The Depolarizing Nature of the Trigger 33 



over firing frequency by the amplitude of the receptor potential 

 could be achieved with some precision. As the authors so vividly 

 described it, * . . . the present preparation provides a cell which 

 can be poised delicately on the verge of activity, maintained in 

 activity, accelerated or depressed — all through mechanisms within 

 the dendrites, not necessarily involving axon-type conduction '.^^ 

 Since the receptor potential spreads decrementally throughout 

 a sensory neuron from the locus of its origin — normally the 

 point(s) on the membrane exposed to stimulus energy, — its 

 influence at any distant point will decrease as the area of the 

 membrane intervening between the potential source and that 

 point increases. In a uniform cylindrically-shaped cell projection, 

 such as an axon, this decrement may be expressed quantitatively 

 by the following exponential relation: 



V=V, exp [-xl{rJr,Y] 



where Vq is the source voltage, V the voltage measured at a 

 distance x from the source, and r^ and r- are, respectively, the 

 electrical resistance of the cell membrane and the internal 

 cytoplasm. The term {rjr^^ is known as the space constant of 

 the axon under examination, and it is numerically equal to the 

 distance within which the original voltage (Fq) falls to i/c, or 

 about 37 per cent, of its source value. It is, however, extremely 

 difficult to describe in quantitative terms the decrement of 

 receptor potential with distance in the stretch receptor neurons, 

 for this occurs in a complex of dendritic branches forming a 

 network with the soma and having unknown axial resistances and 

 varying membrane geometry. From the foregoing, it must be 

 evident that impulse activity cannot arise simultaneously in all 

 parts of the cell, since the receptor potential (or its electrotonic 

 derivative) clearly has a varying amplitude along the membrane. 

 In fact, the impulse almost certainly originates in a fairly restricted 

 region of the electrically-excitable membrane. It seems reasonable 

 to assume that this locus would be in the region of the impulse- 

 supporting membrane which is closest to the generator of the 

 receptor potential; for it is in such a region that threshold values 

 of the latter would be reached first. This most certainly would 

 be true if membrane excitability throughout a neuron were 

 everywhere similar, but this is not so. There is now good 



