Anatomy Histology

Muscle fibers in adults are approx 50 ^m in size. They are polygonal in shape and are bundled into fascicles (Fig. 1). Each fascicle contains approx 20 to 60 muscle fibers, and each

Fig. 1. Schematic diagram of the connective tissue sheaths of muscle. Muscle fibers are bundled into fascicles bordered by perimysial connective tissue. From DeGirolami and Smith, 1982 with permission.

muscle fiber consists of 50 to 100 myofibrils (Fig. 2). The myofibrils are divided into sarcomeres, which are the smallest contractile units. The sarcomere is that portion of the myofibril that extends from Z band to Z band (Fig. 3). The sarcomere consists of two filaments, the thick (myosin) filaments alternating with the thin (actin) filaments. The four major contractile proteins that are present in a myofilament are actin, myosin, troponin, and tropomyosin. The T tubules (Fig. 2) are located near the junction of the A and I bands and transmit the initial depolarization at the motor end plate. They are opposed on each side by the terminal cisterns of the sarcoplasmic reticulum. The terminal cisterns act as the storage space for calcium ions. This sarcoplasmic reticulum-T tubule-sarcoplasmic reticulum complex is called the "triad" and is responsible for converting electrical signals (membrane action potentials) to chemical signals (calcium release).

In a normal muscle, the muscle fibers are organized into functional groups called motor units. The term "motor unit," first used by Liddell and Sherrington in 1929, refers to a single lower motor neuron and the muscle fibers it innervates. One motor neuron innervates multiple muscle fibers, but each muscle fiber is innervated by only one motor neuron. The average number of muscle fibers in a motor unit determines the innervation ratio of the muscle. This ratio varies greatly from one muscle to another and is related to the degree of dexterity required of that muscle. For instance, the innervation ratio of the external ocular muscles is approx 10 muscle fibers to 1 motor neuron, that of intrinsic hand muscles is approx 100 muscle fibers to 1 motor neuron, and that of gastrocnemius is approx 2000 muscle fibers to 1 motor neuron. The muscle fibers belonging to a motor unit are typically distributed widely, reaching over as many as 100 fascicles. They are randomly distributed over a circular or oval

Muscle Innervation Ratio

Fig. 2. Structure of a single muscle fiber cut both horizontally and longitudinally. Individual myofibrils are surrounded and separated by sarcoplasmic reticulum. T tubules are continuous with extracellular fluid and interdigitate with sarcoplasmic reticulum. Note the regular association of T tubule with sar-coplasmic reticulum to form membranous triads. Note the location of myonuclei at the periphery of the muscle fiber and the presence of many mitochondria. From Westmoreland et al., 1994 with permission.

Fig. 2. Structure of a single muscle fiber cut both horizontally and longitudinally. Individual myofibrils are surrounded and separated by sarcoplasmic reticulum. T tubules are continuous with extracellular fluid and interdigitate with sarcoplasmic reticulum. Note the regular association of T tubule with sar-coplasmic reticulum to form membranous triads. Note the location of myonuclei at the periphery of the muscle fiber and the presence of many mitochondria. From Westmoreland et al., 1994 with permission.

region that can reach approx 20 to 30% of the muscle's cross sectional area, with an average diameter of 5 to 10 mm. In general, there is a mosaic pattern of as many as 20 to 50 overlapping motor units in the same muscle, although a slightly higher density of muscle fibers belonging to the same motor unit is observed at the center of the motor unit.

2.2. Physiology

Muscle contraction occurs as a result of a series of steps. The process starts with an action potential traveling along the motor nerve axon and reaching the axon terminal (Fig. 4). This (presynaptic) depolarization opens voltage-gated calcium channels, which increases calcium conductance and leads to release of acetylcholine (ACh) into the synaptic cleft. The ACh receptors (ligand-gated channels) at the postsynaptic membrane bind to this ACh, resulting in an end-plate potential, which, when reaching threshold, triggers opening of the adjacent muscle fiber voltage-gated sodium channels (Fig. 4). This ionic flow induces a voltage change, producing an action potential that initiates the excitation-contraction coupling.

With this outline in mind, let us now consider each step in more detail. In the resting state, small fluctuations in membrane potentials occur at the end-plate region called miniature endplate potentials. The miniature end-plate potentials are produced by the spontaneous release of quanta of ACh from the motor nerve ending that are not sufficient to produce an action

Sarcomere Section

Fig. 3. Organization of protein filaments in a myofibril. (A) Longitudinal section through one sarcomere (Z disc to Z disc) showing the overlap of actin and myosin. (B) Cross section through the A band, where the thin actin filaments interdigitate with the thick myosin filaments in a hexagonal formation. (C) Location of specific proteins in the sarcomere. From Westmoreland et al., 1994 with permission.

