Electromyography is the study of the electrical activity of skeletal muscles and is commonly used to examine the extent of denervation or re-innervation of skeletal muscle
The recording of electrical signals from skeletal muscle fibres can be accomplished using hypodermic needles that have a central electrode insulated from the outside of the needle. The potentials recorded from muscle fibres in contact with the central electrode at the end of the needle are measured relative to the tissue surrounding the exterior surface of the needle.
Normal muscle fibres conduct action potentials that originate from the relase of transmitter at the nerve-muscle junction; denervation causes action potentials to disappear. However the muscle fibres begin to produce small local (non-conducted) potentials known as fibrillation potentials following denervation. Fibrillation potentials are easily distinguished from action potentials because they are smaller, shorter and irregular in time and shape.
When alpha-motoneurones regenerate, fibrillation potentials disappear and are replaced by conducted action potentials that often have different characteristics - they are often larger and irregular in shape.
Regenerating nerve endings do not usually make contact with their original muscle fibres, but tend to innerve closely packed, localised groups of muscle fibres. Action potentials in these groups appear large because potentials arising from each muscle fibre in the group occur almost simulataneously. These large irregular potentials represent large motor units, and are characteristic of reinnervated skeletal muscle.
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Normal skeletal muscle may fire action potentials generated as a result of the arrival of nerve impulses in the alpha motoneurones that innervate them. If the patient is asked to relax that muscle, these potentials often disappear.
Electrical Activity in Skeletal Muscle
When an alpha motoneurone fires and action potential, the latter travels rapidly down every branch of the alpha motoneurone and excites all the muscles fires that are in contact with those nerve terminals. The Motor Unit is defined as the number of muscle fibres innervated by a single motoneurone, and can be as low as 10 or as great as several thousand.
Acetylcholine is released when an action potential arrives at each nerve terminal, where it binds to nicotinic receptors in the post-synaptic membrane. When this occurs, the post-synaptic membrane becomes depolarised (the End Plate Potential)and the surrounding muscle cell membrane fires an action potential that passes along the length of the muscle fibre.
Muscles with small motor units are involved in fine control of movement, as in the extraocular muscles or the small muscles of the hand. Large motor units are found in the powerful muscles involved in producing considerable force.
The EMG needle can pick up this normal electrical activity, but attempts are always made to get the patient to relax the muscles being examined, so that potentials arising spontaneously in abnormal muscle may be observed.
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The recording shows fibrillation potentials from a denervated muscle.
Fibrillation Potentials in Skeletal Muscle
Denervated skeletal muscle fibres produce some spontaneous electrical activity called fibrillation potentials.
Fibrillation Potentials are NOT action potentials, and are distinguished by their small size, their duration and irregularity in both time and voltage.
Unlike action potentials they are not conducted along the length of the muscle fibre, but are local potentials.
Fibrillation Potentials are generated by denervated muscle fibres and appear after the nerves to the muscle have degenerated.
This is believed to occur when the nicotinic receptors (normally confined to the post-synaptic membrane of the nerve-muscle junction) appear along the whole surface of the muscle fibre.
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After reinnervation, the muscle fibres innervated by a single motoneurone tend to be close together: as a result a concentric needle electrode picks up potentials from several muscle fibres that are simultaneously active in the same 'new' motor unit.
The time of arrival of action potentials in the different muscle fibres being recorded is not simulataneous because of variations in the conduction velocity or length of the individual branches of the alpha motoneurone, and the site of the nerve muscle junctions.
Reinnervation of Denervated Skeletal Muscle Fibres
Following denervation some motoneurones may regrow and re-innervate denervated muscle fibres.
When the growth cones of regenerating alpha motoneurones make contact with denervated, these are usually a different set of skeletal muscle fibres, and the contacts are as widely dispersed as in normal muscle.
In a normal muscle the branches of the alpha motoneurones are distributed across a wide area of the muscle so that the muscle fibres of a motor unit that are often separated by significant distances.
The diagram shows some of this variation, and the electrical potentials being recorded from several musle fibres simulataneously.
These motor unit potentials are large because of summation of potentials arising from different muscle fibres, and irregular because of the different times of arrival of the potenials at the electrode.
However the irregularity in shape is constant, indicating that all the components originate from branches of the same motoneurone.
Image source: neuromuscular.wustl.edu
Legend Sweep speed = 10 ms/div; Sensitivity = 1.0mV/div
The components of this complex action potential are consistently related to each other in time, due to individual branches of the axon taking different intervals to excite the muscle fibres with which they are in contact; these skeletal muscle fibres are in close proximity to the recording needle.
Increase in Motor Unit Size after re-innervation.
After axonal regeneration has occurred and a contact between the motoneurone and the skeletal muscle fibre has been established, the muscle begins to function normally again. The fibrillation potentials cease, and the expression of nicotinic receptors is again confined to the nerve muscle junction. The skeletal muscle produces action potentials synchronised wuth action potentials in the alpha motoneurone that innervates them.
Large Motor Unit Potentials. One of the changes that occurs after regeneration is the number of muscle fibres innervated by each motoneurone. Some motor units increase in size considerably.
Electromyography shows the action potentials to be large in size, and irregular (but constant) in shape, because the electrode picks up voltages generated in several neighbouring skeletal muscle fibres, each activated at different times, by the same motoneurone.
In the diagram opposite, complex potentials are seen consistently, with some of the components being separated by up to several tens of milliseconds.
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The recordings show the muscle EMG elicited during a repetitive burst of electrical stimulation of the motor nerve (at the vertical arrows). In myaesthenia gravis, the size of the muscle potential diminishes during the repetitive train of stimuli, indicating a failure of transmission at the NMJ.
Disorders of the Nerve-Muscle Junction
ElectroMyoGraphy (EMG) is an important investigation in the diagnosis of diseases of the nerve-muscle junction, such as Myaesthenia Gravis and the Eaton-Lambert Syndrome.
In these conditions, muscles become weak and voluntary contractions cannot be sustained. Muscles involved in fine movements, such as the extraolular muscles, are affected, so that the direction of gaze of the two eyes can differ, resulting in double vision.
In the small muscles of the hands, the basic investigation involves repetitive electrical stimulation of the median nerve above the flexor retinaculum while recording of electrical activity from the small muscles of the thumb.
Normally, the compound action potentials recorded form the small muscles will be constant in size following supramaximal stimuli to the nerve. However in disorders of the nerve-muscle junction (NMJ), there is a decline in the response of the muscle during repeated stimuli, indicating that the is a problem with transmission across the NMJ.
In the extensor digitorum brevis, repetitive electrical stimulation of the common peroneal nerve at the neck of the fibula results in similar changes.
Image source: yale.edu
In myaesthenia gravis, the folds of the post-synaptic membrane are smaller and the number of nicotinic receptors on these membranes is also reduced, which explains the failure of neurotransmission.
Myaesthenia Gravis and Eaton-Lambert Syndrome
In Myaesthenia Gravis the responses of the muscle get less with successive repeated nerve stimulationdue to changes in the acetylcholine receptor.
There are abnormalities in the molecular structure of the nicotinic receptor within the post-synaptic membrane in this condition.
Electronmicroscopy also shows a loss of folds in the postynaptic membrane .
In the Eaton-Lambert Syndrome, there are again abnormalities in nerve-muscle transmission, but in this condition the problem is in the release of the transmitter acetylcholine from the pre-synaptic membrane.
With every stimulus to the nerve, less acetylcholine is released into the synaptic cleft.