Electricity Within the Body PDF

MEDICAL PHYSICS ELECTRICITY WITHIN THE BODY DR.EKHLAS JAWAD Electricity within The Body Physical phenomena involving electricity and magnetism have been observed since ancient times. Electricity plays an important role in medicine. There are two aspects of electricity...

MEDICAL PHYSICS ELECTRICITY WITHIN THE BODY DR.EKHLAS JAWAD Electricity within The Body Physical phenomena involving electricity and magnetism have been observed since ancient times. 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. Basically, a central computer (the brain) receives internal and external signals and (usually) makes the proper response. The nervous system and the neuron The nervous system can be divided into two parts: 1- The central nervous system. 2- The autonomic nervous system. The central nervous system consists of the brain, the spinal cord, and the peripheral nerves- nerve fibers (neurons) that transmit sensory information to the brain or spinal cord (afferent nerves) and nerve fibers (neurons) that transmit information from the brain or spinal cord to the appropriate muscles and glands (efferent nerves). The autonomic nervous system controls varies internal organs such as the heart, intestines, and glands. The control of the autonomic nervous system is essentially involuntary. Neuron: is the basic structure unit of nervous system. This is specialized for reception, interpretation & transmission of electrical signals or messages. MEDICAL PHYSICS ELECTRICITY WITHIN THE BODY DR.EKHLAS JAWAD Neuron consists of the followings: 1- Dendrites: Short, branched & un myelinated part of the neurons specialized for receiving electrical messages (signals) from other neurons towards the cell body. 2- Soma (Cell body: The cell body, or soma, contains the nucleus of the cell and its associated intracellular structures. Dendrites are specialized extensions of the cell body. They function to obtain information from other cells and carry that information to the cell body. 3- Axon (nerve fiber): Carries (propagates) electrical messages (signals) away from the cell body into the nerve terminals. Axons are ≈ 1 (m) long. 4- Axon terminals: Transmit electrical messages (information) from the neuron to muscles, glands or other neurons. 5- synapses (located on the dendrites or on the cell body): is the basic unit of communication in the brain. At each synapse, a chemical neurotransmitter is released from one cell and binds to receptors on the second cell. This chemical transmission generates electrical and biochemical signals in the second cells that are then passed along to a network of nerve cells. MEDICAL PHYSICS ELECTRICITY WITHIN THE BODY DR.EKHLAS JAWAD Electrical potentials of nerves A human body is combination of multiple cells. These cells are made up of different chemical substances like sodium, potassium, calcium, chloride etc. Cell membrane is used to separate cells from their environment i.e. Intracellular and extracellular. The intracellular environment is rich in potassium ions while extracellular environment has abundance of sodium and chloride ions. Hence, both environments are at different potential, therefore, a flow of ions results between the two environments creating a potential difference. This potential difference is called bio potential. The bio-potential can be measured using electrodes, amplified using instrumentation amplifiers and monitored to study functioning of various organs like heart, brain, eye, muscles etc. There are two types of bio potential: 1-Resting potential 2- Action potential Across the surface or membrane of every neuron is an electrical potential (voltage) difference due to the presence of more negative ions on the inside of the membrane than on the outside. 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. 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 transmission of signals within the body.  MEDICAL PHYSICS ELECTRICITY WITHIN THE BODY DR.EKHLAS JAWAD Electrical signals from muscles (EMG) The record of the potentials from muscles during movement is called the electromyogram, or EMG. A muscle is made up of many motor units. A motor unit consists of a single branching neuron from the brain stem or spinal cord and the 25 to 2000 muscle fibers (cells) it connects to via motor end plates. 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. Or a concentric needle electrode inserted under the skin measures single motor unit. MEDICAL PHYSICS ELECTRICITY WITHIN THE BODY DR.EKHLAS JAWAD Action of EMG: 1 – Action potential appears in EMG after a latency period (Time between stimulation and the beginning of the response). 2 – EMGs of symmetrical muscles of the body are compared to each other, or to those of normal individuals, to determine the action potential & latency periods. 