Fig. 3. Organization of protein filaments in a myofibril. (A) Longitudinal section through one sarcomere (Z disc to Z disc) showing the overlap of actin and myosin. (B) Cross section through the A band, where the thin actin filaments interdigitate with the thick myosin filaments in a hexagonal formation. (C) Location of specific proteins in the sarcomere. From Westmoreland et al., 1994 with permission.

potential. An action potential is generated if this end-plate potential reaches sufficient amplitude. Postsynaptic depolarization causes the action potential to travel along the muscle fiber in both directions at a velocity of approx 4 m/s and to come in contact with the terminal cis-ternae of the sarcoplasmic reticulum, thus, gaining entry into the contractile apparatus.

The T tubules transmit this action potential inward, penetrating the triad complex (sar-coplasmic reticulum-T tubule-sarcoplasmic reticulum complex). This results in a signal transduction through ryanodine receptors releasing calcium ions. The calcium ions then bind to the troponin subunits, causing a conformational change in the tropomyosin and the actin helix configuration. Sliding of the thin actin filaments over the thick myosin filaments produces muscle contraction. The shortening of the sarcomeres and the I band during contraction is not caused by any change in the absolute length of the filaments but rather by the sliding of the filaments themselves. Contraction ceases when calcium is removed from the sarcoplasmic reticulum by active transport.

2.3. EMG Correlates of Normal Muscle Physiology

In muscle at rest, the advancing needle records discharges described as insertional activity and the stationary needle electrode records discharges described as spontaneous activity. When the muscle is voluntarily activated, the needle electrode records groups of individual motor unit action potentials (MUAPs).

Define Emg Records

Fig. 4. The binding of acetylcholine (ACh) at transmitter-gated channels opens channels permeable to both Na+ and K+. The flow of these ions into and out of the cell depolarizes the cell membrane, producing the end-plate potential. This depolarization opens neighboring voltage-gated Na+ channels. To elicit an action potential, the depolarization produced by the end-plate potential must open sufficient Na+ channels to reach the threshold for initiating the action potential. From Kandel et al., 1995 with permission.

Fig. 4. The binding of acetylcholine (ACh) at transmitter-gated channels opens channels permeable to both Na+ and K+. The flow of these ions into and out of the cell depolarizes the cell membrane, producing the end-plate potential. This depolarization opens neighboring voltage-gated Na+ channels. To elicit an action potential, the depolarization produced by the end-plate potential must open sufficient Na+ channels to reach the threshold for initiating the action potential. From Kandel et al., 1995 with permission.

2.3.1. Insertional Activity

In normal muscle, the act of inserting the needle electrode generally evokes only a brief discharge that lasts a little longer than the actual movement of the needle.

2.3.2. Spontaneous Activity

Physiological spontaneous activity is usually restricted to the end-plate regions, observed as end-plate noise and spikes. A more prolonged discharge can occur when the needle electrode is in the end-plate zone (Fig. 5).

2.3.3. Motor Unit Action Potential

The standard concentric needle electrode used in EMG practice has an active recording surface of approx 150 x 600 ^m. This recording surface captures activity of muscle fibers that

END-PLATE NOISE AND SPIKES

END-PLATE NOISE AND SPIKES

SPIKES

NOISE

Fig. 5. Normal spontaneous activity in the end-plate region. Electrical activity recorded from the muscle at rest after insertional activity has subsided, and there is no voluntary muscle contraction. From Daube, 1991 with permission.

SPIKES

NOISE

Fig. 5. Normal spontaneous activity in the end-plate region. Electrical activity recorded from the muscle at rest after insertional activity has subsided, and there is no voluntary muscle contraction. From Daube, 1991 with permission.

are located within a 10-mm diameter. The electrical activity of a motor unit recorded by a needle electrode represents the summation of action potentials of muscle fibers that are firing near the electrode. Approximately 8 to 20 muscle fibers belonging to the same motor unit contribute to the recorded MUAP. It is important to appreciate that the MUAP represents the summated activity of some muscle fibers in a motor unit but not activity of all fibers of a motor unit. Type I muscle fibers are primarily responsible for the generation of the MUAP because low-threshold small motor neurons are preferentially activated on initial minimal voluntary contraction.

Normally, there is some variation of size and form of the MUAPs in a single muscle and of the average size and form of action potential in different muscles. The morphology of the MUAPs is influenced by a number of technical and physiological factors, such as type of needle electrode used, patient's age, muscle temperature, and so on. In general, MUAP duration is shorter in proximal than in distal muscles and their size is larger in adults than in children. MUAPs recorded in normal muscles of the extremities are commonly diphasic or triphasic waves, and they produce a thumping or knocking sound over the loudspeaker.

A variety of parameters define the MUAP (Fig. 6). The amplitude is primarily derived from the muscle fiber action potentials of the motor unit residing within 500 ^m of the active recording surface and usually measures between 100 ^V and 2 mV. It is influenced by the number and diameter of fibers involved, proximity of the needle electrode to motor units, and the synchronicity of their action potentials. The rise time is measured from the initial positive peak to the subsequent negative peak, and is an indicator of the distance between the recording tip of the electrode and the depolarizing muscle fibers. The rise time should be less than 500 ^s before a MUAP is accepted as a genuine near-field MUAP. A short rise time is characterized by a sharp, crisp sound. If the MUAP is associated with a dull sound, the electrode should be adjusted until MUAPs with sharp and crisp sounds are detected. The amplitude and rise time are inversely proportional to the distance between the electrode and the muscle fibers.

