is called influx, while the opposite flow is called efflux. Certain ion channels and
ion pumps extrude ions from the cytoplasm towards the extracellular space,
however here the positive ion efflux (potassium) leads to hyperpolarization, while
the negative ion efflux (hydrogencarbonate) leads to depolarization! Normal
neuronal membrane potentials vary over a range from about -90 mV to +50 mV.
The ion channels in the neuronal membranes are ligand or voltage gated. In the
first case ligands are the neuromediator molecules that bind to the receptor ion
channels and open their gates causing flux of ions. In the second case the ion
channels are sensitive to the voltage across the membrane and their gates open
and close in concert with the membrane potential changes.
Fig. 7 Membrane lipid bilayer and a single ion channel. Every phospholipid
molecule consists of polar head and non-polar tail. The diameter of the ion
channel is 3-6 nanometers. Legend: AP, aqueous pore; SF, selectivity filter; IG,
inactivation gate.
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