Chapter 9. Principles of Electricity
for Electrotherapy (Part C)
© 2008 LWW
Tissue Responses to Electrical
Stimulation
• Four basic responses in the tissue:
–
–
–
–
Chemical
Thermal
Magnetic
Kinetic
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Chemical Effects
Driving ions (of medication) into the body
– Positively charged ions move to the negative pole
(or cathode).
– Negatively charged ions move to the positive pole
(or anode).
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LeDuc’s Potato Experiment
• Cut a hole in a potato
• Filled it with a potassium iodine
solution, which was partially
dissociated into positive potassium
ions and negative iodine ions
• Stuck wires into either end and
attached them to a battery
• As the DC current passed through
the potato, ions were attracted to
the two poles.
– The positive potassium ion to the
negative pole and the negative
iodine ion to the positive pole
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LeDuc’s Potato Experiment
(cont.)
• Illustrates chemical
effect
– The iodine ions of the
potassium iodine solution
migrated to the end of the
potato with the positive
pole.
– The iodine interacted with
the starch near the
positive pole to form blue
starch iodine.
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LeDuc’s Rabbit Experiment
• Placed two rabbits in series of electrical
circuits so current passed through both
rabbits
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LeDuc’s Rabbit Experiment
(cont.)
• Electrode A attached to first
rabbit, with electrode pad
soaked in strychnine sulfate
solution
• Electrode B attached to
second rabbit with electrode
pad soaked in potassium
cyanide solution
• Electrical line between
rabbits attached to electrode
pads soaked in water only
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LeDuc’s Rabbit Experiment
(cont.)
• When electrode A
was negative and
electrode B was
positive, the rabbits
felt the current but
were fine.
• When the electrodes
were reversed, both
rabbits died.
• Why?
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LeDuc’s Rabbit Experiment
(cont.)
• When A was negative
and B was positive
– A attracted the positive
strychnine and repelled
the negative sulfate.
– B attracted the negative
cyanide and repelled
the positive potassium.
– Thus current passed
through the rabbits but
the poisons did not.
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LeDuc’s Rabbit Experiment
(cont.)
• When A was positive
and B was negative
– A attracted the
negative sulfate and
repelled the positive
strychnine.
– B attracted the
positive potassium
and repelled the
negative cyanide.
– Thus current passed
through rabbits and
drove the poisons into
them.
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Requirements for Ion
Migration: Chemical Effects
• Must have continuous monophasic DC
electron flow to cause ion migration.
• Moving electrons against gradient
– Like pushing a car uphill
– When you pause, it rolls back down.
• Why does a twin-pulse high-volt current not
produce a chemical effect?
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Requirements for Ion Migration:
Chemical Effects (cont.)
• Iontophoresis
– Driving ions of
medication into the
body
– Requires long-term
flow of a DC current
• Medication must
ionize in solution and
be placed under
appropriate electrode
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Thermal Effects
• Minimal during muscle and nerve
stimulation
• Discussed in Chapter 16
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Magnetic Effects
• Magnetic field created with electrical
current flow through a conductor (body
part through which therapeutic electrical
stimulation applied)
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Magnetic Effects (cont.)
• Specific responses of tissue are poorly
understood.
• Seem to be related to bone and wound
healing
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Kinetic Effects
• Sensation and muscle contraction,
which result from stimulation of
sensory and motor nerves
• Muscle contraction can be either
– Single contraction
– Twitch
– Multiple contractions fused together
into a tetanic contraction
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Polarization and Action
Potentials
• Stimulation requires a polarized
membrane (between inside and
outside of nerve membrane).
–
–
More positive ions than negative
ions outside nerve and more
negative ions than positive ions
inside membrane
When polarized, membranes
have a potential of −70 to −90 mV
between inside and outside of
membrane
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Polarization and Action
Potentials (cont.)
• Membrane
stimulation causes
depolarization at
point of stimulation.
• Positive ions rush
into the nerve and
negative ones move
out.
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Polarization and Action
Potentials (cont.)
• Change in potential causes adjacent
membrane tissue to depolarize, which
causes adjacent tissue depolarization.
• Depolarization down the nerve is called
an action potential.
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Polarization and Action
Potentials (cont.)
• Nerve action potential eventually causes
– An ascending sensory impulse to the brain
Or
– A muscle action potential
• Muscle action potential causes muscle
contraction.
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Polarization and Action
Potentials (cont.)
• Twigs of a single motor nerve synapse with
multiple muscle fibers.
– A motor nerve and all the muscle fibers it
synapses with are known as a motor unit.