The duration of the MUAP depends on the depolarization of many muscle fibers that are both away from and close to the tip of the needle. It best reflects the number of muscle fibers

Motor Unit Action Potential Muap

Fig. 6. A schematic motor unit potential with characteristics that can be measured. The motor unit potential (also known as the motor unit action potential) is the compound action potential of a single motor unit whose muscle fibers lie within the recording range of an electrode. From Daube, 1991 with permission.

Fig. 6. A schematic motor unit potential with characteristics that can be measured. The motor unit potential (also known as the motor unit action potential) is the compound action potential of a single motor unit whose muscle fibers lie within the recording range of an electrode. From Daube, 1991 with permission.

in a motor unit, and typically measures approx 5 to 15 ms. It is the time from the initial deflection away from the baseline to its final return to the baseline. It varies with the muscle tested, patient's age, and the muscle temperature. The initial deflection reflects the arrival of the fastest muscle fiber action potential and, thus, the arrival time of various action potentials determine the final duration of that particular MUAP. Factors that influence the arrival time of various action potentials include length of individual terminal nerve branches, the distance of individual NMJs from the recording electrode, the diameter of individual muscle fibers, and the conduction velocity along individual muscle fibers (Fig. 7). Thus, unlike the amplitude, the duration of the MUAP is significantly affected by the distant muscle fibers. The number of phases in the MUAP depends on the synchrony of depolarization of the muscle fibers, that is, the extent to which the muscle fibers within a motor unit fire at the same time. MUAPs with more than four phases are called polyphasic. Polyphasia may be observed normally, and up to 12% polyphasic MUAPs has been described in normal muscles.

2.3.4. Recruitment in Normal Muscle

Motor unit recruitment refers to the successive activation of the same and additional motor units with increasing strength of voluntary muscle contraction. The Henneman size principle refers to the orderly successive activation of motor units during an increasing voluntary muscle activation, with the activation of small, weak (type 1) motor units first in an early contraction, and the sequential addition of larger, stronger (type 2) motor units to provide a smooth increase in muscle power. Recruitment frequency refers to the firing rate of a MUAP when a different MUAP appears during gradually increasing voluntary muscle contraction. Recruitment interval refers to the time elapsed between consecutive discharges of a MUAP when a different MUAP first appears during gradually increasing muscle contraction.

Henneman Size Principle

Fig. 7. Schematic representation of motor unit action potential generation and recording by a concentric needle electrode. Individual muscle fiber action potentials numbered one through six arrive at the recording electrode at different times, depending on factors such as terminal nerve branch length, distance between the muscle fiber's neuromuscular junction and the recording electrode, diameter of the individual muscle fiber, and the conduction velocity of muscle fiber action potentials along individual muscle fibers. From Ball 1985 with permission.

Fig. 7. Schematic representation of motor unit action potential generation and recording by a concentric needle electrode. Individual muscle fiber action potentials numbered one through six arrive at the recording electrode at different times, depending on factors such as terminal nerve branch length, distance between the muscle fiber's neuromuscular junction and the recording electrode, diameter of the individual muscle fiber, and the conduction velocity of muscle fiber action potentials along individual muscle fibers. From Ball 1985 with permission.

In the EMG laboratory, motor unit recruitment is commonly assessed with the rule of fives. Let us consider a situation in which the first motor unit begins to fire at 5 Hz. A new MUAP is recruited when the first motor unit reaches a firing frequency of 10 Hz. Subsequent new motor units are added as the previous motor units reach their maximal firing frequency. For instance, if the MUAPs are firing at 25 Hz, there should be at least five different MUAPs on the EMG monitor. If the ratio exceeds six, this indicates that few motor units are present and those are firing rapidly, at more than the usual frequency of 5 Hz. Each muscle also has a characteristic maximal MUAP recruitment frequency. For instance, the facial muscles have high maximal recruitment frequencies (20-30 Hz) in contrast to the extremity muscles, which have maximal recruitment frequencies in the 10- to 12-Hz range.

The other commonly used term for recruitment in the EMG laboratory is interference pattern. A full interference pattern implies that no individual MUAP can be clearly identified. In healthy individuals, one would expect a full interference pattern on voluntary muscle contraction. A reduced interference pattern is one in which some of the individual MUAPs may be identified, whereas other individual MUAPs cannot be identified because of superimposition of waveforms. The importance of early or increased recruitment observed in myopathy will be discussed in Section 3.2.4. If documenting recruitment pattern or the interference pattern, it is important to specify the force of muscle contraction associated with that pattern.

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