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.)

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.

FIGURE 15–8 Relationship of motor unit action potential (MUAP) amplitude to needle position.

Of all MUAP parameters, amplitude is most dependent on needle position. Only muscle fibers very close to the needle contribute to amplitude, as opposed to duration, wherein most muscle fibers contribute. Note change in amplitude as needle is moved to different locations within the same motor unit.

Major Spike

The major spike is the largest positive-to-negative component of the MUAP and usually occurs after the first positive peak (Figure 15–4). The major spike is the highest-frequency component of the MUAP. Because tissue acts as a high-frequency filter, as the needle is moved closer to the MUAP, the major spike increases in amplitude and its rise time shortens, indicating the proximity of the needle to the motor unit. MUAP parameters should be measured only when the needle is very close to the motor unit (Figure 15–9). When the needle is close to the motor unit, the MUAP becomes “sharp.” The sharp sound represents the high-frequency component of the major spike, occurring when the major spike rise time is less than 500 µs, indicating proper needle placement.

FIGURE 15–9 Motor unit action potential (MUAP) morphology and needle electromyography (EMG) position.

The position of the EMG needle influences the morphology of the recorded MUAP. To properly assess MUAP parameters, the major spike must be as steep as possible, indicating the proximity of the needle to the motor unit. Note that needle electrode position E3 has the shortest major spike rise time and is the preferable position in which to assess the MUAP. Also note that although MUAP amplitude changes markedly with needle position (compare position E1 with E3), duration is relatively unaffected.

(From Dimitru, D., DeLisa, J.A., 1991. AAEM minimonograph #10: volume conduction. Muscle Nerve 14, 605. Reprinted by permission of Wiley.)

Stability

MUAPs usually are stable in morphology from potential to potential. This stability is due to the fact that each time a nerve action potential is generated, there is normally effective transmission across the NMJs, and all muscle fibers of the motor unit fire. If there is impaired NMJ transmission, unstable MUAPs may result (Figure 15–10). Unstable MUAPs occur when individual muscle fibers either are blocked or come to action potential at varying intervals, leading to an MUAP that changes in configuration from impulse to impulse. There is a change between potentials in either the amplitude or the number of phases (or serrations), or both. Although unstable MUAPs always indicate unstable NMJs, they occur not only in primary disorders of the NMJ (e.g., myasthenia gravis, Lambert–Eaton myasthenic syndrome) but are also often seen as secondary phenomena in both neuropathic and myopathic disorders. Any disorder associated with denervation may demonstrate unstable MUAPs. During the process of early reinnervation, newly formed, immature NMJs often fail to conduct NMJ transmission faithfully. The result is variability in endplate transmission or intermittent blocking of transmission across some of the muscle fibers within a motor unit (Figure 15–7).

FIGURE 15–10 Unstable motor unit action potentials (MUAPs).

MUAPs that change in either amplitude or number of phases from impulse to impulse are called unstable MUAPs. Unstable MUAPs occur in both primary neuromuscular junction (NMJ) disorders and disorders associated with new or immature NMJs, as commonly occur early in reinnervation. Note change in amplitude from potential to potential.

Firing Pattern (Activation, Recruitment, Interference Pattern)

One of the most important and yet most difficult tasks for the electromyographer is the assessment of firing pattern and its relationship to the number of MUAPs. MUAPs normally fire in a semi-rhythmic pattern, that is, there is slight variation in the time interval between the same MUAP as it fires consecutively (Figure 15–11). This unique firing pattern helps to identify the potential as an MUAP under voluntary control, in contrast to various spontaneous waveforms that are not under voluntary control, and have other distinct firing patterns, such as fibrillation potentials and positive sharp waves, which are regular; complex repetitive discharges, which are perfectly regular or change abruptly; myotonic discharges, which have a waxing/waning amplitude; or fasciculation potentials, which are very slow and irregular.

FIGURE 15–11 Motor unit action potential (MUAP) firing pattern.

Normally, MUAPs fire in a semi-rhythmic pattern, with a slight variation in the interval between potentials. Top: Single voluntary MUAP firing at approximately 6 Hz. Note the variation in interpotential intervals. Bottom: Single voluntary MUAP placed on a delay line and rastered. First potential of each sweep triggers sweep. Note the variation between firing time of the next consecutive MUAP. The pattern is not quite regular (i.e., it is semi-rhythmic). This firing pattern is seen only with voluntarily activated MUAPs.

