MOTOR CONTROL ElectroMyoGraphy (EMG)

Transcription

1 MOTOR CONTROL ElectroMyoGraphy (EMG) Aim: The aims of this laboratory exercise are (a) to show how to measure muscle activity with electromyography (EMG), and (b) to illustrate how the central nervous system (CNS) regulates muscle force to control limb movements by recruitment, and frequency regulation of motor units and by automatic load compensation (stretch reflex). Background: Each muscle cell like any cell is surrounded by a membrane. In a living cell the membrane is charged with intracellular potential around 50 mv in relation to the outside of the cell. Muscle cells like neurons can generate an action potential (a spike), that is a temporary (for a period of about 1 ms) depolarization of the membrane, that is actively propagated along the cell membrane. Obviously, during a spike, different parts of the membrane have different electrical potentials and there are electrical currents flowing inside the cell as well as outside of the cell (Fig. 1). Thus temporary changes of the cell membrane potential produce temporary changes of electrical potential everywhere outside of the cell. outside cell membrane Negatively charged (inactive) part of a cell Depolarized (active) part of a cell Negatively charged (inactive) part of a cell inside cell membrane outside Fig. 1. Origin of the EMG signal: extracellular currents generated by an action potential in a muscle fiber. Electromyography is the recording of extracellular electrical potentials generated by an active muscle. Besides neurons and muscles there are numerous sources of electrical potentials that we do not want to detect. For example the power supply 50 Hz signal, radio waves, electric motors, electromagnets. These sources can create unwanted electric fields even if they are located in another room or building. That is why for EMG the bipolar differential recording method is used: there are 2 recording electrodes. A differential subtracts voltages at the two recording sites and amplifies the difference. This subtraction procedure significantly reduces the magnitude of the

2 signals that come from distant sources. Effectively the radius of the space that produces recordable signals is approximately equal to the distance between the electrodes (Fig. 2). The amplified signal is fed into a computer. To further reduce the noise and assure selective recording of biopotentials, the body surface is usually grounded with a third large indifferent (or ground ) electrode. Another method that helps to pick up the EMG signal is filtering. The action potentials are very fast events with typical times of potential changes of a few milliseconds. Therefore a high-pass filter, which cuts off all the frequencies equal or below 50 Hz, is frequently used. As a result, the power supply 50 Hz noise is reduced, as are possible reactions of the electrodes to purely mechanical factors that are usually much slower than changes in biopotentials. On the other hand, the upper limit of frequency is chosen based on the characteristic times of events that are of interest to the experimenter. If the experimenter is not interested in the microstructure of the EMG signals such as shapes of individual spikes, a low-pass cut-off frequency is usually about a few hundred Hertz. differential inputs effectively recorded signals distant (subtracted) signals Fig. 2. Bipolar recording of biopotentials. A pair of electrodes (dark gray circles) picks up all signals. However subtraction of the two signals by a differential weakens the signals coming from distant sources so that effectively only the signals from the vicinity of the electrodes (light gray area) are recorded. Based on the distance between recording electrodes, there are two basic methods of recording muscle activity: (1) intramuscular or needle electromyography and (2) surface or interferential electromyography. A B V MU1 MU1 MU3 MU3 computer muscle time Fig. 3. Intramuscular EMG with needle electrodes. Activity of individual motor units can be discriminated from the recording (MU1,, MU3 in B). With the first method, a thin needle (with a diameter of less that 1 mm) is inserted into a muscle (Fig. 3A). Inside the needle is a very thin wire that is electrically isolated from the needle. The tip of the wire is not isolated. An picks up the voltage between the tip of the wire and the needle. Since the dimensions of each electrode and the distance between them are very small, the

