EMG PDF: Electromyography

Electromyography (EMG) Mona A Hussain Assistant Prof. in Physiology department FOM- PSU  EMG: Definition Electromyography is a recording of the electrical activity occurring in a muscle during voluntary contraction. It is the sum of action potentials...

Electromyography (EMG) Mona A Hussain Assistant Prof. in Physiology department FOM- PSU  EMG: Definition Electromyography is a recording of the electrical activity occurring in a muscle during voluntary contraction. It is the sum of action potentials of many muscle fibers. The record obtained with such electrodes is the electromyogram (EMG).  EMG: Relevance EMG is useful in the detection of lower motor neuron diseases, and disorders of neuromuscular transmission from certain muscle disorders such as muscular dystrophy.  The motor unit Each single motor neuron and the muscle fibers it innervates constitute a motor unit. The number of muscle fibers in a motor unit varies. In muscles such as those of the hand and those concerned with motion of the eye (ie, muscles concerned with fine, graded, precise movement), each motor unit innervates very few (on the order of three to six) muscle fibers. On the other hand, values of 600 muscle fibers per motor unit can occur in human leg muscles. An average figure for all the muscles of the body is questionable, but a good guess would be about 80 to 100 muscle fibers to a motor unit. The group of muscle fibers that contribute to a motor unit can be intermixed within a muscle. That is, although they contract as a unit, they are not necessarily “neighboring” fibers within the muscle.  Principle It has been shown by electromyography that little if any spontaneous activity occurs in the skeletal muscles of normal individuals at rest. With minimal voluntary activity a few motor units discharge, and with increasing voluntary effort, more and more are brought into play to monitor the recruitment of motor units.  METHODOLOGY A resting muscle is electrically silent, i.e. it does not show any electrical potentials. When it contracts, however, such changes can be recorded. NB***While a normal muscle fiber shows a RMP, and an AP when activated for contraction, a denervated muscle fiber shows unstable potential and spontaneous twitching (Denervation sensitivity to circulating acetylcholine)  Types of electrodes Needle electrode Surface electrode OBSERVATIONS AND RESULTS 1. Insertion activity. There is a brief burst of electrical activity of 0.5–1.0 msec due to mechanical damage by the needle, appearing as positive or negative bursts of high- frequency spikes. 2. Spontaneous activity. There is no spontaneous electrical activity except that when the needle electrode is near the end plate region when miniature end plate potentials (MEPP) may be recorded. They are monophasic negative waves of up to 100 mV and of 1–2 msec duration. ( Abnormal spontaneous activities include: eg. Fibrillation and fasciculation.) 3. Voluntary contractions. With weak contractions, Motor Unit Potentials (MUPs) of 5–15 Hz are recorded. With stronger contractions, potentials of 0.3-2 mV and 5–15 msec are recorded. With still stronger contractions, the potentials run into each other and the resulting confused tracing is called interference pattern. Electromyographic tracings from human biceps and triceps muscles during alternate flexion and extension of the elbow.  Isotonic and isometric systems for recording muscle contractions Muscular contraction involves shortening of the contractile elements. Isotonic contraction occurs when the force of the muscle contraction is greater than the load and the tension on the muscle remains constant during the contraction; when the muscle contracts, it shortens and moves the load. Isometric contraction occurs when the load is greater than the force of the muscle contraction; the muscle creates tension when it contracts, but the overall length of the muscle does not change. Isotonic and isometric systems for recording muscle contractions  Isometric contraction Muscles consist not only of contractile components (contractile proteins) but also elastic and viscous elements (elastic fibers, tendons, connective tissue sheaths, blood vessels, etc.), which are arranged in series with the contractile components. Therefore, if both the ends of a muscle are rigidly fixed, it is possible for the muscle fibers to contract without an appreciable shortening of the muscle as a whole, though there is development of tension. Such a contraction is called isometric (“same measure” or length). (The myofibrils do contract and shorten, and in doing so, they stretch the in-series elastic elements.  Recruitment The process of increasing the number of motor units that are active in a muscle at any given time is called recruitment. It is achieved by activating excitatory synaptic inputs to more motor neurons. The greater the number of active motor neurons, the more motor units recruited and the greater the muscle tension.  Summation Summation means the adding together of individual twitch contractions to increase the intensity of overall muscle contraction. Summation occurs in two ways: (1) by increasing the number of motor units contracting simultaneously, which is called multiple fiber summation, and (2) by increasing the frequency of contraction, which is called frequency summation and can lead to tetanization.  Tetanus: smooth, sustained, and forceful contraction Genesis of tetanus. The approximate rate of stimulation is indicated above each set of recording. At lower rates, single contractions occur, while with increasing rates, subtetanus (clonus) and then tetanus (sustained contraction of the muscle) occur. Fatigue sets in if tetanic stimulation is continued. Question: Is this phenomenon present in the cardiac muscle? Why?  Causes of the muscle fatigue Prolonged and strong contraction of a muscle leads to the well-known state of muscle fatigue. muscle fatigue increases in almost direct proportion to the rate of depletion of muscle glycogen. Therefore, fatigue results mainly from inability of the contractile and metabolic processes of the muscle fibers to continue supplying the same work output. So ATP is used during a muscle contraction faster than it can be produced and lactic acid builds up faster than it can be removed. Transmission of the nerve signal through the neuromuscular junction can diminish at least a small amount after intense prolonged muscle activity, thus further diminishing muscle contraction. Interruption of blood flow through a contracting muscle leads to almost complete muscle fatigue within 1 or 2 minutes because of the loss of nutrient supply, especially the loss of oxygen.