As in many other fields the Operational Amplifier is a very useful block. As salmiakki said the instrumentation amplifier is widely used since it has a great CMRR, which means that cancels out most of the common mode noise. Also the high input impedance of the instrumentation amplifier make it perfect to be interface with sensors.
This website can be of help to you: **broken link removed**
It explain biopotentials in terms of Electrocardiograms (ECG). But you will find the information useful for any type of biopotencial you are interested. This pointer also will show some techniques to reduce electromagnetic pickup.
This other pointer **broken link removed** includes some basic building blocks for biomedical applications.