ACTION POTENTIAL (AP) – Strictly defined, the all-or-none, self-propagating, nondecrementing voltage change recorded from an excitable cell. The source of the action potential should be specified, e.g. nerve (fiber) action potential or muscle (fiber) action potential. Commonly, the term refers to the nearly synchronous summated action potentials of a group of cells, e.g., motor unit potential. To avoid ambiguity in reference to the recording of nearly synchronous summated action potentials of nerve and muscle as done in nerve conduction studies, it is recommended that the terms compound nerve action potential and compound muscle action potential be used.
ACTIVE ELECTRODE – See recording electrode.
AMPLITUDE – With reference to an action potential, the maximum voltage difference between two points, usually baseline to peak or peak to peak. By convention, the amplitude of the compound muscle action potential (CMAP) is measured from the baseline to the most negative peak. In contrast, the amplitude of a compound sensory nerve action potential (SNAP) is measured from the onset to the negative peak, also called “negative peak amplitude”. The amplitude measured from the most positive peak to the most negative peak is called “peak-to-peak amplitude”.
AMPLITUDE DECAY – The percentage change in the amplitude of a compound muscle action potential (CMAP) or a compound sensory nerve action potential (SNAP) between two different stimulation points along the nerve.
ANODE – The positive terminal (+) of a source of electrical current.
ANTIDROMIC – Said of an action potential or of the stimulation causing the action potential that propagates in the direction opposite to physiological (orthodromic) one for that fiber-i.e., conduction along motor fibers toward the spinal cord and conduction along sensory fibers away from the spinal cord. Most sensory nerve conduction studies are performed antidromically because of the higher amplitude of the sensory nerve action potentials (SNAPs) than the orthodromic studies. Contrast with Orthodromic.
ARTIFACT – A voltage change generated by a biological or nonbiological source other than the ones of interest. The stimulus artifact is the potential recorded at the time the stimulus is applied and includes the electrical or shock artifact, which is a potential due to the volume conducted electrical stimulus (a cutaneous spread of a stimulating current to the recording electrode). The stimulus and shock artifacts usually precede the activity of interest. A movement artifact refers to a change in the recorded activity due to movement of the recording electrodes.
BASELINE – The potential difference recorded from the biological system of interest while the system is at rest. In an electrophysiological recording, the baseline is the display of zero potential differences between the two input terminals of the recording instrument. It is also called isoelectric line.
CATHODE – The negative terminal (-) of a source of electrical current.
COMPOUND MUSCLE ACTION POTENTIAL (CMAP) – Evoked response from a muscle by a single electrical stimulus to its motor nerve. By convention, the compound action potential elicited by supramaximal stimulation is used for motor nerve conduction studies. The recording electrodes should be placed so that the initial deflection of the evoked potential is negative. The latency, commonly called motor latency, is the latency, measured in millisecond (ms) to the onset of the first negative phase. The amplitude, measured in millivolt (mV) is the baseline-to-peak amplitude of the first negative phase, unless otherwise specified. The duration, measured in millisecond (ms) refers to the duration of the first negative phase, unless otherwise specified. Normally, the configuration of the compound action potential (usually biphasic) is quite stable with repeated stimuli at slow rates (1-5 Hz).
COMPOUND ACTION POTENTIAL – The summation of nearly synchronous muscle fiber action potentials recorded from a muscle commonly produced by stimulation of the nerve supplying the muscle either directly or indirectly. Baseline-to-peak amplitude, duration, and latency of the negative phase should be noted, along with details of the method of stimulation and recording.
COMPOUND NERVE ACTION POTENTIAL -The summation of nearly synchronous nerve fiber action potentials recorded from a nerve trunk, commonly produced by stimulation of the nerve directly or indirectly. Details of the method of stimulation and recording should be specified, together with the fiber type (sensory, motor, or mixed).
COMPOUND SENSORY NERVE ACTION POTENTIAL (SNAP) – A compound sensory nerve action potential (SNAP) is considered to have been evoked from afferent fibers if the recording electrodes detect activity only in a sensory nerve or in a sensory branch of a mixed nerve, or if the electrical stimulus is applied to such a nerve or a dorsal nerve root, or an adequate stimulus is applied synchronously to sensory receptors. The amplitude, latency, duration, and configuration should be noted. Generally, the amplitude is measured as the maximum peak-to-peak voltage, the latency as the peak latency to the negative peak, and the duration as the interval from the first deflection of the waveform from the baseline to its final return to the baseline.
CONDUCTION BLOCK – Failure of an action potential to be conducted past a particular point in the nervous system. In practice, a conduction block is documented by demonstration of a reduction in amplitude of an evoked potential greater than that normally seen with electrical stimulation at two different points on a nerve trunk; anatomical nerve variations and technical factors related to nerve stimulation must be excluded as the source of the reduction in amplitude.
