The correct statement is this: A substance that decreases the membrane's permeability to sodium will DECREASE the probability of generating a nerve impulse.
Action potential (nerve impulse) is generated by voltage gated ion channels that are located in the plasma membrane of cells. Action potential occurs when a neuron sends information through an axon. As the nerve impulse travels down the axon, changes in polarity occur across the membrane of the axon. These changes make the sodium and potassium ion channels to open and close as the membrane move up to its threshold potential. At the start of the action potential, depolarization occurs as the sodium ions moves into the axon while repolarization occurs as the potassium ions move out of the axon. Any substance that brings about reduction in cell membrane permeability to sodium ions will reduce the probability of generating an action potential