3 – Determining the velocity of the action potential in motor nerves many times, nerve damage may resulting decrease in velocity. Electrical Signals from the Brain – EEG 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. The recording of the signals from the brain is called the electroencephalogram or EEG. Electrodes for recording the signals 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. The amplitude of the EEG signals is low (about 50μV), and interference from external electrical signals often causes serious problems in EEG signal processing.  Medical Physics Electricity Within The Body Dr.Ekhlas jawad Even if the external noise is controlled, the potentials 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: As a person becomes drowsy, particularly with his eyes closed, the frequencies from 8 to12 Hz (alpha wave) dominate the EEG. The amplitude increases and the frequency decreases as a person moves from light sleep to deeper sleep. Occasionally an EEG taken during sleep shows a high frequency pattern called paradoxical sleep or rapid eye movement (REM) sleep because the eyes move during this period. Paradoxical sleep appears to be associated with dreaming.  Medical Physics Electricity Within The Body Dr.Ekhlas jawad The sensorimotor rhythm (SMR) is a brain wave. It is an oscillatory idle rhythm of synchronized electric brain activity. It appears in spindles in recordings of EEG, MEG, and ECG over the sensorimotor cortex. Besides recording the spontaneous activity of the brain, we can measure the signals that result when the brain receives external stimuli such as flashing lights or pulses of sound. Signals of this type are called evoked responses. The EEGs show responses to the first few pulses and the last two pulses. The lack of responses in between is called habituation. MEDICAL PHYSICS ELECTRICITY WITHIN THE BODY DR.EKHLAS JAWAD Electrical Signals from the Eye: I – Electroretinogram ERG: The recording of potential changes produced by the eye when retina is exposed to aflash of light is called ERG. One electrode is located in a contact lens that fits over the cornea & the other oneattached to the ear or forehead to approximate the potential at the back of eye. * ERG is more complicated than a nerve axon signal because it’s the sum of manyeffects taking place within the eye. B wave is the most interesting clinically since it arise in the retina.Its absent in the ERG of patients with inflammation of the retina. II – Electrooculogram EOG: It’s the recording of potential changes due to eye movement. * A pair of electrodes is attached near the eye. * EOG provides. 1 – Information on orientation of eye. 2 – Its angular velocity & its angular acceleration. 8 MEDICAL PHYSICS ELECTRICITY WITHIN THE BODY DR.EKHLAS JAWAD Electrical Signals From the Heart –{Electrocardiogram} ECG: Our heart's electrical system controls all the events that occur when the heart pumps blood. The electrical system also is called the cardiac conduction system. The heart test called an ECG (electrocardiogram) is a graphical picture of the heart's electrical activity. Your heart's electrical system is made up of three main parts:  The Sinoatrial (SA) node, located in the right atrium of your heart.  The atrioventricular (AV) node, located on the interatrial septum close to the tricuspid valve.  The His-Purkinje system, located along the walls of your heart's ventricles. 9 MEDICAL PHYSICS ELECTRICITY WITHIN THE BODY DR.EKHLAS JAWAD A heartbeat is a complex cycle of electrical conductive events. A heartbeat is a single cycle in which the heart's chambers relax and contract to pump blood. This cycle includes the opening and closing of the inlet and outlet valves of the right and left ventricles of the heart. Each heartbeat has two basic parts: diastole and systole. During diastole, the atria and ventricles of your heart relax and begin to fill with blood. At the end of diastole, the heart's atria contract (atrial systole) and pump blood into the ventricles. The atria then begin to relax. Heart’s ventricles then contract (ventricular systole), pumping blood out of the heart. Heartbeat Conduction Cycle (1) The Sinoatrial (SA) node and the remainder of the conduction system are at rest. (2) The SA node initiates the action potential. The electrical signal from SA node initiate depolarization of nerves & muscles of both Atria causing them contraction & pumping blood into ventricles, Repolarization of atria follows. (3) After reaching the atrioventricular node, there is a delay of approximately 100 ms that allows the atria to complete pumping blood before the impulse is transmitted to the atrioventricular bundle. (4) Following the delay, the impulse travels through the atrioventricular bundle and bundle branches to the Purkinje fibers, and also reaches the right papillary muscle via the moderator band. (5) The impulse spreads to the contractile fibers of the ventricle. (6) Ventricular contraction begins& force blood into the two systems, then ventricles are repolized & so forth. 