Unit 8 Electricity Ii

Unit 8 Electricity II

1. Electrical signal transmission through nerves 1.1 Bioelectric potentials Bioelectric potentials are involved in nerve impulses, brain conductivity, heartbeat, muscle contraction, etc. They are caused by ionic activity in excitable cells, such as nerves and muscles. Bioelectric potentials can also be called signals, or electric voltages.

1.2 Nerve cells The nervous system controls the movement of the body. The neuron, or nerve cell (神經細胞, 神經元) is the basic component of nervous system. The nucleus of the nerve cell is located in the spinal cord. The part of the nerve cell which carries messages to the muscle is called the axon. The axon is a long fiber which is usually a few micrometers in diameter and a meter or longer in length. The axon is immersed in body fluid and is enclosed by a selectively permeable (semi-permeable) membrane. The body fluid is an electrolyte solution (like salt water), and contains sodium (Na+),

potassium (K+), and chloride (Cl-) ions. These ions are located both inside and outside of the cell. When the cell is at rest, K+ and Cl- can pass through its membrane easily while sodium ions cannot.

1.3 Resting potential When the cell is at rest, the sodium potassium pump pumps sodium out of the cell, and potassium into the cell. This is an active process. Therefore there is a high concentration of sodium outside the cell and a high concentration of potassium inside the cell. However, the cell membrane 1

is relatively impermeable to sodium, but quite permeable to potassium. Therefore, most of the sodium is kept outside the cell, but potassium can diffuse out of the cell. This is a passive process. This keeps the potential inside the cell negative compared to the outside of the cell. This cell voltage is called resting potential and is usually -70 mV to -90 mV. The cell is said to be polarized in its resting state.

1.4 Action potential When a voltage is applied to the nerve cell membrane, or when other external stimuli are applied to the nerve cell, the membrane becomes permeable to sodium. Sodium rushes into the cell, and the electric potential of the cell becomes positive compared to its environment. The potential reaches about +20 mV. The cell is now depolarized. The membrane then becomes less permeable to Na+ ions and more permeable to K+ ions. Potassium diffuses out of the cell to restore the electric potential (repolarization). The diffusion of sodium and then potassium causes changes in the membrane potential. This is called an action potential. After the action potential is over, the sodiumpotassium pump restores the sodium potassium gradient between inside and outside of the cell. The cell is now ready to respond to another stimulus.

2. The Electrocardiogram (ECG) The heart acts as double pump and consists of four chambers with valves. Blood from the body flows into the right atrium and then the right ventricle. Then, then blood is pumped to the lung. The oxygenated blood from the lung flows into the left atrium and then the left ventricle. Then it is pumped into the body. The heartbeat is synchronized and regulated by electrical impulses (stimuli). The heart beats as a single entity. The normal heartbeat rate is about 70 beats/min.

During each heartbeat, electrical stimuli spreads across the heart, causing potential differences between the polarized and depolarized cells. These potential differences appear collectively on 2

the surface of skin as electric voltages that can be measured by electrodes suitably placed on the skin. They are displayed as specific bioelectric signal patterns, known as electrocardiogram abbreviated ECG. Shown in the figure is a typical ECG pattern recorded from the surface of the body. It is made up of three parts: • The P-wave: representing the depolarization of the atria, causing them to contract. • The QRS-complex: corresponding to the depolarization and contraction of the ventricles. •

The T-wave: resulting from repolarization and relaxation of the ventricles.

The ECG is usually recorded on a chart or displayed on a cathode ray oscilloscope. The wave form measured at the body‘s surface depends not only on the individual person but also on the position of the electrodes. For a typical ECG pattern recorded on the surface of the body, peak amplitude values are as follows: P-wave: QRS-complex: T- wave:

0.2 mV 1 mV 0.1 to 0.5 mV

The shape of the signal at any one location will depend on the state of health of the heart muscles. Consequently the study of an ECG gives diagnostic information about the heart. Some common cardiac disorders that can be diagnosed with an ECG are: • High pulse rate (tachycardia) • Low pulse rate (bradycardia) • Ventricular fibrillation – irregular contraction of heart muscles • Damaged heart muscle – wave heights are reduced •

Heart blockage – part of the trace is missing

3. The defibrillator The heart is able to perform its important pumping function only through precisely synchronized action of the heart muscles. Under the action potentials the two auricles chambers contract and pump blood into two ventricles. After a delay time, the ventricles are synchronously activated to pump blood into body and lungs. If this synchronism is lost, it is called fibrillation. The condition

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of fibrillation is observed on an ECG as a very irregular trace. The fibrillation of ventricles can lead to death in a very short time. To stop defibrillation, two electrodes or paddles are placed on the skin one on each side of the heart. The potential difference across the electrodes is about 5000V and the energy stored in a 10.0 µ F capacitor of the defibrillator is order of 100J. The effect of the electric shock is to make all the heart muscles suffer a major contraction. This should then jolt the heart back to its normal rhythm.

