electric currents by b.nelson. aims of presentation outline the basic principles of electricity...
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ELECTRIC CURRENTS
BYB.Nelson
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Aims of Presentation
• Outline the basic principles of electricity• Explain the physiological responses to
electricity
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What is electric current ?
• Net movement of electrons through a conductor
• Electrons are particles of matter which are negatively charged and have a small mass
• Electrons will move from an area of high potential to an area of lower potential
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Components of electrical current
• Ampere(A) : unit of measure that indicates the rate at which electrical current is flowing
• Coulomb :the number of electrons flowing per second
• 1 ampere = 1 coulomb/ 1 second
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Components of electrical current
• Conductor : materials that permit free movement of electric current (metals)
• Insulators : materials that resist the flow of electric current (wood , glass)
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Basic Types of current
• Direct current (DC/galvanic)
• Alternating current (AC)
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Types of current cont’d
• Direct current :unidirectional flow of electrons from the negative to the positive poles,e.g batteries
• Alternating Current: continuous bidirectional flow of electrons from negative to positive poles ,e.g outlets in the home produce AC
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Waveforms
• Indicates a graphic representation of the shape ,direction ,amplitude ,duration and frequency of the current produced by the device
• Electrical currents may take on a Sinusoidal,• Rectangular, square or spiked waveform
shape
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Waveforms Cont’d
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Components of electrical current
• Pulse amplitude : reflects the intensity of the current and is measured in amperes(amp/A)
• Pulse duration :the length of time current is flowing in one cycle
• Pulse frequency: number of pulses or cycles per second and is measured in Hertz (Hz)
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Current flow through biologic tissue
• Flows through the path of least resistance• Tissue with highest water content & ion
content best conductor of electricity• Skin is considered an insulator as it offers
primary resistance to currentMuscle conducts electric current effectivelyMuscle- tendon unit and fat are poor
conductors
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Physiological responses to electrical currents
• Electrical currents are used to :• Produce muscle contractions• Modification of pain through effects on motor
and sensory nerves
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Effects of electrical impulses
• Nerve Depolarisation• Muscle depolarisation• Ionic effects of electrical currents
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Nerve Depolarisation
• Action potential : basic unit of communicationWithin the nerve Strength – duration curve: a graphic
representation of quantity of current required to produce an action potential in a nerve
SDC varies for different types of nerves
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• Sensory nerves: low current amplitude/intensity and short pulse durations can depolarise- less than 80 microseconds
• Motor nerves : high current amplitude/intensity and longer pulse duration cad depolarise- 150- 300 microseconds
• Pain transmitting C –fibres: higher current intensity and longer pulse duration- 10 milliseconds and over
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• Longer pulses of 10ms are required to depolarise muscle cell of denervated muscle
• This type of stimulation stimulates the A-delta and C-fibres – when present
• Uncomfortable for patient
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• Rheobase: minimum current ampitude/intensity with long duration required to produce an action potential
• Chronaxie: minimum time to stimulate the tissue at twice the rheobase intensity
• Chronaxie and rheobase are used in electromyography
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• Electromyography ; analysing and recording electric activity in skeletal muscles
• Used to diagnose neuropathies, myopathies ect
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Accommodation
• `process where the nerve becomes less responsive to stimulation
• Action potential travels faster in large –diameter myelinated nerves than in small- diameter unmyelinated fibres
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Muscle depolarisation
• Innervated muscle contract due to depolarisation of motor nerves
• Denervated muscles contract due to depolarisation of muscle cell membrane
• Denervated muscles require longer pulse duration (> 10 ms) to produce muscle contraction
• Muscles do not accommodate
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Ionic effects of electrical currents• Cathode (neg. electrode)attracts positively
charged ions, while the anode(+ electrode) attracts negative electrode
Iontophoresis : process where direct current is used to facilitate transdermal drug penetration
• Used to treat during the inflammatory stages, facilitate tissue healing and decrease oedema