A primer to extracellular recording
Electric current flows in the tissue around the neurons during
action potentials can be detected by means of
extracellular microelectrodes as extracellular ‘spikes’. Extracellular
spike potentials recorded from the mammalian
central nervous system have a duration of between 0.2 and 10 ms. Their
amplitudes are typically a few
hundred microvolts although they may vary in amplitude from the noise
level of the electrode (several
microvolts) up to several millivolts, depending on the type of neuron
and the quality of the recording system.
The greatest advantage of extracellular recording is that the activity
of neurons can be recorded without having
to impale and consequently damage them. For this and other reasons,
most in vivo neuronal spike detection is
done with extracellular recording.
Signals picked up by extracellular electrodes are in the microvolt range and they need to be amplified to be
able to be processed in more conventional electronic devices such as oscilloscopes, analyzers or computers.
The usual degree of amplitude amplification in extracellular amplifiers is around 10,000x. The microelectrode
must be connected to a specialized preamplifier (also known as a headstage probe) in order to work properly.
The electrical properties of the headstage and main amplifiers set a limit to the smallness of signals that can be
reliably measured.
Read more>>
The ExAmp-20KB, an affordable battery-operated extracellular amplifier
Back panel view
The ExAmp-20KB is a battery operated, AC-coupled amplifier designed for low-noise extracellular
recording. Its unique headstage probe design puts the first stage of amplification (10x) at the microelectrode
interface, resulting in less external interference noise pickup. The included 60 Hz (50 Hz
optional) reject filter and our special electrode holder adapters virtually eliminate the need for Faraday
cages. Possible applications are single-unit or single-axon recording in research or teaching experiments.
It is not intended for use on human subjects. The ExAmp-20KB ships complete with headstage
probe, operator‘s manual and batteries.
Specifications
Input impedance: 10 TeraOhm
Input leakage current: 0.8 pA
System gain with headstage probe: 2,000x to 20,000x
Filter bandpass frequencies: 300 to 8000 Hz
Output voltage swing: ±10 V, maximum
Power source: 4 standard D size batteries, included
Battery test: Audible tone
Lifetime of batteries: 500 hours (estimated)
Box dimensions: 6 1/8" x 2 1/4" x 6 7/8" (155 x 54 x175 mm) (WxHxD)
Weight: 2 1/4 lbs (1020 gram) (with batteries)
Order codes
For prices click on the catalog numbers.
M2100 ExAmp-20KB extracellular amplifier
Includes 1 headstage probe, 1 main unit, 4 “D” size batteries, 1 manual.
Optional:
M2110 Headstage probe for ExAmp-20KB.
M2331 Electrode holder adaptor for Kation-made CF microelectrodes
M2332 Electrode holder adaptor for metal microelectrodes
M2333 Electrode holder adaptor for 1.5 mm O.D. glass micropipette electrodes
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The ExAmp-20K an affordable high-gain, low-noise extracellular amplifier
More picture>>
The ExAmp-20K is an AC-coupled spike amplifier designed for low-noise, monopolar extracellular
recording. Its unique headstage probe design puts the first stage of amplification at the microelectrode
interface, resulting in less external interference noise pickup. The built-in filters are optimized for single-
unit recordings. The included 60 Hz (or 50 Hz for Europe) reject filter and our special electrode
holder adapters virtually eliminate the need for Faraday cages. Front and rear panel controls are intentionally
simplified for ease of use without compromising the quality of recordings. The ExAmp-20K
can be used for research and teaching purposes only that do not involve human subjects.
Requires an external power supply. Order separately.
Specifications
|
|
Front panel controls
|
| Model and Catalog Number: |
ExAmp-20KB, M2100 |
Power: |
Turns the unit on or off. LED signals on status |
| Input impedance: |
10 TeraOhm |
Probe: |
Receptacle for headstage probe |
| Input leakage current: |
0.8 pA |
Gain: |
Selects the magnitude of amplification |
| Probe gain: |
10x |
Notch filter: |
Turns on 60 Hz (or 50 Hz) reject filter |
| System gain: |
2,000x to 20,000x |
Output: |
Outputs for the amplified signal. |
| Bandpass frequencies: |
300 Hz to 8000 Hz | Rear panel controls |
| Output voltage swing: |
±10 V, maximum |
| Power source: |
External, 12 V DC power supply |
Power In/Out: |
12V DC power connector |
| Power consumption: |
180 mA, maximum |
Gnd: |
Amplifier circuit`s floating ground |
| Amplifier (Gnd) ground: |
Isolated from DC input ground (Case) |
Case: |
Housing and input power ground |
| DC power jack: |
Rear panel, 2.1 mm x 5 mm type |
|
|
| Grounding receptacles: |
Rear panel, miniature (2.64 mm)
banana jack |
|
|
| Dimensions: |
155 x 54 x 175 mm |
|
|
| Weight: |
1.35 lb (614 grams) |
|
|
Order codes
For prices click on the catalog numbers.
