Bioelectrics, tissue healing & EPA
The electrical currents and electrical potentials generated by or occuring within live cells, tissues, and organisms are referred to as tissue bioelectrics (Rizwan, Tse, Nori, Leong, & Yim, 2019). Bioelectricity is concerned with the tissue's ability to create electricity. The energy produced is endogenic (created within an organism or cell) and is generated by the tissue itself. It also describes how the tissue can be regulated by externally delivered electricity (Grimnes & Martinsen, 2015). Every cell in the body is an electric battery, with voltage maintained throughout the membrane and organelles. The active movement of ions, mainly sodium and potassium, creates a charge separation that results in a potential difference of voltage across the membrane (Poltawski & Watson, 2009).
Bioelectrics are produced by a multitude of causes, including the fact that each live cell has a membrane potential that varies in magnitude but averages many tens of millivolts, with the inside of the cell being negative compared to its external surface (Watson, 2010). The cell membrane is a key component when influencing cellular activity levels, the nucleus is critical for genetic control and reproductive function but activity change within the membrane exert a strong influence on cell processes (Watson, 2000). The membrane potential of a cell is strongly related to the cell membrane transport mechanism, in which the passage of material across the membrane is ionic (charged particles), allowing the movement of charged particles to impact the membrane potential (Watson, 2010). Ion transport via bioelectricity regulates cell behaviour (Poltawski & Watson, 2009), with even non-excitable cells including voltage gated channels that control ion transit. Endogenous fields are essential to several metabolic processes, including development, adaptation, and repair; influencing the cells morphology and the growth of body parts during foetal development (Poltawski & Watson, 2009).
Bioelectricity has the ability to impact cell morphology and is produced when connective tissue such as bone and tendon is strained, influencing adaptive changes in the extracellular matrix (Poltawski & Watson, 2009). When tissues are damaged, currents are established to help drive components of the healing response and slow the healing process, with normal values being restored after the healing is complete (Poltawski & Watson, 2009). Studies have shown that when setting up a voltage in opposition to the endogenous one can slow or abolish the healing response in a variety of tissue types (Poltawski & Watson, 2009). Application of microcurrent to tissue has been found to boost the number of organelles responsible for cellular activities and helps increase the concentration of ATP (Adenosine triphosphate), which can facilitate cell proliferation and protein synthesis which was found to increase when microcurrents are applied to constituent cells of skin, tendons, cartilage and bone (Poltawski & Watson, 2009). However, larger current or alternating microcurrents at certain frequencies have been found to reduce cell proliferation or induce cell death in some cases (Poltawski & Watson, 2009). Migration of ion channels within the cell membrane influenced by an applied field can occur resulting in cytoskeletal modification including the creation of membrane projections which enables cell movements (Poltawski & Watson, 2009). Movement of cells within an electric field (known as galvanotaxis) has been observed within leukocytes and macrophages as well as a variety of cells responsible for tissue formation.
Musculoskeletal tissues are generally electrically active in their own right (Watson, 2000), this electrical energy is an essential component of normal physiological function. According to Poltawski and Watson (2009), there are two methods to the application of electrotherapy modalities, with the first giving enough energy to overcome the energy of the membrane and so 'push' it to alter behaviour. The second method uses a considerably lower energy level and, rather than modifying behaviour, stimulates the living cell to achieve an impact (Poltawski & Watson, 2009). The overarching idea of electrotherapy is the use of a low power/energy technique to boost the body's innate ability to stimulate, direct, and control the healing and reparative process. Low level energy applications aim to tickle the cells to stimulate them into some higher activity level or up regulation, thus, use the natural resources of the body to do the work (Poltawski & Watson, 2009). Over the last few years the energy levels applied within electrotherapy have been reduced with modalities like ultrasound being doses at a significantly lower rate than previously thought to be effective, and most recently the development of low intensity pulsed ultrasound has generated a very substantial evidence based in the relation to fracture healing (Watson, 2002). The application of low power/ energy modality can enhance the natural ability of the body to stimulate, direct and control the healing and reparative process instead of the earlier high energy levels which was once used to force the cells to respond the low level applications aim to tickle the cells to stimulate them into a higher activity level and thus using the body's natural resources (Watson, 2002). Watson (2002) also stated that really low levels (microcurrent type therapies) and delivering a current to the tissue which is remarkably similar to the endogenous currents appear to be physiologically effective.
The two general approaches to EPA are: the higher energy approach forcing a tissue response or a low energy approach generating cellular excitement and therefore physiological up-regulation (Poltawski & Watson, 2009). Electrical stimulation (TENS, NMES) work primarily via a nerve mediated mechanism and are linked into the high energy group on the basis that their mode of action is via a forced nerve depolarization mechanism. Ultrasound, laser, pulse shortwave are listed as low energy modalities to achieve their effects via cellular up-regulation. Each modality initiates a tissue response which is a result of cellular excitement rather than a direct effect (Watson, 2000). It is not the modality which induces such changes but rather the modality producing cellular excitement consequencing the activation of a range of physiological processes which are related to tissue healing (Watson, 2000).
