Sensory Cell Function and Architecture 85 



of this cell which separates the tip of the distal process from the 

 impulse-initiating region (at or near the cell body) runs through 

 an extracellular channel which has an extremely high electrical 

 resistance. The current which enters the cell at the transducer 

 locus near the tip of the distal process may consequently attain 

 high outward density in the region of the cell body, where the 

 spikes are probably generated.^®*- ^®^ 



Fig. 37. Diagram of the membrane and myelin sheath in the region 

 of the nerve terminal in a Pacinian corpuscle. I, II indicate, 

 respectively, the first and second nodes of Ranvier. The elements 

 labeled G represent separate independent generators of current on 

 the electrically-inexcitable membrane of the nerve terminal; hypo- 

 thetical lines of current flow due to a stimulus are indicated.'" 



By far the most elusive of the properties of sensory cells which 

 influence impulse initiation are those factors affecting the functional 

 nature of the membrane itself. Little is known concerning the 

 structural differences between electrically excitable and inexcitable 

 membranes. The final classification of a particular region into 

 one of these categories must always depend upon electrophysio- 

 logical analysis. The resolution obtainable by these techniques 

 is not nearly as fine as could be wished for, and no complete 

 functional map of the membrane has yet been executed success- 

 fully for any neuron. Some extreme examples can be cited, 

 however, which may be susceptible to experimental analysis, a 

 likely one being large, completely electrically-inexcitable structures, 

 such as arthropod central somata. Most axons, by definition, 

 possess electrically-excitable plasma membrane, but even this 

 property is, in the strictest sense, a generic term embracing various 

 types of response to identical electrical stimuli. This point is 