M2200 ExAmp-20K extracellular amplifier
Includes 1 headstage probe, 1 main unit, 2 minature banana plugs, 1 manual.
M1116 External power supply with selectable US, EUR, GB or AUST plugs
Optional:
M2210 Replacement headstage probe for ExAmp-20K if needed.
M2331 Electrode holder adaptor for Kation-made CF microelectrodes
M2332 Electrode holder adaptor for metal microelectrodes
M2333 Electrode holder adaptor for 1.5 mm O.D. glass micropipette electrodes
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The ExAmp-20KD, a two-channel extracellular spike amplifier
This modes is a dual-channel version of the ExAmp-20K extracellular amplifier with similar specifications.
Requires an external power supply. Order separately.
More picture>>
Order codes
For prices click on the catalog numbers.
M2300 ExAmp-20KD extracellular amplifier
Includes 1 headstage probe, 1 main unit, 2 minature banana plugs, 1 manual.
M1116 External power supply with selectable US, EUR, GB or AUST plugs
Optional:
M2310 Replacement headstage probe for ExAmp-20K if needed.
M2331 Electrode holder adaptor for Kation-made CF microelectrodes
M2332 Electrode holder adaptor for metal microelectrodes
M2333 Electrode holder adaptor for 1.5 mm O.D. glass micropipette electrodes
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Headstage probes
Our headstage probe design is a classic, non-inverting amplifier with a 10x gain. In order to minimize
interferences by alternating electromagnetic fields, microelectrodes can actually be plugged into the
spark-plug type SMA receptacle of the headstage probe as shown below. Aminiature spring socket
provides an input for the ground lead and mates with pin diameters ranging from 0.66 to 0.84 mm. The
very short distance between the electrode and the first stage of amplification and the PTFE (Teflon)
insulation of the receptacle allows a leakage-free, low-noise recording. The entire circuitry is placed in
a 14 mm x 50 mm (diameter x length) nickel-plated cylindrical brass container for added shielding
from electromagnetic interference. The center pin (inpu) is embedded in a PTFE insulator, whereas the
grounding receptacle is in galvanic contact with the metal parts of the headstage probe.
The headstage probe can be securely fixed in most micromanipulators via its 6.4 mm x 105 mm
(diameter x length) mounting rod, which also serves as a cable guide. The headstage probe is connected
to the main unit by a 1.3-m-long, well-shielded, high-flex cable through the use of a high-tech connector
placed on the free end of the cable.
The positive input of the front amplifier (center pin of the headstage’s input) is referenced to the
ground (grounding socket) providing high-quality, single-ended (monopolar) recordings.
This unique headstage probe design provides an electrode holder in one. See the adaptor section.
Physical layout of probe
Electrode holder adaptors
Three types of electrode holder adaptors can be screwed onto the input SMA connector. The first (No.
2331) is used to attach Kation-made, glass-insulated single or multibarrel carbon fiber electrodes. Microelectrodes
can simply be inserted directly into the center pin of the SMA connector fixed in the front-end
of the probe.
The second type of adaptor (No. 2332) is used to accommodate small-diameter metal electrodes. To configure
this type of electrode holder, the connector pin of the electrode is inserted first into the front of the
adaptor and pulled through it so that it protrudes somewhat from the back-end of the adaptor. This is done
on a flat surface before the adaptor is screwed onto the SMA connector. A small pair of tweezers is used to
grasp the pin of the electrode and insert it into the input socket of the probe. The adaptor is then screwed
onto the probe gently by hand. Next, an about 2.5 cm-long strip of copper foil with conductive adhesive
tape (3M, Part No. 1181) is rolled cylindrically so that one-third of the 1" (25 mm) width of the tape is
around the front-end of the adapter and two-thirds of it is around the shank of the electrode itself. Finally,
the front-end of the tape is flattened with the thumb and forefinger and the excessive parts of the foil are
cut off. The copper foil has a twofold purpose: it holds the electrode in place and it provides an extended
shielding for smoother recording. A common electrical tape can also be used.
