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Repetitive stimulation of optic nerve and lateral geniculate synapses
EXPERIMENTAL
NEUROLOGY
Repetitive
1,
534-555
(1959)
Stimulation of Optic Nerve Lateral Geniculate Synapses
P. 0. BISHOP,~. Brain Research UGversity
and
BURKE, AND W. R. HAYHOW' Unit, Department of Physiology, Sydney, Sydney, Australia
of
Received
August
19, 1959
The effects of repetitive stimulation of the optic nerve on the electrical response recorded in the dorsal nucleus of the lateral geniculate nucleus of the cat have been studied. Observations have been restricted to effects on the fast-conducting group of optic fibers. In the main, tetanic frequency has been 500 to 700 per second, and duration of tetanus either 15 set or 10 min. Tetanic stimulation results in post-tetanic potentiation and depression of both presynaptic and postsynaptic responses. The conditions determining which effect is obtained are described. The presynaptic response during and following the tetanus has been studied, and the excitability of the optic tract and tract nerve endings has been tested by antidromic stimulation. Decrease in excitability of the optic nerve after tetanization is explained on the hypothesis that a hyperpolarization in all parts of the optic fiber follows the tetanus. An increase in amplitude of the presynaptic response also follows the tetanus but can be due only in small degree to an increase in spike height. It is more probably due to slowed conduction and perhaps to a longer duration of action potential in the optic tract nerve endings. The relationship between presynaptic and postsynaptic potential changes after a tetanus is discussed; there is no evidence in these experiments that the size of the presynaptic action potential is the sole or even the main determining factor of the postsynaptic potential changes. Introduction
Repetitive stimulation of a synapse is often a useful way of examining the normal behavior of that synapse, even when the stimulation is at a 1 This study was aided by grants from the National Health and Medical Research Council of Australia, from the Ophthalmic Research Institute of Australia, and from the Consolidated Medical Research Funds of the University of Sydney, Sydney, Australia. We are grateful to Dr. J. Smith for his assistance with some of the early experiments and we wish to acknowledge the skilled help given by the technical staff of the Physiology Department, particularly Mr. J. Stephens, Mr. D. Larnach, and Mr. B. McGee. Our thanks are due to Miss J. Kearsey for considerable secretarial assistance. 534
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frequency and for a duration not experienced under physiological conditions. Hughes, Evarts, and Marshall (23) and Evarts and Hughes (12, 13) have studied the effects of tetanic stimulation of the optic nerve on the response of the cells of the lateral geniculate nucleus in the cat. This paper confirms and extends their observations and examines in more detail the aftereffects of a tetanus on the optic tract nerve endings and the relationship between presynaptic and postsynaptic potential changes. The observations have been restricted to the effects resulting from stimulation of the fast group (ti) of optic fibers (5). Furthermore, the test stimuli have been given at 5-set intervals; changes occurring within 5 set from the end of the tetanus have not been examined. Methods Adult cats were used in all experiments. They were anesthetized with intraperitoneal allobarbitone (Dial, Ciba, 0.5 ml/kg) plus pentobarbitone sodium (Sagatal, May and Baker, 0.1-0.3 ml/kg). The latter drug ensured a reasonably rapid induction; by the time the first records were obtained several hours later, its effect had probably worn off. In a few experiments the cerveuu isole preparation (7) was used, preliminary induction being with ether. The cat’s body temperature, read with a rectal thermometer, was controlled within normal limits (3%39°C) with the aid of an electric heating blanket. One eyeball was resected and the optic nerve prepared for stimulation or recording by being suspended free of orbital tissuee2 The response of the contralateral lateral geniculate nucleus was recorded by means of a stereotaxically-directed steel microelectrode inserted vertically through the intact cerebral cortex. The optic tract was stimulated by means of a pair of steel stimulating electrodes, mounted about 1 mm apart. The position of the tips of these electrodes was checked initially by recording from each separately, the waveform of the response serving as a guide to the position, and finally, by electrolytically depositing iron and staining subsequently by the Prussian blue reaction (30). The recording and stimulating electrodes were prepared by electrolytically pointing steel beading needles (3, 18) and insulating to the tip with Bakelite varnish. The stimuli were rectangular pulses 50 psec in duration, applied once every 5 set except during the period of repetitive stimulation. Repetitive 2 See Bishop,
Jeremy,
and Lance
(5)
for
a fuller
description
of the method.
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stimulation was usually at about 500 to 700 per second for periods of 15 set or 10 min (12, 13, 23). Results and Comments
Tetanus for 15 Sec. Figure 1 illustrates typical changes in electrical responseoccurring in the dorsal nucleus of the lateral geniculate nucleus during and following high-frequency tetanic stimulation of the contralateral optic nerve for 15 sec.
