How to measure microvolts of biopotential with Arduino?
Rather than give specific circuits, I think I should point you in a few directions.

Google on "eeg amplifier", "ecg amplifier" and "ekg amplifier".

In general, your job will be much easier if you can accept AC-coupled amplifiers - that is, if you don't need to measure DC potentials. You will need total gains in the range of about 10,000 to see your signals, and this is generally best done in several modest stages.

You will need to learn about shielding, and the use of coaxial cable and twisted pairs. Searching for application notes on low-noise techniques will also be helpful, especially anything by Robert Pease and Jim Williams.