The third type of adaptor (Cat. No. 2333) is a machined aluminum housing used to hold a glass micropipette
in position and provides an additional shielding for a very low-noise extracellular recording.
To use this adaptor, first, solder a 30 to 40 mm long
chlorided silver wire into the provided gold-plated pin.
Alternatively, solder the wire into the pin and then chloride
it using diluted HCl and a battery. Insert the pin in
the center socket of the headstage probe (panel A).
Carefully screw the body of the adaptor on the probe so
that the silver wire protrudes from it (panel B). Pull the
electrolyte-filled micropipette through the adaptor’s nut
and put on the gasket ring as shown in panel C. Then,
holding the micropipette, nut and gasket together, introduce
the micropipette into the adaptor’s bore so that the
silver wire goes smoothly into the micropipette (panel
C). Finally, screw the nut onto the adaptor’s body by
hand (panel D). Make sure that the micropipette is
firmly positioned in the adaptor.
Order codes
For prices click on the catalog numbers.
M2110 Headstage probe for ExAmp-20KB.
M2210 Headstage probe for ExAmp-20K
M2310 Headstage probe for ExAmp-20KD.
M2331 Electrode holder adaptor for Kation-made CF microelectrodes
M2332 Electrode holder adaptor for metal microelectrodes
M2333 Electrode holder adaptor for 1.5 mm O.D. glass micropipette electrodes
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Sample experiment
Recording was taken from a brainstem
neuron using an ExAmp-20KB in combination with a Carbostar-3 carbon
fiber electrode. Cell firing was evoked by iontophoresed NMDA using
a Union-36 iontophoresis pump. The amplified signal was sampled and
digitized at 50 KHz frequency by a National Instruments PCI-1200 data
acquisition board. |
 |
Grounding and shielding in extracellular recording
In order to minimize external noise pickup and interference, experimental set-ups (‘electrophysiology rigs’)
have to be correctly shielded and grounded. Overall shielding is most often provided by a Faraday cage (a
grounded wire mesh box surrounding the baseplate, microscope, preparation and headstage probe). Faraday
cages, however, do not necessarily remove interference from magnetic fields, which usually cause the most
problem. To minimize magnetic field interference, place the the preparation to be recorded as close as possible
to a large carbon steel (not stainless steel!) baseplate which should be connected to the ground pin of the
headstage probe. The more massive the plate, the more helpful it will be at deflecting magnetic field. A steel
slab1/2" (12 mm) thick and at least 20" (500 mm) on each side greatly improves the quality of extracellular
recordings. Always keep the lead wire from the electrode to the headstage probe as short as possible, less then
2" (5 cm). This is accomplished by our unique headstage probe design where microelectrodes can actually be
plugged straight into the probe.
Keep line-powered equipments as far away from the site of recording as it is possible. Everything near the
preparation (manipulators, microscope, stereotaxic apparatus, microdrives, lamps, heaters, etc.) should be
grounded to a single point, that is the ground pin on the headstage probe and to nothing else. Use of a “star”
formation grounding will minimize ground loops. Isolation of extracellular amplifiers from power line ground
prevents ground loop formation. This is accomplished by employing battery power in our ExAmp-20KB
amplifier. After following these basic rules, the removal of interference noise is normally a process of trial and
error, involving experiments with slightly different patterns of grounding and shielding. Remember that our
ExAmp-20K amplifiers permit a very smooth, low-noise recording. Thus, if your extracellular recording
shows a frustrating level of noise keep trying to find and eliminate the source.
Keep line-powered equipments as far away from the site of recording as it
is possible. Everything near the preparation (manipulators, microscope, stereotaxic
apparatus, microdrives, lamps, heaters, etc.) should be grounded to a single
point, that is the ground pin on the headstage probe and to nothing else.
Use of a "star" formation grounding will minimize ground loops.
Isolation of extracellular amplifiers from power line ground prevents ground
loop formation. This is accomplished by employing battery power in our ExAmp-20KB
amplifier. After following these basic rules, the removal of interference
noise is normally a process of trial and error, involving experiments with
slightly different patterns of grounding and shielding. Remember that our
ExAmp-20K amplifiers permit a very smooth, low-noise recording. Thus, if your
extracellular recording shows a frustrating level of noise keep trying to
find and eliminate the source.
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