10 INVERTEBRATE PHYSIOLOGY 



The evidence against electrical transmission, based on the absence or 

 minute size of the voltage change across the postsynaptic membrane pro- 

 duced by the arrival of the presynaptic impulse (del Castillo and Katz, 

 1954, on muscle; Bullock and Hagiwara, 1955, on squid) may be con- 

 clusive. But this does not mean, as some have supposed, that weak electric 

 fields are without influence on poised or already active neurons. The ex- 

 periments cited on cardiac ganglia, as well as many others of the same sort, 

 classical and recent, are direct and pertinent and, when considered quanti- 

 tatively, impressive in the sensitivity they bespeak. 



The significance of this sensitivity is the enormous integrating poten- 

 tiality, in complex centers, of the fields of current interacting among small 

 and large groups of neurons. Here ordinary synaptic pathways give way 

 in importance to architectonics. And synchronization and desynchroniza- 

 tion of graded subthreshold activity of somata and dendrites take on a para- 

 mount significance both in producing fields effective upon other units and 

 in sensitizing the somata and dendrites themselves to effects en masse (cf . 

 Fessard, 1954). 



Taken together with our earlier conclusion (see above under sub- 

 threshold excitability) about tonic subthreshold influence, these consid- 

 erations also lead us to the suggestion that much of normal nervous func- 

 tion occurs without impulses but mediated by graded activity, not only as 

 response but also as stimulus. 



Pattern Formation in the Discharge of Groups of Neurons 



As a special case of the most general interest we may examine the inte- 

 grative mechanisms capable of organizing patterned bursts of impulses in 

 which the serial order is determined centrally and in which several efferent 

 neurons are coordinated. Since overt behavior consists in just such co- 

 ordinated bursts of impulses, as far as its neurophysiology is concerned, 

 this problem is a large segment of the problem of behavior. It is too much 

 to expect that we can enunciate a satisfactory general solution or even a 

 complete solution of a single case. But I believe there are some things we 

 can say which will carry us quite a way in accounting for simple patterns 

 with only the properties outlined above. Actually there is little difficulty in 

 drawing hypothetical circuit diagrams of neurons with connections and 

 properties within known limits which will produce a given pattern of out- 

 put impulses in space and time. But there has been little effort to discover 

 what actual neurons and connections are employed in real cases, perhaps 

 because the enormous neuron pools in the familiar cases are too complex 

 in sheer number of cells and impulses. A few cases have been studied re- 

 cently in which a very small number of nerve cells control a large muscula- 



