782 Comparative Animal Physiology 



Whatever the function of ACh, it is certain that even the molecular lip- 

 oid layer of squid giant axons is virtually impermeable to ACh (a quater- 

 nary amine), whereas it is readily penetrated by trimethyl amine. In regions 

 where lipoid membranes are absent (or altered), as at junctions, low con- 

 centrations of ACh are exciting and higher concentrations are depressant. 

 Complete (more than 90 per cent) inhibition of cholinesterase abolishes 

 nerve conduction, presumably by permitting accumulation of depressant 

 concentrations of acetylcholine.^"* Quaternary anti-esterases such as prostig- 

 mine and curare fail to penetrate nerve fibers but physostigmine and dipro- 

 pylfluoro phosphate (DFP) penetrate and block conduction. The action 

 of the toxic agent, DFP, and of the insecticides hexaethyl tetraphosphate 

 and tetraethyl pyrophosphate appears to be due to their anti-esterase prop- 

 erties.^'' Cholinesterase is widely distributed, is concentrated in the axon 

 sheath (squid giant fiber), and has very high turnover number.^*'^ 



Two types of cholinesterase have been identified. One (Fig. 295, A) is 

 found in mammalian blood serum and in the dart sac of Helix. This is not 

 specific for acetylcholine; it splits propionylcholine and butyrylcholine at 

 higher rates, and it is not inhibited by high concentrations of acetylcholine. 

 The other type (Fig. 295, B, C) is characteristic of all excitable tissues, 

 nerve, muscle, and electric organs; this splits acetylcholine at a higher rate 

 than other choline esters and shows a sharp optimal ACh concentration.^* 



The chemical events underlying the electrical activity are probably simi- 

 lar in all nerves. However, quantitative differences among nerves and ani- 

 mals may be used in learning the significance of different steps in the com- 

 plex chain of events. 



Speed of Conduction. The speed of conduction in nerve fibers and nerve 

 tracts differs from animal to animal and in different parts of the same ani- 

 mal. Five morphological adaptations of nerves correlate with increased 

 speed of reaction among animals: length of nerve processes, fiber diameter, 

 myelin sheath, nodes, and giant neurones. There are probably intrinsic 

 differences among the nerve fiber membranes as well. 



Length of Nerve Processes. Animals increase their speed of reaction by 

 the lengthening of conducting processes. A message travels more quickly 

 by a single long fiber than when it has to pass through numerous synapses, 

 with a delay at each. In many coelenterates the nervous system consists of 

 a network of multipolar and bipolar neurones whose processes have been 

 traced for several millimeters.^^ Conduction in this network is slow, less 

 than 0.5 M./sec. (Table 75). In some coelenterates through tracts of con- 

 tinuous fibers exist for rapid conduction. ^^^ In general, fast-moving animals 

 have some nerve tracts consisting of long single fibers. Giant fiber conduc- 

 tion in annelids and arthropods is more rapid than conduction through the 

 neuropile or small fiber mass in their ganglion chains. 



Fiher Diameter. In mixed nerves of frogs and mammals speed of con- 

 duction is proportional to some function of the fiber diameter. The fiber- 

 diameter/velocity correspondence has not been investigated for the entire 

 spectrum of any invertebrate nerves. However, there is no doubt that faster 

 animals have some large motor nerve fibers, and in slower animals most 

 nerve fibers are small. 



