Curriculum/Pillar 3 · Needle EMG Physiology

Resting & Spontaneous Activity

Module 9 of 36·30 min readREAL EMG DATA

Learning objectives

1Grade insertional activity and recognise normal endplate phenomena
2Identify fibrillations and positive sharp waves and explain their membrane basis
3Differentiate fasciculations, myotonia, CRDs, and myokymia by their signatures

The resting needle examination is, in essence, an assay of muscle-fibre membrane stability. A healthy muscle fibre at rest holds its membrane potential clamped near −80 mV and fires nothing of its own accord; the only legitimate electrical events arise where motor axons terminate. Every spontaneous potential you record therefore forces a binary decision — is this a normal endplate phenomenon, or is a membrane misbehaving because it has been denervated, inflamed, or rendered hyperexcitable? Mastery of the resting exam is the discipline of making that decision from a waveform's shape, rhythm, and initial deflection alone.

REAL EMG DATATibialis anterior at rest and on minimal activation

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Healthy control: Crisp tri/biphasic MUAPs around 0.5–1 mV that recruit smoothly as effort rises.

44 y/o male · Tibialis anterior · concentric needle · No history of neuromuscular disease. Real recording · 4 kHz.

The healthy 44M concentric-needle recording. Between bursts of voluntary activity the baseline is essentially silent — no regular, initially-positive spontaneous discharges. Drop the sensitivity to 50–100 µV/division and park the needle to appreciate how little a normal resting membrane generates; contrast this with the irritable baselines described below.

Insertional activity: a mechanical membrane test

Advancing or repositioning the needle physically deforms and depolarizes the fibres it pierces, triggering a brief crackle of insertional activity. In normal muscle this discharge is a direct mechanical artifact of the moving electrode and ceases within roughly 300 ms of the needle coming to rest — its duration is a crude bioassay of how readily the sarcolemma depolarizes and then repolarizes. Increased insertional activity, in which trains of potentials outlast needle movement by half a second or more, signifies a hyperexcitable, partially depolarized membrane and is the earliest electrodiagnostic fingerprint of denervation or inflammatory myopathy — often preceding frank fibrillations. Decreased or absent insertional activity means there are simply fewer excitable fibres to discharge: end-stage fibrosis, fatty replacement, or the transiently inexcitable membrane of a periodic-paralysis attack. Quantification is unavoidably subjective, which is why experienced electromyographers weight a clearly prolonged afterdischarge far more heavily than a borderline one.

The endplate zone: normal noise mistaken for disease

Drive the needle into the motor point and you enter the endplate zone, where two normal — and notoriously painful — phenomena appear. Endplate noise is the summated electrical signature of spontaneously released acetylcholine quanta generating miniature endplate potentials (MEPPs): a dense, low-amplitude (10–50 µV), monophasic-negative hiss with no discrete rhythm, classically likened to a seashell murmur. Riding on this noise are endplate spikes — intermittent, irregularly firing biphasic potentials that are initially negative because the needle sits at the depolarizing endplate itself, where the impulse originates rather than propagating toward the electrode. Both are entirely physiological. The single most important discriminator is rhythm and initial polarity: endplate spikes are irregular and initially negative, whereas the pathological discharges that mimic them are regular and initially positive.

Fibrillations and positive sharp waves

When a muscle fibre is severed from its axon, denervation supersensitivity and unstable resting potential cause it to self-discharge at a metronomic rate. Recorded as a propagating single-fibre action potential, this is a fibrillation potential: brief (<5 ms), low amplitude (10–300 µV), di- or triphasic, and crucially initially positive, firing with near-perfect regularity at 1–15 Hzand a crisp, clock-like "tick." The positive sharp wave (PSW) is the same denervated membrane recorded against a needle that has mechanically injured it — a steep initial positive deflection followed by a long, low negative tail, firing at the same regular rate. The two are pathophysiologically identical markers of active denervation (or, in myopathy, of fibre splitting and segmental membrane necrosis). Their regularity and obligatory initial positivity are precisely what separate them from the irregular, initially-negative endplate spike.

