Lecture Seven: Electricity within the body PDF

Dr. Nisreen Khalid Fahad Medical Physics Lecture Seven Electricity within the body Electricity plays an important role in medicine. There are two aspects of electricity and magnetism in medicine: - 1)...

Dr. Nisreen Khalid Fahad Medical Physics Lecture Seven Electricity within the body Electricity plays an important role in medicine. There are two aspects of electricity and magnetism in medicine: - 1) Electrical and magnetic effects generated inside the body. 2) Applications of electricity and magnetism to the surface of the body. The electricity generated inside the body serves for the control and operation of nerves, muscles, and organs. The nervous system plays a fundamental role in nearly every body function. A central computer (the brain) receives internal and external signals and (usually) responds properly. -The nervous system can be divided into two parts: 1) The central nervous system consists of the brain, the spinal cord, and the peripheral nerves. The nerve fibers are of two types: A) Afferent nerves transmit sensory information to the brain or spinal cord. B) Efferent nerves transmit information from the brain or spinal cord to the appropriate muscles and glands. 2) The autonomic nervous system controls various internal organs (heart, intestine, glands etc). The control of autonomic nervous system is essentially involuntary. 1 Dr. Nisreen Khalid Fahad Medical Physics Neuron is the basic structural unit of nervous system. It is specialised for reception, interpretation and transmission of electrical messages. Neuron is basically composed of: 1) The cell body receives the signal from other neurons through contacts called "synapses" which are located on the dendrites or the cell body. 2) Dendrites: are part of neurons specialized for receiving information from stimuli or other cells. 3) Axon (or nerve fibers): This carries the electrical signal to muscles, glands, or other neurons. It's usually covered by a myelin sheath, except some parts called "Nodes of Synapse: which permit the transparency of a signal in one direction and prevent it from going back. As shown in Fig. (1). 2 Dr. Nisreen Khalid Fahad Medical Physics Electrical potentials of nerves Resting Potential of the Neuron The cell membrane divides the intracellular and extracellular regions, in neurons and other cells. There are Na+, K+, Cl−, negatively-charged proteins, and other charged species both in the neurons (intracellular) and in the extracellular medium. Across the surface or membrane of every neuron, there is an electrical potential (voltage) difference due to the presence of more negative ions inside the membrane than in the extracellular fluid. The neuron is said to be polarized. The inside of the cell is typically 60 to 90 mV more negative than the outside. This potential difference is called the resting potential of the neuron. The Action Potential (electrical signal transmission) When the neuron is stimulated, a large momentary change in the resting potential occurs at the point of stimulation. This potential change called the action potential propagates along the axon. The action potential is the major method of signals transmission within the body. 3 Dr. Nisreen Khalid Fahad Medical Physics Suppose an axon has a resting potential of about -80 mV. If the left end of the axon is stimulated, the membrane walls become porous to Na+ ions and these ions pass through the membrane, causing it to depolarize. The inside momentarily goes positive to about 50mV. The reversed potential in the stimulated region cases ions movement which in turn depolarizes the region to the right. Meanwhile the point of original stimulation has recovered (repolarized) because K+ ions have moved out to restore the resting potential. Types of Nerve Fibers There are two different types of nerve fibers: 1) Myelinated nerves: the membranes of their axons are covered with a fatty insulating layer called myelin and have small uninsulated gaps called nodes of Ranvier every few millimeters. Myelinated nerves conduct action potentials much faster than unmyelinated nerves. The myelinated segments of their axons have very low electrical capacitance. Myelinated fibers have outer radii of 0.5–10 μm and a conduction speed of u (in m/s) ≈ 12(a + b) ≈ 17a, where (a) is the radius of the axon 4 Dr. Nisreen Khalid Fahad Medical Physics (in μm) and (b) is the myelin sheath thickness (in μm). The spacing between the nodes of Ranvier is ≈280a. 2) Unmyelinated nerves: Approximately 2/3 of the axon fibers in the body are unmyelinated. They have radii of 0.05–0.6 μm and a conduction speed of u (in m/s) ≈ 1.8 √a, where (a) is the radius of the axon (in μm). The axons of unmyelinated nerves have no myelin sheath. -Factors Affecting the Propagation Speed of Action Potential Two primary factors affect the speed of propagation of the action potential: 1) The resistance within the core of the membrane 2) The capacitance (or the charge stored) across the membrane.  