During muscle contraction, there are only two ways to increase muscle force: either motor units can increase their firing rate (up to tetanic fusion frequency which is approximately 50 Hz), or additional motor units can fire (Figure 15–12). Normally, one increases force using a combination of these two processes, resulting in an orderly recruitment of motor units. With the smallest contraction, a single motor unit action potential normally begins firing semi-rhythmically at 4 to 5 Hz. Any potential that fires more slowly than 4 to 5 Hz cannot be an MUAP under voluntary control and must be a spontaneous potential. As one increases force, the first motor unit action potential increases its firing rate, and then a second motor unit action potential begins to fire, and so forth. This process continues, with the firing rate increasing and additional motor unit action potentials being recruited as force is increased. Normally, the ratio of firing frequency to the number of different MUAPs firing is approximately 5 : 1. Thus, by the time the first MUAP firing frequency reaches 10 Hz, a second MUAP should begin to fire; by 15 Hz, a third motor unit action potential should fire, and so forth. During maximal contraction, multiple MUAPs normally overlap and create an interference pattern in which no single motor unit action potential can be distinguished (Figure 15–13A). For most muscles, the maximal firing frequency is 30 to 50 Hz. Important exceptions include quick ballistic contractions, in which the firing frequency may transiently reach 100 Hz, and muscles that are predominantly slow twitch (e.g., soleus), in which the maximal firing frequency is approximately 15 Hz.

FIGURE 15–12 Relationship of force to firing frequency.

Top traces of each pair show twitch forces; bottom traces show motor unit action potentials (MUAPs) firing at different frequencies. Note increased force with increased firing frequencies. To increase muscle twitch force, either motor units must fire faster or additional motor units must be added. Although the electrical MUAP lasts only 5 to 15 ms, the mechanical twitch lasts more than 100 ms. As MUAP firing rate increases, twitch forces summate. Force increases up to a frequency of approximately 50 Hz (tetanic fusion frequency). Near that frequency, the maximal overlap occurs between muscle myosin and actin filaments. Firing above that frequency may result in more even firing but does not appreciably change the amount of force generated.

(Adapted with permission from Kandel, E.R., Schwartz, J.H., Jessell, T.M. (Eds.), 1991. Principles of neural science, third ed. Appleton & Lange, Norwalk, CT.)

FIGURE 15–13 Interference patterns.

A: Normal. B: Neuropathic. C: Myopathic. D: Central. In each trace, the patient is asked to contract maximally. In normal subjects, so many motor unit action potentials (MUAPs) fire during maximal contraction that differentiating individual motor unit action potentials is difficult. In neuropathic recruitment, a reduced number of MUAPs fire at a high frequency, resulting in an incomplete interference pattern (often referred to as the “picket fence” pattern, when only one MUAP is firing). In myopathic recruitment, although the number of MUAPs is normal, the interference pattern consists of short-duration, small-amplitude MUAPs, which fire with a small amount of force. In central disorders, the primary problem is the inability to fire faster (i.e., decreased activation); although the number of MUAPs is reduced, it is appropriate for the level of firing.

One of the key questions to answer in assessing MUAPs is the following: Are the number of different MUAPs firing appropriate for the firing rate? That is, is the ratio of firing rate to MUAPs approximately 5 : 1? To answer that question, one must understand that increasing force depends on two processes: activation and recruitment. Activation refers to the ability to increase firing rate. This is a central process. Poor activation may be seen in diseases of the central nervous system (CNS) or as a manifestation of pain, poor cooperation, or functional disorders. Recruitment refers to the ability to add motor unit action potentials as the firing rate increases. Recruitment is reduced primarily in neuropathic diseases, although rarely it may also be reduced in severe end-stage myopathy.

An incomplete interference pattern may be due to either poor activation or poor recruitment. Consider the two different incomplete interference patterns shown in Figure 15–14. In both cases, the patient has been asked to maximally contract the muscle of interest. In the first case (top trace), note that the same MUAP is firing rapidly at 30 Hz. Thus, although the firing rate is maximal, only one MUAP is seen firing at 30 Hz (30 : 1 ratio). In a normal muscle, by the time the firing rate reaches 30 Hz, one should see five or six different MUAPs firing (a ratio of approximately 5 : 1). Thus, in this case, the interference pattern is reduced because of decreased recruitment, but activation (firing rate) is normal. Decreased recruitment occurs when there has been loss of MUAPs, usually through axonal loss or conduction block. In the unusual situation of end-stage myopathy, if every muscle fiber of an MUAP is lost, the number of MUAPs also will effectively decrease, leading to reduced recruitment.

