Final answer:
When ouabain inhibits the Na+-K+ ATPase pump in a neuron, it disrupts the ion concentration gradients necessary for action potentials, thereby leading to a scenario where no action potential will likely be able to fire. Option 4 is the correct answer.
Step-by-step explanation:
The Na+-K+ ATPase pump plays a crucial role in maintaining the resting membrane potential and the repolarization phase of the action potential in neurons. The pump actively transports Na+ out of the neuron and K+ into the neuron against their respective concentration gradients, using ATP as an energy source. When ouabain, a specific inhibitor of the Na+-K+ ATPase pump, is applied to a neuron, it prevents the pump from functioning normally. As a result, the neuron cannot maintain its resting membrane potential since the concentration gradients of Na+ and K+ ions necessary for the generation of an action potential are disrupted.
This blockage means that over time, Na+ ions will accumulate inside the neuron, and K+ ions will not be effectively pumped back into the cell after an action potential. Consequently, the ability of the neuron to repolarize and return to its resting state is compromised.
Therefore, after ouabain is applied, the likelihood increases that no action potential will fire because the membrane potential will be less negative than normal and may not reach the threshold needed to start an action potential, leading to the neuron's inability to fire.
Given the understanding that ouabain disrupts the action potential process by inhibiting the Na+-K+ ATPase pump, the correct option answer in the final answer is: 4) No action potential will fire after ouabain is applied.