Bioelectricity
Principle
Bioelectricity refers to the electrical and/or magnetic fields produced by living material (from cells to organisms). Bioelectricity should better be equated with bioelectromagnetism but this is easily confused with bioelectromagnetics, which deals with the effect on living matter from external electromagnetic fields, such as the diverse types of radiobiology, transcranial magnetic stimulation, but also animal migration and navigation by means of electroreception or magnetoreception. Some animals (sharks, rays, catfishes etc.) have bioelectric sensors, the sensory cells of their electroreceptive system, with sensitivities down to less than 1 μV/m (skates). Other animals, such as migratory birds, are believed to navigate in part by orienting with respect to the Earth magnetic field.
Bioelectric phenomena are for example the cell membrane potential and the electric currents that flow in nerve axons and muscle fibers (as a result of action potentials) and dendritic currents. In addition to aims of communication (signal one another) biological cells use bioelectricity to store metabolic energy, to do work or trigger internal or external changes or events (e.g. the electrical field due to the electric discharge of an electric ray or eel).
Bioelectromagnetism is an aspect of all living things, including all plants and animals. It is studied predominantly through the techniques of electrophysiology in animals. It is based at the propagation of electrical activity along nerve cell or muscle cell membranes. The resulting evoked electrical fields can often been recorded non-invasively when enough cells are active.
More recently, also physiological measurements of magnetic fields evoked by the electric activity of the brain and the heart can be measured (magnetography).
Many living cells of animals and micro-organisms have a potential difference over their cell membrane. For nerve cells including receptor cells and muscle cells this is basic for their performance. The membrane potential is maintained by the cell metabolism. The following holds for (most) nerve cells and muscle fibers.
The nerve potential is a consequence of the semi-permeability of the cell membrane, which is ca. 10 nm thick with a voltage of 0.1 V across it, yielding a field strength of 10 kV/mm, 10 times the break down voltage in air. The measured capacity of nerve membranes is large due to their thinness: ≈1 μF/cm2, in accordance with the equation of the size of a capacitor:
Cm = εmε0S/d
where εm the relative dielectric constant (ca. 5), ε0 the dielectric constant in vacuum, S the surface and d the thickness of the membrane. Nerve fiber diameters range from 0.1μm (unmyelinated sensory nerves) to I mm (giant unmyelinated axon of squid). Axon length ranges from mm to meters. Due to the squid giant axon’s large size intracellular recordings can be made simultaneously at several places.
The charge of the membrane is caused by an unbalance in the concentrations between the cell interior and exterior, and as is later explained by the conductivities of these ions. In mammals, the concentrations of the ions are given in Table 1.
Table 1 Approximate concentrations of relevant small ion of mammalian nerve cells
|
ion |
Intra-cellular mM |
Extra-cellular mM |
Plasma
mM |
Equilibrium potential E mV |
|
Na+ |
12.5 |
145 |
141 |
+60 |
|
K+ |
135 |
4 |
4.2 |
─ 90 |
|
Cl─ |
9 |
115 |
104 |
─ 80 |
Table 1 (which is slightly different for muscle cells and heart muscle cells) does not give the concentration of the large organic anions for which the membrane is impermeable. Their negative charge is compensated by especially K+ for which, in rest, the membrane is well permeable. (In rest means that there is no disturbance of the membrane potential.) A disturbance can give rise to an action potential or dendritic potential which is propagated along the membrane. Then the cell is active.) The cell interior is electrically neutral. The charge of Cm at a potential difference of 0.1 V is due to only a fraction (10─3 - 10─5) of the number of ions present in the cell's interior. This means that a nerve can generate many action potentials before the concentrations of the relevant ions change significantly.
For each ion, given the inside and outside concentrations, there exists an equilibrium potential according to the thermodynamical law of Nernst, which is based on Boltzmann's distribution:
(1)
where n1 and n2 are the number of ions in the two volumes, V1 and V2, E the equilibrium potential, k the Boltzmann constant, T the absolute temperature, z the valence of the ion and F Faraday's constant.
