The study of the muscular contractions which take place during speech. Muscles produce tiny amounts of electrical activity when they contract. The activity is recorded by applying electrodes to the individual muscles of the vocal tract, and displaying the signals on a screen or on paper.
Electromyography (EMG) is a medical technique for evaluating and recording physiologic properties of muscles at rest and while contracting. An electromyograph detects the electrical potential generated by muscle cells when these cells contract, and also when the cells are at rest.
Electrical Characteristics
The electrical source is the muscle membrane potential, about -70mV.
Typical repetition rate of muscle unit firing is about 7-20Hz, depending of the size of the muscle (eye muscles versus seat (gluteal) muscles), previous axonal damage and other factors.
Procedure
To perform EMG, a needle electrode is inserted through the skin into the muscle tissue. The insertional activity provides valuable information about the state of the muscle and its innervating nerve. Abnormal spontaneous activity might indicate some nerve and/or muscle damage.
A motor unit is defined as one motor neuron and all of the muscle fibers it innervates. When a motor unit fires, the impulse (called an action potential) is carried down the motor neuron to the muscle. The area where the nerve contacts the muscle is called the neuromuscular junction, or the motor end plate. After the action potential is transmitted across the neuromuscular junction, an action potential is elicited in all of the innervated muscle fibres of that particular motor unit. The composition of the motor unit, the number of muscle fibres per motor unit, the metabolic type of muscle fibres and many other factors affect the shape of the motor unit potentials in the myogram.
Normal results
Muscle tissue at rest is normally electrically inactive. a muscle at rest should be electrically silent, with the exception of the area of the neuromuscular junction, which is normally electrically very spontaneously active). As the strength of the muscle contraction is increased, more and more muscle fibers produce action potentials.
Abnormal results
Abnormal results may be caused by the following medical conditions (please note this is nowehere near an exhaustive list of conditions that can result in abnormal EMG studies):
Alcoholic neuropathy Axillary nerve dysfunction Becker's muscular dystrophy Brachial plexopathy Carpal tunnel syndrome Centronuclear myopathy Cervical spondylosis Charcot-Marie-Tooth disease Common peroneal nerve dysfunction Denervation (reduced nervous stimulation) Dermatomyositis Distal median nerve dysfunction Duchenne muscular dystrophy Facioscapulohumeral muscular dystrophy (Landouzy-Dejerine) Familial periodic paralysis Femoral nerve dysfunction Fields condition Friedreich's ataxia Guillain-Barre Lambert-Eaton Syndrome Mononeuritis multiplex Mononeuropathy Motor neurone disease Myasthenia gravis Myopathy (muscle degeneration, which may be caused by a number of disorders, including muscular dystrophy) Myotubular myopathy Peripheral neuropathy Poliomyelitis Polymyositis Radial nerve dysfunction Sciatic nerve dysfunction Sensorimotor polyneuropathy Shy-Drager syndrome Spinal Stenosis Thyrotoxic periodic paralysis Tibial nerve dysfunction Ulnar nerve dysfunction
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