ELECTRIC CURRENTS

BYB.Nelson

Aims of Presentation

• Outline the basic principles of electricity• Explain the physiological responses to

electricity

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

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

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)

Basic Types of current

• Direct current (DC/galvanic)

• Alternating current (AC)

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

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

Waveforms Cont’d

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)

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

Physiological responses to electrical currents

• Electrical currents are used to :• Produce muscle contractions• Modification of pain through effects on motor

and sensory nerves

Effects of electrical impulses

• Nerve Depolarisation• Muscle depolarisation• Ionic effects of electrical currents

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

• 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

• 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

• 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

• Electromyography ; analysing and recording electric activity in skeletal muscles

• Used to diagnose neuropathies, myopathies ect

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

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