Physiology

The basic component of the peripheral nervous system is the motor unit, defined as an individual motor neuron, its axon, and associated neuromuscular junctions (NMJs) and muscle fibers. The extracellular needle EMG recording of a motor unit is the MUAP (Figure 15–1). The number of muscle fibers per motor unit varies greatly, from 5 to 10 in laryngeal muscles to a couple of thousand in the soleus. The transverse territory of a motor unit usually ranges from 5 to 10 mm in adults, with many motor unit territories overlapping with one another. Because of this overlap, two muscle fibers from the same motor unit rarely lie adjacent to each other. Transverse motor unit territory increases greatly with age, doubling from birth to adulthood, mostly because of the increase in individual muscle fiber size.

FIGURE 15–1 The motor unit.

The basic component of the peripheral nervous system is the motor unit, defined as an individual motor neuron, its axon, and associated neuromuscular junctions and muscles fibers. The extracellular needle electromyography recording of a motor unit is the motor unit action potential (MUAP).

When a motor neuron depolarizes to threshold, a nerve action potential is generated and propagates down the axon. Under normal circumstances, this results in all muscle fibers of the motor unit being activated and depolarizing more or less simultaneously. Any variability between muscle fiber depolarization times is due to differences in the length of the terminal axons and in NMJ transmission times.

The “size principle” governs many of the properties of motor units (Figure 15–2). The size of the motor neuron is directly related to (1) the size of the axon, (2) the thickness of the myelin sheath, (3) the conduction velocity of the axon, (4) the threshold to depolarization, and (5) the metabolic type of muscle fibers that are innervated. The larger motor neurons have larger axons, with the thickest myelin sheath (hence, the fastest conduction velocity), highest threshold to depolarization, and connections to type II, fast twitch muscle fibers. Conversely, the smaller motor neurons have smaller axons, less myelin sheath, slower conduction velocity, lower threshold to depolarization, and, in general, connections to type I, slow twitch muscle fibers. Thus, with voluntary contraction, the smallest motor units with the lower thresholds fire first. As contraction increases, progressively larger motor units begin to fire. The largest type II motor units fire with maximum contraction. During routine needle EMG, most MUAPs analyzed are thus from the smaller motor units that innervate type I muscle fibers.

FIGURE 15–2 Size principle and motor unit properties.

During the needle EMG examination, each MUAP recorded represents the extracellular compound potential of the muscle fibers of a motor unit, weighted heavily toward the fibers nearest to the needle. A MUAP recorded just outside a muscle membrane is 1/10 to 1/100 the amplitude of the actual transmembrane potential and the amplitude decreases rapidly as the distance between the needle and the membrane increases. The classification of an MUAP as normal, neuropathic, or myopathic rests on no single finding. As is true of spontaneous activity, recorded MUAPs must be assessed for morphology (duration, polyphasia, amplitude), stability, and firing characteristics before any conclusions can be reached.

Morphology

MUAP properties vary widely both within and between different muscles. Even within a muscle, there is a wide range of normal motor unit morphology, with MUAP size following a bell-shaped distribution curve (Figure 15–3). Due to this normal variability, normal values of MUAP morphology are based on the mean of many different MUAPs. The analysis of MUAP morphology can be performed on either a qualitative or a quantitative basis. To perform quantitative MUAP analysis, one must isolate 20 different MUAPs for each muscle being studied and measure their individual durations, amplitudes, and number of phases. From these values, the mean duration, amplitude, and number of phases are calculated and compared with a set of normal values for that particular muscle and age group. MUAP morphology varies depending on the muscle being studied and the patient’s age. This is particularly true of MUAP duration (Table 15–1). In general, MUAPs in proximal muscles tend to be shorter in duration than those in more distal muscles. MUAP size in adults is larger than in children, primarily because of an increase in the size of muscle fibers during development. In addition, MUAP size is generally larger in older individuals, probably as the result of dropout of motor units from the normal effects of aging, leading to some compensatory “normal” reinnervation. The loss of motor units has been estimated to be approximately 1% per year, beginning in the third decade of life, which then increases rapidly after age 60.