A decrease in either (resistance or capacitance) will increase the propagation velocity.  The internal resistance of an axon decreases as the diameter increases, so an axon with a large diameter will have a higher velocity of propagation than an axon with a small diameter.  The greater the stored charge (the capacitance) the longer it takes to depolarize it and thus the slower the propagation speed. Electrical Signals from Muscles  One means of obtaining diagnostic information about muscles is to measure their electrical activity. The record of the potentials from muscles during movement is called the electromyogram or EMG.  Motor unit consists of a single branching neuron (from brain stem or spinal cord) and 25 to 2000 muscle fibers (cells) which are connected together by motor end plates. 5 Dr. Nisreen Khalid Fahad Medical Physics  EMG can be obtained from single or many motor units that are stimulated electrically.  The resting potential across the membrane of a muscle fiber is similar to the resting potential across a nerve fiber. Muscle action is initiated by an action potential that travels along an axon and is transmitted across the motor end plates into the muscle fibers, causing them to contract.  Single muscle cells are usually not monitored in an EMG examination because it is difficult to isolate a single fiber. Instead, EMG electrodes usually record the electrical activity from several fibers. 1) Either a surface electrode attached to the skin measures the electrical signals from many motor units. 2) A concentric needle electrode inserted under the skin measures single motor unit activity using insulated wires connected to its point.  In EMG of a motor unit, an electrical stimulation is applied and the response will appear after a latency period. The velocity of the action potential in the motor nerve can also be determined. Stimuli are applied at two locations, and the difference in latency period between the two responses is the time for action potential to travel. The velocity of action potential is the distance divided by time. 6 Dr. Nisreen Khalid Fahad Medical Physics  EMG can also be obtained for sensory nerves. The conduction velocity for sensory nerves can be measured by stimulating at one site and recording at several locations that are known distances from the point of stimulation.  Typical velocities are 40 to 60m/sec, and below 10 m/sec indicate a problem. Electrical Signals from the Heart (the electrocardiogram) The rhythmical action of the heart is controlled by an electrical signal initiated by spontaneous stimulation of special muscle cells located in the right atrium. These cells make up the sinoatrial (SA) node, or the pacemaker. The SA node fires at regular intervals about 72 times per minute. The electrical signal from the SA node initiates the depolarization of the nerves and muscles of the atria, causing the atria to contract and pump blood into the ventricles. Repolarization of the atria follows. The electrical signals then pass into the atrioventricular (AV) node, which initiates the depolarization of the right and left ventricles, causing them to contract and force blood into the pulmonary and general circulations. The ventricle nerves and muscles then repolarize and the sequence begins again. 7 Dr. Nisreen Khalid Fahad Medical Physics Obviously it is not practical to make direct electrical measurements on the heart; diagnostic information is obtained by measuring at various places on the surface of the body the electrical potentials generated by the heart. The record of the heart's potentials on the skin is called the electrocardiogram or ECG. The surface electrodes for obtaining the ECG are most commonly located on the left arm (LA), right arm (RA), and left leg (LL), although the location of the electrodes can vary in different clinical situations; sometimes the hands or positions closer to the heart are used. The standard limb leads: - These leads are obtained in the frontal plane. 1) The measurement of the potential between RA and LA is called Lead I. 2) The measurement of the potential between RA and LL is called Lead II. 3) The measurement of the potential between LA and LL is called Lead III. 8 Dr. Nisreen Khalid Fahad Medical Physics The augmented lead configurations: - These leads are also obtained in the frontal plane. 