34 The Physiology of Sense Organs 



evidence*^ to indicate that inherent differences in the excitability 

 of the membrane are Hkely to occur from one part of the cell to the 

 next. Thus, the site of impulse initiation cannot be inherently 

 obvious from the geometry, and attempts to determine its locus 

 on a geometrical basis, seem unwarranted without sound experi- 

 mental evidence for corroboration. 



The measurement of impulse threshold — or firing level — of a 

 sensory neuron by an intracellular electrode is critically dependent 

 not only on the absolute value of depolarization necessary at the 

 site of initiation to trigger the spike, but also on the spatial 

 relationship of the recording pipette to this site. If the recording 

 electrode is inserted directly beneath the membrane region which 

 first generates the impulse, it will of course measure the absolute 

 value of depolarization needed at threshold. If, however, the 

 electrode is spatially removed from the locus of origin of the spike, 

 the depolarization recorded at threshold may be quite a different 

 value. It will be less if the zone of initiation is between the 

 electrode and the source of the receptor potential, and greater if 

 the electrode intervenes between these two regions. In some 

 cases, it may be that the region of electrically-excitable membrane 

 having the lowest threshold is close to, or forms a mosaic with, the 

 electrically-inexcitable membrane which generates the receptor 

 potential. In such cases, the spike-generating mechanism will 

 detect the true magnitude of the receptor potential, since the 

 latter will show no relative degradation by current loss through 

 intervening membrane. 



The absorption of stimulus energy in a receptor cell or a 

 primary sensory neuron does not always result in a depolarizing 

 potential change. A graded hyperpolarizing response has been 

 observed, for example, in the blowfly contact chemoreceptor 

 (fig. 13). This effect presumably originates in a single neuron or 

 group of primary sensory neurons. The existence of hyper- 

 polarizing receptor potentials may also be inferred from the 

 results of investigations on primary photosensory neurons in 

 Bivalve molluscs. Hartline*' succeeded in recording activity 

 from the optic nerve of a scallop. This nerve contains two 

 branches, one of which innervates only the sensory neuron 

 population in the distal layer of the retina. This branch was found 

 to produce spontaneous discharge of impulses when the eye was 



