Introduction to Microelectrode Amplifiers
Molecular Devices offers a complete line of microelectrode
amplifiers for voltage recording, current and voltage clamping, and patch clamping.
The wide choice of headstages designed for specific applications and the various features
incorporated into these amplifiers make them ideal instruments upon which to base an
electrophysiology workstation.
This introduction provides basic characteristics and requirements of microelectrode amplifiers,
and the many important features that enable MDC's Axon Instruments amplifiers to meet your requirements.
Amplifier Functions
Electrophysiological measurements involve amplification of small, fast signals. In order
to be useful, these measurements must be accurate, low in noise, stable and reliable.
A microelectrode amplifier measures current or voltage and passes that measurement on for
recording and analysis. Most often, this requires that the amplifier reliably amplify a
signal, since the voltages and currents of interest are usually very small. Microelectrode
amplifiers must also control current and voltage in current-clamp and voltage-clamp mode.
MDC amplifiers incorporate many features that take them far beyond these minimal requirements.
Additional features such as series resistance compensation, capacity compensation, low-pass
filters, square-wave generators, holding potential control, automatic telegraphing of gain
and capacitance settings to acquisition equipment, and audio monitors make Axon amplifiers
the central component of microelectrode and patch-clamp workstations.
Extracellular Voltage Recording
A common requirement in electrophysiological research is voltage recording. Single-unit
recordings and field potentials in brain or brain slice, electrocardiograms, myograms,
encephalograms and oculograms all require measurements of small biological potentials.
These are often less than a millivolt in amplitude. The Axoclamp-2B, GeneClamp 500B
and MultiClamp 700B microelectrode amplifiers may be used for such experiments.
The CyberAmp signal conditioner is also designed for voltage recording.
Voltage Clamp
The goal of a voltage clamp experiment is to measure membrane current. To do this, one
monitors the membrane voltage and injects current to attain and maintain the desired voltage.
Hence, a voltage-clamp amplifier must be able to: 1) measure voltage and 2) pass current
in order to regulate the cellular voltage. The Axoclamp-2B, Axopatch 200B, GeneClamp 500B,
and MultiClamp 700B amplifiers are all excellent voltage-clamp amplifiers.
Patch-Clamp
Patch-clamp refers to the technique of using a blunt pipette to isolate a patch
of membrane. Patch-clamp recording can measure the individual ion channel currents
that contribute to whole-cell currents. The patch-clamp technique is compatible with
current-clamp and voltage-clamp recording modes. If the patch of membrane underneath
the pipette is ruptured or otherwise made permeable, the technique is further referred
to as "whole-cell patch-clamping." In this case, the currents passing through the entire
cell membrane are recorded. This is equivalent to intracellular recording with sharp
microelectrodes, but has the advantage that it can be applied to even very tiny or flat
cells that would be impossible to impale otherwise.
The Axopatch 200B and MultiClamp 700B are excellent amplifiers for both modes of patch-clamp
recording. A separate patch-clamp headstage is available for the GeneClamp 500B to enable it
for single-channel measurements. The Axoclamp-2B is capable of performing whole-cell voltage
clamp with its continuous Single-Electrode Voltage Clamp (cSEVC) mode.
Most single-channel patch-clamp amplifiers, including the MultiClamp 700B and GeneClamp 500B,
use resistive headstages. For the ultimate in low-noise recording, a capacitor-feedback
(or integrating) headstage is standard with the Axopatch 200B. This amplifier offers the
dual advantages of lower noise and wider bandwidth than resistive headstages. With the
introduction of the innovative active cooling of headstage circuit components, the
Axopatch 200B achieves the lowest noise levels ever.
Two-Electrode Voltage Clamp
The magnitude of the transmembrane current varies greatly between cell types. Two electrodes,
one for passing current and one for measuring voltage, are best for clamping large cells with
large currents. The Axoclamp-2B and GeneClamp 500B amplifiers are specifically designed for
such measurements using the Two-Electrode Voltage-Clamp (TEVC) mode.
Current Clamp
Current-clamp amplifiers are designed to control the current and measure the corresponding
membrane voltage. It is common to pass current to stimulate a cell or modify its resting
potential during intracellular voltage recording. The Axoclamp-2B, GeneClamp 500B and
MultiClamp 700B amplifiers are able to pass current while in voltage-sensing (i.e.,
current-clamp) mode. The Axopatch 200B patch-clamp also offers a current-clamp mode, which
can be used to monitor membrane potentials and to follow (with some distortion) action
potentials. The current-clamp performance of the Axopatch is not as fast as the
aforementioned amplifiers, because the circuitry was not specifically designed for
following voltage.
Discontinuous Clamp
The Axoclamp-2B amplifier offers special recording modes referred to as "discontinuous,"
applicable to both voltage clamp (dSEVC) and current clamp (dCC). In this mode the
instrument divides its time in passing current and recording voltage. The advantage of
this mode is that the recording is free from the usual error due to the voltage drop across
the electrode resistance. It also can be used with a conventional intracellular microelectrode.
On the other hand, dSEVC is much harder to set up than cSEVC and requires frequent fine-tuning
of the controls as the microelectrode resistance drifts. The amount of noise in dSEVC is
about two to three times greater than in cSEVC.
Ion-Selective Electrodes and Electrochemistry
Ion-selective electrodes, voltammetry and constant-voltage amperometry have been used to
measure levels and small changes in ion, neurotransmitter and hormone concentrations in
tissues or in and near cells. These techniques require the ability to record small
potentials and pass large currents.