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Action Potential vs. Muscle
Contraction
• Action potential is much quicker than muscle
contraction.
–
Think of a boulder on the side of a mountain: One quick push (the action
potential) causes the boulder to tumble down the mountain for minutes
(muscle contraction).
Nerve
AP
Muscle twitch
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Polarization and Action
Potentials (cont.)
• All-or-none law
– All muscle fibers of a motor unit contract in
response to a single action potential in its
nerve.
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Polarization and Action Potentials
(cont.)
• All-or-none law
– All muscle fibers of a motor unit contract in response
to a single action potential in its nerve.
• Gradation of contraction comes from the
number of motor units stimulated.
– Not by the strength of stimulation of individual motor
units
• Action potential will occur if the stimulation is
great enough.
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Polarization and Action
Potentials (cont.)
• After the action potential passes, the nerve
must repolarize before another action potential
can be generated.
– Time during which membrane repolarizes known
as refractory period
• Absolute refractory period: Nothing can be done to
make the membrane fire.
• Relative refractory period follows the absolute
refractory period.
• Membrane can fire but requires a much greater
stimulus.
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Polarization and Action
Potentials (cont.)
• Nerve repolarizes
quickly.
• Absolute refractory
periods vary from 0.4
to 2 msec
– Depends on specific
nerve
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Nerve Excitability
• Differentiate between stimulating
– An individual, isolated nerve fiber
– Part or all of a mixed nerve within other tissue
– Nerve threshold vs. nerve excitability
• Action potential is generated when the
stimulus meets or exceeds a nerve’s
threshold.
• Nerve excitability
– Amount of current applied to the surface necessary to
elicit an action potential in a specific nerve
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Nerve Excitability (cont.)
• Nerve excitability
– Amount of
current applied to
the surface
necessary to
elicit an action
potential in a
specific nerve
• Factors that
influence nerve
excitability follow.
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Excitability: Stimulation Parameters
• Therapeutic goal
– Stimulate numerous fibers of a mixed
nerve
– Current must pass through the skin,
subcutaneous tissue, and usually muscle.
• Amount of current applied to the skin to
evoke action potentials in the target
nerves
– Depends on numerous factors
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Excitability: Nerve Size and Depth
• The larger the nerve, the easier it can be stimulated.
• The more superficial the nerve, the easier it can be
stimulated.
• In a practical sense:
–
–
Large sensory nerves are more excitable than motor nerves.
Motor nerves are more excitable than pain fibers.
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Excitability: Tissue Resistance
•
•
•
•
The nerve stimulated depends on
tissue resistance.
The horny layer of skin is a good
insulator.
Hair and oils add to the insulation.
Coupling medium is used to decrease
resistance.
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Excitability: Current Density
•
•
•
•
A measure of the quantity of charged ions moving through an
electrode and the tissue immediately beneath it.
= current flowing/area of the electrode
If electrode pair are the same size, the current density is equal in
and beneath the two electrodes (a)
If electrodes of a pair are unequal size, the current density will
be greater in the smaller electrode (b)
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Excitability: Current Density
• Pulse rise time
– Time to go from 0 to maximal amplitude
– Slow rise time allows for accommodation
– Accommodation: gradual increase of
threshold
– Sodium pump may work to reverse
depolarization.
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Excitability: Current Amplitude
and Pulse Duration
• Current strength is a product of amplitude and
duration.
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Excitability: Rate of Stimulation
• Relationship between action potential and
muscle contraction
– Very brief nerve stimulation causes much
longer muscle contraction
Nerve
AP
Muscle twitch
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Excitability: Rate or Frequency
• Doesn’t effect individual threshold
– Effects torque as it approaches tetany
– Increased rate means increased fatigue rate.
tetany
5/sec
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Excitability: Motor Point
•
•
•
•
Point at which given amount of current will
elicit greatest muscular contraction
Point at which motor nerve enters muscle
Locate motor points by trial and error.
Do not confuse motor points with trigger
points.
– Trigger points are localized areas that are
extremely sensitive to palpation, electrical
stimulation, and ultrasound.
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Excitability: Other
•
•
Duty cycle (on–off cycle)
Muscle conducts four times better
longitudinally than transversely.
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Electrodes
• Devices attached
to the terminals of
a generator or
electrical stimulator
• Come in a variety
of sizes, shapes,
and materials
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Electrodes (cont.)
• Cross-contamination of reusable electrodes
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Electrodes (cont.)
• One-use electrodes
– Come on a roll like postage stamps
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Electrodes: Physical Dimensions
• Shape is unimportant
– Most are round or square or rectangular.
