2nd Lecture 2024.pdf

Mælitækni og lífsmörk 2024 The Origin of Biopotentials, part I Dr. Paolo Gargiulo, Professor All biological issues are related to electrical activities  Physiological Effects of Electricity Electrolysis: separation of a chemical compound using an electric current If a DC cu...

Mælitækni og lífsmörk 2024 The Origin of Biopotentials, part I Dr. Paolo Gargiulo, Professor All biological issues are related to electrical activities  Physiological Effects of Electricity Electrolysis: separation of a chemical compound using an electric current If a DC current is passed through body tissues for a period of minutes, ulceration begins to occur. Such ulcers, while not normally fatal, can be painful and take long periods to heal.  Physiological Effects of Electricity Burns: When an electric current passes through any substance having electrical resistance, heat is produced. The amount of heat depends on the power dissipated (I²R). Whether or not the heat produces a burn depends on the current density.  Muscle cramps Cramps are unpleasant, often painful sensations caused by muscle contraction or overshortening When an electrical stimulus is applied to a motor nerve or a muscle, the muscle contracts. The prolonged involuntary contraction of muscles (tetanus) causes fatigue  Respiratory arrest The muscles between the ribs (intercostal muscles) need to repeatedly contract and relax in order to facilitate breathing. Prolonged tetanus of these muscles can therefore prevent breathing.  Ventricular fibrillation The heart is a muscular organ, which needs to be able to contract and relax repetitively in order to perform its function as a pump for the blood. Tetanus of the heart musculature will prevent the pumping process. A domestic power supply voltage (110 or 230 V), 50 or 60 Hz AC current through the chest for a fraction of a second may induce ventricular fibrillation at currents as Ventricular low as 60 mA. fibrillation With DC, 300 to 500 mA is required. probability If the current has a direct pathway to the heart (e.g., via a cardiac catheter or other kind of electrode), a much lower current of less than 1 mA (AC or DC) can cause fibrillation The Effects of Current and Voltage on the Human Body  Let go current With sufficiently high current there can be a muscular spasm which causes the affected person to grip and be unable to release from the current source. The maximum current that can cause the flexors of the arm to contract but that allows a person to release his hand from the current's source is termed the let-go current. For DC, the let-go current is about 75 mA for a 70-kg man. For alternating current, the let go current is about 15 mA, dependent on muscle mass and frequency.  Electric shock vs. frequency Low-frequency (50- to 60-Hz) AC is used in US (60 Hz) and European (50 Hz) households; it can be more dangerous than high-frequency AC and is 3 to 5 times more dangerous than DC of the same voltage and amperage. Low- frequency AC produces extended muscle contraction (tetany), which may freeze the hand to the current's source, prolonging exposure. DC is most likely to cause a single convulsive contraction, which often forces the victim away from the current's source. Electric shock vs. frequency Physiological effects of electricity: Threshold or estimated mean values are given for each effect in a 70 kg human for a 1 to 3 s exposure to 60 Hz current applied via copper wires grasped by the hands.  Nervous system Central nervous system containing the brain and spinal cord, and the peripheral nervous system which is a network of nerves and neural tissues branching out throughout the body Neurons are the nerve cells, the structural and functional units of the nervous system The motor neurons are cells of the central nervous https://images.app.goo.gl/wDp5jXFjmC5UARJu9 system. Motor neurons transmit signals to muscle cells or glands to control their functional output. Sensory neurons do not have true dendrites. They are attached to sensory receptors and transmit impulses to the central nervous system, which then stimulate the interneurons, and then motor neurons. https://images.app.goo.gl/NqLcsVS9erQquGFP8  Biopotentials Biopotentials are electrical signals (voltages) that are generated by physiological processes occurring within the body. Biopotentials are produced by the electrochemical activity of a type of cell, called an excitable cell. Excitable cells are found in the nervous, muscular and glandular systems in the body. When an excitable cell is stimulated, it generates an action potential, which is the essential source of biopotentials in the body. Excitable cell types, that contribute to biopotential generation, are: https://images.app.goo.gl/NqLcsVS9erQquGFP8 1. Afferent (receptor or sensory) neurons that transmit signals from tissues and organs to the central nervous system 2. Efferent (motor) neurons that transmit signals from the central nervous system to effector cells 3. Effector cells, which include muscle cells and neuroendocrine cells, that instigate a physical effect on the basis of a received signal https://images.app.goo.gl/BcZY4pw7Af6Nwq4i8 4. Interneurons that exist entirely within the central nervous system, including the brain. Interneurons convey signals between afferent neurons and efferent neurons.  