FIGURE 15–14 Incomplete interference patterns.

In both traces, the patient is asked to contract the muscle maximally with the electromyography needle in place. The top trace demonstrates an incomplete interference pattern due to reduced recruitment. The bottom trace demonstrates an incomplete interference pattern due to reduced activation (see text for details).

Contrast this with the pattern of the second patient (bottom trace), in which one also sees a single MUAP firing. In this case, however, the single MUAP is firing at 5 Hz. Thus, the firing rate (activation) is clearly submaximal, although the number of MUAPs firing (recruitment) is normal for the firing rate (a ratio of approximately 5 : 1). In this case, the interference pattern is reduced primarily because of decreased activation, but recruitment (i.e., the number of different MUAPs) is appropriate for the level of firing. The patient’s weakness is reflected in the decreased activation of motor unit action potentials, judged by the submaximal sustained firing rate. This pattern of reduced activation may be seen if a patient cannot fully cooperate, perhaps because of pain, or has a CNS lesion (e.g., stroke, multiple sclerosis).

Of course, both decreased activation (i.e., an upper motor neuron disorder) and decreased recruitment (i.e., a lower motor neuron disorder) may be present in the same patient. This situation occurs most classically in amyotrophic lateral sclerosis, a disorder of both upper and lower motor neurons. More commonly, though, it occurs in patients with neuropathic disorders who also have difficulty moving a limb because of pain (e.g., abducting the hip in a painful L5 radiculopathy). In this latter situation, there is decreased recruitment due to loss of L5 nerve root fibers, and decreased activation due to pain.

The last concept to understand is “early recruitment.” In diseases in which there is dropout of individual muscle fibers from a motor unit (e.g., myopathies or NMJ diseases with block), the motor unit becomes smaller and subsequently can generate less force. Because each motor unit generates less force, many motor units must fire to generate even a small amount of force. This is known as early recruitment, which refers to the inappropriate firing of many motor unit action potentials to generate a small amount of force. On the monitor, many MUAPs will appear to fire almost simultaneously, with small amounts of force. Usually, only the electromyographer performing the study can assess early recruitment; it requires knowledge of how much force is being generated. To reiterate, early recruitment refers to the inappropriate (i.e., increased) number of MUAPs firing for the degree of force generated; it does not refer to the number of MUAPs firing for the level of activation or for the firing rate. An early recruitment pattern is typically seen in muscle disorders and in some disorders of the NMJ.

Many electromyographers will judge recruitment only during maximal contraction, by examining the interference pattern. However, not as well appreciated is that recruitment is more easily evaluated during moderate levels of contraction. Remember, the key question that must be answered is the same: Are the number of different MUAPs firing appropriate for the level of activation (firing rate)? If only one MUAP is seen firing at 15 to 20 Hz (medium level of activation), recruitment is decreased, regardless of the interference pattern. There is no need to increase the firing rate using maximal contraction in order to make this determination. Maximal contraction with the EMG needle in the muscle often is perceived as more painful by the patient, and is best avoided or at least minimized. Indeed, during maximal contraction, judging the relationship between the number of MUAPs firing and the firing rate can actually be more difficult.

Patterns of Motor Unit Abnormalities

MUAP morphology and firing patterns usually can discriminate among the various disorders affecting the motor unit. No single parameter identifies an MUAP as myopathic, neuropathic, or associated with an NMJ disorder. Specific patterns of abnormalities in MUAP morphology and firing rate reflect whether the underlying disorder is (1) acute, chronic, or end stage; (2) neuropathic, myopathic, or associated with an NMJ transmission defect; and, if neuropathic, (3) whether the primary pathophysiology is axonal loss or demyelination (Table 15–2).

