Membrane response in nerve or muscle after reaching excitation threshold. The complete response is the same regardless of the type of stimulus and is referred to as an “all-or-none response.”

The components of a circuit that provide amplification or that control the direction of current flow (e.g., diodes, transistors, and vacuum tubes).

In digital data storage systems, the description of a location (stated in system notation) where information is stored. Also, as a verb, to select or to designate the location of information in a storage system.

A flow of current in which the direction of current flow reverses periodically. When the reversal occurs cyclically, two current reversals are termed one cycle. The number of complete cycles per second is the frequency, and it is stated in Hertz.

A device that multiplies its input voltage, current, or power by a fixed or controllable factor, usually without altering its waveform.

This amplifier responds to alternating current (AC) signals only and not to an input potential that does not vary. This type of amplifier is used in EMG apparatus. Sometimes termed resistance-capacitance (RC)- or AC-coupled amplifier.

This amplifier responds to direct current (DC) signals, pulsating DC signals, and alternating current signals. This type of amplifier is not used in clinical EMG. It is used in force and tension measurements, as well as in the recording of intracellular resting potentials in which fixed, slowly, and rapidly changing phenomena are measured.

An amplifier used in EMG preamplifiers. It has two recording electrode input terminals (instead of the single input terminal of a conventional amplifier) and a ground or zero-potential terminal. It rejects unwanted potentials originating at a distance and presenting at both input terminals (common-mode or in-phase potentials).

The potential measured in volts for any type of recorded response in electrodiagnostic testing.

Systems of signal transmission, recording, or processing that use an alternating current carrier potential of peak amplitude that varies proportionally with the instantaneous amplitude of the signal.

A term applied to signals and devices capable of accommodating continuous change and assuming an infinite number of values with finite limits. An analog signal may be a current or a voltage that varies in time continuously, simulating and representing a natural phenomenon.

A device that converts an analog signal, usually a varying voltage or current, to a digital output (see “Digital System”).

A positive terminal. The terminal through which “electron current” enters a device. “Conventional current” flow, however, is said to be away from the anode and toward the cathode (negative) terminal.

In nerve conduction testing, refers to a test situation where the nerve action potential signal propagates in the reverse of the normal physiologic direction.

All unwanted potentials that originate outside the tissues examined. They are also called “noise” when they appear in measurement. An artifact may arise from biologic activity, the electrode or apparatus used in the examination, the power line, or the extrinsic electricity surrounding the apparatus or patient (see “Noise”).

In electrical circuits, an arrangement that introduces a definite reduction in the magnitude of a voltage current or power. Attenuators may be fixed or adjustable either continuously or in steps.

A signal-processing method that aids in the recording of small-stimulus evoked potentials that are obscured by noise or artifact. The stimulus is repeated a number of times, and the responses are subjected to a special summation technique that causes the random noise portion of the response to become smaller in proportion to the evoked potentials that are coherent in time with each stimulus.

Nerve injury with loss of axon membranes and myelin, preserving other support structures.

The amplifier frequency response limits, defined by the high- and low-frequency filters, beyond which the amplification falls to 70% of full power (see “Frequency Response”).

A fixed electrical or mechanical input to a device or a system that is distinct from the input signal. The bias brings the system to a desired operating range.

A digital logic system that operates with two distinct states, variously called “one and zero,” “high and low,” and “on and off.”

A binary numeral, the “one and zero” or “high and low,” and so on, of binary logic. A group of bits make up a binary word or byte.

An effect that results when a large transient input potential is applied to an amplifier, temporarily causing the disappearance or severe distortion of the output signal.

Intermittent loss of a component of a single-fiber EMG recording.

A binary word containing a system-defined number of bits.

A device that identifies units of measurement by reference to a known standard.

A measure of electric charge that can be stored within the insulation separating two conductors when a given voltage is applied to the conductors. A capacitor or a condenser uses conductors of large surface area separated by air or by various insulators (dielectrics) that enhance capacitative effects. The unit capacitance is the farad. Direct current is not conducted by capacitors; alternating current or pulsating direct current signals are conducted to an extent proportional to frequency.

A potential, usually alternating current, of sine or pulse waveform used in signal transmission, recording, or processing systems that in itself carries no information, but is modified most commonly in amplitude (amplitude modulation), frequency (frequency modulation), or timing by the signal. The carrier is at least a number of times higher in frequency than the highest-frequency component in the signal.

A negative terminal. The terminal through which “electron current” leaves a device. Conventional current flow is said to be toward the cathode or away from the anode (positive) terminal.

A vacuum tube used to visualize electrical waveforms. It generates X-Y traces on its screen by means of a moving fluorescent spot on its screen.

This occurs when signals of excessive amplitude are applied to an amplifier, with a resultant waveform at the amplifier output that faithfully reproduces the shape of the input waveform only up to a level at which the signal becomes excessive (clipping level). All portions of the waveform that exceed the clipping level appear at the output at a fixed level that does not vary with time and are therefore seriously distorted.

An important property of differential amplifiers that expresses their ability to discriminate against artifact potentials that appear equally at both amplifier input terminals (common mode signals) and to amplify the desired potentials (differential or series mode signals) that appear as different signals at the two input terminals.

A calculation performed to measure the effectiveness of the differential amplifier.

A system that cyclically switches a number of signals sequentially to a single device amplifier, transmission, or recording channel. Also termed multiplexing.

High-frequency recording in needle EMG, usually with abrupt onset and ending.

The incursion of information from one channel into any other channel of a multichannel information-handling system. The presence of crosstalk in a multichannel EMG study can be seriously misleading.

A complete sequence of values of an alternating quantity repeated as a unit. Cycles per second (CPS) is also called Hertz.

A dimensionless unit for comparing the ratio of signal levels on a logarithmic scale. Positive
decibel values represent a signal increase with respect to a reference. Negative decibel values represent signal decrement with respect to a reference signal.

Diminished quality of response with repeated identical stimulations.

A short-term electrical dynamic storage device that delays potentials applied to its input so that they appear at its output as if they had occurred (1 to 20 ms) later in time. This permits events preceding action potentials to be seen on the monitor or CRT screen when the sweeps are triggered by the potentials.

See “Amplifier, Differential.”

A device or circuit with an output waveform that is proportional to the rate of change (e.g., speed, velocity) of the input waveform.

A system or circuit for handing or processing information in terms of numbers and utilizing circuits that operate in the manner of switches, having two (on–off) or more discrete positions. The simplest and most common digital system is the binary system.

A circuit that accepts the discrete coded signal voltages of a digital system and generates, at its output, voltages of amplitudes analogous to the numbers represented by the digital codes at its input. The analog output may then be directly interpreted by viewing a CRT or reading a meter or graphic recording.