SYNAPTIC AND EPHAPTIC TRANSMISSION 



159 



FIG. II. Desensitization of the synaptic membrane of frog sartorius muscle fibers by sustained 

 applications of acetylcholine. The drug was applied through each of two pipettes close to the surface 

 of the endplate. From one pipette it was released at regular intervals in brief jets of approximately 

 constant quantity. These testing stimuli are signaled by dots on the lower line in each set. The upper 

 line shows the response of the endplate recorded with an internal microelectrode. The e.p.p.'s in 

 these records are compressed on the slow time scale. In the course of the recordings a larger longer- 

 lasting jet of diflTerent amounts of acetylcholine was also applied to the endplate as a conditioning 

 stimulus. Lejt: An otherwise normal preparation, a: The conditioning stimulus was a weak dose of 

 acetylcholine applied for a long time, b to d: The concentration was higher, and the drug was applied 

 for different times. The testing responses diminished progressively during the depolarization pro- 

 duced by the conditioning stimulus. Their amplitudes recovered gradually after the conditioning 

 depolarization had ended. Note that the recovery from desensitization is not associated with further 

 change in potential. The recovery process therefore is not controlled by the membrane potential. 

 Right: The muscle was immersed in isotonic potassium sulfate which depolarized the fibers and 

 rendered them unresponsive to electrical stimuli. The tested muscle fiber was made inside-positive 

 by about 15 mv by means of an intracellularly applied current. The synaptic membrane remains 

 excitable to acetylcholine following these procedures, but the sign of the response is now reversed 

 for reasons that will be discussed in the third subsection of this portion of this chapter. The membrane 

 still exhibits desensitization to different intensities of the excitant drug (jop to bollom). The desensiti- 

 zation process itself therefore is also not controlled by the membrane potential. At the end of the 

 lower record the internal recording electrode was withdrawn from the muscle fiber (at the arrow}, the 

 trace going from a level of internal positivity to that of the reference zero potential. [From Katz & 

 Theslefr(i27).] 



develop. Thus, the sustained depolarization at 

 sensory receptor terminals or at synaptic junctions, 

 which is a property of electrically inexcitable mem- 

 brane while initially excitatory for the associated 

 electrically excitable spike generator can, secondarily, 

 inactivate the latter and thereby block further conduc- 

 tile or transmissional activity. 



This effect accounts for Wedensky inhibition, the 

 failure of transmission produced by stimulating the 

 presynaptic nerve at high frequency. Summated and 



sustained by this synaptic drive, the depolarizing 

 p.s.p.'s at first evoke a few spikes which then cease to 

 develop while the large p.s.p.'s continue to be pro- 

 duced by the afferent stiinulation (fig. 9). Although 

 Weden.sky inhibition is probably of little importance 

 in physiological activity of organisms, the phenom- 

 enon has long interested physiologists because the 

 attempt to explain it in terms of electrical excitability 

 has proved uncon\incing (cf 81, 141). The presence 

 of electrically excitable and inexcitable electrogenesis 



