Individual Motor End-plates Research Articles

Chapter 13 Quantitation and standardization in EMG and neurography

The electrophysiology of the human motor unit (MU) may be studied with various electromyographic methods. Because of differences in the size, shape, and construction, the signals recorded by different electrodes differ in shape and amplitude. Quantitation of the various neurophysiological parameters is highly demanded. With modern techniques, this has now become possible even for routine use. In routine electromyography (EMG) investigations, the muscle is studied during rest, during slight voluntary activation for single MU studies and during strong voluntary contraction for the assessment of the interference pattern. Single fiber EMG (SFEMG) is used to study the muscle fiber membrane characteristics, muscle fiber propagation velocity, function of individual motor end-plates, organization of muscle fibers in the MU territory, reflexes, and central influences. Surface EMG recordings with spike triggering are used to assess MU size, propagation velocity and end-plate distribution. The cross-section of the MU, the territory can be studied with multi-electrodes or with scanning EMG. This technique has not been used routinely, but for research purposes only. Standardization of nerve conduction studies (NCS) involves two main topics: standardization of the recording technique itself including parameters and the standardization of the study protocol in a given clinical situation. Values of NCS have to be comparable between different investigators and different laboratories for practical and scientific reasons.

The study of normal and abnormal neuromuscular transmission with single fibre electromyography

The use of single fibre electromyography (SFEMG) in the study of neuromuscular transmission across individual motor endplates in situ is reviewed. The neuromuscular jitter can be studied both during voluntary contraction and electrical activation of the muscle fibre. The differences, pitfalls and advantages of these methods are discussed. Findings in myasthenia gravis and other disorders of the neuromuscular transmission are examined.

Jitter measurement by axonal micro-stimulation. Guidelines and technical notes

Single fiber EMG (SFEMG) with axonal micro-stimulation is a convenient method to study the neuromuscular jitter at the individual motor end-plates. Compared to the original method of jitter measurement in voluntarily activated muscle, it has the advantage of perfect control of the discharge rate, including pauses in activity, useful in quantitative estimation of the neuromuscular transmission defect. It obviates the need to search for muscle fiber pairs. It can be used in young children and in uncooperative patients, as well as those with impaired voluntary motor control. It is useful in animal experiments as well as veterinary medicine. The technique eliminates the possibility of overestimating the jitter due to unrecognized interdischarge interval dependent jitter, as well as that of underestimating it due to unrecognized low jitter in split muscle fibers. The technique has certain pitfalls causing under- or overestimation. The paper gives practical guidelines and hints as to how to avoid some of these, particularly errors due to overlooked threshold stimulation and to unrecognized direct muscle fiber stimulation.

Electrical Microstimulation with Single-Fiber Electromyography

This article reviews the essential technical principles and findings with electrical microstimulation as used in conjunction with single-fiber electromyography (SFEMG). It highlights the details in the microphysiology of the motor unit that can be conveniently studied with these methods, particularly the neuromuscular transmission at the individual motor endplates, muscle fiber recovery functions, and a variety of late responses. The differences, advantages, and disadvantages of the stimulation method of jitter measurement, compared to the original method, in the voluntarily activated muscle are described. Special attention is given to the pitfalls of the technique. Indications for clinical use of stimulation SFEMG in the electromyography laboratory are suggested.

Neuromuscular junctions in adult and developing fast and slow muscles.

Functional changes that occur just before hatching in future fast muscles of the chicken are thought to be influenced by the pattern of innervation. We have compared the neuromuscular junctions of two fast muscles, the posterior latissimus dorsi (PLD) and the pectoralis, which differ in their myosin composition at 18 days in ovo. We have also presented new information on the neuromuscular junctions of the adult fast muscles and an adult slow muscle, the anterior latissimus dorsi (ALD). Both categories of adult muscles were heterogeneous, and there was little difference between endplates of the two fast muscles or between the fast and slow muscles. In contrast, there were significant structural differences between the two fast muscles during embryonic development. In early embryonic muscle fibers, which synthesize embryonic forms of myosin, individual motor endplates were contacted by multiple axon terminals. At 18 days in ovo, the majority of the neuromuscular junctions in the pectoralis continued to be multiterminal, whereas all but one of the terminals had been withdrawn from each endplate in the PLD. This single terminal had a unique form that distinguished it from the embryonic pectoralis and also from the two adult muscles. By 7 days after hatching, the neuromuscular junctions of both muscles had single terminals. They were different from the embryonic terminals, though not necessarily equivalent to adult terminals. The results show that multiple terminals persist at 18 days in ovo in the muscle that continues to express an embryonic myosin, but they have been withdrawn from the muscle that has lost this myosin. It is concluded, from combined data on the two muscles, that maturation of the neuromuscular junction during embryonic and late posthatch development is correlated with transitions in the myosin pattern and in contractile properties.