10 MEDICAL PHYSICS ELECTRICITY WITHIN THE BODY DR.EKHLAS JAWAD The cells of the SA node at the top of the heart are known as the pacemaker of the heart.The normal heart rate at rest ranges between 60 and 100 beats per minute (average 72 beat /min.). Your heart rate can adjust higher or lower to meet your body's needs. Your brain and other parts of your body send signals to stimulate your heart to beat either at a faster or a slower rate. Although the way all of the chemical signals interact to affect your heart rate is complex, the net result is that these signals tell the SA node to fire charges at a faster or slower pace, resulting in a faster or a slower heart rate. For example, during periods of exercise, when the body requires more oxygen to function, signals from your body cause your heart rate to increase significantly to deliver more blood (and therefore more oxygen) to the body. Your heart rate can increase beyond 100 beats per minute to meet your body's increased needs during physical exertion. Similarly, during periods of rest or sleep, when the body needs less oxygen, the heart rate decreases. Changes in your heart rate, therefore, are a normal part of your heart's effort to meet the needs of your body. 11 MEDICAL PHYSICS ELECTRICITY WITHIN THE BODY DR.EKHLAS JAWAD Graphical ECG recording on paper In the normal ECG waveform the P wave represents atrial depolarization, the QRS complex ventricular depolarization and the T wave ventricular repolarization. The P - R Interval is taken from the start of the P wave to the start of the QRS complex. The Q - T interval is taken from the start of the QRS complex to the end of the T wave. This represents the time taken to depolarize and repolarize the ventricles. The S - T segment is the period between the end of the QRS complex and the start of the T wave. All cells are normally depolarized during this phase. 12 MEDICAL PHYSICS ELECTRICITY WITHIN THE BODY DR.EKHLAS JAWAD ECG Normal values P - R interval 0.12 - 0.2 seconds (3-5 small squares of standard ECG paper). QRS complex duration less than or equal to 0.1 seconds (2.5 small squares). Q - T interval corrected for heart rate (QTc) QTc = QT/ RR interval less than or equal to 0.44 seconds. Measuring ECG: Notice:- 1- The potentials measured on the surface of the body depend on the location of the electrodes. 2- Surface electrodes for obtaining ECG are most commonly located in left arm (LA), right arm (RA), & left leg (LL). 3- ECG graphing contains 12 sections :- (6) Of them are in the frontal plane. The other (6) are in the transverse plane. In each section an electrode is located. Surface Electrodes in the Frontal Plane:- Lead I - measures the potential difference between the right arm electrode and the left arm electrode. The third electrode (left leg) acts as neutral. Lead II - measures the potential difference between the right arm and left leg electrode. Lead III - measures the potential difference between the left arm and left leg electrode * The potential between any two electrodes gives the relative amplitude & direction of the electric dipole vector in the frontal.. 13 MEDICAL PHYSICS ELECTRICITY WITHIN THE BODY DR.EKHLAS JAWAD The Rest Augmented lead configurations aVR: one side of the electrodes connect to RA the other connect to the center of the resistance between LL & LA aVL : one side of the electrodes connect to LA& the other connect to the center of the resistance between LL & RA aVf : one side of the electrodes connect to LL and the other connect to the center of the resistance between RA & LA The six chest electrodes:  V1 - placed in the 4th intercostal space, right of the sternum  V2 - placed in the 4th intercostal space, left of the sternum  V3 - placed between V2 and V4  V4 - placed 5th intercostal space in the nipple line. Official recommendations are to place V4 under the breast in women.  V5 - placed between V4 and V6  V6 - placed in the mid axillary line on the same height as V4 (horizontal line from V4, so not necessarily in the 5th intercostal space) 14 MEDICAL PHYSICS ELECTRICITY WITHIN THE BODY DR.EKHLAS JAWAD The ECG can be used to diagnose problems with the heart which include: Arythmia, an irregular pumping pattern which is quite common in young people, increased by exercise. Blockage of part of the blood supply for the heart. This can lead to the heart muscle getting tired, or in extreme cases death of the heart muscle, myocardial infarction (heart attack). Fibrillation in which there is no co-ordinate pumping activity, which will lead to death if not, treated very quickly. 15