The Dielectrophoresis Network
at the University of Surrey
Professor Luigi Galvani of the University of Bologna, Italy was the first person to observe electrical effects on tissue. He observed that electricity applied to muscles of dead frogs caused movement (famously, using a scalpel that had acquired static charge). He coined the term animal electricity to describe the force that activated the muscles, and regarded their activation as being generated by an electrical fluid that is carried to the muscles by the nerves (something not too far from the truth – dissolved ions are fundamental). The phenomenon was dubbed Galvanism - later bioelectricity, and now electrophysiology.
Electrophysiology describes the relationship between electricity and physiology (and biology, and medicine). Electric fields are observed principally in muscles and nerves; electrical effects from organs can be detected outside the body; electrical recordings of the heart (ECG), brain (EEG) and muscles (EMG) are used in routine medicine.
The electric fields are generated due to the movement of charged atoms (ions) moving across the membranes of cells. All cells contain voltages, but electrophysiology is usually concerned with cells which generate their own electric fields. There are two types of cell that require electrical conduction to work – muscle and nerve.
Both use the fact that at rest, every cell has an imbalance of positively- and negatively-charged ions with respect to the outside environment. This is maintained by ion channels – protein structures on the cell surface which pump specific ion types across the membrane. Most common types are sodium (pumped out) and potassium (pumped in), with calcium pumped outwards in the heart. At rest, cells are maintained with a negative voltage with respect to the outside- called a resting potential, typically -70mV. When an input is detected, sodium or calcium flood in to reduce this to zero (depolarizaton) which spreads across the cell. Other (voltage gated) ion channels respond to this and spread the effect further. Potassium channels then restore the balance.
In neurons, this allows conduction of a signal along the axon to the next neuron, allowing signal conduction. In muscle, it causes the release of calcium in the cell, which in turn causes muscle contraction. However, electrophysiology plays a role far beyond muscle and nerve cells, but the cost and complexity associated with measurement make it only worth doing on cells which are electrically active. Normally, electrophysiology only looks at the flow of ions across the membrane – effectively, a change in resistance. However, if we regard the cells from an electrical perspective, there are many more interesting things to observe, such as the capacitance of the membrane or the conductivity of the cytoplasm. These parameters give insights into other aspects of electrophysiology which have applications far beyond muscle and nerve, and give insights into cancer, drug discovery, stem cells and fundamental cell biology.
Technology for observing electrophysiology
There are numerous techniuqes which use electrically derived signals for clinical use - the EEG and ECG among them. However, for the determination of electrophysiological responses in the lab, measurement of cellular response is more complicated. Early measurements of cell electrical activity used needles inserted into the cytoplasms of giant squid axons, allowing the first understanding of the action potential; however, such a technique is limited where human cells are involved, as the diameter of the neuron is significantly smaller.
Invented in the 1970s/80s, Patch Clamp electrophysiology uses a saline-filled pipette with a1 micron inner diameter that is brought close to cell, then pressure used to suck cell onto tip. A tight seal (>1Gohm) formed around junction; electrical recordings can be taken from ion channels under the tip. The process is slow – only one cell can be analysed at a time, and setting the apparatus up for successful recording is time-consuming. However, the time resultion is excellent. Automated patch clamp systems do exist, which use microfluidics to sort cells, place them on pipettes inside a chip, and automatically take recordings from up to 20 cells simultaneously whilst also releasing chemicals to observe effects on cells. Such systems are very expensive, though widely used in pharmaceuticals research.
Another method is the use of voltage-sensitive dyes to measure changes in cell polarisation, though not absolute values. This is useful for e.g. observing interaction of neurons. Dyes can also be used for relative values in flow cytometry – expensive both for equipment and dyes.
Finally, microelectrodes can be used e.g. for observing electrical activity suh as cardiac activity or neural communication – again, looking for changes rather than absolute values. However, they are used for testing drug action on such tissues, for example, and are particuarly useful for determining negative drug side effects on heart function. Such measurements can be used on single cells, or more commonly on dissected tissue strips. Such techniques can also be used to measure the conventional impedance presonse of the tissue as an alternative method of electrophysiology to dielectrophoresis.
Key dates in Electrophysiology
•1666 Francesco Redi Electric eel mechanism
•1791 L Galvani De viribus electricitatis in mortu musculari commentarius
•1849 Emil du Bois-Reymond Electrical signals observed in muscle contraction
•1876 Etienne-Jules Marey First ECG recording in animals
•1887 Augustus D Waller First ECG in humans
•1892 Willem Eindhoven Modern ECG
•1922 Herbert Gasser and Joseph Erlanger* Observation of EMG by oscilloscope; discrimination of muscle fibres
•1939 Andrew Huxley* Single nerve conduction
•1952 Alan Hodgkin* and Andrew Huxley *Action potentials
•1970s Erwin Neher* and Bert Sakmann* Patch Clamp
*Nobel Prize for Physiology of Medicine