Neuropathic

Acute Axonal Loss

After an acute axonal injury to a nerve, the process of wallerian degeneration occurs in motor nerve fibers within the first 3 to 5 days, followed by denervation of the distal muscle fibers of the involved motor units. Reinnervation normally occurs as surviving nearby axons form sprouts that grow and eventually reinnervate the denervated fibers. When this occurs, the number of muscle fibers in the reinnervated MUAP is larger than normal, leading to an MUAP with increased duration, amplitude, and number of phases (Figure 15–15). However, this process takes time, usually many weeks to months. In the acute setting, MUAP morphology remains normal. The only abnormality seen on EMG in an acute neuropathic lesion is a decreased recruitment pattern in weak muscles due to the initial loss of motor units. Thus, in acute axonal loss lesions on needle EMG, there is a pattern of decreased recruitment of MUAPs with normal morphologies. This pattern does not occur in slowly progressive or chronic conditions (e.g., most polyneuropathies). In those conditions, changes in MUAP morphology are always present by the time the patient presents with symptoms. The acute neuropathic pattern associated with axonal loss characteristically occurs in the first several weeks after trauma, compression, or nerve infarction. The only other situation in which a similar pattern is seen is in pure demyelinating lesions with conduction block (discussed in section on Neuropathic: Demyelinating).

FIGURE 15–15 Motor unit action potential (MUAP) morphologies.

Normal MUAPs have two to four phases. In chronic neuropathic lesions that occur after reinnervation, the number of muscle fibers per motor unit increases, resulting in long-duration, high-amplitude, and polyphasic MUAPs. In myopathies or in neuromuscular junction disorders with block, the number of functional muscle fibers in the motor unit decreases. This leads to short-duration, small-amplitude, and polyphasic MUAPs.

Chronic Axonal Loss

After axonal loss and denervation, the process of reinnervation can occur by one of two mechanisms. If there has been complete denervation, the only possible mechanism for reinnervation is axonal regrowth from the point of injury (see section on Early Reinnervation Following Severe or Complete Denervation). Typically, this regrowth is quite slow (no more than 1 mm per day) and may take months to years, depending on the length of the nerve. For the regrowth to occur, however, the anterior horn cells must remain intact. For example, the original nerve fibers can regrow following transection of a nerve, but not after poliomyelitis, which results in the death of anterior horn cells.

In contrast, in cases of partial or gradual denervation, reinnervation usually occurs through collateral sprouting by adjacent surviving motor units (Figure 15–5). As the number of muscle fibers per motor unit increases, MUAPs become prolonged in duration, with a high amplitude, and polyphasic. These MUAP changes, in conjunction with decreased recruitment, are the hallmarks of reinnervated motor unit action potentials and nearly always imply chronic neuropathic disease (i.e., disorders of the anterior horn cell, nerve root, or peripheral nerve). Similar to other neuropathic conditions, during maximal contraction, the interference pattern will not be full, secondary to decreased recruitment of MUAPs (Figure 15–13B). Long-duration, high-amplitude, polyphasic MUAPs are never seen in acute conditions. When present, they always imply that the process has been present for at least several weeks and more often for months or years.

Demyelinating

Loss of axons results in denervation and ultimately reinnervation, with resultant changes in MUAP morphology. If, however, the pathology is purely or predominantly demyelinating, the underlying axon remains intact. Thus, there is neither denervation nor subsequent reinnervation. In pure demyelinating lesions, MUAP morphology remains normal. If demyelination results in conduction velocity slowing alone, the nerve action potential will still reach the muscle, albeit more slowly, and the number of functioning motor units will remain normal. Accordingly, there will be no change in either MUAP morphology or recruitment pattern on needle EMG. If demyelination results in conduction block, however, the number of available MUAPs effectively decreases. Although the MUAP morphology remains normal, the firing pattern shows decreased recruitment. This pattern of reduced recruitment with normal MUAP morphology is seen only in demyelinating lesions with conduction block (e.g., some cases of Guillain–Barré syndrome, carpal tunnel syndrome) or in cases of acute axonal loss before enough time has passed for reinnervation to occur.

Early Reinnervation following Severe or Complete Denervation

Reinnervation most often occurs from collateral sprouting by adjacent surviving motor units. If there is severe or complete denervation, with no nearby surviving axons, the only possible mechanism for reinnervation is regrowth of the axon from the site of injury. As the axon regrows, at some point in time it will reinnervate some, but not all, of the original muscle fibers. At that point, the MUAP will be short duration, small amplitude, and polyphasic, similar to an acute myopathic motor unit action potential (Figure 15–16). Early reinnervated motor unit action potentials following severe denervation are known as nascent motor units. The key factor that differentiates nascent motor unit action potentials from myopathic motor unit action potentials is the recruitment pattern. Nascent MUAPs are always seen in the context of markedly reduced recruitment, whereas myopathic MUAPs are seen in the context of normal or early recruitment. Although nascent motor units are uncommon, they emphasize that not all short-duration, small-amplitude, polyphasic MUAPs are myopathic.