4. The artificial pacemaker The pumping action of the heart is controlled by sino-atrial (SA) and atrio-ventricular (AV) nodes. For a resting adult, the heart has pulse rate of about 70 per minute. If the AV node become damaged, the heart does not stop but instead it goes into automatic control at a pulse rate of 30 per minute. This rate is sufficient to maintain life, but only if the person is very inactive. Artificial pacemakers can provide electrical pulses directly to the heart and replaces the action of the AV node. The pacemaker is inserted into the body and the wire is fed through a vein to appropriate position in the heart. The batteries powering the pacemaker last for several years. Early pacemaker produces pulses at a fixed rate of 70/min. Modern pacemakers can change their pulse rate, dependent on the oxygen demands of the body.

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An artificial pacemaker, in-situ, shown by X-ray imaging.

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5. Physiological effects of current Biological tissue contains free charge so that it is meaningful to consider it as an electrical conductor. Bound charges are also present in tissue so that dielectric properties also exist. These properties might arise as polarization of molecules in the tissue. The electrical conduction is the dominant factor when relatively low frequency (lower than 100 kHz) electric current is applied to the tissue. Electric current has three mains effects on the tissue; they are electrolysis, neural stimulation and heating. Neural stimulation is potentially the most dangerous effect, as the nervous system controls the two important systems: the circulation of blood and respiration. 5.1. Electrolysis Electrolysis will take place when a direct current (current with a frequency below 0.1 Hz) is passed through tissues which contain free ions. The positively charged ions will migrate to the negative electrode, and the negatively charged ions to the positive electrode. If two electrodes are placed on the skin, and a direct current of 100 µ A is passed beneath them for a few minutes, small ulcers will be formed beneath the electrodes. These ulcers take a very long time to heal. 10 µ A is the limit of safety for such dc current.

5.2. Neural stimulation At frequencies above 10 Hz, electrolysis appears to be reversible and the dominant effect is neural stimulation. The co-ordinated pumping activity of the heart can be disrupted by electric currents which pass through the heart. This is called fibrillation and can continue after the current is removed. Stimulation becomes increasingly difficult at frequencies above 1 kHz. There are two major ways of stimulation: indirect stimulation (stimulation through the skin), and direct stimulation (direct stimulation of the heart).

Indirect stimulation For main commercial supply frequencies (50-60 Hz), the threshold of the sensation is about 1 mA. A 5 mA current is the maximum harmless current above which sensory nerves are stimulated. At about 15 mA, the skeletal muscles will be stimulated to contract continuously, and it will not be possible to release an object held in the hands (can’t let go). As the current is further raised, it becomes increasingly painful, and difficult to breathe; at about 100 mA ventricular fibrillation begin. Currents up to 500 mA will 6

cause ventricular fibrillation which will continue after the current stops flowing, and burns will be caused by the heating of the tissue. At currents above 500 mA the heart will restart spontaneously after the current is removed-this is the principle of the defibrillator.

Direct stimulation The direct passage of current through heart causes ventricular fibrillation even the amount of current is as small as 100µ A, which is much smaller than the threshold current by indirect stimulation. 5.3. Tissue heating At frequency above 10 kHz, electrical currents will not stimulate nerves but they will cause heating through I2R. Radio frequency energy (usually 27 MHz) is applied between two metal plates so that part of the energy is absorbed in the tissue, this is known as diathermy. High frequencies between 400 kHz and 30 MHz are used in surgical diathermia/electrosurgery to give either coagulation or cutting.

6. Electrical safety Electromedical equipment is a possible source of hazards to the patient. In many cases the patients directly connected to the equipment so that a fault current may flow through the patient. The skin can have a resistance as high as 1 M Ω for dry skin and falling to 1k Ω for wet skin. Internally the body resistance is about 50 Ω. For a person with wet skin touching both terminal of a 240 V voltage source, the current would gives rise to I=V/R=240/2050=117mA, which is enough to cause ventricular fibrillation. Hints for the electric equipment safety: • Never overload circuits. • Check for UL (Underwriter’s Laboratory) approval. • Ensure that appliances are grounded. • Study instructions on use of equipment. • Disconnect appliances when not in use. • Inspect all electrical equipment regularly. • Keep wires, plugs, and equipment in good repair. Use one plug per outlet. • Avoid contact with water or metal when using electricity. • Keep cords off floor to avoid tripping. • Do not leave room when patient is connected to any electrical device. • Do not attempt to clean around electrical outlets while equipment is plugged in. • Disconnect appliances by pulling plug, not cord. • Do not attempt to repair electrical appliances unless you are qualified. • Do not step on or set objects on electrical cords. 7

Safety devices

Checklist Unit_8 • recall electric charge, electrostatic forces • recall the unit of charge and charge of electrons • Understand conductors and insulators • Recall the definition of electric potential difference • Recall capacitor: charge stored on a capacitor, energy stored on a capacitor, calculation of capacitance • Recall the definition of electric current • Application of Ohm’s law • Recall resistance and resistivity of resistors • Recall electric power and energy • Dc current and ac current, average power and peak power, effective (rms) and peak current or voltage • Recall resting/action potentials, related polarization /depolarization of a nerve cell • Describe the basic features of a typical ECG waveform • Know the basic electric safety precautions

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