Match the following: Some K+ channels remain open,
and Na+ channels rest.
1. Threshold stimulus
Voltage-gated Na+ channels open.
2. Depolarization
3. Hyperpolarization
Na+ channels are inactivating, and
voltage-gated K+ channels open.
4. Repolarization
Any stimulus at or above this level
will result in action potential.

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when you breathe in, or inhale, your diaphragm contracts (tightens) and moves downward. this increases the space in your chest cavity, into which your lungs expand. the intercostal muscles between your ribs also enlarge the chest cavity. they contract to pull your rib cage both upward and outward when you inhale.