- to? -
with
a satiafactoxy recording eyatam* aotlon p o te n tia ls a ria s on the peak o f a voltage o s c illa tio n when they achieve a c r i t i c a lthreshold potential# I
O sc illa tio n s a t th e term ination of th e re p e titiv e response are found in a l l type U a axons# This i s seen in fig u re 47# which coopères h i ^ gain records
tram
the beginning and end o f are p e titiv e train# In th is fig u re a lin e i s drawn to pass th r o u ^ the le v el o f d ep o larisatio n a t which e a ^ successive actio n p o te n tia l develops# The c r i t i c a l le v el o f d ep o larisatio n r is e s throughout th e re p e titiv e response* and the o s c illa tio n s a t th e end o f the tr a in o f actio n p o te n tia ls f a i l to reach th is now elevated potential# The progre ssiv e r i s e in the threshold p o te n tia l re s u lts in a corresponding increase in th e in tersp ik e intervals* since i t tak es longer fb r th e subthreshold p o te n tia l to reach threshold level#
Th* Raeoywy Qyol*.
The recow zy qd.e of an
taxm
of th is type* as determined by a dual shock method* i s shown in fig u re 46# The absolute refracto zy period la s ts fo r about 2 msec* a f te r which a period o f markedBupem om ality develops* having i t s peak a f te r 3 to 4 msec* and
declining th e re a fte r u n til a t between 10 and 14 msee the threahold i s normal# Bziring th e follow ing 20 msec a sligÿit supem orm ality i s ^ observed# As noted by Hodgkin (1948)* i f th e recovery cycle alone determined th e re p e titiv e fzwquency* then a Ju st threshold current should y ie ld a tr a in o f actio n p o te n tia ls a t over 500/seo* which i s close to th e maximum th a t crab axons are capable# At threahold th e observed frequency i s between 50 and 90/seo while th e highest
frequencies w ith d ire s t current are only 250/sec (fig# 45)# The upper frequency lim it o f the r e p e titiv e response a t 250/sec shows
•103 -
th a t actio n p o te n tia ls t«i& to develop a t the peak of the
supem orm ality (3-4 msec) only to strong currents* so th a t w ith weaker currents th e stqM m oraality a c ts to inorease the ra te o f development of the subthreshold potential# The absence o f low frequency disdiarges to d ire c t current can presumably be re la te d to the r is e in the c r i t i c a l le v el of d ep o larisatio n fo r th e spike* and to the subnom ality la te in the reoovezy cycle#
Trains of Pulses#
S h o rt p u lses#
To tra in s o f short pulses* th is type o f
uocun
y ie ld s complete tr a in s o f actio n p o te n tia ls over a wide range o f frequencies* when the strength o f the sin g le pulse i s adjusted to be near threshold# Hcwevsr* a t stim ulation frequencies between 20 and dO/sec inoomplete tra in s are sometimes seen# These incomplete tr a in s are due to the fa ilu re o f the axon to respond to eveiy pulse* so th a t breaks appear* often q u ite reg u larly in the response# Complete tr a in s o f actio n p o te n tia ls develop w ith stim ulus frequw&oies as h i ^ as 3oo c/s*w ith re la tiv e ly l i t t l e increase in the cu rren t stren g th above ^ threshold fb r th e sin g le pulse* again dem onstrating the depressant actio n o f maintained depolarisation# When the effectiv en ess o f
tra in s o f short spikes i s compared to th a t o f d ire c t current (fig# 49)* i t i s seen th a t tra in s of spikes in q u ire a re la tiv e ly sm all increase in curr ent stren g th w ith in creasin g response A equencies w hile d ire c t current req u ires a more marked increase in cu rren t# Beyond 300/sec th e amplitude o f the actio n p o te n tia ls shows some decline# He
incomplete tr a in s o f the type described fo r type lb axons are obtained when the stren g th o f the pulse i s reduced below threehold fb r a
- 1 0 4 -
Long pulses#
Figure 50 I s of a continuous record of a se rie s o f repeated long duration current pulses each 250 msec* which s tin u la te the axon a t in te rv a ls o f 1 second# % ere i s a gradual progressive lengthening o f th e latency to the f i r s t spike to each
successive pulse# This shows th a t th e depression due to
prolonged cuzrent is cumulative* and can extend over a period a t le a s t three tim es the duration o f the stimulus# With such experiments* actio n p o te n tia ls are progressively lo s t
from
the fro n t and re a r o f th e re p e titiv e response* and subthreshold voltage o s c illa tio n s appear a t a frequency lower than the ^ ik e sthey replace# This change in frequency i s presumably sim ila r to the one th a t occurs a t th e boundary between o s c illa tio n s and
actio n p o te n tia ls in th e normal response to d ire c t cu rren t (fig#42)# No change in the c r i t i c a l le v e l d f d ep o larisatio n fo r the
spike was found in type 1 axons* where prolonged d ep o larisatio n has been shown to have a depressant effect** g re a te st follow ing an actio n potential# A sim ila r depression due to prolonged
d ep o larisatio n has been demonstrated in type U a axons* even though they possess a marked supem orm ality during th e ir recovery cycle# The subthreshold o s c illa tio n s fbllowing a re p e titiv e tr a in f a i l to reach the c r i t i c a l le v e l o f d ep o larisatio n fb r the spike because the l a t t e r i s rising# I t can be questlôned whether o r not the c r i t i c a l threahold p o te n tia l i s ris in g during the development o f the f i r s t subthreshold potential# The HodgldjHQfuxley equations permit a r is e in in a c tiv a tio n and potassium conductance as a re s u lt o f depolarisation* but such changes must be tra n s ito ry i f
subthreshold o s c illa tio n s are seen# In type 11a axons the c r i t i c a l lev el o f d ep o larisatio n fo r the f i r s t spike i s constant fo r current
^ :rfv^ =■, - 105 ■€ - I . . # %
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S " i l '
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Figur# 48# The reoovezy curve of a ty p io a l type U a axcm# Axon 79* bridge ^ 4 mymtem# Ordinate* threehold/threm hèld during reoovezy#
Aheoiaaa* in te rv a l between shock In msec# ^ ,i L Î T ” ' ' %*
i
itfS Î r--'-A./'y
: 'i ". A
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