Electrical Stimulating Currents

1. Electrical Stimulating Currents Jennifer Doherty-Restrepo, MS, LAT, ATC FIU Entry-Level ATEP PET 4995: Therapeutic Modalities

2. Physiologic Response To Electrical Current • #1: Creating muscle contraction through nerve or muscle stimulation • #2: Stimulating sensory nerves to help in treating pain • #3: Creating an electrical field in biologic tissues to stimulate or alter the healing process

3. Physiologic Response To Electrical Current • #4: Creating an electrical field on the skin surface to drive ions beneficial to the healing process into or through the skin • The type and extent of physiologic response dependent on: • Type of tissue stimulated • Nature of the electrical current applied

4. Physiologic Response To Electrical Current • As electricity moves through the body's conductive medium, changes in the physiologic functioning can occur at various levels • Cellular • Tissue • Segmental • Systematic

5. Effects at Cellular Level • Excitation of nerve cells • Changes in cell membrane permeability • Protein synthesis • Stimulation of fibroblasts and osteoblasts • Modification of microcirculation

6. Effects at Tissue Level • Skeletal muscle contraction • Smooth muscle contraction • Tissue regeneration

7. Effects at Segmental Level • Modification of joint mobility • Muscle pumping action to change circulation and lymphatic activity • Alteration of the microvascular system not associated with muscle pumping • Increased movement of charged proteins into the lymphatic channels • Transcutaneous electrical stimulation cannot directly stimulate lymph smooth muscle or the autonomic nervous system without also stimulating a motor nerve

8. Systematic Effects • Analgesic effects as endongenous pain suppressors are released and act at different levels to control pain • Analgesic effects from the stimulation of certain neurotransmitters to control neural activity in the presence of pain stimuli

9. Physiologic Response To Electrical Current • Effects may be direct or indirect • Direct effects occur along lines of current flow and under electrodes • Indirect effects occur remote to area of current flow and are usually the result of stimulating a natural physiologic event to occur

10. Muscle and Nerve Responses To Electrical Current • Excitability dependent on cell membrane's voltage sensitive permeability • Produces unequal distribution of charged ions on each side of the membrane • Creates a potential difference between the interior and exterior of cell • Potential difference is known as resting potential • Cell tries to maintain electrochemical gradient as its normal homeostatic environment

11. Muscle and Nerve Responses To Electrical Current • Active transport pumps: cell continually moves Na+ from inside cell to outside and balances this positive charge movement by moving K+ to the inside • Produces an electrical gradient with + charges outside and - charges inside

12. Nerve Depolarization • To create transmission of an impulse in a nerve, the resting membrane potential must be reduced below threshold level • Changes in membrane permeability may then occur creating an action potential, which propagates impulse along nerve in both directions causing depolarization

13. Nerve Depolarization • Stimulus must have adequate intensity and last long enough to equal, or exceed, membrane's basic threshold for excitation • Stimulus must alter the membrane so that a number of ions are pushed across membrane exceeding ability of the active transport pumps to maintain the resting potential, thus forcing membrane to depolarize resulting in an action potential

14. Depolarization Propagation • Difference in electrical potential between depolarized region and neighboring inactive regions causes the electrical current to flow from the depolarized region to the inactive region • Forms a complete local circuit and makes the wave of depolarization “self-propagating”

15. Depolarization Effects • As nerve impulse reaches effector organ or another nerve cell, impulse is transferred between the two at a motor end plate or a synapse

16. Depolarization Effects • At the motor end plate, a neurotransmitter is released from nerve • Neurotransmitter causes depolarization of the muscle cell, resulting in a twitch muscle contraction Differs from voluntary muscle contraction only in rate and synchrony of muscle fiber contractions!

17. Strength - Duration Curves • Represents the threshold for depolarization of a nerve fiber • Muscle and nerve respond in an all-or-none fashion and there is no gradation of response

18. Strength - Duration Curves • Shape of the curve • Relates intensity of electrical stimulus (strength) and length of time (duration) necessary to cause depolarization of muscle tissue

19. Strength - Duration Curves • Rheobase • Describes minimum current intensity necessary to cause tissue excitation when applied for a maximum duration

20. Strength - Duration Curves • Chronaxie • Describes length of time (duration) required for a current of twice the intensity of the rheobase current to produce tissue excitation Manufacturers select preset pulse durations in the area of chronaxie!

21. Strength - Duration Curves • Aß sensory, motor, A sensory, and C pain nerve fibers • Durations of several electrical stimulators are indicated along the lower axis • Corresponding intensities would be necessary to create a depolarizing stimulus for any of the nerve fibers Microcurrent intensity is so low that nerve fibers will not depolarize

22. Nonexcitable Tissue and Cells Response To Electrical Current • Cell function may speed up • Cell movement may occur • Stimulation of extra-cellular protein synthesis • Increase release of cellular secretions

23. Nonexcitable Tissue and Cells Response To Electrical Current • Gap junctions unit neighboring cells • Allow direct communication between adjacent cells (forms electrical circuit) • Cells connected by gap junctions can act together when one cell receives an extracellular message • The tissue can be coordinated in its response by the gap junction’s internal message system

24. Nonexcitable Tissue and Cells Response To Electrical Current • All structures within the cell, membrane, and microtubes are dipoles • Molecules whose ends carry opposite charge • Therefore, all cell structures carry a permanent charge and are capable of… • Piezoelectric activity • Electropiezo activity

