptors found in the postsynaptic terminal. When this happens, the sodium channels found in the postsynaptic open letting sodium ions flow inside the cell and starting a new action potential.
Around 2 milliseconds after the action potential started potassium channels open and the ion starts flowing outside the cell. Since potassium is a positive ion (cation), the efflux of these ions causes the inside of the cell, which was positively charged, to return to its starting charge (negative). This process is called repolarization of the cell membrane. Once the inside of the cell reached its negative charge, it enters a resting state.