Tools for physiology labs: an inexpensive high-performance amplifier and electrode for extracellular recording

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Abstract

The cost of electronic equipment can be a critical barrier to including neurophysiology exercises in biology teaching programs. We describe the construction of a simple and inexpensive AC preamplifier with performance comparable to that of commercial products. The amplifier consists of two integrated circuits in five stages: differential input, fixed gain, variable gain (100 or 1000), low-pass filter (5 or 20 kHz), and 50 or 60 Hz notch filter. We compared our amplifier with two commercial units, the A-M Systems Model 1700 and the Grass P15. The quality of extracellular recording from a typical student preparation (spontaneously active crayfish motor nerve) was the same for all three amplifiers, although our amplifier has slightly higher internal noise than the P15 and slightly lower common-mode rejection than the 1700 and P15. In addition, we describe a simple suction electrode for extracellular nerve recording. It is easily constructed from readily available materials and uses a disposable plastic pipette tip, instead of the traditional glass tip, to contact the nerve. This tip is easily replaced if broken or clogged, and can be adapted to different recording conditions by selecting a different tip size or stretching the plastic. Development of this equipment is part of an ongoing project to promote neuroscience education by expanding the neurophysiology options available to laboratory instructors.

Introduction

We are developing teaching material for the undergraduate neuroscience laboratory. Our goal is to expand the instructional capabilities of faculty teaching physiology, and in particular, to increase the resources for introducing neuroscience into biology curriculum. We have previously presented laboratory exercises that use invertebrates as model systems to teach general principles of nervous system physiology, instead of more traditional vertebrate preparations (Wyttenbach et al., 1999; www.crawdad.cornell.edu). A limitation of adopting these and other neurophysiology exercises in many teaching environments is the cost of the electronic equipment required to record electrical potentials from nerves and muscles. Most commercially available equipment is designed for research and is priced accordingly. This can be a significant barrier to setting up a neurophysiology teaching lab. To help overcome this barrier, we are designing inexpensive instrumentation that can be built by faculty, support staff, or even students. In this paper, we describe a low-cost extracellular amplifier with performance comparable to that of research units. We also describe a simple extracellular electrode that our students use with this amplifier to record action potentials from motor and sensory nerves. Both pieces of equipment can be used with either invertebrate or vertebrate preparations.

An extracellular amplifier for use in the student laboratory should have the following characteristics: (1) gains of 100 and 1000, sufficient for most nerve and muscle activity; (2) good frequency response from 300 Hz to 5 kHz, matching the band width of nerve and muscle spikes; (3) 60 or 50 Hz notch filter to reduce interference from line voltages; (4) good common-mode rejection, permitting use in electrically noisy environments; (5) low internal noise; (6) high input impedance to permit use with a variety of electrode types; and (7) low power requirements to allow battery operation for extended periods. To be a practical alternative to commercial units, a home-made amplifier should (1) be significantly less expensive; (2) be straightforwardly constructed, with as few components as possible; and (3) require no adjustments for best performance.

There are many published designs for physiological amplifiers (e.g. Hamstra et al., 1984, MettingVanRijn et al., 1994), but most are designed for specific uses and either do not meet the needs listed above or are complicated to build. The circuit we describe uses only two integrated circuits (ICs) and has the entire differential input stage in a single IC, eliminating the need for component matching and calibration. Details of the circuit design and construction are found in Section 2.

There are many ways to record nerve, neuron, and muscle potentials from the extracellular fluid surrounding excitable tissue (Sykova, 1992). Suction electrodes, for example, are commonly used to record action potentials from exposed nerves because they contact the nerve relatively gently but can form a tight seal between the electrode tip and the preparation, giving excellent signal-to-noise ratios. In fact, modern electrophysiological methods of patch clamp recording are descended from cruder suction electrode recording techniques (Sakmann and Neher, 1995). A variety of suction electrode designs are documented in the literature and some can be bought commercially (Easton, 1960, Florey and Kreibel, 1966, Delcomyn, 1974, Stys, 1992). Most rely on a pulled, broken, and polished piece of glass tubing as an electrode tip. A glass tip has major disadvantages in the teaching laboratory: it is easily broken, is difficult to quickly make and replace during a laboratory exercise, and is difficult to make in consistent sizes. To avoid these problems, we designed a simple and inexpensive suction electrode that uses commercial plastic pipette tips instead of glass tubing.

Section snippets

Amplifier design

The amplifier consists of five logical sections: an input stage, two amplification stages, and two filtering stages (Fig. 1). It is a standard operational-amplifier (op-amp) design using two ICs (for explanation of basic op-amp use, see Horowitz and Hill, 1989). Gains of the sections are 10, 10, 0.63 or 6.3, 1.58, and 1, giving a total gain of 100 or 1000. Keeping the gain below 10 at each stage improves the band width of the circuit and its ability to respond to fast events.

The input section

Results

The internal noise of our circuit was comparable to that of the A-M Systems Model 1700 at all connecting resistances, slightly higher than that of the Grass P15 at 0 and 100 kΩ, and twice that of the Grass P15 at 1 MΩ (Fig. 5). The internal noise of all three amplifiers is negligible relative to the physiological signals they are intended to record. The CMRR of our circuit was 75 dB. A CMRR of 20 dB means that the subtraction eliminates 90% of the noise voltage that is applied to both inputs. A

Discussion

We present an amplifier design that meets the requirements for high quality extracellular recordings of nerve and muscle in the teaching laboratory. In particular, it has the gain and frequency response needed to reproduce action potentials; has adequate common-mode rejection, notch filtering, and internal noise; has high input impedance; and has low enough power consumption that a pair of batteries should last for an entire semester. In addition, the cost of parts is very low and construction

Notes

Most amplifier components are readily available from a variety of suppliers, including Radio Shack (www.radioshack.com), Allied Electronics (www.alliedelec.com), and DigiKey (www.digikey.com). Precision resistors and the Burr-Brown INA121 are stocked by DigiKey. Readymade amplifiers based on our design can be purchased from Edvotek, Inc. (www.edvotek.com).

Parts for the suction electrode are available from most scientific supply dealers, since none of the specifications are critical. Ultra

Acknowledgements

Supported by NSF Grant 9555095 to Dr. R.R. Hoy and by the Department of Neurobiology and Behavior, Cornell University. We thank the anonymous reviewers for their suggestions to improve the manuscript, Dr. Hoy for his support of our educational projects and Dr. S.J. Zottoli for suggesting a preliminary suction electrode design.

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