ELECTRIC CURRENTS BY B.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 current Muscle conducts electric current effectively Muscle- 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 communication Within 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

+++++++++++++ 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