CELLS OF THE NERVOUS SYSTEM AND ELECTRICAL STIMULATION - Advanced Biomaterials: Fundamentals, Processing, and Applications - page 611

tioned in regular curriculum. That phenomenon is known as the injury potential
and occurs when a nerve or other tissue is damaged. The injury potential is a
voltage gradient established within the extracellular and intracellular space due
to current fl owing out of a wound.
In intact epithelium, a uniform potential is maintained perpendicular to the
plane of the epithelium. Due to asymmetrically distributed membrane-bound ion
channels and pumps on epithelial cells, dissimilar concentrations of ionic species
are separated by the epithelium. Na + channels are more numerous at the apical
side of the epithelium, while K + channels and Na + /K + - ATPase (pump) are local-
ized at the basolateral membrane of epithelial cells. This causes the concentration
of K + and Na + at the basolateral side of the epithelium to be larger than the apical
side, establishing a potential difference across the epithelium. Thus, a concentra-
tion gradient drives ions to the apical side where possible, such as between the
cells. Gap junctions unite epithelial cells, creating a resistance to the fl ow of ionic
species from the basolateral to the apical side of the epithelium (Nuccitelli 2003).
A break occurs across multiple cell layers, as occurs in injury, creates a low
resistance pathway for ionic current to fl ow between the basolateral and apical
sides of the epithelium. Therefore, a potential gradient (or EF) with a positive
vector pointing toward the wound is created. The EF that arises due to a wound
is found in the subepithelium or at the basolateral side of the epithelium and runs
parallel to the plane of the epithelium. The EF or injury potential guides wound
healing by making cells migrate in the direction of the wound. Furthermore, the
rate of mitosis increases, the mitotic spindle aligns perpendicularly to the EF, and
neurite outgrowth is directed in the direction of the EF vector (Nuccitelli 2003).
For most cell types that respond to EFs or the injury potential, there seems to
be a minimum EF strength that will elicit a cellular response and an EF strength
that maximizes the response. In the case of the human corneal endothelial cells,
alignment of their mitotic spindles perpendicularly to the vector of a DC EF is most
prominent at 200 mV/mm (Zhao et al. 1999). There are two recent, illustrated,
thorough and well-written articles about the injury potential related to wound
healing and the history of the injury potential (McCaig et al. 2005; Nuccitelli 2003).
18.5.2 Electrical Stimulation of the Nervous System
Electrical stimulus has been applied therapeutically to improve the regeneration
of nerve cells in the PNS. The type of EFs used can be direct (DC), alternating
current (AC), and electromagnetic. The sciatic nerve of rats is a common model
used to study the regeneration of the PNS. The two types of trauma commonly
studied are crush lesions and transections. After the animal is injured, the electri-
cal stimulation is applied and the results are evaluated.
18.5.3 DC Electric Fields In Vivo
The use of DC EFs to treat nervous system injury is effective at stimulating re-
growth of axons as long as the cathode is present distally, otherwise no benefi ts
are observed (Pomeranz et al. 1984). When an EF is created in air, electrons fl ows
form the anode to the cathode. However, when an EF is applied through tissue