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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



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FIG. 3. Responses at the cathode of single carcinus a,\ons to 

 constant currents. A-H: increasing currents as indicated. /-A .■ 

 near threshold currents at higher amplification and faster sweep 

 speed. L. potential change at anode, conditions as /-A. [From 

 Hodgkin (49).] 



critical potential remains the same at all but the 

 highest values of impulse frequenc\ . 



The initiation of impulses b\ the receptor potential 

 generated in the muscle spindle of the frog has also 

 been observed (58). In this preparation the critical 

 potential remains constant for all except the first im- 

 pulse in a discharge at a constant frequency, but the 

 value is different for different frequencies; in fact 

 there is a direct and linear relationship between the 

 value of the critical potential and the frequency of 

 the discharge. An explanation of this phenomenon 

 has been given as follows (58) : recovery after a nerve 

 impulse depends on two processes o) a restoration of 

 membrane resistance and 6) a return of excitability 

 (see 48). If the first of these processes is the more 

 rapid in the frog muscle spindle fibers, but not in 

 the crustacean fiber, then results such as have been 

 observed would be expected. 



Modifuation 0/ Afferent Discharges by Current 



Afferent discharges can be modified by the applica- 

 tion of currents to the regions in which such dis- 

 charges are set up. This can be seen in the frog's 

 muscle spindle (25); if the spindle is made to dis- 

 charge at a suitable frequency by stretch and a current 

 is applied between an electrode on the afferent nerve 

 and another on the muscle, the frequency is increased 

 if the electrode on the muscle is the cathode and de- 

 creased if this electrode is the anode. The increase or 

 decrease of frequency is related to the intensity of 

 current, though the relation is not a simple one. Other 

 preparations exhibit similar effects. Current passed 

 through the nerve terminals of the isolated labyrinth 

 of the rav causes an increase in the frequency ot 

 discharge in these fibers when the cathode is on the 

 tissue surrounding the .sensory endings and the anode 

 on the afferent nerve fibers; a current in the opposite 

 direction causes a reduction in the discharge fre- 

 quency (71). These changes caused by the flow of 

 current summate with those due to angular accelera- 

 tion in the appropriate direction. Similar results can 

 be observed by polarization of the lateralis organs of 

 Xenopus laevis. It has been shown that when the applied 

 current flows along the nerve fiber, as in the instances 

 already described, an increase in frequency occurs 

 when the cathode is on the terminal and the anode on 

 the nerve; however, if the current flows between elec- 

 trodes placed on either side of the skin, the frequency 

 is increased when the cathode is on the inside and the 

 anode on the outside (77). The frequency of discharge 

 in the nerve fibers from the lateral line organ of the 

 Japanese eel is also increased by a current passed 

 between an anode on the outside of the skin and a 

 cathode on the inside (55); the passage of a current in 

 the same direction has been shown to excite afferent 

 fibers from touch receptors in frog skin (73). Currents 

 can al.so modifv the discharge from a compound eye 

 (40). 



These results are important in two respects. First, 

 depolarization of the terminal parts of the axon 

 membrane can summate with end organ activity 

 which suggests that the latter involves a depolariza- 

 tion of the terminals. This is in fact known to occur 

 in manv instances which will be considered below. 

 Second, it can be argued from the results obtained 

 with currents pas.sed across the skin instead of along 

 the ner\e that, during sensory activity, impulses are 

 initiated away from the terminal (77)- Direct evi- 

 dence that this is so in certain instances will be given 

 later. 



