CHAPTER 10 



NERVOUS SYSTEM AND BEHAVIOUR 



In the multicellular animal, especially for those higher reactions which 

 constitute its behaviour as a social unit in the natural economy, it is 

 nervous reaction which par excellence integrates it, welds it together 

 from its components and constitutes it from a mere collection of organs 

 an animal individual. 



Sir Charles Sherrington, 1947. The Integrative Action of the 



Nervous System. 



Co-ordination in metazoans is accomplished by two mechanisms, 

 chemical (hormonal) and nervous. In a broad sense, all chemical substances 

 which penetrate into the body, or are released by the cells in the normal 

 course of metabolism and circulate in the body fluids, bring about changes 

 in the organism. Many higher animals, however, have evolved endocrine 

 glands which discharge chemical mediators into the blood stream, and 

 various aspects of hormonal co-ordination are noted in other chapters. 



All animals above the sponges possess a specialized conducting tissue, 

 the nervous system, for controlling the visceral activities of the organism, 

 and for mediating and co-ordinating responses to environmental events. 

 In simpler animals the nervous system is diffuse and has the appearance 

 of a network. In more complex organisms there is a tendency for the 

 integrative functions of the nervous system to be localized in special centres 

 — ganglia, brain and nerve cords — which contain most of the nerve cell 

 bodies, and from which nerve fibres go to and from the periphery and the 

 various organs. The most advanced nervous systems are found in active, 

 highly organized animals, arthropods, cephalopods and vertebrates. In 

 their highest expression, such nervous systems permit complex behaviour, 

 variability of response to change of environmental circumstances and 

 utilization of past experience in the life-span of the animal. In short, 

 animals endowed with complicated nervous systems are able to perceive 

 a greater range of environmental stimuli, and react to them in intricate 

 and varied ways. 



CO-ORDINATION IN ABSENCE OF A NERVOUS SYSTEM 



Excitation and conduction are regarded as generalized properties of 

 protoplasm, occurring in all cells. Conduction of an excitatory state 

 across the cell surface, or even from cell to cell, has been described in 

 plant cells, eggs and other non-nervous elements. Transmission across 

 ciliary fields by non-nervous mechanisms is well known in animals, and 

 special conditions are found in protozoans and sponges, which lack any- 

 thing that can be termed nervous tissue. During transmission a wave of 



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