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 transmits equally well in either direction, but in the intact 