The temporal rule: fibrillations are not immediate

Denervation does not announce itself at once. After an acute axonal injury, fibrillation potentials require roughly 1–3 weeksto emerge — appearing earlier in proximal muscles (as little as 7–10 days) and later in distal ones (up to 3–5 weeks), as a function of the distance over which the distal axon stump must degenerate and the denervated membrane must develop supersensitivity. They peak in density over the subsequent weeks to months and recede only as fibres are either reinnervated or fibrose. This latency is itself diagnostic data: a silent resting exam three days after a traumatic nerve lesion does not exclude a complete axonotmesis, and the timing of the study must be factored into every interpretation.

The other spontaneous discharges

Fasciculation potentials — spontaneous, whole-motor-unit discharges firing slowly and irregularly. Their morphology mirrors the unit of origin (simple in benign fasciculations; large and complex in motor neuron disease), but their irregular rhythm distinguishes them from the regular single-fibre fibrillation. Benign in isolation; ominous when combined with widespread fibrillations and large, unstable units.
Myotonic discharges — runs of single-fibre potentials whose amplitude and frequency wax and wane, producing the unmistakable accelerating-decelerating dive-bomber sound. They reflect a primary muscle-membrane channelopathy (chloride or sodium) and occur in myotonic dystrophy, myotonia congenita, paramyotonia, and acid maltase deficiency.
Complex repetitive discharges (CRDs) — abrupt-onset, abrupt-offset trains of identical polyphasic complexes propagating ephaptically from fibre to fibre, with a uniform, machine-like rhythm. A non-specific marker of chronicity in either neurogenic or myopathic disease.
Myokymic discharges— grouped, rhythmically recurring bursts of the same motor unit ("marching soldiers"), classic for radiation plexopathy, demyelination (facial myokymia in multiple sclerosis and Guillain–Barré syndrome), and brainstem pathology.
Neuromyotonic discharges— very-high-frequency (150–300 Hz) decrementing bursts of a single unit, the signature of peripheral nerve hyperexcitability (Isaacs syndrome, often CASPR2/VGKC-complex antibody mediated).

Clinical Pearl

Initial polarity plus rhythm resolves nearly every resting-exam dilemma. Initially positive and regular → fibrillation or positive sharp wave (denervation or myopathic membrane damage). Initially negative and irregular → endplate spike (normal). Whole-unit and irregular → fasciculation. Waxing/waning single fibre → myotonia. Train the ear as much as the eye: the metronomic tick of fibrillation, the dive-bomber of myotonia, and the marching rhythm of myokymia are diagnostic before the waveform is fully resolved on screen.

Common Pitfall

The most common error in electrodiagnostic medicine is reporting endplate spikes as fibrillations. They are recorded in the same painful motor-point zone, are of similar brevity and amplitude, and produce a comparable crackle. Anchoring on a busy, noisy baseline — rather than waiting to judge initial deflection and regularity — manufactures false denervation and, with it, spurious diagnoses of radiculopathy or neuropathy. When the baseline is irritable, withdraw fractionally from the endplate, isolate a single potential, and ask only two questions: which way does it deflect first, and is it regular?

Key points

Normal insertional activity ceases within ~300 ms; prolonged afterdischarge is the earliest sign of membrane irritability; reduced/absent activity means fibrosis, fat, or an inexcitable membrane.
Endplate noise (MEPPs, seashell murmur) and endplate spikes (irregular, initially negative) are both normal motor-point phenomena.
Fibrillations and positive sharp waves are regular and initially positive single-fibre discharges signifying active denervation (or myopathic membrane damage).
Fibrillations require ~1–3 weeks to appear after axon loss (proximal sooner, distal later) — a silent early exam does not exclude denervation.
Fasciculations (whole-unit, irregular), myotonia (waxing/waning dive-bomber), CRDs (machine-like), and myokymic/neuromyotonic discharges each carry a specific differential.