Glossary of EPS terms


Action potential-The all or none, self -propogating, non-decrementing voltage change recorded from an excitable cell.
Amplitude-The maximal voltage difference between two points, usually measured baseline to peak
Anode-The positive terminal of a source of electrical current.
Antidromic-Said of an action potential or of its stimulation causing the action potential that propogates opposite to the normal (dromic or 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. 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 potential due to the volume conducted electrical stimulus. 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 electrode.
Axonotmesis- Nerve injury characterized by disruption of the axon and myelin sheath but with preservation of the supporting tissues, resulting in axonal degeneration distalk to the injury site.
Baseline-The potential difference recorded form the biological system of interest while the system is at rest.
Bipolar needle electrode-A recording electrode with two insulated wires side by side in a metal cannula whose bare tips act as the active and reference electrodes. The metal cannula may be grounded.
Cathode-The negative terminal of a source of electrical current.
Chronaxie- See Strength-Duration curve.
Complex repetitive discharge-Polyphasic or serrated action potentials that may begin spontaneously or after needle movement. They have uniform frequency, shape, and amplitude, with abrupt onset, cessation, or change in configuration. Amplitudes range form 100 uV to 1 mV and frequency of discharge from 5-100 Hz.
Compound Muscle Action Potential (CMAP)-The summation of nearly synchronous muscle fiber action potentials recorded from a muscle commonly produced by stimulation of the nerve supplying the muscle.
Compound Sensory Nerve Action Potential- The action potential recorded by stimulation of mixed nerve with sensory stimulation, or by stimulating sensory fibers and recording from mixed nerve, or stimulating and recording from sensory fibers.
Concentric needle- Recording electrode that measures potential difference between the bare tip of the central wire (active electrode) is flush with the bevel of the cannula (reference electrode).
Conduction block- Failure of an action potential to be conducted past a particular point in the nervous system.
Conduction Velocity-Speed of propagation of an action potential along a muscle or nerve.
End plate activity- Spontaneous electrical activity recorded with a needle near or in the muscle end plate; packets of acetylcholine are mechanically disrupted by the needle.
End plate noise- Graded, non- propogating potential recordings resulting from mechanical disruption of packets of acetylcholine.
Fasciculation- The random, spontanous twitching of a group of muscle fibers which may be visable from the skin.
Fasciculation potential- The electrical potential associated fasciculation which has the dimensions of a motor unit potential that occurs spontaneously as a single discharge.
Fibrillation- The spontanous discharge of individual muscle fiber which are not ordinarily seen through the skin.
Fibrillation potential- The electrical activity associated with fibrillating muscle fibers, reflecting the action potential of a single muscle fiber, occurring spontaneously. Fibrillation is commonly seen with axonal denervation, but is also reported in myogenic pathology, in particular inflammatory muscle disease.
F wave- A late compound action potential evoked from a muscle with supramaximal electrical stimulation to the nerve. Compared with the M wave of the same muscle, the F wave has a reduced amplitude and variable configuration and a longer, more variable latency. The F wave is due to the antidromic activation of motor neurons.
Ground electrode- An electrode connected to a large conducting body (such as earth), use as a common return for electrical circuit and as an arbitrary zero potential reference point.
H reflex (H wave)- A late compound muscle action potential having a consistent latency evoked regularly, when present, form a muscle by an electrical stimulus to the nerve. The H wave is recorded by stimulation of the sensory Ia fibers, which synapse and cause orthodromic activation of the alpha motor neuron. With supramaximal stimulation, the H wave is inhibited.
Insertional activity- Electrical activity caused by insertion or movement of a needle electrode.
Interference pattern- Electrical activity recorded from a muscle with a needle electrode with voluntary contraction.
Late response- A general term used to describe an evoked potential having a longer latency than the M wave. See H or F wave.
Latency- Interval between the onset of a stimulus and the onset of the evoked response.
Membrane instability- Tendency of a cell membrane to depolarize spontaneously or after mechanical irritation or voluntary activation.
Miniature end plate potential- when recording with microelectrodes, discharges recorded at the myoneural junction; thought to be due to small quanta of acetylcholine released spontaneously.
