The temporary hyperpolarization that occurs at the end of the action potential is caused by the

This hyperpolarization is due to the exiting of potassium ions or the influx of chloride ions through the channels in the neuron. During this time, the neuron enters a refractory period (approximately 2ms), during which an action potential is unable to be transmitted. After this timeframe, the neuron resets to around -70mV and the cell becomes able to re-transmit an action potential.