Download ELECTRIC CURRENTS

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 smalldiameter 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