CONDUCTION VELOCITY – Speed of propagation of an action potential along a nerve or muscle fiber. The nerve fiber studied (motor, sensory, autonomic, or mixed) should be specified. For a nerve trunk, the maximum conduction velocity is calculated from the latency of the evoked potential (muscle or nerve) at maximal or supramaximal intensity of stimulation at two different points. The distance between the two points (conduction distance) is divided by the difference between the corresponding latencies (conduction time). The calculated velocity represents the conduction velocity of the fastest fibers and is expressed as meters per second (m/s). As commonly used, the term conduction velocity refers to the maximum conduction velocity.
CONTRACTION – A voluntary or involuntary reversible muscle shortening that may or may not be accompanied by action potentials from muscle.
CYCLES PER SECOND – Unit of frequency. Preferred equivalent is Hertz (Hz).
DEPOLARIZATION – A decrease in the electrical potential difference across a membrane from any cause, to any degree, relative to the normal resting potential.
DURATION – The duration of individual potential waveforms is defined as the interval from the first deflection from the baseline to its final return to the baseline of an action potential or waveform, unless otherwise specified. Motor potentials have longer duration than sensory potentials.
ELECTRODE – A device capable of conducting electricity. The material, size, configuration (disc, ring, needle), and location (surface, intramuscular, intracranial) should be specified. Electrodes may be used to record an electrical potential difference (recording electrodes) or to apply an electrical current (stimulating electrodes). In both cases, two electrodes are always required (bipolar configuration).
ELECTROMYOGRAPH – An instrument for detecting and displaying action potentials from muscle and nerve.
ELECTROMYOGRAPHY (EMG) – Strictly defined, the recording and study of insertional, spontaneous, and voluntary electrical activity of muscle. It is commonly used to refer to nerve conduction studies as well.
ELECTRONEUROGRAPHY (ENG) – It is the recording and study of the action potentials of a peripheral nerve. It is synonymous with nerve conduction studies.
EVOKED COMPOUND MUSCLE ACTION POTENTIAL – The electrical activity of a muscle produced by stimulation of the nerves supplying the muscle. Baseline-to-peak amplitude of the negative phase, duration of the negative phase, and latency should be measured, details of the method of stimulation should be recorded.
FILTERS – Filters are electronic circuits that perform the function of processing the body signals removing unwanted electrical noise. Electrodiagnostic studies use low-frequency (high-pass) and high-frequency (low-pass) filters to exclude high- and low-frequency electrical noise to reproduce the signal of interest. Specific filter ranges have been selected to ensure exact reproduction of the responses, and in practice the frequency range (according to the type of conduction test) should be set as recommended because a change in the filter range will affect the amplitude and latency of the potentials. Several differences have been observed between the motor and sensory responses: The amplitude is affected more by narrowing the lower frequency limit than the higher frequency limit in the former (because of the longer duration of the motor potentials); conversely in the latter, in which the high frequency limit is more important because of the shorter duration of the sensory potentials. In the motor nerve conduction study, the recommended filter range is 2 Hz (high-pass) to 10 kHz (low-pass), while in the sensory nerve conduction study, the filter range to be used is 20 Hz (high-pass) to 2–3 kHz (low-pass). For the electromyography (EMG), the suggested filters are 20 Hz for the low frequency filter and 10 Hz for the high-frequency filter.
FREQUENCY – Number of complete cycles of a repetitive waveform in 1 second. Measured in Hertz (Hz), a unit preferred to its equivalent, cycles per second (c/sec).
F-WAVE – A late compound action potential evoked intermittently from a muscle by a supramaximal electrical stimulus to the nerve. Compared with the maximal amplitude compound action potential of the same muscle, the F-wave has a reduced amplitude and variable configuration and a longer and more variable latency. It can be found in many muscles of the upper and lower extremities, and the latency is longer with more distal sites of stimulation. The F-wave is due to antidromic activation of motor neurons. It was named by Magladery and McDougal in 1950.
GROUND ELECTRODE – An electrode connected to a large conducting body (such as the earth) used as a common return for an electrical circuit and as an arbitrary zero potential reference point. In general, to avoid shock artifact due to current stimulation spread, the ground electrode should be placed between stimulation and recording sites.
HERTZ (Hz) – Unit of frequency representing cycles per second.
