Physiological Effects of Electrical Currents

1. Physiological Effects of Electrical Currents

There are three primary physiological responses to electrical current:

1.1. Electrochemical Effects

• Mechanism: Movement of ions (primarily sodium and chloride) in bodily fluids (excluding blood) in response to electrical stimulation.

  • Sodium (Na+) migrates to the cathode, forming Sodium Hydroxide.$(\text{Na}^+ + \text{H}_2\text{O} \to \text{NaOH} + \text{H}^+)$

  • Chloride (Cl-) migrates to the anode, forming Hydrochloric Acid.$(\text{Cl}^- + \text{H}_2\text{O} \to \text{HCl} + \text{OH}^-)$

• Result: Alkaline reaction at the cathode and acidic reaction at the anode.

• Consequences: Excessive or prolonged reactions (long-time application or high current intensity) can lead to corrosive effects and skin damage/burns, especially with direct current (DC) or monophasic currents which produce ionization.

1.2. Electrothermal Effects

• Mechanism: Charged particles moving through bodily tissues (conductors) produce heat due to friction and vibration of tissues.

• Consequences: Skin acts as an insulator; excess heat from high current intensity can cause skin burns.

1.3. Electrophysical Effects

• Mechanism: Capability of electrical currents to depolarize nerve and muscle cell membranes.

  • Membrane permeability to Na+ increases in the presence of an electrical stimulus.

  • This reduces the resting membrane potential, leading to depolarization and ion movement across the membrane, stimulating the nerve.

• Electrode Efficiency: When using monophasic or biphasic asymmetrical current, the cathode (negative electrode) is more efficient at depolarizing nerves.

2. Action Potentials and Stimulation Parameters

• Stimulus Requirements: Electrical stimuli must have sufficient amplitude and duration to produce cell membrane depolarization and action potentials.

• Strength-Duration Curve: Shows combinations of current strength (amplitude) and duration required to stimulate nerves or muscles.

• Rheobase: The minimal amplitude (current strength) required to stimulate tissue when the pulse duration is infinite.

• Chronaxie: The minimal pulse duration required to stimulate tissue when the amplitude is double the Rheobase.

  • Nerves: Chronaxie is typically below $1\text{ ms}$.

  • Denervated Muscles: Prolonged Chronaxie (e.g., $10\text{ to }20\text{ ms}$) indicates denervation or other excitable tissue diseases.

• All-or-Nothing Principle: Action potentials are generated in an “all-or-nothing” manner. Stimuli greater than the minimal required will not produce a larger action potential. Insufficient stimuli will not cause stimulation.

3. Stimulation of Denervated Muscles

• Denervated muscles have a significantly larger Chronaxie compared to innervated muscles.

• Requires a larger pulse duration for effective stimulation. Using short pulse durations (ideal for nerves) on denervated muscles will not be effective.

4. Nerve Fiber Specificity and Recruitment

• A-Beta Fibers: (Touch and pressure) Require shorter pulse durations/widths (e.g., for acute pain TENS).

• Motor Fibers: Require larger pulse durations (e.g., for chronic pain TENS to induce muscle twitches).

• Overall: During electrical stimulation, larger-diameter nerve fibers are preferentially stimulated.

• Location: More superficial fibers closer to electrodes are stimulated first. Increasing intensity/amplitude stimulates deeper and more fibers.

5. Common Current Types in Physical Therapy

• Various types of current, waveforms, and parameters are used (detailed in subsequent lectures).

• TENS: Standard TENS often uses symmetrical biphasic waveforms. Ramp-up and -down parameters are not typically used with TENS.