Certain species of frogs in the genus Phyllobates have a powerful defensive adaptation—their skin can secrete a milky fluid that contains an extremely toxic compound called batrachotoxin (BTX). These frogs, which are found in Colombia, are known as poison dart frogs because some indigenous Colombian hunters coat the tips of their blowgun darts with the frogs' skin secretions. An animal hit by one of these darts dies quickly. What is the mechanism of action of BTX? Predict the effects of each of the following on the membrane potential of a neuron simultaneously poisoned with BTX: (a) removing extracellular sodium ions; (b) increasing the intracellular potassium ion concentration; and (c) adding tetrodotoxin from puffer fish.