The Depolarizing Nature of the Trigger 29 



could be drawn between the spike generating membrane, on the 

 one hand, and those membrane regions responsible for stimulus 

 transduction, on the other. Important questions still remained 

 as to the functional relationship between receptor potential 

 magnitude and impulse threshold and frequency. The definitive 

 answers to these questions could only be revealed with the 

 development of suitable preparations using intracellular electrode 

 techniques, which would enable reliable measurements of the 

 absolute values for receptor potentials to be made at various 

 regions of the sensory neuron. It was not long before such a 

 preparation was found. It consisted of the large primary sensory 

 neurons which function as stretch detectors and are a characteristic 

 feature of the neuroanatomy of Macruran Decapod crustaceans. 

 Especially notable are those which are associated with the 

 abdominal muscle receptor organs (MROs), first described by 

 Alexandrowicz^ and commonly found in lobsters and crayfish. 

 There is a pair of these organs on each abdominal hemisegment, 

 making a complement of four in each segment. Each organ 

 consists of a primary sensory neuron and a small dorsally-located 

 muscle, which are so arranged that flexion of the abdomen 

 stretches the muscle and, if tension is strong enough, excites the 

 sensory neuron.*^' ^"^ A representation of a single pair of these 

 MROs is shown in figure lo, which shows the large dendritic 

 branches which proliferate from each roughly-pyramidal cell 

 body and imbed themselves in the structure of the muscle. The 

 organs are readily accessible for dissection purposes and remain in 

 functional condition for several hours when isolated in an appro- 

 priate physiological solution. Moreover, the muscles associated 

 with the sensory neurons can be removed in an intact condition, so 

 that passive and active stimulation of the sensory dendrites can 

 occur very much as happens under natural circumstances. It is 

 interesting to find that there are physiological diff^erences, not only 

 between the two sensory neurons of a pair, but also (and compar- 

 ably) between the muscle fibers of the two organs. Thus, one 

 neuron adapts to mechanical stimuli quite rapidly and is most 

 effectively activated by rapid changes in the tension of its associated 

 muscle (which consists of phasic twitch-type fibers); the other 

 neuron adapts very slowly, often maintaining nearly constant 

 firing frequencies for periods of an hour or longer. The muscle 



