420 THE BIOLOGY OF MARINE ANIMALS 



Conduction Speed 



The speed of conduction in nerve tracts and in nerve fibres depends on a 

 number of variables. There are probably intrinsic differences in axon 

 velocities of different species, but these are not sufficient to mask the part 

 played by other factors. In any given fibre, conduction speed varies with 

 the temperature, the Q 1Q for molluscan nerve, for example, lying between 

 1*4 and 1-8(114). Some of the older data, derived from mechanical record- 

 ing, are too low when compared with more precise measurements by 

 electronic methods, but they still indicate the relative velocities of different 

 fibre-systems. The principal factors determining the overall speed of con- 

 duction are: continuity of fibres; axon diameter; thickness of myelin 

 sheath (89). 



Conduction tends to be slow in nerve-nets and in neuropile. The nervous 

 system of coelenterates consists of a network of neurones, most of which 

 extend only short distances before making contact with other elements. 

 Similarly, in the nerve cords of higher animals (annelids, crustaceans), the 

 neuropile contains many small neurones, whose processes extend through 

 only one or two segments before establishing synaptic contact with other 

 neurones in series. Nerve fibres in nerve-net and neuropile are generally 

 very small, at most a few micra in diameter, and this alone keeps conduc- 

 tion velocity at a low level. Apart from this factor, impulses which have 

 to pass across many synaptic junctions are subjected to synaptic delay at 

 each junction, and the speed of transmission in such a system is reduced 

 correspondingly. Some data for transmission speeds in individual fibres, 

 nerve-nets and neuropiles are presented in Tables 10.1 and 10.2. 



Transmission velocity is also related to axon diameter and thickness of 

 myelin sheath. Large nerve fibres conduct faster than smaller nerve fibres 

 in the same animal. The exact numerical relationship between speed and 

 size is still uncertain. In giant fibres of squid and cuttlefish, which provide 

 a wide spectrum of axon diameters, velocity has been found to increase as 

 the diameter raised to the 0-61 power (91), or linearly with diameter 

 (approximate increase of 6*5 m/sec for 100 /li increase in diameter (45)). 

 In the fanworm Myxicold infundibulum, which possesses a tapering giant- 

 axon, the velocity varies approximately as the square root of diameter (77). 



Many worms are highly extensible (nemertines, various polychaetes), 

 and during contraction and extension giant-axons in the nerve cord under- 

 go twofold alteration in diameter. Stretched giant axons, however, show 

 no change in conduction rate (Marphysa, Lumbriconereis, etc.), a condition 

 suggestive of compensatory changes in submicroscopic structure and 

 impedance in the axonic membrane (26). 



The majority of invertebrate nerve fibres are non-medullated, i.e. they 

 possess no osmophilic myelin sheath. By the use of suitable techniques, 

 nevertheless, it can be shown that all nerve fibres are invested by a layer 

 of oriented lipoid. This lipoidal layer is only some 1 % of total fibre 

 diameter in giant axons of squid and sabellid polychaetes. A few inverte- 