Acetylcholine synthesis at the rat neuromuscular junction.

Acetylcholine (ACh) synthesis in homogenates of rat soleus muscles had two components. One component, specifically inhibited by bromoacetylcholine (BrACh), had a Km for choline of 0.26 mM; the other, resistant to BrACh, had a Km for choline of 45 mM. The component with a low Km was absent from denervated muscle, and was identical in kinetic terms to ACh synthesising activity in homogenates of sciatic nerve. It is therefore considered choline acetyltransferase (ChAT)-specific. The use of BrACh as a specific inhibitor of ChAT activity allowed the calculation of ACh synthesis at individual motor end-plates in the soleus muscle of the rat: 2.1 X 10(-3) nmol h-1. Since the number of muscle fibres and the number of motor units are known for this muscle, ACh synthesis per motor unit could be calculated: 0.15 nmol h-1. It is concluded that BrACh can be used as a specific inhibitor of ChAT activity in homogenates of skeletal muscle and that its use will obviate the necessity of dividing biopsied muscle or small rodent muscles into neural and aneural segments.

Neuromuscular transmission in myasthenia gravis studied with single fibre electromyography.

Patients with myasthenia gravis have been investigated with single fibre electromyography. In all myasthenic muscles recordings were obtained where there was an increase of the jitter and occasional blocking of different degree of single fibre action potentials. The jitter is the variability at consecutive discharges in the time interval between action potentials from two muscle fibres from the same motor unit. During prolonged activity the jitter increased and blockings occurred more and more frequently. The size of the jitter and the degree of blocking also depended on the discharge rate. At a low rate the jitter was lower and the degree of blocking less than at the high rate. Injection of edrophonium could decrease or increase the jitter or leave it unaffected. Single fibre electromyography is a sensitive way of studying the transmission in individual motor end-plates in myasthenia gravis and is also a valuable diagnostic aid.

Dystrophic chicken muscle: altered synaptic acetylcholinesterase.

Individual motor endplates in the skeletal muscles of chickens genetically homozygous for muscular dystrophy have been compared with those in normal chickens. Measurements were made there, by specific autoradiographic techniques, of the numbers of total cholinesterase-like molecules and of acetylcholinesterase molecules. The acetylcholinesterase is distinctly decreased at the endplates in dystrophic muscles. The various data available on these muscles are compatible with the concept that a neural factor which determines the synaptic acetylcholinesterase, along with a number of other characters in the muscle cell, is defective in this disorder.

Ischemic effects on impulse transmission to muscle fibers in man

Single fiber electromyography has been used to study the influence of ischemia on the impulse transmission to individual muscle fibers in man. The jitter (the variability in time interval between the action potentials from 2 muscle fibers belonging to the same motor unit) has been used to study the neuro-muscular transmission in individual motor end-plates. Ischemia increased the jitter and caused more and more frequent blocking of the neuromuscular transmission until total block occurred. About 3,500–7,000 impulses could be transmitted before total block developed. When circulation was restored, the formerly blocked potential reappeared within seconds and the jitter was normal within some minutes, even if ischemia had lasted for 30 min.

The effect of non-paralytic doses of d-tubocurarine on individual motor end-plates in man, studied with a new electrophysiological method

The authors have used a new method for the study of synaptic transmission in individual motor end-plates in man. Single muscle fiber action potentials are recorded and the method is based upon the following principle: when two muscle fibers, belonging to the same motor unit, are recorded at the same time, there is always a variability in the time interval between the two action potentials for consecutive discharges. This variability, of 10–30 μsec, is called the jitter phenomenon and has been used in this investigation to study d-tubocurarine. It has been found that the jitter increases with doses of d-tubocurarine far below those required to give any neuromuscular block. With a dose of 15 μg/kg d-tubocurarine the jitter increased by 5–15 μsec from the normal. A dose of 30 μg/kg increased the jitter to more than 70 μsec, when occasional blocking occurred. This seems to be the first in situ method for obtaining quantitative information concerning synaptic transmission in individual motor end-plates in man.