. Sciences biophysiques. La biophysique. 4 : 5/ La conduction des impulsions par les nerfs 83 diagramme illustré à la Figure 8. Tout ce processus est actif, de sorte que l'épi potentiel n'est pas atténué comme il se déplace le long de l'axone, mais plutôt se construit à nouveau à chaque point le long du chemin. La vitesse à laquelle un potentiel épi voyage le long d'une fibre est limitée par le diamètre de la fibre. Les plus gros diamètres correspondent à de plus grandes vitesses. Dans le grand "vertébrés" myélinisées fibres synthétiques, spike se déplace à un taux supérieur à celui prévu par le diamètre de l'axone. Il Spike se reposer. A/o + d'Intercell

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. Biophysical science. Biophysics. 4 : 5/ The Conduction of Impulses by Nerves 83 diagram shown in Figure 8. This entire process is an active one, so that the spike potential is not attenuated as it travels along the axon but, rather, is built up anew at each spot along the way. The velocity at which a spike potential travels along a fiber is limited by the diameter of the fiber. Larger diameters correspond to larger velocities. In the large "myelinated" vertebrate fibers, the spike travels at a rate in excess of that predicted from the axon diameter. It Spike Resting. A/o+ Intercellular Fluid Na* Membrane Metabolism Ax op I asm .. Regenerative Na* — Phase Recovery Phase Figure 8. Ion movements across axon surface. After A. L. Hodgkin and R. D. Keynes, "Active Transport in Nerve, " J. Physiol. 128: 28 (1955). is believed that the regenerative and recovery phases described above occur only at the nodes. In between, the spike is simply conducted, the entire segment acting as a single conductor. The spike potential thus is attenuated between the nodes and restored to its characteristic height at the nodes. The myelin sheaths of the "nonmyelinated" axons may act primarily as electrical insulation between fibers. This limits the probability that a spike potential along one axon will stimulate its neighbor. Muscle fibers have similar spike potentials but lack myelin insulation. In the muscle, unlike the nerve, it may be desirable for one fiber to stimulate other parallel ones, although this has never been demonstrated to occur. 5. Synaptic Conduction Along the axon, the information is transmitted as an electrical spike potential. This transmitted spike is maintained at a constant height by renewal and amplification, either continuously or at certain nodes. There are thus tw6 symbols for coding transmitted information: either a spike or its absence. In other words, all neural information is coded as binary digits. (See Chapter 25.) The axon t

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