FIGURE 15–3 Range of normal motor unit action potential (MUAP) duration and amplitude.

Histogram of MUAP duration and amplitude in the biceps brachii of a normal subject. Note that both MUAP duration and amplitude vary markedly in normal muscles, with small and large units in the same muscle. MUAP duration or amplitude should not be classified as abnormal based on one or two MUAPs but requires a mean of many motor units.

(Reprinted with permission from Buchthal, F., Guld, C., Rosenfalck, P., 1954. Action potential parameters in normal human muscle and their dependence on physical variables. Acta Physiol Scand 32, 200.)

Table 15–1 Mean Motor Unit Action Potential Duration Based on Age and Muscle Group

Only by comparing mean MUAP morphology in each muscle studied to normal values for that particular muscle and age group can one determine whether the morphology is truly abnormal. Previously, quantitative MUAP analysis was tedious and time consuming. However, many modern EMG machines now have programs that largely automate the procedure. With experience over time, however, the well-trained electromyographer usually can perform qualitative MUAP assessment with the same precision as can be achieved using quantitative methods. Essentially the same procedure is used. The needle is moved to several locations within the muscle until approximately 20 different MUAPs have been examined, qualitatively analyzed, and compared to the expected normal values for that particular muscle and age group.

Duration

MUAP duration is the parameter that best reflects the number of muscle fibers within a motor unit (Figure 15–4). Typical MUAP duration is between 5 and 15 ms. Duration is defined as the time from the initial deflection from baseline to the final return of the MUAP to baseline. It depends primarily on the number of muscle fibers within the motor unit and the dispersion of their depolarizations over time. Dispersion in turn depends on the longitudinal and transverse scatter of endplates and on variations in terminal distances and conduction velocities. Duration lengthens as the number of fibers and the territory of a motor unit increase; it varies directly with age (increased age, increased duration) and inversely with temperature (decreased temperature, increased duration) and depends on the individual muscle being studied. Proximal and bulbofacial muscles in general have MUAPs of shorter duration. When performing EMG, it often is more rewarding to listen to the potential than to see it. This is especially true when evaluating MUAP duration, because duration correlates with pitch. Long-duration MUAPs (low frequencies) sound dull and thuddy, whereas short-duration MUAPs (higher frequencies) sound crisp and static-like. As the electromyographer gains experience, the sound of a long-duration versus a short-duration MUAP becomes unmistakable.

FIGURE 15–4 Motor unit action potential (MUAP) measurements.

Duration is measured as the time from the initial deflection of the MUAP from baseline to its final return to baseline. It is the parameter that best reflects the number of muscle fibers in the motor unit. Amplitude reflects only muscle fibers very close to the needle and is measured peak to peak. Phases (shaded areas) can be determined by counting the number of baseline crossings and adding one. MUAPs are generally triphasic. Serrations (also called turns) are changes in direction of the potential that do not cross the baseline. The major spike is the largest positive-to-negative deflection, usually occurring after the first positive peak. Satellite, or linked, potentials occur after the main potential and usually represent early reinnervation of muscle fibers.

Polyphasia, Serrations, and Satellite Potentials

Polyphasia is a measure of synchrony, that is, the extent to which the muscle fibers within a motor unit fire more or less at the same time. This is a nonspecific measure and may be abnormal in both myopathic and neuropathic disorders. The number of phases can be easily calculated by counting the number of baseline crossings of the MUAP and adding one (Figure 15–4). Normally, MUAPs have two to four phases. However, increased polyphasia may be seen in up to 5 to 10% of the MUAPs in any muscle and is considered normal. The one exception is the deltoid, where up to 25% polyphasia may be normal. Increased polyphasia beyond 10% in most muscles and 25% in the deltoid is always abnormal. Through the speaker, polyphasic MUAPs are recognized as a high-frequency “clicking” sound.

