MNB.17 Body Surface Potential Measurements PDF

Musculoskeletal System, Nervous System & Bioelectricity MNB.17 Body surface potential measurements INGMAR SCHOEN DAT E : 1 2 - N OV- 2 0 2 4 Learning outcomes At the end of this lecture, the learner will be able to List electrically active cells in the body. Explain how electrical potential...

Musculoskeletal System, Nervous System & Bioelectricity MNB.17 Body surface potential measurements INGMAR SCHOEN DAT E : 1 2 - N OV- 2 0 2 4 Learning outcomes At the end of this lecture, the learner will be able to List electrically active cells in the body. Explain how electrical potentials arise outside of electrically active cells and at the body surface. Explain the vectorial nature of measurements of the body’s electrical activity. Define and differentiate between EEG (electroencephalogram), EMG (electromyogram) and ECG (electrocardiogram). Briefly describe how an EMG is measured. Name applications of EMG for the diagnosis of neurological/neuromuscular problems and in management/rehabilitation of motor disability. Briefly describe how an EEG is measured using scalp electrodes and different montages. Name clinical applications of EEG including its role in the diagnosis of epilepsy. MNB.17 Body Surface Potential Measurements 2 Origin and measurement of extracellular potentials MNB.17 Body Surface Potential Measurements 3 Electrical activity of cells Definitions All cell types contain ion channels and establish a resting potential. A cell is called electrically active when the opening and closing of ion channels leads to a transient change of the membrane voltage, often called an action potential. Not all cells with ion channels can produce an action potential. MNB.17 Body Surface Potential Measurements 4 Nerve cells Neuronal action potential The opening of voltage-gated sodium channels and an influx of Na+ ions start the action potential. The opening of voltage gated potassium channels and the efflux of K+ ions stop the action potential. The duration of the action potential is short (0.5- 1 milliseconds). MNB.17 Body Surface Potential Measurements 5 Muscle cells and heart cells Skeletal muscle action potential Cardiac action potential The action potential of a muscle cell is very The AP of a cardiac cell is much longer. similar to a neuron, only slightly longer. Slow voltage-gated Ca2+ channels provide a continued influx of calcium ions which prolongs the depolarisation phase. MNB.17 Body Surface Potential Measurements 6 Extracellular electrical potentials What causes them? electrical Ions flowing across the membrane change the potential extracellular electrical potential on both sides. decreased Na+ An influx of positively charged ions makes the inside more positive but also leaves the outside less positive (=becomes more negative). Because ions can move freely through a larger increased volume around the cell, the change in electrical intracellular potential outside the cell is much smaller than inside and always of opposite sign. MNB.17 Body Surface Potential Measurements 7 Extracellular potential differences membrane voltage How can they be measured? The membrane voltage measures the difference mV between the electrical potential inside vs outside of the cell. In an analogue way, the difference between extracellular potentials at two spatial locations can be measured as a voltage. When all cells are at rest, there is no extracellular potential difference. Only when cells at one location depolarise, a extracellular voltage voltage signal arises. mV MNB.17 Body Surface Potential Measurements 8 Body surface potential measurements How are they actually measured? Wet electrode Dry electrode The extracellular space around cells has a low resistance. By placing electrodes on the skin, these electrodes can sense changes of the extracellular potential of the tissue even far away from the source causing them. MNB.17 Body Surface Potential Measurements 9  Correspondence between surface potentials vs intra-tissue electrical activity Example: EMG Signal recorded from Expanded time window surface electrodes electrodes within tissue MNB.17 Body Surface Potential Measurements 10 Vectorial nature of body surface measurements What is actually measured? The difference in extracellular potentials can be represented by a dipole, i.e. it has a direction. If the vector points in the direction of the 11 electrodes, the full amplitude is measured. If the vector forms an angle with the electrode direction, only the vector component parallel to the direction of the electrodes is detected. If the vector is at 90° to the electrode direction, no signal is measured. The sign also depends on the direction. MNB.17 Body Surface Potential Measurements Applications: measurements of muscle, brain or heart activity MNB.17 Body Surface Potential Measurements 12 Overview ElectroMyoGraphy (EMG) Measures the activity of myocytes during muscle contraction. ElectroEncephaloGraphy (EEG) Measures the activity of neurons in the brain. ElectroCardioGraphy (ECG) Measures the activity of cardiomyocytes during a heartbeat. (see MNB.18) MNB.17 Body Surface Potential Measurements 13 Relation between surface EMG, EEG, and ECG Commonalities and differences All techniques are noninvasive when using body surface electrodes. No current is applied. Endings ’-graphy’ (=the technique) and ’-gram’ (=the reading) are used interchangeably. Each signal (EMG, EEG or ECG) has characteristic patterns of different frequencies. EEG signals are much weaker than ECG or EMG signals. MNB.17 Body Surface Potential Measurements 14 Physiological basis of EMG measurements What activity is captured by an EMG? EMG = study of muscle function through analysis of the electrical signals controlling muscular contractions. When input signals from the motor neuron reach a depolarisation threshold, the muscle fibre produces an action potential. Excitation-contraction coupling leads to the contraction of the muscle fibre, while the action potential spreads along the muscle. The EMG signal is the algebraic summation of action potentials of all muscle fibres and motor units within the pick-up area of the electrodes being used. MNB.17 Body Surface Potential Measurements 15 Electrode positioning for EMG example Electrode positioning EMG signals reflect the local muscle activity between the two locations of the electrodes. The electrodes are placed over the muscle belly, i.e. to one side of the motor point. There are specific anatomical reference points for the correct placement of electrodes. MNB.17 Body Surface Potential Measurements 