Overview of Electricity and Physiology

 

All use of electricity to produce a change in body tissue is directly related to the properties of the cell membrane. Electricity can stimulate ionic flow, and therefore action potentials, which result in activation of nerve pathways, muscle tissue, and chemical changes

Review the process of an action potential at the cell membrane.

(Approximately 7 minutes)

This will refresh your understanding of cellular physiology and help make connections between physiology and electrical stimulation as a rehabilitation modality

 

Key Terms and Law's of Electricity

Approximately 9 minutes

• charge - Addition or removal of electrons. A gain of electrons is a net negative (-) charge, and removal of electrons is a net positive (+) charge.
• recall from A&P that cells depolarize to initiate or transmit signals to other cells by the movement of ions; ionic flow is the movement of ions across the membrane
• the separation of charges (ions) across the resting cell membrane is maintained by a greater concentration of sodium (Na+) the outside of the cell, and a greater concentration of potassium (K+) inside the cell
• the charge imbalance at the cell membrane is maintained by the sodium-potassium pump
• cathode - The side of the cell membrane with excess electrons (net negative charge)
• anode - The side of the cell membrane with a deficiency of electrons (net positive charge)
• when electricity is introduced into tissue, there is a localized effect in the extracellular space in response to a sustained charge
• When we introduce electrical stimulation, the salt water in our bodies (NaCl) dissociates into its respective ions: Na+ and Cl-, which then migrate toward the cathode (-) and anode (+) respectively. This creates a chemical reaction in the extracellular space that effects the protein density in the tissue.
• This response supports the use of electrical stimulation in wound and tissue healing
• Voltage
• driving force that moves electrons; the force that pushes charge
• like charges repel, opposite charges attract; energy from attraction and repulsion creates an electrical field (Coulomb's Law)
• High Volt = 100-150V
• Low Volt <100 V
• Conductors - a material that permits the movement of ions
• metals
• water
• biological tissue example: nerve
• Insulators - a material that restricts the movement of ions.
• rubber
• plastic
• biological tissue example: fat (recall that the myelin sheath that surrounds nerve axons has high lipid content)
• Current - measured in amperes (A) of electron flow per second
• movement of ions in response in a conductor in response to a voltage force
• determines depth of penetration in biological tissue
• Watt - measure of power or work (W)
• Frequency - number of pulses or wave forms per unit of time
• Hz = 1 cycle per second
• Resistance (Impedance) - opposing the flow of current; The following are biological sources of high resistance:
• skin, adipose (fat), hair, fascia, ligament, callus, bone, tendon, scar
• more that 99% of the resistance to current flow in the body is from the epidermis (top layer of skin)
• adipose (fat) is a resistor, causing increased impedance to current flow. A body part covered with a thick layer of adipose tissue may require an increase in current to elicit the desired response, which can lead to discomfort and pain due to activation of nociceptive nerve endings in the skin.
• High water content decreases impedance and increases conductivity. Tissues that have high water content are muscle and nerve
• Low water content increases impedance. Bone, fat, tendon, and fascia are poor conductors due to low water content
• Accommodation - decreased sensitivity to tissue excitability
• Capacitance and Impedence ability to store a charge
• capacitance is the "capacity" for a system, like the human body, to store an electrical charge
• impedence is a form of resistance; it is a frequency-dependent resistance to flow of current
• gels and adhesive material found on electrodes decrease impedence and increase conductivity between the electrode and skin
• Ohm's Law - relationship between electrical current, voltage, and resistance
• Current = Voltage/Resistance
• INCREASE Voltage = INCREASE Current, DECREASE Voltage = DECREASE Current
• INCREASE Resistance = DECREASE Current, DECREASE Resistance = INCREASE Current
• Ohm's law is important because the amount of current that flows through biological tissues determines changes in the physiological effects

More about Ionic Flow

Approximately 3 minutes

This excerpt from the American Society of Neurological Monitoring has one of the best explanations I've seen to help understand ionic flow in biological tissue:

"How an Electrical Stimulator Works

In an electrical stimulator, the flow of anions (-) and cations (+) is controlled by the mechanics of the circuitry within the stimulator.  The stimulator is unique in that the cathode is the negative pole (-) because it discharges anions (-), and the anode is the positive pole (+) because it discharges cations (+). At the end of the day, that's the fundamental difference between a battery and a stimulator.

Depending on how we configure the polarity, the stimulator will discharge either cations or anions into the body part being stimulated.

In cathodal stimulation, anions (-) are discharged into the body as current flows from the cathode (-), through the tissue, and back to the anode (+).

In anodal stimulation, cations (+) are discharged into the body as current flows from the anode (+), through the tissue, and back to the cathode (-).

Now, let's imagine that we place an electrical stimulator on the surface of the skin with a nerve bundle running underneath (Figure 2). Within the nerve bundle is a single nerve fibre (axon) upon which we will focus.

At rest, the inside of a cell is more negative than the outside of a cell.

This occurs because there is a slightly greater number of negative charges than positive charges inside of the cell (intracellular space), and a slightly greater number of positive charges than negative charge outside of the cell (extracellular space). Because of the electrical difference, the cell is said to be polarized- just like a magnet, one side is more positive and the other side is more negative. If the electrical gradient were suddenly reversed, the cell would be depolarized, and we might see an action potential."

(Reference: Vogel, R. W. (2017, December). Understanding Anodal and Cathodal Stimulation [Blog post]. American Society of Neurophysiological Monitoring. Retrieved from

https://www.asnm.org/blogpost/1635804/290597/Understanding-Anodal-and-Cathodal-Stimulation

 

Below is a table view of the difference between a contraction elicited through normal central nervous system function as compared to that via electrical stimulation.

Motor Unit Recruitment - Central Nervous System	Motor Unit Recruitment and Contraction - Electrical Stimulation
Active	Passive
Small type I motor units are recruited first then larger type II motor units for smooth and gradual tension	Large superficial fatigable type II motor units are recruited first, then smaller motor units
Asynchronous firing in off and on pattern - energy efficient and slower onset of fatigue	Synchronous firing - motor units stimulated continue to fire until stimulus removed, causing quick onset of fatigue
Action potential moved away from the nerve cell body	Action potential generated in two directions, away from the cell body and back toward the cell body

  

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