H-REFLEX – A late compound muscle action potential having a consistent latency evoked regularly, when present, from a muscle by an electrical stimulus to the nerve. It is regularly found only in a limited group of physiologic extensors, particularly the calf muscles. The reflex is most easily obtained with the cathode positioned proximal to the anode. Compared with the maximal amplitude compound action potential (CMAP) of the same muscle, the H wave has a reduced amplitude, a longer latency, and a lower optimal stimulus intensity; its configuration is constant. The latency is longer with more distal sites of stimulation. A stimulus intensity sufficient to elicit a maximal-amplitude CMAP reduces or abolishes the H wave. The H wave is thought to be due to a spinal reflex, the Hoffman reflex, with electrical stimulation of afferent fibers in the mixed nerve to the muscle and activation of motor neurons to the muscle through a monosynaptic connection in the spinal cord. The reflex and wave are named in honor of Hoffman’s description (1918). Compare with F-Wave.
LATENCY – Interval between the onset of a stimulus and the onset of a response unless otherwise specified. Latency always refers to the onset unless specified, as in peak latency. The onset latency is the interval between the onset of a stimulus and the onset (usually the negative phase) of the evoked potential. The peak latency is the interval between the onset of a stimulus and a specific peak (usually the negative peak of the negative phase) of the evoked potential. The sensory onset latency represents the time for conduction through the largest and fastest cutaneous sensory fibers, while the sensory peak latency represents a mix of large and small fibers. The motor distal latency (MDL) represents the conduction time from the stimulation site to the neuromuscular junctions, the time delay across the neuromuscular junction, and the depolarization time across the muscle.
MIXED NERVE ACTION POTENTIAL – A compound nerve action potential is recorded from a mixed nerve when an electrical stimulus is applied to a segment of the nerve that contains both afferent (sensory) and efferent (motor) fibers. Some parameters, like amplitude, latency, duration, and number of phases, should be noted for every mixed nerve action potential.
M-WAVE – It is a compound muscle action potential (CMAP) evoked from a muscle by an electrical stimulus to its motor nerve. By convention, the M-wave elicited by a supramaximal stimulus is used for motor nerve conduction studies. Ideally, the recording electrodes should be placed so that the initial deflection of the evoked potential from the baseline is negative. The measurement commonly includes (1) the latency, commonly called the motor latency, which is the time from stimulation (ms) to the onset of the first phase (positive or negative) of the M wave; (2) the amplitude (mV), which is the baseline-to-peak amplitude of the first negative phase; (3) the duration (ms), which refers to the duration of the first negative phase. It is also referred to as the motor response.
MOTOR UNIT – The anatomical unit of an anterior horn cell, its axon, the neuromuscular junctions, and all the muscle fibers innervated by the axon.
MOTOR POINT – Also called the “belly point,” it is the place where muscle depolarization first occurs. The motor point is very important in motor nerve conduction studies because if the recording electrode is not placed here, nerve conduction studies can be artificially abnormal because the CMAP amplitude may appear artificially reduced. It can be useful to ask the patient an appropriate voluntary muscle contraction. If an initial positive deflection (which makes the onset latency difficult to accurately measure) is observed, the recording electrode can be moved slightly in order to obtain a clear negative deflection of the CMAP.
NEEDLE ELECTRODE – It is an electrical device used for recording or stimulating that is positioned near the tissue of interest by penetration of the skin. Needle electrodes are used in electroneurography (near-nerve recording. technique). The coaxial needle electrode is indeed the electrical device that is positioned inside the muscle, and it measures an electrical potential difference between a centrally insulated wire and the cannula of the needle through which it runs. It is synonymous with a concentric needle electrode, and it is used mainly in electromyography.
NERVE CONDUCTION STUDIES (NCSs) – Refers to all aspects of electrophysiologic evaluation of peripheral nerves. However, the term is generally used to refer to the recording and measurement of compound nerve and compound muscle action potentials (CMAPs) elicited in response to a single supramaximal electrical stimulus under standardized conditions that permit establishment of normal ranges of amplitude, duration, and latency of evoked potentials and the calculation of the maximum conduction velocity of individual nerves.
NOISE – Strictly defined, an artifact consisting of low-amplitude, random potentials produced by an amplifier and unrelated to the input signal. It is the electrical activity not related to the signal of interest (i.e., all waveforms generated by electrodes, cables, amplifiers, or storage media and unrelated to potentials of biological origin). The signal-to-noise ratio is the ratio of the desired signal power to the background noise signal power. A common background noise is 50–60 Hz (depending on the countries) from electrical devices in the surrounding environment. Since the signals recorded during nerve conduction studies (NCS) are based on the differences between the active and references electrodes, making sure that the two electrodes have the same impedance (which is the opposition to current flow) and placing the ground electrode between the stimulator and recording sites will decrease the background noise. A good signal-to- noise ratio can be obtained using electrodes of the same type and having intact wires, good connections, and the underlying skin clean and intact.
