As opposed to metal conductors, the tissue resistance varies depending on its functional state.
In a biological medium, the carriers of electric current are almost exclusively ions. The function of ions depends on their charge.
In a greatly simplified tissue model we can distinguish two types of electrical conductivity:
- Ion Conductivity
- Macromolecules behaving like electric dipoles.
The basic cytoplasm and the intercellular medium behave as a conductor characterised by an ohmic resistance R, independent of frequency.
Membrane structure posses capacitance properties. They are characterised by impedance Z, which depends both on resistance R and capacitive reactance – capacitance X L.
Due to the capacitance properties of membranes, the flow of electric current through these structures depends on frequency. The electric properties of tissues are most often characterised by specific electrical resistance (resistivity).
Macromolecules behaving like electric dipoles:
The dipoles of molecules have various orientations in the tissues – they interact such that the total dipole = 0. However, when acted on by an external electric field the dipoles become oriented to their charge – they become polarized. As a result, the internal electric field (of an opposite polarity to that of the external field) reduce the intensity of the external field. This is called the dielectric loss.
The biophysical effects depend on the electric current type.
- DC current exhibits polarisation effects.
- Interrupted DC and Low-Frequency AC have excitation effects.
- High-Frequency AC has thermal effects.
Excitability, i.e. the ability to respond to stimulation, is a general property of living systems. In mammals, nervous and muscle tissues are called excitable tissues because of their high response to stimuli.
Excitation will not occur until a specific threshold intensity – the rheobase – is reached. The time necessary to evoke excitation at a current intensity equal to twice that of the rheobase is called chronaxy. The shorter the chronaxy, the greater the excitability of the tissue.
Each skeletal muscle has a typical level of chronaxy which can be used to determine the degree of excitability impairment in case of damage to the muscle.
Muscles can be sorted by the length of chronaxy, from low to high, as follows:
- skeletal muscles – chronaxy < 1 ms
- heart muscles – chronaxy = 5 ms
- smooth muscles – chronaxy = 50 to 500 ms
Uninterrupted DC is capable of initiating excitability alterations – by changing the ion environment. This effect is called electrotonus.
- Motor nerves become increasingly irritable – this local state is called catelectrotonus.
- Sensory nerves become less irritable – this local state is called anelectrotonus.
The excitation effect of AC are heavily dependent on frequency:
- At low frequencies (up to 100 Hz) excitation increases linearly with the frequency
- At higher frequencies (500 – 3000 Hz) excitation increases with the square root of the frequency
- At frequencies > 3000 Hz excitation begins to drop until it completely ceases at 10 kHz. There is no more excitation, but heat begins to develop. This is due to induced eddy current which result in dielectric heating.
Elektrokinetic Phenomena: DC sets in motion both small ions and electrically charged macromolecules.
We distinguish two methods that take advantage of eletrokinetic phenomena:
- Electrophoresis – the motion of charged particles in a DC electric field
- Electroosmosis – the motion of solvent molecules due to the effect of DC
An analysis of electric properties of cell membranes has confirmed that their resistance drops with the increasing frequency of AC. Membranes behave as biological capacitors with a relatively steady specific capacitance (1 micro Faraday). In addition to the functional state, the passage of current through tissues is affected by their anatomical, histological and molecular structures. The resistance measurement using direct current is influenced by polarization phenomena – as a result we only use this method to study polarization of cell membranes.