70 The Physiology of Sense Organs 



at least one of these is open to the external environment at the tip 

 of the sensillum. Within this canal are the distal processes of 

 sensory nerve cells. The somata of these cells are located beneath 

 the cuticle of the insect, close to the base of the sensillum. The 

 distal processes of the neurons must thus traverse the articulation 

 at the base of the structure, and at this point the inner canal 

 through which they run is highly constricted. As a result, the 

 electrical resistance of the canal between the base and the tip of the 

 sensillum is extremely high, due to the very small cross-sectional 

 area of the column of extracellular fluid in this region and the 

 close packing of the four or five distal processes of the sensory 

 neurons. Now recordings from these neurons are usually 

 obtained by making contact with the tip of the hair (and thus with 

 the neurons inside the tip) by means of an electrolyte-filled glass 

 capillary recording electrode. Due to the basal constriction, this 

 electrode is isolated from the distantly-located impulse-generating 

 membrane of the neuron by an unusually high extracellular 

 resistance. Since the value of this resistance apparently exceeds 

 the combined cell membrane resistance and internal longitudinal 

 axoplasmic resistance, action potentials are recorded by the tip 

 electrode as positive-going changes, as suggested by the equivalent 

 circuit detailed in figure 31. So long as absolute spike amplitude 

 and locus of closest approach of the impulse to the tip electrode 

 do not change during variations in stimulus strength, increases in 

 recorded spike amplitude can best be explained by decreases in 

 the electrical resistance of membrane areas beneath the recording 

 electrode, i.e. the membrane of the transducer region of the cell. 

 It is not surprising that the crustacean stretch-receptor 

 preparation, which has proved so useful in other studies of sensory 

 neuron properties, has also been used for an examination of 

 membrane-resistance changes during the application of a stimu- 

 lus. By passing constant current pulses through an intracellu- 

 lar recording pipette, evidence has been provided ^^ that such 

 changes also occur as a direct result of the absorption of stimulus 

 energy in these sensory neurons. Conclusive evidence is there- 

 fore available from several sensory systems that the immediate 

 consequence of stimulus application is an increase in the ionic 

 conductance of the sense cell membrane. The amplitude and 

 time-course of these conductance changes are also directly 



