Assuming sodium channels are functioning normally, there would still be an action potential. Threshold would be unaffected, because it is a property of sodium channels. A peak voltage will be reached when the sodium channels become inactivated; this voltage may be higher than normal since usually there is a partial canceling of the rising voltage as potassium exits. Without a potassium current, the falling phase would be much slower, being dependent on the sodium-potassium pump removing the sodium ions that came in. The undershoot would be absent because it is a result of potassium current.