Monopolar needle electrode- A solid wire, usually of stainless steel, coated, except at its tip, with insulating material; usually used as a recording cathode, used with a reference anode electrode place on the skin.
Motor Unit- The anatomic unit of an anterior horn cell, its axon, the neuromuscular junction, and all the muscle fibers innervated by the axon.
Motor unit action potential (abbr. MUAP)- Action potential reflecting the electrical activity of a single anatomic motor unit. It is the compound action potential of those muscle fibers within the recording range of an electrode.
Motor unit territory- The area in a muscle over which the muscle fibers belonging to an individual motor unit are distributed.
Myotonia- The clinical observation of delayed relaxation of muscle after voluntary contraction or percussion. The delayed relaxation may be electrically silent or accompanied by propagated electrical activity such as myotonic discharge.
Myotonic discharge- Repetitive discharges at rates of 20-80 Hz; Potential forms are recorded after voluntary muscle contraction, or after muscle percussion, and are due to independent, repetitive discharges of single muscle fibers. The amplitude and frequency of the potentials must both wax and wane to be identified as myotonic discharges. This change produces a characteristic music sound in the audio display of the EMG due to corresponding change in pitch, which has been likened to the sound of a “dive bomber”
Neuropraxia- Failure of nerve conduction, usually reversible, due to metabolic or microstructural abnormalities without disruption of the axon.
Neurotmesis- Partial or complete severance of a nerve, with disruption of the axons, myelin sheaths, and the supporting connective tissue, resulting in degeneration of the axons distal to the injury site.
Orthodromic- Propagation of an impulse in the direction the same as physiologic conduction; e.g., conduction along motor nerve fibers towards the muscle and conduction along sensory nerve fibers toward the spinal cord.
Phase- That portion of a wave between the departure from, and the return to, the baseline.
Polyphasic potential- an action potential with five or more phases. Seen with EMG exam, usually consistent with re-innervation. Reportedly seen also with denervation, and on with volitional exam of myogenic disorders.
Postive sharp wave- A biphasic, positive-negative actin potential initiated by needle movement and recurring in uniform, regular pattern at a rate of 1-50 Hz. The initial positive deflection is rapid (sharp), followed by a longer duration, smaller amplitude positive deflection. Positive sharp waves are seen in areas of fibrillating muscle fibers, and may represent fibrillation. Commonly seen in axonal injury and inflammatory muscle disease.
Recruitment- The successive activation of the same and additional motor units with increasing strength of voluntary muscle contraction.
Reference electrode- The anode electrode, used in conjunction with a cathode recording (active) electrode. Both anode and cathode allow for measuring potential difference.
Rheobase- See Strength-Duration curve.
Spontaneous activity- Electrical activity recorded from muscle or nerve at rest after insertion activity has subsided and when there is no voluntary contraction or external stimulus.
Stimulus- Any external agent, state, or change that is capable of influencing the activity of a cell, tissue, or organism. In clinical NCS, an electrical stimulus is generally applied to a nerve or a muscle. The electrical stimulus may be described in absolute terms or with respect to the evoked potential of the nerve or muscle. In absolute terms, its waveform, duration, and amplitude define the electrical stimulus. A stimulus that evokes the maximum response is considered a maximal stimulus, with 20% additional voltage not producing increased amplitude of the evoked response labeled a supra maximal stimulus.
Strength-Duration curve- Graphic presentation of the relationship between the intensity (Y axis), and various durations (X axis) of the threshold stimulus for a muscle with the stimulating cathode positioned over the motor point. The rheobase is the intensity of an electrical current of infinite duration (300 ms in clinical practice) necessary to produce a minimal visible twitch of a muscle when applied over a motor point. The chronaxie is the duration of the pulse at twice the rheobase to elicit the first visible muscle twitch.
Temporal Dispersion- Relative dysychronization of components of a CMAP due to different rates of conduction of the synchronously evoked component from the stimulation point to the recording electrode.
Volitional activity- Examining a muscle with an EMG electrode during voluntary contraction.
Volume Conduction- Spread of current from a potential source through a conducting medium, such as the body tissues.

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