Hyperpolarization has several meanings:
In biology, hyperpolarization occurs when a cell's membrane potential dips below its resting level.
hyperpolarization
An electrical state whereby the inside of the cell is made more negative relative to the outside than at the resting membrane potential.
[edit] Hyperpolarization ("Undershoot")
Closing of voltage-gated potassium channels is both voltage- and time-dependent. As potassium exits the cell, the resulting membrane repolarization initiates the closing of voltage-gated potassium channels.
This hyperpolarization is called an inhibitory postsynaptic potential (IPSP).
Although the threshold voltage of the cell is unchanged, it now requires a stronger excitatory stimulus to reach threshold.
After opening, the sodium channels become inactivated as the potential becomes more positive, and they cannot open again until they are "reset" by hyperpolarization at the end of an action potential.
 See also: Membrane, Trans, Cell, Cells, Neuron
  
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