FIGURE 15–16 Nascent motor units.

After a severe axonal loss lesion, wallerian degeneration occurs distal to the injury, resulting in denervation B. If there are no surviving nearby axons, reinnervation can occur only from axonal regrowth from the terminal stump. Early in this reinnervation process, there will be a point at which some but not all of the muscle fibers are reinnervated C. At that point, motor unit action potentials (MUAPs) will be short duration, small amplitude, and polyphasic, resembling myopathic units. Compare the nascent MUAP C with the normal MUAP A. Nascent MUAPs are differentiated from myopathic MUAPs by the reduced recruitment pattern compared with normal or early recruitment seen in myopathy.

Central Nervous System Disorders

In CNS disorders, normally there is no loss of anterior horn cells and, accordingly, no denervation or reinnervation. MUAP morphology and recruitment remain normal. On needle EMG, weakness is demonstrated as the inability to fire motor unit action potentials rapidly (i.e., reduced activation). Thus, although the interference pattern is incomplete, with a reduced number of motor unit action potentials firing, the actual number of motor unit action potentials (i.e., recruitment) is appropriate for the reduced level of activation (Figure 15–13D).

Occasionally, other patterns may be seen with CNS disorders. In spinal cord lesions, motor units may be lost at the level of the lesion because of segmental loss of anterior horn cells. For example, in a C6 spinal cord lesion, denervation, reinnervation, and decreased recruitment of MUAPs may be seen in the C6-innervated muscles. In the weak lower extremities, however, only decreased activation, but not decreased recruitment, of MUAPs will be seen. In those muscles that receive partial innervation from C6 (e.g., pronator teres, C6–C7 innervated), there may be a combination of decreased recruitment and decreased activation of MUAPs.

Only rarely are other EMG abnormalities seen in CNS disorders. In some reported patients with multiple sclerosis, signs of denervation and reinnervation have been seen, presumably due to involvement of motor fibers as they leave the anterior horn cell in the spinal cord prior to exiting and becoming motor roots. Whether EMG abnormalities can be seen in other CNS disorders, especially stroke, remains controversial. Stroke patients are susceptible to entrapment and compression palsies because of poor mobility, which more often explains any EMG abnormalities.

Tremor may occur in some CNS disorders and can complicate the interpretation of both spontaneous activity (see Chapter 14) and MUAP morphology. Tremor is recognized as a bursting pattern of voluntary MUAPs separated by relative silence. When tremor occurs at rest (e.g., in Parkinson’s disease), the spontaneous bursting discharges may be mistaken for myokymic discharges. Although both tremor and myokymia result in a bursting pattern of MUAPs, the major difference is that in myokymia the same MUAP fires repetitively in a burst, whereas in tremor the burst is composed of many different MUAPs. In addition, most patients can voluntarily alter their tremor by changing their limb position or action, whereas myokymia cannot be voluntarily influenced by the patient. Most tremors, however, worsen with activation. Because multiple MUAPs fire simultaneously in tremor, the morphology of individual MUAPs may be difficult to assess, and polyphasia may appear to be increased. In general, it is very difficult to accurately judge MUAP morphology, stability, or recruitment if the patient has a tremor when activating their muscles.

Lastly, persistent involuntary contraction can be seen during the needle EMG as the result of central disorders, including dystonia, stiff-person syndrome, and tetanus. In all of these disorders, MUAP morphology will be normal, and the EMG pattern will be one of involuntary persistent firing of MUAPs, characterized by delayed relaxation and co-contraction of muscles. Normally, individuals can easily relax their muscles and stop contracting. In these CNS disorders, however, this often is not possible. In addition, co-contraction of agonist and antagonist muscles occurs. Normally, antagonist muscles are quiet while agonists are contracting (e.g., the triceps is relaxed while the biceps is contracting and flexing the elbow). In dystonia, MUAP firing often actually increases in the antagonist muscle when the patient is instructed to move the agonist muscle (e.g., increased firing in the tibialis anterior when the patient plantar flexes the ankle) (Figure 15–17).

FIGURE 15–17 Dystonic firing pattern.

Recording from the tibialis anterior muscle in a patient with dystonia. Top trace: At rest. Note the persistent firing of motor unit action potentials. Bottom trace: The patient is asked to plantar flex the ankle (i.e., activate an antagonist muscle). Note that the firing markedly increases. This pattern of co-contraction of agonist and antagonist muscles occurs in dystonia and other central nervous system disorders.