150 THE PHYSIOLOGY OF EARTHWORMS 



Pharmacological evidence as to the response of the body wall to 

 various transmitter substances shows that there is a relative lack of 

 sensitivity to acetylcholine, the threshold being between 10~4 g/L, 

 compared with the activity of the gut. The body muscle response is 

 greatly potentiated by eserine which lowers the threshold to 10"^- 

 10"^ g/1. As eserine inhibits the action of choHnesterase by com- 

 petitive action this prompts one to postulate a fairly large concen- 

 tration of this enzyme present in the normal earthworm body wall 

 preventing acetylcholine action (and also in the body wall of Hiriido 

 and Arenicola). Nicotine causes a single large contraction and com- 

 pletely abolishes the muscular response to acetylcholine in high 

 concentrations. This, coupled with the observations of Mendes and 

 Nonato (1957) that acetylcholine causes vasoconstriction in the 

 body wall suggests that acetylcholine may be a transmitter in the 

 peripheral nervous system. 



Electrophysiology 



The electrical activity of the nervous system associated with the 

 normal locomotion of earthworms has only once been described, 

 by Gray and Lissmann (1938). Recordings were made from the 

 ventral nerve cord along the length of which, adjacent to the 

 recording site, the segmental nerves had been cut. When the 

 animal was still no nerve action potentials were seen in the central 

 nervous system. If peristalsis started, however, a well-defined 

 rhythm of impulses was recorded, the frequency of which was 

 identical to that of the body movement. So far as could be judged 

 the electrical potentials coincided with the phase of longitudinal 

 contraction but the observation is not conclusive. It is less easy to 

 explain the spontaneous activity of the nerve cord when isolated 

 from the body. Rhythmic bursts of potentials occur as in the 

 in situ recordings, but in the majority of cases the frequency and 

 duration of the bursts is not like that of peristalsis. The variation in 

 rhythm may be partly due to independent movement of the isolated 

 cord on the electrodes, since the nerve fibres are enclosed in a 

 muscle sheath which contracts with a rhythm of its own. 



Although these electrical rhythms seem to be the basal nervous 

 activity of the animal Gray and Lissmann (1938) were unable to 

 conclude that these initiate movement in the earthworm. They 

 offered two explanations, between which they could not choose. 