1) aVR lead, one side of the recorder is connected to RA and the other side is connected to the center of two resistors connected to LL and LA. 2) aVL lead, one side of the recorder is connected to LA and the other side is connected to the center of two resistors connected to LL and RA. 3) aVF lead, one side of the recorder is connected to LL and the other side is connected to the center of two resistors connected to LA and RA. Six precordial, unipolar chest leads: In a clinical examination, six transverse plane ECGs are usually made in addition to the six frontal plane ECGs. Electrode is moved across the chest wall to the six different positions. (V1, V2, V3, V4, V5, V6). 9 Dr. Nisreen Khalid Fahad Medical Physics The major electrical events of the normal heart cycle are: 1) The atrial depolarization, which produces the P wave. 2) The atrial repolarization, which is rarely seen and is unlabeled. 3) The ventricular depolarization, which produces the QRS complex. 4) The ventricular repolarization, which produces the T wave. Electrical signals from the brain If you place electrodes on the scalp and measure the electrical activity, you will obtain some very weak complex electrical signals. These signals are due primarily to the electrical activity of the neurons in the cortex of the brain. 10 Dr. Nisreen Khalid Fahad Medical Physics The recording of the signals from the brain is called the electroencephalogram or EEG. Electrodes for recording the signals are often small discs of chlorided silver. They are attached to the head at locations that depend upon the part of the brain to be studied. The reference electrode is usually attached to the ear (A1 or A2). Since asymmetrical activity is often an indication of brain disease, the right side signals are often compared to the left side signals. Interference from external electrical signals often causes serious problems in EEG signal processing. Even if the external noise is controlled, the potential of muscle activity such as eye movement can cause artifacts in the record. The frequencies of the EEG signals seem to be dependent upon the mental activity of the subject. For Example: - 1) A relaxed person usually has an EEG signal composed primarily of frequencies from 8 to 13 Hz, or alpha waves (α). 2) When a person is more alert a higher frequency range, the beta wave (β) or fast range (above 13Hz). 3) Delta (δ) or slow (0.5 to 3.5 Hz). 4) Theta (θ) or intermediate slow (4 to 7 Hz). 11 Dr. Nisreen Khalid Fahad Medical Physics Electrical signals from the eye The recording of potential changes produced by the eye when the retina is exposed to a flash of light called the electroretinogram or ERG. One electrode is located in a contact lens that fits over the cornea and the other electrode is attached to the ear or forehead to approximate the potential at the back of the eye. An ERG signals is more complicated than a nerve axon signal because it is the sum of many effects taking place with the eye. The B wave is the most interesting clinically since it arises in the retina. The B wave is absent in the ERG of a patient with inflammation of the retina. 12 Dr. Nisreen Khalid Fahad Medical Physics The electrooculogram or EOG is the recording of potential changes due to eye movement. For this measurement, a pair of electrodes is attached near the eye. Electrooculograms provide information on the orientation of the eye, its angular velocity, and its angular acceleration. Magnetic signals from the heart and brain Since a flow of electrical charge produces a magnetic field, a magnetic field is produced by the current in the heart during depolarization and repolarization. The recording of the heart's magnetic field is the magnetocardiogram or MCG (Magnetocardiography measures these very weak magnetic fields around the heart). The magnetic field around the heart is about 5x10-11 tesla (T), or about one-millionth of the earth's magnetic field. To measure fields of this size it is necessary to use magnetically shielded rooms and very sensitive magnetic field detectors (magnetometers). One such detector is called a SQUID (Superconducting QUantum Interference Device). 13 Dr. Nisreen Khalid Fahad Medical Physics The SQUID magnetometer has also been used to record the magnetic field surrounding the brain. The recording of this field is called the magnetoencephalogram or MEG. The magnetic field from the brain is about 1x10-13 tesla (T). This is almost one billionth of the earth's magnetic field. Not all magnetic fields produced within the body are due to ion currents; the body can be easily contaminated with magnetic materials. For Example, asbestos workers inhale asbestos fibers, which contain iron oxide particles. The size of the magnetic field from the iron oxide in a worker's lungs can be used to estimate the amount of inhaled asbestos dust. Typical magnetic fields from asbestos workers' chests are about one-thousandth of the earth's magnetic field (5x10-8T). 14