Serrations (also called turns) are defined as changes in the direction of the potential that do not cross the baseline. Increased polyphasia and serrations have similar implications, indicating less synchronous firing of muscle fibers within a motor unit. Often, a serration can be changed into an additional phase with needle movement.

Satellite potentials (also known as linked potentials or parasite potentials) are interesting phenomena seen in early reinnervation. After denervation, muscle fibers often are reinnervated by collateral sprouts from adjacent intact motor units. The newly formed sprout often is small, unmyelinated or thinly myelinated, and therefore very slowly conducting. Because of the slow conduction time and increased distance, reinnervated muscle fibers are seen as time-locked potentials that trail the main MUAP (Figures 15–5 and 15–6). These satellite potentials are extremely unstable (see section on Stability) and may vary slightly in their firing rate or may block and not fire at all (Figure 15–7). Over time, the sprout matures, and the thickness of the myelin and consequently the conduction velocity increase. The satellite potential then fires more closely to the main potential and ultimately will become an additional phase or serration within the main complex. It is usually necessary to put the main MUAP on a delay line to appreciate a satellite potential and to demonstrate that it is time locked to the main potential.

FIGURE 15–5 Collateral sprouting and satellite potentials.

A: Normal state. B: Following partial denervation, the injured axon(s) undergoes wallerian degeneration. C: Reinnervation commonly occurs from sprouting by adjacent surviving axons. In early reinnervation, sprouts are small and thinly myelinated and conduct slowly. Because of the slow conduction time and increased distance, these reinnervated fibers initially occur as time-locked potentials (satellite potentials) trailing the main motor unit action potential (MUAP). As sprouts mature and conduct more quickly, the time-locked potentials are eventually incorporated into the main MUAP, resulting in an MUAP with increased amplitude, duration, and number of phases.

FIGURE 15–6 Satellite potential.

Note in the tracing the small potential that is time locked to the main motor unit action potential (MUAP). This is a satellite potential, a sign of early reinnervation. After denervation, muscle fibers often are reinnervated by collateral sprouts from adjacent intact motor units. The newly formed sprout often is small, unmyelinated or thinly myelinated, and therefore very slowly conducting. Because of the slow conduction time and increased distance, reinnervated muscle fibers are seen as time-locked potentials that trail the main MUAP.

FIGURE 15–7 Unstable satellite potential. Note the satellite potential in the first and fourth firing of the MUAP.

However, the satellite potential is not present in the second and third firing of the MUAP. In early reinnervation, collateral sprouts attach to nearby denervated fibers which results in satellite potentials. However, the newly formed NMJs are immature, and do not always reach threshold, resulting in intermittent firing of the satellite potential. Eventually the satellite potential becomes incorporated into the main MUAP. This is the basis of unstable MUAPs that occur following reinnervation.

Amplitude

MUAP amplitude varies widely among normal subjects. Most MUAPs have an amplitude greater than 100 µV and less than 2 mV. Amplitude is generally measured from peak to peak of the MUAP (Figure 15–4). Amplitude is essentially a high-frequency response. Tissue between the needle and muscle fibers effectively acts as a high-frequency filter. Thus, unlike duration, most muscle fibers of a motor unit contribute little to the amplitude. MUAP amplitude reflects only those few fibers nearest to the needle (only 2–12 fibers). Hence, amplitude is not as helpful as duration in judging motor unit size. Several factors are associated with increased amplitude, including (1) the proximity of the needle to the motor unit (Figure 15–8), (2) increased number of muscle fibers in a motor unit, (3) increased diameter of muscle fibers (i.e., muscle fiber hypertrophy), and (4) more synchronized firing of the muscle fibers. Listening to the EMG, the amplitude of MUAPs is correlated not with pitch but with volume.