Multichannel Systems MEA-System Manual - Manual (Page 13)

Appendix
13
3 Extracellular Recording from MEAs
3.1 Introduction
Over the last 30 years, non-Invasive extracellular recording from multiple electrodes has
developed into a widely used standard method. Systems and methods have been greatly
improved, leading to more features, higher throughput, and lower costs. Almost all excitable
or electrogenic cells and tissues can be used for extracellular recording in vitro, for example,
central or peripheral neurons, heart cells, retina, or muscle cells.
3.2 Background
The semi permeable lipid bilayer cell membrane separates different ion concentrations (charges)
on the inner and outer side of the membrane. Therefore, the cell membrane has the electrical
properties of a plate capacitor. The electrochemical gradient results in a membrane potential
that can be measured directly with an intracellular electrode. When ion channels are opened
due to chemical or electrical stimulation, the corresponding ions are moving along their
electrochemical gradient. In other words, the resistance of the membrane is lowered,
resulting in an inward or outward flow of ions, measured as a transmembrane current.
The extracellular space is conductive as well, and though the resistance is very low, it is not zero.
According to Ohm's law (U=R*I), the extracellular current results in a small voltage that can be
measured with extracellular electrodes. Extracellular signals are smaller than transmembrane
potentials, depending on the distance of the signal source to the electrode. Extracellular signal
amplitudes decrease with increasing distance of the signal source to the electrode. Therefore,
a close interface between electrode and cell membrane is very important for a high
signal-to-noise ratio.
The transmembrane current and the extracellular potential follow the same time course and
are roughly equal to the first derivative of the transmembrane potential.
A microelectrode array (MEA) is an arrangement of several (typically 60) electrodes allowing
targeting several sites for stimulation or recording at once.
The following components are important for an extracellular recording system:
 Signal source (cells / tissue);
 Cell / sensor interface;
 Biosensor (MEA);
 Filter amplifier (MEA2100, USB-MEA256, MEA1060);
 Recording hardware (MEA2100, USB-ME or USB-MEA devices) and software
(MC_Rack, LTP-Director, Cardio2D).