Answered By Emily D

Hey there, you forgot to add the context and possible answers for your question, so it's hard to answer it! I'm going to take a guess and say that the voltmeter is measuring the electric gradient across a neuron's membrane. Here are some of the possible answers:

 

Resting Potential: around -70 mV and there is no signal being sent through the neuron, the concentration of negative (Cl-) ions inside the cell is larger than the concentration of positive (K+) ions

 

Threshold: usually around -55 mV the stimulus is great enough to cause the sodium gates in the neuron membrane to open, allowing Na+ in

 

Action Potential: higher than the threshold, usually up to +40 mV the gates are open, Na+ is following its concentration gradient (there is more outside the neuron than inside, so it flows in)

 

Hyperpolarization/Repolarization: lower than -70 mV, K+ gates opened during the second half of the action potential, allowing it to flow out along its concentration gradient (more K+ inside the neuron than out, so it flows out and makes the charge more -ve) this period of hyperpolarization stops the action potential from traveling backward along the neuron (the threshold is farther away than normal). I've included a drawing to hopefully clear up my explanation