398 



THE BIOLOGY OF MARINE ANIMALS 



Shocks of the electric skate (Raja clavata) are rather weak, about 4 V 

 maximum (measured in air). 



When an electric torpedo discharges, each electric organ fires off a train 

 of some 2-7 pulses at intervals of a few msec (Fig. 9.19). Individual 

 responses are of constant height and obey the all-or-nothing law. Each 

 spike lasts 1-75-2-25 msec, with a rising phase of 0-5-0-7 msec; latencies 

 vary from 1-4 msec (Torpedo torpedo). In skates (Raja) they are much 

 longer, about 12 msec, with rising phase 2-2-5 msec. The spike is mono- 

 phasic and hence non-propagated in the ray and torpedo. 



The electric plates of Electrophorus are directly excitable, whereas those 



Fig. 9.19. Records of Discharge of the Electric Organ of Torpedo torpedo 



Upper record, natural discharge; five following traces, discharge provoked by stimula- 

 tion of the electric lobes with alternating current, at frequencies of 1,000, 500, 200, 150 

 and 100 c/s. (From Albe-Fessard (5).) 



of Torpedo are not and can be excited only through the nerve. In the latter 

 animal nerve section and degeneration render the organ inexcitable. Each 

 time the electric lobe of the medulla or the nerve is stimulated a single 

 discharge is produced in the electric organ. Reflex stimulation produces 

 a volley of impulses in the nerve to the electric organ, accompanied by a 

 corresponding series of electric discharges (Fig. 9.20). Functional analysis 

 of the reflex pathway has revealed a relay centre (oval nucleus) in the 

 medulla, where ascending fibres from the cord terminate. From this 

 bulbar nucleus impulses are relayed to motor neurones of the electric 

 organ in the electric lobe. All parts of both organs discharge almost 

 synchronously, within a period of 1 msec, thereby providing maximal 

 power output. It appears that this synchrony is due to different latencies 



