Sensory Cell Function and Architecture 87 



Summary 



The impulse-initiating regions of a sensory neuron are determined 

 both by gross neuron geometry and by functional divisions within 

 the membrane of the cell. The transducer locus of a cell must be 

 connected to those regions where impulse initiation occurs by a 

 low resistance pathway, for sufficient current must pass outward 

 across the membrane of the latter regions to depolarize the 

 membrane to threshold values. Large areas of expanded mem- 

 brane, such as neuron somata, tend to draw current by their large 

 capacitances; this process may deprive adjacent axonal regions 

 of sufficient current to trigger conducted impulses. Usually, 

 however, membrane in the axon hillock region of various nerve 

 cells is modified (by as yet unknown parameters) so that it has a 

 greater excitability to outward currents than other parts of the 

 cell; such areas of low-threshold membrane normally constitute 

 the site of initiation of orthodromic impulses. 



