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Electromagnetic fields in vivo

Neuronal membranes as excitable units

Physiologically, the electrical signal of relevance to the nervous system is the
difference in electrical potential between the interior of a neuron and the
surrounding extracellular medium (Schneidman, 2001). The ionic concentration
gradients across the cell membrane and the membrane permeability to these
ions, determine the membrane potential. The cell membrane is a lipid bilayer,
which is impermeable to most ions. Electrically, the membrane is a capacitor
separating the charges residing along its inner and outer surface, from both
sides. While the resistance of the lipid bilayer by itself is quite high, the
resistance of the membrane is significantly reduced by the numerous aqueous
pores in the membrane, termed ion channels.

The ions flow into and out of the cell due to both voltage and concentration
gradients. However, without external stimuli, these different forces drive the cell
to an equilibrium point - the resting membrane potential Vm of a neuron, which
can be explained from basic physical chemistry principles. Under these resting
conditions, the electrical gradient and the ionic concentration gradient balance
each other for each of the ion types. The potential inside the cell membrane of a
neuron, resulting from the accumulation of charges on the membrane, is then
about -70 mV relative to that of the surrounding bath, and the cell is said to be
polarized.

The positive ions (sodium, calcium) that enter the cytoplasm decrease the
electronegativity of the membrane potential i.e. they lead to depolarization of the
membrane, while the negative ions entering the cytoplasm (chloride) increase the
electronegativity of the membrane potential i.e. they hyperpolarize the
membrane. The flow of ions from the extracellular space towards the cytoplasm

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