456 THE BIOLOGY OF MARINE ANIMALS 



critical value is reached. Nerve cells with high threshold may fire only a 

 single pulse when stimulated, others may respond with repetitive discharge, 

 e.g. many peripheral ganglionic neurones. Alterations in the chemical 

 environment greatly alter excitability. Spontaneously active cells possess 

 high levels of intrinsic excitability, resulting in instability and spontaneous 

 discharge. 



Spontaneous and rhythmic discharge of single neurones is well recog- 

 nized in peripheral receptors (e.g. lateral line of fish). In Limulus the 

 electrocardiogram shows slow waves accompanied by a burst of nervous 

 impulses for each heart beat. The slow waves correspond to the activity of 

 pacemaker cells, and these synchronize discharges from smaller neurones 

 regulating heart beat. Slow waves in central ganglia may also represent 

 synchronous slow beating of cells. Fast spikes probably represent the 

 asynchronous discharge of disparate neurones, each rhythmically but 

 independently active. Spontaneous activity in the bodies of nerve cells may 

 give rise to efferent impulses; these pass out to the somatic muscles in the 

 intact animal, and maintain or regulate tonicity, and initiate periodic 

 visceral activities, e.g. respiratory movements. Incoming messages, from 

 afferent pathways, are superimposed on this background of spontaneous 

 activity, and modify it in complex ways (18, 20, 44, 90). 



Apart from rhythmic oscillations controlling periodic visceral functions, 

 and rhythmic discharges forming background activity of peripheral recep- 

 tors, we may well conjecture as to the physiological significance of constant 

 ganglionic electrical activity. Is it a novel form of nervous activity, distinct 

 from the transitory potentials seen in nervous transmission? Is it the 

 physical manifestation of continuous patterns of nervous functioning, 

 regulating inherent phasic activities and behaviour patterns characteristic 

 of each species? Circus conduction in closed circuits, and slow fluctuations 

 in master pacemaker centres, may maintain and regulate long-term be- 

 haviour activities which are set or modified by periodic environmental 

 stimuli. But as yet we possess no clear concept of the underlying mechan- 

 isms determining phasic behaviour, modification of response, establish- 

 ment of the memory trace and utilization of past experience, to wit, those 

 particular ethological patterns characteristic of each species. 



Instinctive and Plastic Behaviour. The general pattern of behaviour 

 possessed by an animal is predetermined by heredity and restricted by 

 limits of specific organization. Among lower animals much instinctive 

 behaviour is of a relatively inflexible type, in which a given stimulus evokes 

 a predictable type of response and range of responses. Under natural con- 

 ditions it is probably only on rare occasions that an animal is subjected to 

 the simple form of stimulation employed in the laboratory for studying 

 behaviour. In nature an animal is exposed to a complex of different sensory 

 stimuli, the intensities and durations of which affect the pattern of response. 

 Analysis of sensori-neuro-response mechanisms, however, is greatly 

 facilitated by the analytical approach. 



Instinctive behaviour refers to certain patterns of activity characteristic 



