CHEMORECEPTION 149 



the side-wall spike peaked more quickly, and they took this to mean 

 that the differences in height were not due to a gradient of electrotonic 

 spread. They also concluded from this result that there is back firing, 

 and they argued that impulses are recorded extracellularly and not 

 intracellularly as Wolbarsht (1958) concluded. 



Hanson and Wolbarsht (1962) re-examined the situation in 

 PJwrmia and found that the impulse is indeed conducted some dis- 

 tance down the dendrite, that distance being a large fraction, if not 

 the entire length, of the short ones, but only a small fraction of the 

 length of longer ones. Electrodes located at various places along the 

 dendrite all showed an initially positive phase of the impulse some- 

 times followed by a longer negative phase. The following interpreta- 

 tion was proposed : the action potential is generated at a considerable 

 distance from the recording electrode and is recorded as a positive 

 swing. As the action potential invades the dendrite, the active region 

 comes beneath the recording electrode and appears as a negative 

 deflection. Simultaneous records from several electrodes placed 

 along the dendrite show that the active region of the membrane pro- 

 ceeds from the cell body toward the trip. The record resulting from 

 an algebraic sum of the two phases shows the positive phase being 

 abruptly terminated by the longer lasting negative phase. Cocaine, 

 xylocaine, procaline, and chloral hydrate reversibly depress or abolish 

 this negative phase with a correspondingly longer lasting positive 

 phase. Continuous chemical stimulation or injury accentuates and/or 

 speeds up the onset of the negative phase. 



The Sugar Receptor 



Since different receptor cells in a contact chemoreceptive hair respond 

 specifically to different chemicals, the question of mechanism of 

 stimulation, that is, how the chemicals depolarize the dendrite mem- 

 brane, should be investigated separately in the various types. The types 

 are most clearly delineated in the blowfly. In this insect, as already 

 pointed out, there is one cell specifically sensitive to sugars and one to 

 monovalent salts. A modestly detailed story of the operation of the 

 sugar receptor has been constructed from behavioural studies. The 

 task now presented to electrophysiology is that of proof-reading the 

 story, filling in the details, and revising where necessary. 



Carbohydrates are the most effective, if not the only, stimuli; how- 

 ever, this cell is highly specific and unequally sensitive to the various 

 compounds. Behavioural tests in which eighty-two different carbo- 

 hydrates were applied to the chemoreceptive hair housing the receptor 



