Medical Electronics Lec (5)-1 (PDF)

# The origin of bio-potentials ## Out-Lines 1. Basic Concepts of Medical Instrumentation. 2. Basic Sensors and Principles. 3. Origin of Biopotential and Biosignals. 4. Amplifiers and Signal Processing. 5. Biopotential Electrodes. 6. Biopotential Amplifiers. 7. Electrical Safety. ## The origin of b...

# The origin of bio-potentials ## Out-Lines 1. Basic Concepts of Medical Instrumentation. 2. Basic Sensors and Principles. 3. Origin of Biopotential and Biosignals. 4. Amplifiers and Signal Processing. 5. Biopotential Electrodes. 6. Biopotential Amplifiers. 7. Electrical Safety. ## The origin of bio-potentials ### Introduction Neurology (علم دراسة المخ والأعصاب) is the branch of medicine dealing with all aspects of the nervous system. - Electroencephalography (EEG) measurements can help are useful in the investigation of brain tumours and accidental damage to the brain. - Electromyography (EMG) is usually taken to include both the recording of electrical signals from muscle and the measurement of neural function. - Cardiology (علم دراسة وظائف القلب) deals with all aspects of cardiac function. - Electrocardiography (ECG/EKG) measurements describe the electrical signal of the heart muscle. ### Electrical Activity of Excitable Cells - The Nervous System - Resting Potential - Action Potential ## The Nervous System The nervous system includes three major components: - The brain - Nerves - Muscles The brain is supplied with information along sensory or afferent nerves such as heat, touch and pain. On the basis of the information received, the brain can make decisions and pass instructions down the motor or efferent nerves to produce an effect by causing muscles to contract. The human nervous system is a highly parallel system and there are many nerves in a single nerve trunk. **Example:** A mixed peripheral nerve might contain 30,000 nerve fibers. The basic component of both the brain and nerves is the neuron. The basic neuron consists of: - A cell body - Dendrites - An axon which is a long cylindrical structure arising from the cell body. The dendrites are considered as the means of information input to the cell, and the dendrites enable short-distance interactions with other cells. The axon is the channel for the output of information. The axon allows a cell to operate over a long distance. The cell body of the neuron may be within the brain or within the spinal cord such that the nerve axon might supply a muscle or pass impulses up to the brain. The brain itself is a collection of neurons which can interact electrically via the dendrites and axons like an electronic circuit. ## Electrical Activity of Excitable Cells Bioelectric potentials (Ionic voltages) are produced as a result of electrochemical activity of a certain class of cells, known as excitable cells, that are components of nervous, muscular, or glandular tissue. **Measurement:** Using transducers to convert ionic potentials into electronic potentials Electrically they exhibit a resting potential and an action potential when appropriately stimulated. A neuron is enclosed by a membrane which has properties in terms of ionic permeability such that the inside of the neuron is normally about $-40$ to $-90$ mV negative with respect to the outside An impulse is conducted along a nerve axon by means of a propagating wave front of depolarization followed by repolarization. ### The Resting State The resting potential of internal medium is -40 to -90 mV to the external medium. | Species | Intracellular | Extracellular | |---|---|---| | Na+ | 12 | 145 | | K+ | 155 | 4 | | Cl- | 4 | 120 | The cell membrane is a very thin (7-15 nm) lipoprotein complex that is essentially impermeable to intracellular protein and other organic anions. The properties of the membrane normally give rise to a high potassium ion concentration and low sodium ion concentration inside the nerve fiber, which results in a potential of about -80 mV between the inside and the outside of the fiber. The nerve is said to be polarized. The membrane potential in the polarized state is called resting potential, which is maintained until some kind of disturbance upsets the equilibrium. **Membrane and Nerve Conduction** - Measurement of the resting potential is made with respect to the potential of the surrounding extracellular body fluids. - **The Goldman-Hodgkin-Katz (GHK) formula:** $E = \dfrac{RT}{F} ln \left( \dfrac{P_K [K]_o+P_{Na} [Na]_o+P_{Cl} [Cl]_i}{P_K [K]_i+P_{Na} [Na]_i+P_{Cl} [Cl]_o} \right) \ (V)$ where: - $[K]_i$ and $[K]_o$, $[Na]_i$ and $[Na]_o$, $[Cl]_i$ and $[Cl]_o$ are the intracellular and extracellular concentrations of $K^+$, $Na^+$, and $Cl^-$ in moles per liter. - $R$ is the universal gas constant (8.31 J/mole.K) - $T$ is absolute temperature in K - $F$ is the Faraday constant 96500 C/(mole/valence) | Species | Intracellular | Extracellular | P (cm/sec) | |---|---|---|---| | Na+ | 12 | 145 | 2x10-8 | | K+ | 155 | 4 | 2x10-8 | | Cl- | 4 | 120 | 4x10-8 | - The Goldman-Hodgkin-Katz (GHK) formula: - $E_{Na} = \dfrac{RT}{F} ln\left( \dfrac{[Na]_o}{[Na]_i} \right) = +60 \ mV$ - $E_K = \dfrac{RT}{F} ln\left( \dfrac{[K]_o}{[K]_i} \right) = -85 \ mV$ - $E_{Cl} = \dfrac{RT}{F} ln\left( \dfrac{[Cl]_i}{[Cl]_o} \right) = -66 \ mV$ - Active transport mechanism (Sodium-Potassium Pump) ### The Action State When a section of the nerve membrane is excited, either by the flow of ionic current or by an externally supplied stimulus, the membrane characteristics change and begin to allow sodium ions to enter and potassium ions to leave the nerve axon. This causes the trans-membrane potential to change which, in turn, causes further changes in the properties of the membrane. This process is called depolarization and it results in the inside of the nerve becoming positive with respect to the outside; the process of depolarization is the beginning of a nerve action potential. Depolarization is not a permanent state because the properties of the semi-permeable membrane. The all-or-none property of the action potential means that the membrane potential goes through a very characteristic cycle: a change in potential from the resting level of a certain amount for a fixed duration of time. The action state: - Na+ channels close - K+ channels open - Sodium ions diffuse in - Depolarisation - Repolarisation - Potassium ions diffuse out - Na+ channels open - Refractory period - Diffusion through membrane - K+ channels close ## Applications ### Definitions - Electrocardiograph is the instrument. - Electrocardiogram is record. ### Electroencephalogram (EEG) - A record of the electrical activity of the brain - Electrical potentials 10 to 100 µV peak amplitude - Frequency response is 100 Hz - Measured with surface electrodes on the scalp and with needle electrodes just beneath the surface ### Electrocardiogram (ECG/EKG) - A record of the electrical activity of the heart - Electrical potentials is 0.1 to 4 mV peak amplitude - Frequency response is 100 Hz - Used to measure heart rate: Arrhythmia and abnormalities of the heart - Measured with electrodes at the surface of the body ### Electromyogram (EMG) - A record of muscle potentials - Electrical potentials: 0.05 mV to 1 mV - Frequency Response: 10 to 3000 Hz - Used as indicator of muscle action for measuring fatigue. - Measured with surface electrodes or needle electrodes penetrating the muscle fibers - Electroretinogram (ERG) is a record of potentials from شبكة العين the retina. - Electro-oculogram (EOG) is a record of corneal قنية retinal potentials associated with the eye movements.