• Size and placement determine the
number of motor units stimulated.
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Electrodes: Physical Dimensions
(cont.)
• Small electrode placed over single
muscle stimulates only that muscle
• Size has a bearing on current density
under electrode
– The smaller the electrode, the greater the
current density.
• As long as the current output is the
same
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Electrodes: Material
• Conductivity effects amount of current
flow
• Carbon or silicon electrodes offer better
conductivity than metal-sponge
electrodes
– The carbon or silicon will leach out with
use, reducing the electrodes’ conductivity.
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Electrode Function
• Active electrode
– Electrode under which the current density
is great enough to elicit the desired
response
• Indifferent (dispersive) electrode
– Electrode under which the current density
is not great enough to elicit the desired
response
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Electrode Function (cont.)
– When electrode is much larger than
electrode(s) from opposite terminal
– Used to complete the circuit
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Electrode Placement Technique
• Three techniques
– Bipolar
– Unipolar
– Quadrapolar
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Electrode Placement Technique
(cont.)
• Bipolar technique
– Electrodes from terminals are of equal size,
resulting in nearly equal current density under
them
– Both active
– Applied to treatment area in relative proximity
to each other
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Electrode Placement Technique
(cont.)
• Unipolar technique
– Electrodes of unequal size, creating active and
indifferent electrodes
– May have multiple active electrodes
– Active electrode(s) applied to treatment area
– Indifferent electrode applied to remote location
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Electrode Placement Technique
(cont.)
•
Quadrapolar technique
–
–
–
–
Four electrodes of equal size
Pair from each of two channels.
Generally crisscross the target tissue
Most popular use is with interferential
stimulation
• Two currents of differing frequencies are
applied.
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Polarity
• Voltage (positive or negative) on active
electrode compared to voltage on
indifferent electrode
• Different from unipolar and bipolar
techniques
• Applies only with unipolar technique
• Affects excitability of nerves
– Some people respond better to negative
polarity and others to positive polarity
© 2008 LWW
Tissue Responses to Electrical
Stimulation
• Five general tissue responses
–
–
–
–
–
Ion migration
Sensory twitch
Sensory fused
Twitch contraction
Tetanic contraction
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Tissue Responses to Electrical
Stimulation (cont.)
• DC
– Continuous DC stimulation drives ions into
tissue.
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Tissue Responses to Electrical
Stimulation (cont.)
• Sensory twitch
– Single sensation followed by nothing
– Caused by moderate-amplitude, lowfrequency pulsed stimulation
– Not used therapeutically
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Tissue Responses to Electrical
Stimulation (cont.)
• Sensory fused
– Sensory response that feels like pins and
needles
– Caused by moderate-amplitude, highfrequency pulsed or AC stimulation
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Tissue Responses to Electrical
Stimulation (cont.)
• Twitch contraction
– Isolated brief muscular contraction followed by
relaxation
– Can occur in an individual muscle fiber or in
an entire muscle group
– Caused by low-frequency, high-amplitude
pulsed stimulation
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Tissue Responses to Electrical
Stimulation (cont.)
• Tetanic contraction
– Sustained muscular contraction that occurs in
response to repetitive stimulation of 20–30 or
more pulses per second
– Caused by high-frequency, high-amplitude
pulsed or AC stimulation
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Tissue Responses to Electrical
Stimulation (cont.)
Response
Amplitude
Frequency
Ion migration
Sensory twitch
Sensory fused
Twitch contraction
Tetanic contraction
?
Low
Low
High
High
None
Low
High
Low
High
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Tissue Responses to Electrical
Stimulation (cont.)
Response
Amplitude
Frequency
Current
Ion migration
Sensory twitch
Sensory fused
Twitch contraction
Tetanic contraction
?
Low
Low
High
High
None
Low
High
Low
High
DC
?
HV, IF
HV, LV
LV
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Tissue Responses to Electrical
Stimulation (cont.)
Most Commonly Used Wave Forms
DC: DC
IF: Polyphasic
HV: Twin pulse
LV: Biphasic and polyphasic burst (Russian)
TENS: Biphasic
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Tissue Responses to Electrical
Stimulation (cont.)
Response
Amplitude Frequency Current
Ion migration
Sensory twitch
Sensory fused
Twitch contraction
Tetanic contraction
?
Low
Low
High
High
None
Low
High
Low
High
DC
HV, LV
HV, IF
HV, LV
LV
Use
Iontophoresis
?
Wound healing, pain
Muscle with US
Spasm, strength
HV, twin pulse
IF, interferential
LV, biphasic, Russian (polyphasic, burst)
TENS, biphasic
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