ORTHODROMIC – Action potentials or stimuli eliciting action potentials propagated in the same direction as physiological conduction, e.g., motor nerve conduction away from the spinal cord and sensory nerve conduction toward the spinal cord. The orthodromic nerve conduction studies were performed using a stimulus directed in the way the nerve normally depolarizes, for example, stimulating a sensory nerve away from the sensory receptor. In clinical practice, sensory antidromic nerve conduction studies are the preferred methods because of higher sensory nerve action potentials (SNAPs) amplitude than orthodromic sensory nerve conduction studies. Contrast with antidromic.
PEAK LATENCY – Interval between the onset of a stimulus and a specified peak of the evoked potential.
POLARIZATION – As used in neurophysiology, the presence of an electrical potential difference across an excitable cell membrane. The potential across the membrane of a cell when it is not excited by input or spontaneously active is termed the resting potential; it is at a steady state with regard to the electrical potential difference across the membrane. Depolarization describes a decrease in polarization to any degree, relative to the normal resting potential. Hyperpolarization describes an increase in polarization relative to the resting potential. Repolarization describes an increase in polarization from the depolarized state toward, but not above, the normal or resting potential.
RECORDING ELECTRODE – Device used to monitor electrical current or potential. All electrical recordings require two electrodes. The electrode close to the source of the activity to be recorded is called the active electrode, and the other electrode is called the reference electrode. By current convention, a potential difference that is negative at the active electrode relative to the reference electrode causes an upward deflection.
REPETITIVE STIMULATION – The technique of utilizing repeated supramaximal stimulation of a nerve while quantitatively recording compound action potentials from muscles innervated by the nerve. It should be described in terms of the frequency of stimuli and number of stimuli (or duration of the total group).
SENSORY PEAK LATENCY – Interval between the onset of a stimulus and the peak of the negative phase of the compound sensory nerve action potential. Note that the term “latency” refers to the interval between the onset of a stimulus and the onset of a response.
STIMULATING ELECTRODE – Device used to apply electrical current. All electrical, stimulation requires two electrodes; the negative terminal (-) is termed the cathode and the positive terminal (+), the anode. Electrical stimulation for nerve conduction studies generally requires application of the cathode to produce depolarization of the nerve trunk fibers. If the anode is inadvertently placed between the cathode and the recording electrodes, a focal block of nerve conduction (anodal block) may occur and cause a technically unsatisfactory study. By convention, the stimulating electrodes are called bipolar if they are encased or attached together (e.g., pad electrodes fixed on a plastic bar) and are called monopolar if they are not.
STIMULUS – Any external agent, state, or change that is capable of influencing the activity of a cell, tissue, or organism. In clinical nerve conduction studies, an electrical stimulus is generally applied to a nerve or a muscle. In absolute terms, the electrical stimulus has a strength or intensity measured in voltage (volt, V) or current (milliampere, mA) and a duration (milliseconds, ms). With respect to the evoked potential, the stimulus may be graded as subthreshold, threshold, submaximal, maximal, or supramaximal. A threshold stimulus is that electrical stimulus just sufficient to produce a detectable response. Stimuli less than the threshold stimulus are termed subthreshold. The Maximal Stimulus is the stimulus intensity after which a further increase in the stimulus intensity causes no increase in the amplitude of the evoked potential. Stimuli of intensity below this and above threshold are submaximal. Stimuli of intensity greater, than the maximal stimulus are termed supramaximal. Ordinarily, supramaximal stimuli are used for nerve conduction studies. By convention, an electrical stimulus of approximately 20% greater voltage than required for the maximal stimulus maybe used for supramaximal stimulation.
SWEEP TIME – Also called “time base,” it represents the linear time scale with which the potentials are displayed. In clinical practice, the sweep speed usually ranges from 2 to 5 ms/division for motor nerve conduction studies and from 1 to 2 ms/division for sensory nerve conduction studies. For late responses, commonly a sweep speed of 5 to 10 ms/division is used for F-waves or 10 ms/division for H-reflex study. In electromyography, a higher sweep rate (10–100 ms/division) is used than in electroneurography in order to analyze muscle activity at rest and during minimal and maximal contraction.
TEMPORAL DISPERSION – A waveform of longer duration than normal. Commonly used to refer to an increase in the duration of an evoked potential with more proximal sites of stimulation of a greater degree than that normally seen.
VOLUME CONDUCTION – Spread of current from a potential source through a conducting medium, such as the body tissues.
WAVEFORM – A transient change in voltage represented as a line of differing directions over time, describing the shape of a wave.