Sensory Cell Function and Architecture 79 



output frequency of the cell had actually decreased. Thus, there 

 are distinct advantages in having the location of impulse initiation 

 on the final common pathway of information flow, i.e. the axon. 

 Difficulties encountered by the interaction of separate regions 

 initiating regenerative activity are thereby avoided, and the soma 

 and dendritic tree form an integrative unit which summates 

 graded, electrical activity from all available transducer loci. 



In all cases in which the question has been examined, the region 

 of membrane in a primary sensory neuron which is concerned 

 with the transduction of stimulus energy to graded electrical 

 activity appears to be electrically inexcitable, and spike initiation 

 occurs at a more proximal location along the neuron. In the 

 crayfish slowly-adapting stretch-neuron, the finer dendritic 

 branches (i.e. those imbedded in the receptor muscle and exposed 

 to mechanical deformation) apparently do not support impulse 

 activity^' and spike initiation occurs on the axon proper, at a 

 point several hundred microns central to the cell body. The data 

 illustrated in figure 35 provide unambiguous evidence that spikes 

 arise first at a point central to the soma. Now the extracellular 

 records of the electrical changes surrounding an axon immersed 

 in a volume conductor (saline) show the approaching impulse at 

 the active electrode first as a positive-going potential change 

 relative to a distant point in the medium; for action currents just 

 ahead of the spike cross the membrane in an outward direction. 

 As the impulse itself passes underneath the electrode the current- 

 flux is inward (negative-going) but again becomes outwardly- 

 directed as the spike passes by. Thus, the sequence ' source- 

 sink-source ' can be detected only when an impulse is actively 

 propagated past an external recording point on the cell. An 

 impulse which approaches, but never actually reaches, the region 

 of membrane beneath the recording electrode will be detected 

 only as a positive deflexion; while an electrode at the site of origin 

 of the spike will record a negative potential followed by a positive 

 one. These concepts can be related to the records in figure 35. 

 At some point on the axon, the configuration of the action potential 

 generated by an adequate stimulus to the receptor was found to be 

 a diphasic one (with negative-positive phases) in contrast to the 

 triphasic waveform obtained at all other loci. Presumably, 

 impulses are initiated at one point on the axon and then propagate 