Membrane potential All the living cells are electrically neutral with having an equal number of positive and negative charges. The cells try their best to maintain this electrical neutrality. Although the cells are neutral, there exists a difference in the concentration of charges on the inner and outer sides of the cells. This difference in charges creates a potential difference across the membrane called the membrane potential. Some electrical potential exists across all types of cells present in the human body. Depolarization is the process by which potential difference is brought to zero by allowing certain ions to diffuse through the membrane. https://images.app.goo.gl/uG2iEVwC1SFF6iaFA The Resting State A neuron at rest is negatively charged: the inside of a cell is approximately 70 millivolts more negative than the outside (−70 mV, note that this number varies by neuron type and by species). This voltage is called the resting membrane potential; it is caused by differences in the concentrations of ions inside and outside the cell. If the membrane were equally permeable to all ions, each type of ion would flow across the membrane and the system would reach equilibrium. Because ions cannot simply cross the membrane at will, there are different concentrations of several ions inside and outside the cell  Sodium-Potassium Pump Keeping the cell at resting state requires active transport of ionic species against their normal electrochemical gradients. Sodium-potassium pump is an active transport that transports Na+ out of the cell and K+ into the cell in the ratio 3Na+ : 2K+. Energy for the pump is provided by a cellular energy source: 2.5 mmol/liter of K+ 140 mmol/liter of K+ adenosine triphosphate (ATP). 2K+ 3Na+ + - + - Electric Field + - + - External media Internal media https://images.app.goo.gl/4VgkBwf7y8vGe6Si8 Frog skeletal muscle membrane  Equilibrium Potential- Nernst Equation Ek = RT ln [K ]o = 0.0615 log10 [K ]o At 37 oC nF [K ]i [K ]i Where n is the valence of K+. So for all ions: RT ì PK [K ]o + PNa [Na ]o + PCl [Cl ]i ü E= ln í ý F î PK [K ]i + PNa [Na ]i + PCl [Cl ]o þ E: Equilibrium transmembrane resting potential, net current is zero PM : permeability coefficient of the membrane for ionic species M [M]i and [M]o : the intracellular and extracellular concentrations of M in moles/liter R: Universal gas constant (8.31 Joule/mol.oK) T: Absolute temperature in degrees Kelvin F: Faraday constant (96500 Coulomb per mol = e NA)  The Active State: the generation of an action potential Membrane at resting state is polarized (more negative inside the cell). Depolarization : lessening the magnitude of cell polarization by making inside the cell less negative. Hyperpolarization : increasing the magnitude of cell polarization by making inside the cell more negative. A stimulus that depolarizes the cell to a potential higher than the threshold potential causes the cell to generate an action potential. An action potential is defined as a sudden, fast, transitory, and propagating change of the resting membrane potential. Only neurons and muscle cells are capable of generating an action potential; that property is called the excitability. Action Potential: https://images.app.goo.gl/Hb59WUU5Fw8zK2LM9 - Typical firing rate: 1000 action potentials per second for nerves - All-or-none - DV = 120 mV for nerves  Action Potential Action potential travels at one direction. External medium Local closed (solenoidal) lines of current flow + + + + + + + + - - - - - - - + + + + + + + + - - - - - - - - + + + + + + + - - - - - - - - Active region Axon - - - - - - - - + + + + + + + - - - - - - - - + + + + + + + + - - - - - - - + + + + + + + + Resting Repolarized membrane membrane Direction of Depolarized propagation membrane Myelin sheath Active node Periaxonal space Axon + - Schwann Cell Node of Ranvier Myelination reduces leakage currents and improves transmission rate by a factor of approximately 20. https://images.app.goo.gl/xibFznpNoQUYxyzT9  Bioelectric Signals Bioelectrical signals are very low amplitude and low frequency electrical signals that can be measured from biological beings, for example, humans. Bioelectrics signals can measured with electrodes: Ions / Electrodes or skin-electrode interface ENG Electroneurogram, nerve activity EMG Electro-Myogram, Muscle movement ECG Electro-Cardiogram, Heart activity EOG Electro-Oculogram, Eye movement EEG Electro-Encephalogram GSR Galvanic Skin Response Measured with sensors / transducers: NTC (temperature sensor), LDR (photosensor), piezo-crystal, hall-sensor, Accelerometer, Goniometer …Breathing, temperature, movement etc.  Electroneurogram (ENG) An electroneurogram is a method used to visualize directly recorded electrical activity of neurons in the central nervous system (brain, spinal cord) or the peripheral nervous system (nerves, ganglions). The acronym ENG is often used. An electroneurogram is usually obtained by placing an electrode in the neural tissue. The electrical activity generated by neurons is recorded by the electrode and transmitted to an acquisition system, which usually allows to visualize the activity of the neuron. Parameters for diagnosing peripheral nerve disorder - Conduction velocity - Latency - Characteristic of field potentials evoked in muscle supplied by the stimulated nerve (temporal dispersion)  Electroneurogram (ENG): Conduction velocity Conduction velocity in a peripheral nerve can be measured by stimulating a motor nerve at two points a known distance (D) apart along its course. These are done through subtraction of shorter from longer latency, which gives the conduction time (L1-L2) Recording of these signals can be done using concentric needle electrode or surface electrodes. Diagnose motor neuron abnormality  Electroneurogram (ENG), conduction velocity Using the known distance, the conduction velocity can be obtained. D v= L1 - L2 Where, v : Velocity D : Distance between electrodes L1 : Time of longer latency L2 : Time of shorter latency  Sensory Nerve Field Potentials obtained by electrically stimulating sensory fibres and recording the nerve action potential at a point further along that nerve Desirable stimulus: i. 100V amplitude. ii. Duration of 100-300µs. Why do we need these characteristics? i. Excites large, rapidly conducting sensory nerve fibers. Ulnar nerve ii. Does not elicit pain fibers and surrounding muscle. Diagnose peripheral nerve disorders.  