Final answer:
The selectivity filter is part of a voltage-gated channel that determines which ions can pass through when the channel opens in response to changes in membrane voltage, based on the electrochemical exclusion principle.
Step-by-step explanation:
The selectivity filter is a critical component of a voltage-gated channel, which allows the channel to select for specific ions to pass through while the channel is open. These channels open in response to changes in the transmembrane voltage. When the internal membrane potential becomes less negative, the amino acids in the protein structure that are sensitive to charge will undergo a conformational change, causing the gate within the channel to open. Consequently, only ions that are the appropriate size and charge will be able to pass through, due to the electrochemical exclusion principle. This process is vital for physiological functions such as the propagation of action potentials in neurons, where for instance, voltage-gated K+ channels and voltage-gated Na+ channels play a key role in the depolarization and repolarization phases of an action potential.