16 Measurement configuration of EMG Electrode configuration Two electrodes are placed along the direction of the muscle. Needle electrodes or surface electrodes are used. Optional: An additional reference electrode is placed further away. MNB.17 Body Surface Potential Measurements 17 Working principle of a differential amplifier The amplifier subtracts the voltage signal from one electrode from the other and amplifies the difference by a constant factor (from mV to ~1V). This subtraction cancels out contributions which are common to both electrodes. The reference electrode can help to further suppress background noise. MNB.17 Body Surface Potential Measurements 18 Analysis of EMG measurements Extraction of key parameters The raw EMG signal is processed by timing (within signal or computer programs to measure relative to movement) amplitude, duration, timing, integral, raw amplitude root mean square (RMS), frequency, processing etc. (example) integral duration MNB.17 Body Surface Potential Measurements 19 Interpretation of EMG: Amplitude Force of isometric contraction The amplitude correlates soft medium stiff with muscle tension during isometric contraction. Eccentric contractions produce lower amplitude than concentric contractions. For normalising amplitude measurements, often a control task is performed. MNB.17 Body Surface Potential Measurements 20 Interpretation of EMG: Frequency Muscle fatigue As the muscle fatigues, tension decreases despite constant or even larger EMG amplitude. There is a loss of the high-frequency component of the EMG signal as one fatigues, leading to a decrease in the mean/median frequency. MNB.17 Body Surface Potential Measurements 21 Interpretation of EMG: Conduction velocity Advanced measurements High density electrode arrays measure signals between many adjacent surface electrode pairs. Using this technique, it can be seen how the muscle action potential propagates along myofibrils, away MFCV = distance/time from the motor point. Used for measurement of muscle fibre conduction velocity (MFCV). MNB.17 Body Surface Potential Measurements 22 Applications of EMG Clinical diagnosis Physiotherapy Distinguish myopathic from Monitoring of rehabilitation progress, neurogenic causes of muscle i.e. regaining of muscle strength. weakness or loss. Kinesiology Movement analysis to optimise exercise effectiveness & physical performance. MNB.17 Body Surface Potential Measurements 23 Clinical (diagnostic) applications of EMG Spontaneous activity Examples: Relaxed muscles are usually silent. Fasciculation (visible): ‘muscle twitching’; Spontaneous discharges may arise from single discharge at irregular intervals. Due to incomplete relaxation (voluntary) or different problem in nerve endings. neuropathies (involuntary). Fibrillation (invisible): short recruitment of myofibril (6-10 Hz) as result of hypersensitivity after motor neuron degeneration or due to electrolyte imbalance. MNB.17 Body Surface Potential Measurements 24 Clinical (diagnostic) applications of EMG Reduced amplitude or MFCV Most neuromuscular disorders cause a reduction in electromechanical efficiency (=ratio between force and electrical activity). Example: Duchenne muscular dystrophy Complication: Reduced surface EMG signals can be caused by active changes (i.e. loss of myofibrils) or passive changes (i.e. increased thickness of skin fat layer). Solutions: use of MFCV (if established) or fine needle electrodes. MNB.17 Body Surface Potential Measurements 25 Physiotherapeutic applications of EMG Usage of EMG in rehabilitation Examples: Two main areas: Neurological or Muscular Assess muscle spasticity Functional changes based on muscle activation Regain motor coordination post-stroke data can monitor progress and inform (changes Rebuild muscle strength after sports injuries or to) the rehabilitation program. surgery (e.g. rotatory cuff) Increasing trend towards wearable devices Detect muscle fatigue to prevent sport injuries … rotatory cuff MNB.17 Body Surface Potential Measurements 26 Physiological basis of EEG measurements What activity is captured by an EEG? EEG = study of brain function through analysis of the electrical signals mediating information processing and action planning. Brain regions are specialised in different tasks. These tasks invoke the activity of ensembles of neurons in particular spatiotemporal patterns, currents resulting in brain currents. The EEG signal is the algebraic summation of action potentials of all nerve fibres within the pick-up area of the electrodes. MNB.17 Body Surface Potential Measurements 27 Electrode positioning for EEG Scalp electrodes Scalp electrodes are placed in a grid over the skull, aided by a cap Wet electrodes with conductive jelly are used for best contact Additional reference electrodes (ear clips, forehead) A = Ear lobe, C = central, Pg = nasopharyngeal, P = parietal, F = frontal, Fp = frontal polar, O = occipital MNB.17 Body Surface Potential Measurements 28 Measurement configuration of EEG Scalp electrodes The skull is a poor electrical conductor and attenuates the signal amplitude. Only extracellular potentials/dipoles from cortical neurons close to the skull are large enough to be measurable. Typical signal strength is 50 µV. For deeper areas and better spatiotemporal resolution (for Brain- Computer Interfaces), implantable electrodes have to be used. MNB.17 Body Surface Potential Measurements 29 Analysis of EEG measurements power Extraction of key parameters To cancel out background signals from ECG, EMG & other sources, ear or vertex (Cz) electrodes or a global average are used as reference, ears or forehead as ground. The raw EEG signal is processed by a Fourier transformation to extract characteristic frequencies and their relative strength. Measuring the strength and timing of the same signal by different electrodes allows to determine the strength and relative position of electrical activity. MNB.17 Body Surface Potential Measurements 30 Interpretation of EEG: Location Mapping of brain activity centers intentional & motivational to electrode locations emotional impulses rational sensory & motor functions perception & differentiation emotional processors memory functions primary visual areas MNB.17 Body Surface Potential Measurements 31 Interpretation of EEG: Frequency Classification vs cognitive activity levels Gamma (>35 Hz): motor function, heavy cognitive task Beta (13-35 Hz): normal waking state, concentration Alpha (8-13 Hz): relaxed, light meditation, closed eyes, conscious Theta (4-8 Hz): light sleep, deep meditation, hypnosis Delta (