3 electrodes selectively pick up the signals (spikes) in closest proximity to the tips. Such electrodes record the patterns of activity of individual motor units. Note that each motor unit contains many muscle fibers but all fibers of one unit generate spikes synchronously, so the electrode picks up the compound spike of the whole motor unit. Typically, a needle electrode is able to record the electrical activity of a few motor units whose muscle fibers happen to be in close proximity to it. However, because each motor unit has a somewhat different number of muscle fibers with different location in relation to the electrode, each motor unit will have a unique pattern of voltage changes (Fig. 3B). Thus, it is possible to identify compound spikes of individual motor units with a high degree of certainty. Needle EMG is frequently used in clinical tests. The second method is surface EMG, which is more frequently used in studies of voluntary movements of healthy people. The main idea of surface EMG is to sum up the activity of as many motor units as possible across a muscle. Typically, the two recording electrodes are taped on the skin over a muscle belly (Fig. 4). On the one hand, to sum up the activity of a maximal number of motor units, the electrodes should be very large and positioned as far from each other as possible. On the other hand, if the activity of only one muscle is to be recorded selectively from activities of the other muscles, then the electrodes should be smaller and positioned closer. So there is a trade-off that is resolved differently by the experimenter in each particular case. For example, if one wants to record the activity of a relatively small forearm or facial muscle, it is not good to use very large electrodes as they will pick up signals from many neighboring muscles. However, if one wants to record the activity of a large postural muscle such as latissimus dorsi or biceps femoris, large electrodes will be appropriate. Typically, the diameter of electrodes for surface EMG vary between 1 and 20 mm, while the distance between them varies from 5 mm to 50 mm or even more. A B V muscle computer ground time Fig. 4. Surface EMG. Summed activity of many motor units are recorded. Preparations: Choose a test subject. Palpate m. biceps brachii and m. triceps brachii on the right arm. Apply one drop of electrode cream on four EMG surface electrodes, remove the protective paper covering the adhesive surface, and affix the electrodes pairwise, longitudinally on the muscle bellies approximately 3 cm apart. Apply some cream on the ground electrode and attach it to the wrist. The test subjects flexes the elbow joint to 90 with the palm upwards. Connect the electrodes to the s (biceps to channel A, triceps to channel B). Start the data acquisition program Axoscope (double-click the icon Axoscope). Define file names (=your group name) a) open menu File

4 b) choose Set Data File Names c) write your group number in the box File Name (the box Date Prefix should not be checked) d) press OK. Choose the acquisition protocol a) open menu Acquire b) choose Open Protocol c) do not save changes made to previous protocol press No and then OK d) find directory C:\axon\axoscope e) double-click the file emglab1 f) press OK. Test the data acquisition by starting the data display feature of Axoscope (press Play). Ask the subject to contract biceps and triceps while moving the arm up and down to see which muscle is attached to which set of electrodes and to adjust amplification of the recording. Experiment: 1. EMG-patterns at different levels of contraction Ask the test subject to keep the elbow joint relaxed but steady at 90, with the palm upwards. Start recording (press Record). Record the EMG activity for 15 s. A member of the group applies a small even force downward on the wrist of the test subject while he/she tries to keep the angle 90 in the elbow joint. Record the EMG-activity for 15 s. Apply a larger force downwards. Record the EMG-activity for 15 s. Stop recording (press Stop). N.B! Don t stop recording before you have applied different forces. Having episodes with different loads in the same file will make easier to compare the EMG amplitudes. 2. Activation of the stretch reflex by increased load Change data acquisition protocol in Axoscope (as before, but choose the file emglab2 instead of emglab1). Connect the weight to the A/D-converter (channel 4). Remove the paper spacer under the weight. Check that the contact triggers the acquisition program when the weight falls down (start the data display by pressing Play first). Instruct the test subject to hold the weight with straight wrist but the elbow relaxed at 90, and with closed eyes. A member of the group covers the subject s ears to prevent the subject to react to the sound of the sliding weight. Start acquisition (press Record). Lift the weight and drop it. Repeat at uneven intervals 10 times. The program will record the EMG response each time, and display an updated mean response. The recording will stop automatically after 10 successful trials (otherwise press Stop). 3. Reaction time The test subject is instructed to keep the elbow relaxed at 90 with weight as before. Every time the weight is released the task is to react to the increased load as quickly as possible by jerking the arm upward with just a small movement. Eyes and ears should be closed as before. Use the same protocol as in experiment 2 and repeat 10 times.

5 Check with supervisor that all experiments have been properly recorded before removing electrodes and disconnecting the test subject. Analysis: 1. EMG-pattern at different levels of contraction Print representative parts of the recordings in both high and low time resolution. Study the change in the amplitude of the peaks at different levels of contraction. 1) How is the amplitude of the EMG-recording changed for different loads? 2) What are the physiological changes behind the changes observed in the recording? 2. Activation of the stretch reflex by increased load Print the average EMG response to the increased load as well as overdrawn individual responses. 3) Mark the monosynaptic stretch reflex (M1) and the polysynaptic stretch reflex (M2) on your recording. What is the latency to each of them? 4) Explain the physiological background to the different peaks (M1 and M2) in the EMG-response. What parts of the nervous system are involved in the two reflexes? 5) Calculate the theoretical latency for the monosynaptic EMG responses in biceps and triceps (conduction velocity through a nerve is 70 m/s and use 3 ms delay for each synapse). Compare with your own data and explain eventual differences. How can one affect the latency? 6) Estimate the theoretical latency for functional stretch reflex responses to the increased load. 3. Reaction time 7) How does the EMG response differ from the previous experiment? Mark the voluntary response (V) in your recording and measure the latency. What different factors can influence and change the latency? 8) Describe the pathway for the signal underlying the voluntary response as detailed as you can. How does it differ from the reflex response (i.e. which new parts of the nervous system are activated)?