Resting Potential in AP Psychology

Where -70 mV fits in the firing sequence

A neuron sitting at about -70 mV with no signal going out is at resting potential, and that's where every firing sequence starts and ends. In the course, this sits in Unit 1: Biological Bases of Behavior, Topic 1.3 The Neuron and Neural Firing, which connects the step-by-step process of neural transmission to behavior and mental processes.

Here's the order the course expects you to know:

• Resting potential: the neuron sits at about -70 mV, polarized and waiting.
• Depolarization: excitatory input lets positive charge in, so the inside becomes less negative.
• Threshold: if the charge reaches about -55 mV, the neuron fires.
• All-or-none principle: once threshold is reached, the action potential fires at full strength. A stronger stimulus doesn't make a bigger action potential.
• Refractory period: right after firing, the neuron can't fire again until it resets toward resting potential.
• Reuptake: at the synapse, leftover neurotransmitter is reabsorbed by the sending neuron.

The chemistry that keeps the neuron at rest (the sodium-potassium pump) is outside the scope of the AP Psychology Exam. You need to know what the resting state is and where it sits in the sequence.

Reading the millivolt numbers in a question stem

If a question gives you membrane-potential numbers, pay attention to the direction of the change. Moving from -70 mV toward zero is depolarization. Moving to a value more negative than -70 mV makes firing less likely, because the neuron is now farther from threshold.

Worked example: a neuron at -70 mV receives excitatory neurotransmitters at its dendrites, and its membrane potential climbs to -50 mV. The change is depolarization. Because -50 mV is past the -55 mV threshold, the neuron fires an action potential, and it fires at full strength.

Now change the input so the charge only rises to -60 mV. That's still depolarization, but it stops short of threshold. No action potential fires, and the neuron drifts back to rest. A half-fired action potential never happens.

How a shifted resting state changes excitability and behavior

How close the resting potential sits to threshold affects how easily a neuron fires. Suppose, hypothetically, a neuron's resting potential shifts from -70 mV to -60 mV. If the threshold remains about -55 mV, the neuron is now closer to threshold. An input that was previously too small to reach threshold might now trigger firing, and more firing in a circuit can mean changes in behavior or mental processes.

The same logic explains the two message types in 1.3.B. Excitatory neurotransmitters like glutamate push the charge toward threshold, making an action potential more likely. Inhibitory neurotransmitters like GABA push it away from threshold, making an action potential less likely. Psychoactive drugs (Topic 1.3.C) work on these same steps. Depressants such as alcohol generally lower neural activity, and stimulants such as caffeine generally raise it.

The course also links breakdowns in the firing sequence to disorders such as multiple sclerosis and myasthenia gravis. If a question names one of these, connect the disrupted transmission to its behavioral symptoms, like weakness or poor coordination. Don't claim the disorder is specifically a resting potential problem, since the course frames it as a disruption of the sequence as a whole.

Resting potential vs. action potential, and why "resting" isn't neutral

If a stem describes a neuron holding steady at -70 mV and you pick "action potential," you've mixed up the two states. Resting potential is the neuron waiting at a steady -70 mV with no signal being sent. An action potential is the brief electrical impulse that travels down the axon after threshold is reached. If a stem describes a neuron that is polarized, unstimulated, or waiting, choose resting potential. If it describes a neuron sending a signal, choose action potential.

A second trap is assuming a resting neuron has no charge. It does carry a charge (negative inside), and that charge difference is what lets the neuron fire. Also watch for the refractory period. A neuron that just fired and can't fire again is in its refractory period, even though it's heading back toward rest.

You could encounter this term in a standalone or set-based multiple-choice question, or use it to explain neural activity in a free-response answer. When you explain how a biological process leads to a behavior, trace the steps from rest to threshold to firing, and leave out the sodium-potassium pump, since it's outside the course.