A primer to extracellular recording
Electric current flow 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 20 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 10,000.
The main difficulties with extracellular recording is the electrical "noise".
This term refers to spontaneous voltage fluctuations which appear as a thickening
of the baseline when viewed on an oscilloscope at low sweep speed. The noise
may result from external interference from electrical sources in the vicinity
of the recording set-up (mains line hum pickup) and/or from the intrinsic
properties of the substances making up the electrode and electrical circuit
(thermal noise) used to amplify electrode signals. Noise in the input circuit
sets a limit to the smallness of signals that can be reliably measured. See
our noise eliminating tips below.
Microiontophoresis is most often used in conjunction
with extracellular recording of neuronal firing. Spikes can be recorded through
the center barrel of a multibarrel pipette assembly if one barrel is filled
with a suitable electrolyte solution such as sodium chloride. However, electrolyte-filled
glass micropipettes are electrically very 'noisy'. The solid-conductor microelectrodes
such as the tungsten or carbon fiber electrodes, in contrast, show significantly
less noise in extracellular recordings. Carbon fibers are 5-8 micrometers
in diameter and they provide excellent signal-to-noise ratio recordings. Here,
we introduce three versions of a simple yet high-performing extracellular
amplifier proved to be excellent in low-noise extracellular recording of neuronal
spike potentials with carbon fiber or other solid-conductor electrodes.
The ExAmp-20KB, an affordable high-gain, low-noise
extracellular amplifier
Back panel view
The ExAmp-20KB is battery operated, AC-coupled amplifier
designed for low-noise 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 solid-conductor microelectrodes such as carbon fiber, tungsten
or stainless steel. 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, field
potential, cord dorsum potential recording, bipolar electrocardiography, EMG
and EEG. The ExAmp-20KB ships complete with headstage probe, operator's manual
and batteries.
Headstage probe and electrode
holder in one. Using our probe.
Standard tungsten or stainless steel electrodes as well as
Kation-made glass insulated single- and multibarrel carbon fiber electrodes
can directly be plugged into this probe. The No. 2331 and No. 2332 screw-on
adapters eliminate the need for additional electrode holders as they firmly
secure these solid conductor electrodes in the probe. The 0.25" (6.4
mm) diameter mounting rod serves as a cable guide in one. This solution is
not only a very space saving design but also provides an extremely low-noise
recording quality seldom seen before.
More probe pictures
Specifications
Main unit
|
|
| Model and Catalog Number: |
ExAmp-20KB, M2100 |
| Input impedance: |
10 TeraOhm |
| Input leakage current: |
0.8 pA |
| Gain: |
200x to 20,000x |
| Filters built-in: |
Two 6-pole, tuned circuit bandpass |
| Filter bandpass frequencies: |
300 to 8000 Hz |
| Output voltage swing: |
±11 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) |
| Headstage probe |
|
| Catalog Number: |
M2110 |
| Dimensions: |
5/32"x 2" (14.4 x 50 mm) Diam. x Length |
| Mounting rod: |
1/4" x 4" |
| Material: |
Nickel-plated brass |
| Cable length: |
52" (132 cm) |
| Input sockets mates with: |
0.0315" - 0.0370" (0.80 - 0.94 mm) diameter pins |
To place an order for an ExAmp-20KB amplifier
see Kation Scientific's Order and Price list page.
The ExAmp-20K provides switchable
direct electrode access
Front panel view
 |
Back panel view
 |
Signal routing in ExAmp-20K
 |
The ExAmp-20K is a modification of our popular ExAmp-20KB
model designed to provide a direct access to the electrode attached to its
headstage probe. Istead of batteries, this model is powered by an external
12 VDC power supply without compromising recording quality. Its headstage
probe is identical to the ExAmp-20KB's but contains a miniature mechanical
relay to flip-flop the electrode connection between the extracellular amplifier
and the 'Electrode Access' BNC jack located on the back panel. The electrode
is normally connected to the extracellular amplifier circuit. The electrode
can be accessed when the relay is activated either manually (Manual/Ext.Gate
switch) or by an external TTL or CMOS (+5V) gating signal through the 'Gate
In' BNC jack. The gating signal is optically isolated from the rest of the
circuitry. The front panel 'Gate' LED is activated whenever the direct electrode
acess is activated.
Direct electrode access function can be used
for stimulation, electrical tissue coagulation or electrochemical measurements.
Specifications
| Model and Catalog Number: |
ExAmp-20K, M2200 |
| Headstage probe's Cat. No.: |
M2210, included |
| Main unit's dimensions: |
6 1/8" x 2 1/4" x 6 7/8" (155 x 54 x175 mm) (WxHxD) |
| Probe' dimensions: |
5/32"x 2" (14.4 x 50 mm) Diam. x Length. Identical to ExAmp-20KB's |
| Gate In signal: |
TTL or CMOS (+5V), optically isolated up to 5000 V. |
| Swithing (gating)time: |
1.5 ms |
| Electrode Access voltage: |
200 V maximum! DO NOT EXCEED! |
| Weight: |
1.12 lbs (500 grams) |
| Power source: |
12 V DC external power supply, purchase separately |
To place an order for an ExAmp-20K amplifier
see Kation Scientific's Order and Price list page.
The ExAmp-20KD, a dual-channel extracellular amplifier
This amplifier is a two-channel version of our ExAmp-20KB
model. Its front panel input and output connectors are miniaturized to fit.
Istead of batteries, this model is powered by an external 12 VDC power supply.
The included headstage probes, apart from their connector plugs, are identical
to the ExAmp-20KB's. The output SMA-type connectors are interfaced with standard
BNC jacks by the included SMA/BNC cables.
Specifications
| Model and Catalog Number: |
ExAmp-20KD, M2300 |
| Headstage probe's Cat. No.: |
M2310, included |
| Main unit's mechanical and electrical properties: |
Identical to ExAmp-20KB's |
| Probe's mechanical and electrical properties: |
Identical to ExAmp-20KB's |
| Dimensions: |
Identical to ExAmp-20KB's |
| Weight: |
1 lb (450 grams) |
| Power source: |
12 V DC external power supply, purchase separately |
To place an order for an ExAmp-20KD amplifier
see Kation Scientific's Order and Price list page.
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.
References
Conn, P.M., ed. (1991) Electrophysiology and microinjection. Methods
in Neurosciences, Vol. 4., Academic Press: San Diego.
Gerhardt, G.A. and M.R. Palmer (1987) Characterization of the Techniques
of Pressure Ejection and Microiontophoresis Using In-Vivo Electrochemistry.
Journal of Neuroscience Methods, 22 (2): p. 147-160.
Gottschaldt, K.M., T.P. Hicks, and C. Vahle-Hinz (1988) A combined
recording and microiontophoresis technique for input-output analysis of single
neurons in the mammalian CNS. Journal of Neuroscience Methods, 23 (3): p.
233-240.
Land, B.R., R.A. Wyttenbach, and B.R, Johnson (2001) Tools for physiology
labs: an inexpensive high-performance amplifier and electrode for extracellal
recording. Journal of Neuroscience Methods, 106, 47-55.
Purves, R.D. (1981) Microelectrode methods for intracellular recording
and iontophoresis. New York: Academic Press.
Stamford, J.A., ed. (1992) Monitoring neuronal activity: a practical
approach. 1992, Oxford University Press: Oxford.
