Electrical Signal Generation

From single fibres to the motor unit potential

A needle in muscle does not record one fibre; it records the algebraic sum of the action potentials of every fibre of the nearest motor unit that lies within its pickup volume. Because the fibres of one unit are scattered across a 5–10 mm territory and the conduction velocity along each fibre differs slightly, their individual spikes arrive at the electrode at marginally different times. The recorded motor unit action potential (MUAP) is therefore a temporal summation of asynchronous single-fibre potentials, and its features map directly onto this geometry:

• Amplitude is dominated by the few fibres nearest the electrode tip, because extracellular potential falls off steeply with distance. Amplitude is thus a local measure and varies wildly with needle position over millimetres.
• Duration reflects the temporal spread of arrivals across the whole unit and therefore samples a larger fibre population; it is the most reliable index of motor-unit territory and the quantity that lengthens in chronic reinnervation.
• Phases and turns index the synchrony of firing. Loss of synchrony — from variable reinnervated conduction times or from fibre splitting — produces the polyphasia (> 4 phases) seen in both neurogenic and myopathic remodelling.

The waveform at the electrode: a moving dipole and the triphasic shape

To understand the shape of any propagating potential, picture the depolarized region of membrane as a moving electrical source. Ahead of the advancing front the membrane is at rest and the extracellular space is relatively positive; under the front it is intensely negative; behind it, repolarization restores positivity. To a stationary electrode that the impulse approaches, passes, and leaves, this travelling dipole reads as a triphasic waveform: an initial positive deflection, a large negative peak, and a terminal positive deflection — the canonical signature of a potential propagating past a recording point.

Two corollaries are clinically load-bearing. First, when the depolarization originates directly under the active electrode (as at the motor point, or with the active electrode optimally placed over the endplate zone for a CMAP), there is no initial positivity — the potential is biphasic with an initial negativity, and the negative onset marks the true latency. Second, this is why the recording convention matters: by agreement, negativity at the active electrode deflects the trace upward, so the dominant negative peak is the upward peak you measure.

Recruitment and rate coding: building the interference pattern

A single motor unit can grade its own tension only modestly by firing faster (rate coding); large increments of force require activating additional units (recruitment), and the two mechanisms operate together across the voluntary range. By Henneman's size principle, units are recruited in a fixed order of increasing size — small, fatigue-resistant type I units first, large type II units last — because small motor neurons have higher input resistance and reach threshold for less synaptic current. As more units fire and each fires faster, their MUAPs overlap on the screen into a dense, unresolvable interference pattern. Two quantitative readouts summarize the balance:

• Recruitment frequency — the firing rate of the first unit at the moment a second unit appears (normally ~10 Hz). A first unit firing much faster than this before any second unit joins signals that too few units are available — reduced recruitment.
• Recruitment ratio — the firing rate of the fastest unit divided by the number of active units (normally ≈ 5). A ratio well above 5 is the neurogenic pattern; an abnormally full screen at trivial force is the myopathic pattern.

Temporal and spatial summation in the recorded field

The macroscopic signals of nerve conduction studies are governed by the same two summations, now applied across an entire population of axons or fibres firing in near-synchrony. Spatial summationis the superposition of many sources' fields at one electrode at one instant; temporal summation is the integration over time of arrivals that are slightly desynchronized. The critical consequence is that summation of asynchronous sources is lossy: when the constituent potentials are not perfectly aligned in time, the negative peaks of some coincide with the positive tails of others, and the summed amplitude is smaller than the arithmetic sum of individual peaks. This phase cancellation is the hidden mechanism behind several core phenomena and is most extreme in sensory recordings.

The CMAP: a summed, synchronized motor response

The compound muscle action potential (CMAP) is the summated electrical response of all the muscle fibres activated when a motor nerve is stimulated supramaximally. Because supramaximal stimulation recruits every axon and the resulting muscle spikes are tightly synchronized, the CMAP is large — typically several to > 10 mV — and its negative-peak amplitude (or area) is a quantitative surrogate for the number of functioning motor axons and fibres. Onset latency times the fastest-conducting motor axons plus neuromuscular transmission and a short muscle-activation delay. Two failure modes are diagnostic: uniform axon loss lowers amplitude with little change in shape, whereas demyelination between stimulation and recording sites desynchronizes arrivals, producing temporal dispersion (a broadened, lower, polyphasic CMAP) and, when current leak is severe enough, conduction block — a drop in amplitude and area across the affected segment.

The SNAP: why sensory responses are microvolt-scale

The sensory nerve action potential (SNAP) records the travelling potential of sensory axons directly, without the enormous amplifying relay of muscle. Three physical facts collapse its amplitude into the microvolt range (typically ~5–50 µV, versus the millivolt CMAP):

• No muscle amplification — each motor axon commands hundreds of large muscle fibres, so the CMAP is a vastly amplified readout; the SNAP is the bare nerve signal with no such gain.
• Phase cancellation over distance— sensory fibres span a range of conduction velocities, so the longer the conduction distance, the more the fast and slow fibres' biphasic potentials drift out of phase and cancel. SNAP amplitude therefore declines with recording distance, a normal, distance-dependent attenuation that is far more severe for the brief sensory spikes than for the broader CMAP.
• Reference proximity — the active and reference electrodes sit only a few centimetres apart, so both record part of the same travelling potential; the SNAP is the small difference between two similar signals, magnifying its sensitivity to inter-electrode distance.

The practical upshot is that the SNAP is exquisitely vulnerable to background noise and demands averaging, meticulous impedance control, and standardized montage. Its great compensating virtue is localizing power: because the sensory cell body sits in the dorsal root ganglion distal to the root, a lesion proximal to the ganglion (a root avulsion or radiculopathy) leaves the peripheral sensory axon intact and the SNAP preserved, even when the dermatome is clinically numb — a discriminator examined in the localization chapters.