The movement of inorganic ions across a cell membrane that initiates an electrical signal is only one step in signal transmission in organisms. The rapid transmission of this signal, over distances of up to 1 m, by the axon of cells is discussed next.

How is the signal that was initiated by neurotransmitter-mediated reactions propagated? In general, cell membranes are more permeable to potassium ions, which are at a higher concentration inside the cell membrane than outside and bound to immobile cations, mainly amino acids and proteins. The outflow of potassium ions along their concentration gradient is counteracted by the transmem-brane voltage that is created by this outflow. The equilibrium potential for potassium ions, for instance, EK+, is given by the Nernst equation (Eq. 1). The resting transmembrane potential of neurons is around -60 mV.

When the flux of ions through neurotransmitter receptor channels results in a change (of ~20 mV) in the transmembrane voltage to a more positive Vm value, the electrical signal is propagated along the axon of the cell within 1 msec. This occurs in the following way. At a critical value of Vm, specific voltage-dependent Na+ transmem-brane channels in the axonal membrane open, allowing sodium ions to flow inside the axon (Fig. 1). As the inward flow of sodium ions changes Vm to even more positive values, the Na+-specific channels are inactivated. Trans-membrane K+ channels open, changing Vm again to more negative values. This in turn leads again to the opening of voltage-dependent Na+-specific channels. These changes in transmembrane voltage, called action potentials, are propagated along the axon to the nerve terminal adjacent to another cell. Axons range from 0.1 mm to over 1 m in length and can convey electrical signals within 1 msec. When the signal arrives at the nerve terminal, Ca2+-specific transmembrane channels open and the influx of calcium ions leads to the secretion of neurotrans-mitter. Typically, the neurotransmitter diffuses across the synaptic cleft, over a distance of 20-40 nm, and binds to the neurotransmitter receptors on the adjacent cell. Thus the signal is transmitted between the ~1012 cells of the mammalian nervous system.

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