Per wiki"

Electrocardiography (ECG or EKG*) is the process of recording the electrical activity of the heart over a period of time using electrodes placed on the skin. These electrodes detect the tiny electrical changes on the skin that arise from the heart muscle's electrophysiologic pattern of depolarizing and repolarizing during each heartbeat. It is a very commonly performed cardiology test.

"It is a bioelectric signal. Nerve and muscle cells generate bioelectric signals that are result of electrochemical changes within and between cells. If a nerve or muscle cell is stimulated by a stimulus that is strong enough to reach a necessary threshold, the cell will generate an action potencial. The action potencial, which rrepresents a brief flow of ions across the cell membrane, can be measured with intracellular or extracellular electrodes. Action potencials generated by an excited cell can be transmitted from one cell to adjacent cells via its axon. When many cells become activated, an electric field is generrated that propagates through to the biologic tissue. These changes in extracellular potencial can be measured on the surface of the tisue or organism by using surface electrodes. The ECG, EGG, EEG and EMG are examples of this."

Chapter 11 Biosignal Processing; Monty Escabí, p. 668 in 11.2 Physiological origins of biosignals; 11.2.1 Bioletric Signals INTRODUCTION TO BIOMEDICAL ENGINEERING, 3a Ed. J.D.ENDERLE & J.D. BRONZINO

Moujza,

There is an interaction between the electrode and the biological environment during recording and stimulation (These are essentially similar but different processes btw). Electrons are negatively charged particles that can freely move in conductive materials such as silver and copper. And it is a subject of electricity. However, in a conductive media, such as saline or biological tissue, the charge carrying particles are ions like Na and K. Now, this is subject of volume conduction. Ions can be negatively or positively charged.

During stimulation, very complex electrochemical processes occur. Some of these processes are reversible (capacitive mechanisms) and some of them are irreversible (faradaic mechanisms), meaning that the recording/stimulating electrode loses some of its material during the electrochemical reactions. As I said, very complex phenomena and needs a little bit of research to comprehend.

The same mechanisms, but mostly capacitive, work during recordings. When the ions move inside the tissue for some reason (for example the activation of excitable cells) they generate ionic currents. These ionic currents generate an electric field. If you place an electrode close enough to the electric field, you can pick up the changes in the potential around the electrode, and this appears as a varying voltage. I am not sure if it is called "conversion", but yes there is some electron trading happens between the electrode and ionic solution around it. But mostly, the electric field generated by the ionic currents "influence" the electrons inside the metal electrode. (Search for electrode-electrolyte interface for details).

Below is a paper talking about neural recording and stimulating electrodes authored by a highly respected person in the field. It is a bit long but I am sure that you will have a good understanding after reading it.  

http://www.annualreviews.org/doi/abs/10.1146/annurev.bioeng.10.061807.160518

Hi,

John G. Webster, Medical Instrumentation, 2nd ed., p.227:

"In order to measure and record potentials and, hence, currents in the body,

it is necessary to provide some interface between the body and the electronic

measuring apparatus. This interface function is carried out by biopotential

electrodes..."

and

"... the electrode must serve as a transducer to change an ionic

current into an electronic current. This greatly complicates electrodes and

places constraints on their operation..."

I recommend you to study chapter 5 (Biopotential Electrodes) of this book.

Good luck

-Hassan