CHAPTER 23 



Nervous Systems 



INTRODUCTION 



■ HE METABOLIC FUNCTIONS of an animal can proceed without spe- 

 I cialized conducting tissue. The speciaHzation of conducting 



m tissue, particularly of nervous tissue, gives an animal a certain 



freedom from the environment which has survival value, permitting a com- 

 bination of large size and high motility. Many Protozoa and free-swimming 

 larvae are small and have little rapid conduction from one part to another, 

 yet are motile. Many sessile animals have slightly developed conducting 

 mechanisms and may be large, but they are not motile. All of those animals 

 which are both large and motile have rapidly conducting nervous tissue. 



In the evolution of nervous tissues four trends are apparent: (1) increas- 

 ing speed of conduction; (2) integration, or making one impulse count 

 for more or less than one; (3) cephalic dominance; and (4) modifiability 

 of the pattern of response. 



NON-NERVOUS CONDUCTION 



Among the animals which do not have nervous systems some coordinat- 

 ing mechanisms have been described. Probably all living cells have the 

 ability to conduct waves of excitation. Conduction in many plant cells, eggs, 

 and body cells of higher animals has been described as a wave of depolariza- 

 tion passing along the plasma membrane. Conduction from cell to cell, as 

 in ciliated epithelium, is well known. Conduction by way of many intra- 

 cellular fibrils has been claimed for a few Protozoa. ^^^ 



Many ciliates have a regular network of fibrils connecting the basal gran- 

 ules of the cilia. The form of this network varies, but in general the basal 

 granules are connected in longitudinal rows with some cross connections. ^^^ 

 This peripheral network is connected to the central neuromotor system, 

 which may have a center or motorium and has internal fibrils which extend 

 to membranelles, cytopharynx, and cirri when these are present. The entire 

 fibrillar system has been variously claimed to be supporting, contractile, 

 and conducting. Experimental evidence for a conducting function came 

 from Taylor,'^'-'^ who with microneedles cut fibrils between the motorium 

 and the cirri in Euplotes (and cut the peripheral fibrils as well). Coordina- 

 tion between the cirri and membranelles in the oral region disappeared. Co- 

 ordination of membranelles was disrupted after destruction of the motorium 

 in Chlamydodon,^"^^ and ciliary movement was disorganized after fibrillar 

 injury in Ichthyophthicus.'^'^^ However in Parameciuvt lesions near the peri- 

 stome did not destroy ciliary coordination,-"^^ even though a motorium is 

 said to be located there.^'^^ 



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