25. Nonexcitable Tissue and Cells Response To Electrical Current • Piezoelectric activity • Mechanical deformation of the structure causes a change in surface electrical charge • Electropeizo activity • Change in surface electrical charge causes the structure to change shape • Important concepts regarding the effects electrical stimulation has on growth and healing

26. Nonexcitable Tissue and Cells Response To Electrical Current • As a structure changes shape, strain-related potentials (SRP) develop • Results due to tension or distraction on the surface of the structure • Compression = negative SRPs • Tension = positive SRPs

27. Strain-Related Potentials (SRP) • Bone (Wolff’s Law) • Stimulates osteoblast, osteocyte, and osteoclast activity to assist in bone growth and healing • Skin Wounds • Normal Biolelectric Field: skin is negatively charged relative to dermis • Current of Injury: skin will change to positive charge producing a bioelectric current • Stimulates growth and healing • At the conclusion of the healing process, the Normal Bioelectric Field will be re-established

28. Nonexcitable Tissue and Cells Response To Electrical Current • Based on THEORY rather than well-proven, researched outcomes • More research is needed on the effects of electrical current on nonexcitable tissue and cells

29. Effects of Changing Current Parameters • Alternating versus Direct current • Tissue impedance • Current density • Frequency of wave or pulse • Intensity of wave or pulse • Duration of wave or pulse • Polarity of electrodes • Electrode placement

30. Alternating vs. Direct Current • Nerve doesn’t know the difference between AC and DC • With continuous DC, a muscle contraction occurs only when the current intensity reaches threshold for the motor unit DC current influence on a motor unit

31. Alternating vs. Direct Current • Once the membrane of the motor unit repolarizes, another change in the current intensity would be needed to force another depolarization to elicit a muscle contraction DC current influence on a motor unit

32. Alternating vs. Direct Current • Biggest difference between the effects of AC and DC is the ability of DC to cause chemical changes • Chemical effects usually occur only when continuous DC is applied over a period of time

33. Tissue Impedance • Impedance = resistance of tissue to the passage of electrical current. • Bone and Fat = high-impedance • Nerve and Muscle = low-impedance • If a low-impedance tissue is located under a large amount of high-impedance tissue, the intensity of the electrical current will not be sufficient to cause depolarization

34. Current Density • Current density refers to the volume of current in the tissues • Current density is highest at the surface and diminishes in deeper tissue

35. Altering Current Density • Change the spacing of electrodes • Moving electrodes further apart increases current density in deeper tissues

36. Altering Current Density • Changing the size of the electrode • Active electrode is the smaller electrode • Current density is greater • Dispersive electrode is the larger electrode • Current density is less

37. Frequency • Effects the type of muscle contraction • Effects the mechanism of pain modulation

38. Frequency • Frequency of the electrical current impacts… • Amount of shortening in the muscle fiber • Recovery time allowed the muscle fiber • Summation: shortening of myofilaments caused by increasing the frequency of membrane depolarization • Tetanization: individual muscle-twitch responses are no longer distinguishable, results in maximum shortening of the muscle fiber • Dependent on frequency of electrical current, not intensity of the electrical current!

39. Frequency • Voluntary muscle contraction elicits asynchronous firing of motor units • Prolongs onset of fatigue due to recruitment of inactive motor units • Electrically induced muscle contraction elicits synchronous firing of motor units • Same motor unit is stimulated; therefore, onset of fatigue is rapid

40. Intensity • Increasing the intensity of the electrical stimulus causes the current to reach deeper into the tissue

41. Recruitment of Nerve Fibers • An electrical stimulus pulse at a duration intensity just above threshold will excite the closest and largest fibers • Each electrical pulse at the same intensity at the same location will cause the same fibers to fire

42. Recruitment of Nerve Fibers • Increasing the intensity will excite smaller fibers and fibers farther away • Increasing the duration will also excite smaller fibers and fibers farther away

43. Duration • More nerve fibers will be stimulated at a given intensity by increasing the duration (length of time) that an adequate stimulus is available to depolarize the membranes • Duration is typically adjustable on low-voltage stimulators

44. Polarity • Anode • Positive electrode • Lowest concentration of electrons • Cathode • Negative electrode • Greatest concentration of electrons • AC: electrodes change polarity with each current cycle • DC: polarity switch designates one electrode as positive and one as negative

45. Polarity • With AC and Interrupted DC, polarity is not critical • Negative polarity used for muscle contraction • Facilitates membrane depolarization • Usually considered more comfortable • Negative electrode is usually positioned distally

46. Positive Pole Attracts (-) ions Acidic reaction Hardening of tissues Decreased nerve irritability Negative Pole Attracts (+) ions Alkaline reaction Softening of tissues Increased nerve irritability Polarity With Continuous DC • Important consideration when using iontophoresis

47. Electrode Placement • On or around the painful area • Over specific dermatomes, myotomes, or sclerotomes that correspond to the painful area • Close to spinal cord segment that innervates an area that is painful • Over sites where peripheral nerves that innervate the painful area becomes superficial and can be easily stimulated

48. Electrode Placement • Over superficial vascular structures • Over trigger point locations • Over acupuncture points • In a criss-cross pattern surrounding the treatment area • If treatment is not working, change electrode placement

49. Therapeutic Uses of Electrically Induced Muscle Contraction • Muscle re-education • Muscle pump contractions • Retardation of atrophy • Muscle strengthening • Increasing range of motion • Reducing Edema

50. Therapeutic Uses of Electrically Induced Muscle Contraction • Muscle fatigue must be considered • Variables that influence muscle fatigue: • Intensity • Frequency • On-time • Off-time