Med. & Biol. Eng. & Comput., 1978, 16, 219-220

Technical note

Easy recording of extracellular action potentials

Keywords--Amplifiers, Electrodes, Nerve action potentials

1 Introduction

THE RECORDING o f nerve action potentials is known to be difficult because there are many possible sources of interference. This interference may be of an electrical nature and induced by the electrical wiring and appliances in the building, or it can be mechanical and occur when the nerve is moving on the electrodes. Interference can also be caused by the surgical manipulation and often pulling on the nerve to lift it away from nearby tissue provokes dehydration and inactivity.

Since none of the usual methods for eliminating these interferences (Faraday cages, mineral oil) are com- pletely satisfactory, a new electrode has been developed and a preamplifier has been adapted for the purpose.

Modern differential amplifiers have, as a rule, a very good common-mode rejection ratio (in our preamplifier around 80 dB) and should allow, in normal rooms, the recording of potentials down to +30 t~V without a Faraday cage. The reason why this is generally impossible lies in the arrangement of the electrodes, their wiring and the earthing of the surrounding tissue. Electrostatic and magnetic fields are the main causes of the electrical interference. The electrostatic induction is produced by the differences in capacitance of the electrode wires towards the earthing point on the animal, while the magnetic induction occurs through the unequatity of lengthy wires and their positioning.

Pulling on the nerve, its dehydration and its move- ments on the electrodes are difficult to avoid when micromartipulators are used. In fact, the manipulator has Fig. a fixed position and holds the electrodes firmly while the

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animal changes its position continuously towards the electrodes (respiratory, cardiac and other movements),

The new electrode takes care of these facts, reduces considerably the disadvantages of the conventional technique and gave us remarkable results in our ex- periments.

2 The electrode and the amplifiers

The electrode consists of a small perspex rectangle in which three 0.5 mm diameter holes were drilled. Fig. 1

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Layout of the electrode (dimensions are in mHlimetres)

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The preamplifier used in our set- up (Kindly constructed f o r us by C. Mabilde, Department of Physiology, Rijksuniversiteit Gent)

Medical & Biological Engineering & Computing March 1978 219

shows, the dimensions of the perspex block and the arrangements of the holes. Three silver wires 0- 5 mm in diameter were glued in the holes and very flexible insulated leads of 4- 25 cm of length were soldered to the free, rear, end of the silver wires. The nonsoldered, front ends of the silver wires were filed flush with the perspex. The solder points and the filed ends of the silver were insulated with epoxy resin. The flexible leads were twisted and soldered to a three pole male connector (European standard P 231).

The earthing wire, which is the silver wire in the middle bottom end of the electrode, was exposed by filing away the perspex. Placing the electrode, is easily done by gently detaching the nerve from the binding tissue over a length of about 10 mm and gliding the nerve over the electrode until it touches the two silver electrode contacts. Earthing is automatically achieved through the bottom wire. The electrode moves together with the surrounding tissue and the nerve doesn't move in relation to the pick-up wires. There is no pulling on the nerve and dehydration has never been observed.

The connector is plugged into a preamplifier which is a modified version of the one described b y FRYER et al. (Fig. 2). The amplifier is battery powered and has a gain of 1 000. Since the action potentials must be dis- played on an oscilloscope it seemed reasonable to us to use the oscilloscope preamplifier for the observation as well as for the further amplification necessary for the processing of the action potentials. The preamplifier in our case was the Tektronix 2A63. It has a gain of another 1 000 times which brings the total gain to 106. The amplified signals were sampled at the output of this latter preamplifier through two 0-1 /zF capacitors for further recording or analysis.

This set-up has been used in many experiments on the cat for the recording and spectrum analysis of the action potentials of the coeliac plexus. No Faraday cage or mineral oil was used. No interference of any kind was encountered and action potentials could be recorded for several hours without deterioration of the signal or loss in activity (Fig. 3).

A. L. DELAUNOIS

J. F. and C. Heymans Institute of Pharmacology Bijksuniversiteit, Gent, Belgium

Fig. 3 An example of a recording of postganglionic action potentials from the coeliac plexus

Reference FRYER, F. B., SANDLER, H., FREUND, W., McCUTCHEON,

E. P. and CARLSON, E. L. (1975) A multichannel implantable telemetry system for flow, pressure and ECG measurements. J. Appl. PhysioL, 39, 318.

220 Medical & Biological Engineering & Computing March 1978