A Miniature EPG Rig

Pictures of the Minirig and its use

The Electropharyngeogram (EPG):

Parts List:

Dissecting scope
Amplifier
Computer Stuff
Manipulator
Oscilloscope
Faraday Cage
Get a 60 X 36 inch sheet of 1/8 in mesh chicken wire (also called poultry wire or hardware cloth) from a hardware store. Cut out two 20 X 20 inch corners to form a sort of squat T: 


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Fold the three tabs and wire the edges together. Connect a wire from the ground of the P15 amplifier to the cage. If you touch the cage while you look into the microscope, your body will shield the open side. If you want to move around while recording, lean one of the 20 inch squares you cut out against the front of the cage.

Total cost: $7678

Notes:

Data Acquisition Hardware and Software

 We've tried one other software package and talked to people who've used yet others, and we think the National Instruments package is the best around. You can get cheaper computers than the Dell, but the folks at NI use Dell and say the cheaper ones aren't as well made. The NI software is also available for Macs. (As far as we know, NI is the only data acquisition software that works with both Macs and IBM). They even sell a Unix version, but everything is twice as expensive. The VIs we use for data acquisition, written by Wayne Davis, are available here.

Oscilloscope

The Tek 2201 oscilloscope is listed above as an example of the sort of scope you want. But don't buy an oscilloscope new unless you're rolling in money. Electrophysiology is an undemanding application; you can get something adequate from an electronics surplus store. The one feature that's worth paying extra for is storage. If you don't want to buy surplus, call three or four companies and ask about reconditioned scopes. These are as good as new and come with warranties, but cost a lot less.

Micromanipulator

The WPI M3301 is more micromanipulator than you need. All you really need is Z-axis control. If you're strapped for cash, check out some more catalogs (try Narashige or Newport Optical); you can probably come up with something for <$500.

Dissecting Scope and Shielding

There's nothing special about the Chinese SM1 microscope except for its incredible cheapness (in every sense of the word) and, less obviously, the external power supply for the lamp. You cannot have a power cable running from outside into the Faraday cage, because the hum from the power lines will swamp your signal. Thus dissecting scopes with a built-in power supply will need to be modified. Even with an external power supply, the light causes problems. First, the wire from the supply to the scope is an antenna, picking up noise outside the cage and bringing it in. Solve this by grounding one of the two wires leading to the scope. Before you do, though, make absolutely certain you won't cause a short circuit. Unplug the power supply, then flip on the power switch. Now use an ohmmeter to check that neither tab on the plug is connected to either of the lines leading to the bulb. The second problem is that the power supply itself generates noise. You solve this by turning the light off while recording. Alternatively, if you want to go deluxe, buy a DC power supply (about $200), and then you'll be able to look at your worms while you record.

Other things you may need to buy:

Needle Puller

Chances are someone where you are has a needle puller you can use. You'll only need it for a few minutes a day. If not, you'll have to buy one. Vertical pullers (the one I know is from Kopf) cost $1-2000, but they're hard to use. We use a horizontal puller, the Sutter P-87, which costs $5725. (Sutter makes an even more expensive puller with a laser-heating element, but that has no advantages for this purpose.)

Data Backup

You'll need some kind of back-up system for the computer. Five minutes of recording at 2 kHz is 1.2 MB (500 kB with compression): you're going to use up a LOT of space. This price hasn't been included because it is assumed that somewhere in the lab you have a way of backing up computer files. If not, add in the cost of a tape drive. Alternatively, you can get a modified videotape recorder for about $500 that will store your voltage records, and then play them back into the computer for analysis. And there are many other options.

Voltage-clamp Amplifier

The P15 microelectrode amplifier can't do voltage clamp. If you want to measure meaningful absolute amplitudes, you'll need a voltage- clamp. The best (although some would dispute this) are the Axoclamp and the Axopatch from Axon Instruments. They cost $7000 and have lots of features, most of them actually useful. I'm told you can get voltage-clamp amplifers for much less ($2000).

Leon Avery, (leon@eatworms.swmed.edu)