CHAPTER 10 



The Nervous System, Sense Organs and 

 Behaviour 



The nervous system is composed essentially of elongated cells which 

 transmit electrical disturbances or impulses from one part of the body 

 to another. These nerve cells or 'neurones' are derived in develop- 

 ment from the ectoderm. Each consists of a nucleated cell body and a 

 long filament or 'axon'. Where the axon filaments run freely through 

 the body cavity they constitute the nerves, which may be afferent 

 (sensory) or efferent (motor) . The sensory neurones have their cell bodies 

 situated near the periphery. Indeed, they arise throughout post- 

 embryonic development by differentiation from ordinary ectodermal 

 cells which give off an axon process that grows inwards to join other 

 sensory nerves. The cell bodies of the motor neurones are situated in 

 the central nervous system, as also are the 'association neurones'. 



The sensory and motor nerves, with the association neurones, 

 provide the anatomical basis for behaviour. The disturbance or im- 

 pulse which is propagated along them consists in a change in electri- 

 cal potential, due to a momentary depolarization of the axon surface, 

 which allows a momentary out-flow of potassium ions, passing like 

 a wave throughout the neurone. These waves succeed one another 

 at a rate which varies with the intensity of the stimulus that is being 

 transmitted. The neurones, however, are not continuous with one 

 another. The branched terminations of the axon of one neurone 

 come into intimate association with the arborizations of another 

 neurone to form a 'synapse'. It is probable that the electrical dis- 

 turbance does not itself cross the synapse but causes the liberation 

 of some chemical substance, such as acetylcholine, which sets off a 

 fresh disturbance in the succeeding neurone. 



The simplest type of conduction in the central nervous system will 

 consist in the transmission of impulses by a sensory neurone from a 