Ion-selective electrodes require differential input, low leakage current and high-impedance
voltage following. With appropriate headstages, the Axoclamp-2B amplifier is well suited to
this application. The electrochemical techniques of voltammetry and constant-voltage
amperometry are used to measure fast changes in neurotransmitter concentrations. These
techniques require a voltage-clamp amplifier with a command voltage range extended to ±1V.
The Axopatch 200B, GeneClamp 500B and MultiClamp 700B amplifiers are all capable of delivering
such a command.
Amplifier Features
In addition to their unequaled low-noise amplification performance, Axon amplifiers offer
many features that provide exceptional versatility and convenience. Some of these features
are described below. For more detailed information, see specifications for each amplifier
on their respective web pages. Also consult The Axon Guide, an indispensable reference that
is downloadable from this web site.
Series Resistance Compensation
This is a standard feature of the Axoclamp-2B, Axopatch 200B and MultiClamp 700B that
reduces the voltage-clamp error due to the voltage drop across the resistance of the electrode.
Capacitance Compensation
This function compensates for the capacitance associated with the pipette and the input of
the operational amplifier. If left uncompensated, the capacitance results in a reduction of
the recording bandwidth. This feature is standard on the Axoclamp-2B, GeneClamp 500B and
Axopatch 200B amplifiers and contributes to their excellent noise and bandwidth
characteristics. The Axopatch and MultiClamp amplifiers offer two pairs of pipette
capacitance compensation controls, and in addition, compensation for cell capacitance in
the whole-cell mode.
Low-Pass Filters
Filters allow the recording bandwidth to be optimized for a particular acquisition rate or
phenomenon of interest. The Axopatch 200B, GeneClamp 500B and MultiClamp 700B amplifiers
include built-in low-pass filters with a wide range of available settings that allow digital
acquisition directly from the amplifier without need for additional external signal conditioning.
Telegraphs
Telegraph information simplifies experimental procedures and ensures accurate scaling and
record keeping. These outputs convey gain, low-pass filter and whole-cell capacitance
settings to a computer from the Axopatch 200B or GeneClamp 500B amplifiers. The MultiClamp
700B offers additional information, including scaling factors and operating mode (voltage
or current clamp). Telegraphs are supported in Axon's pCLAMP data acquisition software.
Buzz, Clear and Zap
These features make cell penetration and patch rupture easier and more reproducible.
Buzz drives a brief, high-frequency oscillatory current through the microelectrode
which assists penetration. Clear drives a large positive or negative current through
the microelectrode, either to assist in cell penetration or to clear blocked microelectrode
tips. These features are available with the Axoclamp-2B and MultiClamp 700B amplifiers.
Zap applies a large potential to the membrane patch, which is often sufficient to
rupture the patch and begin recording in whole-cell mode. Zap is available with the
Axopatch 200B and MultiClamp 700B amplifiers.
Blanking
This prevents the stimulus artifact from entering the amplifier circuit, and enables
recording to begin immediately after the stimulus terminates. This feature is offered
with the Axoclamp-2B and Axopatch 200B amplifiers.
Bridge Balance
This offset procedure eliminates the voltage drop that would otherwise occur across an
uncompensated electrode resistance in current-clamp mode. The Bridge Balance feature is
available on the Axoclamp-2B and MultiClamp 700B amplifiers. With the Axopatch 200B, the
Series Resistance knob can be used in current-clamp mode to accomplish the equivalent.
Bath Headstages
Allow for measurement, compensation and control of the voltage drop across the bath
resistance, an important consideration when measuring large currents. The Axoclamp-2B,
GeneClamp 500B and MultiClamp 700B amplifiers offer such headstages, allowing accurate
measurement of currents as large as 10 microamperes.
Capacitance Measurements
The Axopatch 200B includes a capacitance dithering feature that provides the capability to
measure small changes in cell membrane capacitance. With the included series resistor
dither, one can use phase tracking to continuously monitor cell capacitance.
Choosing the Best Amplifier for Your Application
Molecular Devices makes the best microelectrode and patch-clamp amplifiers available.
To quickly compare the features of all the amplifiers, go to the
Amplifier Comparison Table.
MultiClamp 700B
resistor-feedback patch clamp and true current clamp
A computer-controlled patch-clamp and current-clamp amplifier can be switched in either of
two headstages. Offsets, series-resistance and capacitance compensation can be performed
automatically in software. Built-in command generators (including Seal Test), a
variable-cutoff low-pass Bessel filter, leak subtraction, variable output gain, RMS noise
monitor, Track, Zap, and an audio monitor are all standard features. The MultiClamp 700B
has four different feedback resistors for single-channel or whole-cell voltage clamp, and
three different current-passing ranges in current clamp.
Axopatch 200B
capacitor-feedback patch clamp
The Axopatch 200B patch-clamp amplifier achieves ultra-low noise in single-channel mode with
its cooled, capacitor-feedback design. It features excellent bandwidth in single-channel,
whole-cell and current-clamp modes - all with one standard headstage. Membrane capacitance
and electrochemical measurements capability are standard. An enhanced series resistance
compensation circuit allows separate control of prediction and correction. Its improved
current clamp circuit includes series resistance compensation and a choice of "normal" and
"fast" for increased stability. The Axopatch 200B's features include Zap, scaled output gains,
variable low-pass filters and leak subtraction.
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