H-reflex (or Hoffmann's reflex) is a reflectory reaction of muscles M-wave early response, occurs 3-6 ms after the onset of after electrical stimulation of sensory fibers in their innervating nerves stimulation The H-reflex test is performed using an Stimulate popliteal nerve and record at triceps sural. electric stimulator, which gives usually a square-wave current of short duration and small amplitude and an EMG set, to record the muscle response. Low intensity stimulus, only high- amplitude H-wave observable. Medium intensity stimulus, both M- and H-wave of moderate amplitude can be seen. High intensity stimulus, only high- The H-waves are later responses amplitude M-wave observable.  Electromyogram, Skeletal muscle Skeletal muscle is organized functionally on the basis of the single motor unit (SMU). SMU is the smallest unit that can be activated by a voluntary effort where all muscle fibers or the unit are activated synchronously. SMU may contain 10 to 2000 muscle fibers, depending on the location of the muscle. Factors for muscle varying strength: 1. Number of muscle fibers contracting within a muscle 2. Tension developed by each contracting fiber  Muscle Fiber (Cell) http://www.blackwellpublishing.com/matthews/myosin.html  Junctional Transmission Synapses: intercommunicating links between neurons Neuromuscular junctions: communicating links between neurons and muscle fibers at end-plate region. Neuromuscular junction (20 nm thickness), releases neurotransmitter substance acetylcholine (Ach). Time delay due to junction is 0.5 to 1 msec Excitation-contraction time delay due to muscle contraction Neuron Muscle end-plate region At high stimulation rates, the mechanical response fuses the twitches into one continuous contraction called a tetanus (mechanical response summates).  Innervated-denervated muscles Healthy Denervated Vastus lateralis Vastus lateralis: 3 years denervated Courtesy of Prof. F. Protasi. CeSi Center for Research and Ageing, University G. D’Annunzio Chieti (Italy)  Electromyogram (EMG) Skeletal muscle functions via contraction of a motor unit. The motor unit can be activated by volitional effort, where all constituent muscle fibers are activated synchronously. In cross section, the motor unit are interspersed with fibers of other motor units. The muscle fibers of a single motor unit (SMU) constitute a distributed, unit bioelectric source located in a volume conductor. The volume conductor consists of all other active and inactive muscle fibers.  Electromyogram (EMG) Field potential of the active fibers of an SMU: 1- tri-phasic form 2- duration 3-15 msec 3- discharge rate varies from 6 to 30 per second 4- amplitude range from 20 to 2000 µV Surface electrodes record the field potential of surface muscles and over a wide area. Monopolar and bipolar insertion-type needle electrode can be used to record SMU field potentials at different locations. The shape of the SMU potential is considerably modified by disease, such as partial denervation. THE FIGURE SHOWS (a)Motor unit action potentials from normal dorsal interosseus muscle during progressively more powerful contractions. (b) In the interference pattern (c ), individual units can no longer be clearly distinguished. (d) Interference pattern during very strong muscular contraction. Time scale: 10 ms per dot. Data Analysis  Filtration In a first moment the filtration occurs when the signal is been collected. With the objective to avoid interferences the EMG signal passes through a 50 to 60 Hz filter (notch filter), if it’s necessary the EMG signal must pass through a pass-band; this pass- band frequency must depend on the aim of the study. Usually, this frequency is fixed between 10/20 and 450 Hz, because normally 80% of the muscular energy is concentrated  Rectification This procedure has the purpose to turn all the signal values integrative, submitting them to the cut of all negative values, that means, to delete the values that are under the baseline, or to turn all this negative values to positive adding the values, making them integrative.  Noise in EMG Signal: Smoothing The Smoothing procedure takes out the extremes of the signal, the parts that are considering noises. A way to understand what a noise is, is looking at it, the figure 1 under is an EMG signal with a closer look in the burst moment. Notice that the areas surrounded with black circles have a peculiar difference, it has a horizontal straight shaped line, which means that those parts don’t have a corresponding negative part, and so, it is considered a noise. Of course in this same image you can find some more of those, not only the surrounded ones, but the intention here is only to show how a noise appears inside an EMG signal.  Burst and silence The Burst moment is the muscle contraction moment, easily noticed by the sudden break in the baseline, and so, the Silence moment is when no contraction is occurring, so the signal stays at zero, or at least should stay. That’s another important reason to maintain a baseline close to zero; it is easier to separate the onset and the end of the Burst from the Silence moment.  Time windows The term “time windows” is used to determine the size of the cuts that will be made in the EMG signal for further analyzes. The most normal is to use the 1 second window, and in case of short tasks it can easily be done once that the signal is short and it is easy to separate the total task time in 1 second parts EMG spectral analysis