Exactly what is meant by signal

[Krimsonking]

Hi, I have a simple question that I haven't had answered to my complete understanding.

In graphs depicting the effect of whatever variable you want the y-axis is usually signal intensity.

Figure 1: http://www.google.com/imgres?imgurl=http...CEQMygEMAQ

But I have also seen this y axis detailed as Mo

Figure 2: https://www.google.com/search?q=Signal+i...B381%3B242

My question is: What exactly is the y axis? Time after excitation is letting the excited protons relax back into the longitudinal plane. Wouldn't this mean that signal intensity is going down? Aren't we measuring the rotation of the magnetic moments through the RF coil that is all around the transverse plane? How do I relate [/align]Mo and signal intensity?

Thank you!