Rewriting (1) for K+ yields:
EK+ = (RT/zF) ln(K+out]/[ K+in]) (2)
where R the gas constant (see Gas laws). For the various ions this yields E as given in Table 1.
In reality, E is ─80 mV meaning that the equilibrium for K+ and Cl─ is nearly reached. In the following the influence of all other permeable anions (e.g. HCO3─) are taken together with Cl─.
For ENa which is strongly positive, so reversed, there is no equilibrium at all. From this follows that the concentration gradient of Na+ as well as the actual Em, mainly determined by the K+ ions, will give an inflow of Na+.
The ions pass the membrane via specific ionic pores or gates. Their passage is regulated by the membrane potential itself (see Bioelectricity: action potential). Although in rest the permeability for Na+ is small, on the long run the nerve would lose its potential if the slowly intruding Na+ would not be ejected by an active process, the so-called sodium Na+ pump. If this stops working, finally, due to osmotic laws (see Osmosis) dictating equal osmolarity at the two sides of the membrane, the cell will then swell and eventually burst.
Fig. 1 Basic electric model of the nerve membrane. Routside is small and therefore often ignored. The RC-network drawn with solid lines is thought to be repeated many times to model a fiber.
Since the ionogenic sources (batteries) have a negligible resistance, the membrane conductance (1/Rm) equals:
1/Rm, = 1/RNa + 1/RK + 1/RCl, (3)
with the variable RNa+ and RK+ indicating the voltage controlled passage. They are constant in rest and variable during action. Sometimes the voltage source (the ionogenic batteries) is placed in series with their resistors, but this is less realistic since the sources are located at different sites in the membrane than the ionic gates. All other ions do practically not contribute to the membrane potential.
Taking the action of the three ions together, Em in rest can be found:
Em,rest = (RT/F) ln((gKK+out] + gNaNa+out] + gCl[Cl─out])/ (gKK+in] + gNaNa+in] + gCl[Cl─in])), (4)
where the conductance g = 1/R. Obviously, the effect of the sources is weighted by their conductances. This is the reason why Em,rest is dominated by the K+ equilibrium potential.
Application
Bioelectricity can be divided in:
· Single cell physiology in situ, isolated organs (e.g. retina), with brain slices or cell cultures (with phenomena such as membrane potential, resting potential, action potential, clamp potential, clamp current, excitatory postsynaptic potential, inhibitory postsynaptic potential, dendritic potential)
· Multi-unit recordings
· Electrophysiology of small pieces of neuronal tissue in situ and in vitro such as brain slices (brain waves, field potentials, compound action potentials)
· Electro(magnetic)physiology of whole or great parts of an organ, used diagnostically in situ, for instance Electroencephalography, Electromyography, Electro-ocolography, Electrocardiography (see ECG: basic electrocardiography), electroneurography (for recording nerve activity, such as brain the auditory stem potentials, the SEP (somatosensory potential of e.g. the ulnar or tibial nerve). Magnetography can be performed with humans (and animals) and of human fetuses and also organs of tissues. Magnetoencephalography and cardiomagnetography are most known.
Electroreception and magnetoreception
Predatory electric field generation.
Therapeutical equipement, for instance pacemakers for the heart, bladder, anus, heart defibrillators, generators for voluntary muscles, for pain release (spinal cord), cochlear and cortical implants (e.g. visual or auditory) and magnetic induction coils in neurology are formally bioelectromagnetics devices.
More Info
The cell membrane can be considered as a capacitor with the membrane as the ‘non’-conductive medium between the two conduction ‘plates’, the outside and inside cell interior close to the membrane. The membrane charge is however so small, that the cell interior can be considered as electric neutral. Since in the exterior space the field strength of an electric (monopole) source decreases with the square of the distance, at a distance of 1 cm the field strength generated by one cell is diminished to the order of 0.1 nV. This is far beyond the thermal noise level, so non-measurable.
For a description of the propagation of a disturbance of the rest potential along the cell membrane, for instance that of a dendritic potential or an action potential see Bioelectricity: electronic propagation and Bioelectricity: action potential.