FIG. 1. Effect of 15-set tetanus (stimulus supramaximal for tl fibers) to optic nerve on lateral geniculate nucleus response; a-g, test stimulus submaximal for t, fibers; a, before tetanus (670/set) ; b, 5 set; c, 15 set; d, 35 set; e, 110 set; f, 6 min, 40 set; g, 129 min, after end of tetanus; time-base, o. h-n, supramaximal test stimulus in another preparation; a, before tetanus (SOO/sec) ; i, j, t, response during tetanus (i, fifth second; j, fifteenth second) ; k, 5 set; 1, 35 set; m, 16 min; n, 78 min, after end of tetanus; time-base, p. In this and all other figures, negative is upwards.
Figure la showsa typical responsein the normal preparation, resulting from a single stimulus to the fast group (tl) of optic fibers (5). The large positive-negative wave (tI) following the artifact represents activity in theseoptic tract fibers; the negative wave ( rl) is the responseof geniculate somata and radiation axons innervated by tl fibers. The method of measuring the amplitude of these responseswas the sameas that adopted by Bishop and Evans (4). Records a to g show the responsesto a test shock submaximal for tI fibers, obtained (a) before and (b-g) at various times after a high-frequency tetanus; records h to n are from another preparation in which the test shock was supramaximal. The tetanic stimuli were
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always at least maximal in strength in these and all other experiments. The complete sequence of changes over a period of several minutes has been plotted in Figs. 2 and 3, respectively, for the two experiments illustrated in Fig. 1.
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FIG. 2. sponses in which are dotted lines
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Effect of Ssec tetanus on presynaptic (tI) and postsynaptic (rl) relateral geniculate nucleus. Graph of amplitudes of responses, some of shown in Fig. 1, a-g. Vertical line marks end of tetanus. Horizontal indicate mean level of pretetanic response.
Provided that the test shock is supramaximal for tl fibers, the posttetanic response of these fibers is greater for about a minute before returning to normal (Figs. lk, 3, 4). The amplitude increased by an average value of 117 per cent (measured 5 set from the end of the tetanus; fifteen experiments, range 100-142 per cent). If the test shock is submaximal, the tl response shows post-tetanic depression (Figs. 1, 2). This depression is usually no greater than to 9Q per cent but may take 10 to 20 min to disappear. In the case of a submaximal test shock, or of one which is barely maximal, the post-tetanic response may show both effects, i.e., an early brief potentiation followed by a prolonged depression (Fig. 2 ; also Figs. 5, 7). The increase in amplitude in the tl response occurs solely in the negative portion of the waveform, and even when the total amplitude is decreased the negative phase is usually increased (Fig. lc). Although the positive
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phase increases slightly in duration, the negative phase increases markedly in this respect (Figs. lk, 10a). Following a 15set tetanus, the response of the geniculate ceils (rl) is depressed for a few seconds, may then be larger than normal (1 to 2 min), and finally is markedly reduced for a period lasting for hours (Figs. 2,
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FIG. 3. Graph (t l(9) ), responses endings.
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similar to Fig. 2 from results partly illustrated in Fig. 1, h-n; in the optic nerve to a submaximal stimulus applied to tract nerve
3).3 In no case has the recovery from this delayed depression been followed to completion. In one instance the rl response, which had been depressed to 24 per cent, recovered to 45 per cent in 5 hours. In Fig. 2 the recovery seems more rapid than this. The average increase in amplitude of the rl response in four experiments in which the test shock was submaximal (50 to 75 per cent maximal) was to 112 per cent of the pretetanic value. When the test shock was supramaximal (five experiments) there was a potentiation to an average value of 103 per cent. It is difficult to compare these two sets of results be3 This period of potentiation corresponds to the “late potentiation” of Lloyd (29). The depression has been termed “second subnormality” by Hughes, Evarts, and Marshall (23). We prefer to call this “delayed depression.” The term “late depression” has been used (29) to refer to depression preceding “late potentiation.”
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cause, as already mentioned, with submaximal testing the t1 response is reduced in amplitude for 10 to 20 min, whereas there is no depression with supramaximal testing. One other change in the response of geniculate cells following a 15set tetanus should be mentioned. There is frequently a considerable fluctuation in the response, even when the pretetanic response is very steady (Fig. 3). In several experiments this fluctuation has taken on a regular oscillatory character (Fig. 4). These changes are not associated with any
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FIG. 4. Graph similar to Fig. 2 from another experiment; ulus. Note oscillatory behavior of rl following tetanus (630/set).
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change in the presynaptic response. In the experiment of Fig. 4, the period averaged 200 sec. In another experiment in which the oscillation was very marked, the period was about 500 set; tetanic stimulation was repeated four times in this preparation and on each occasion a similar periodicity was observed. In other cases (e.g., Fig. 3)) although some oscillation is evident, it is not possibleto be precise about the periodicity. This irregularity of responsehas not been seen after a IO-min tetanus (Figs. 5, 7). The effect has not been investigated further. Tetanus for 10 Min. The responsein the lateral geniculate nucleus following a lo-min tetanus has been graphed in Fig. 5. In this case, the test stimulus was submaximal for tl fibers and the t1 responseafter an early period of near-recovery was again depressedfor about 10 min. When the test stimulus is supramaximal there is a post-tetanic increase in tl amplitude, the mean value being 144 per cent (six experiments) at 5 set from
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the end of the tetanus. It is often the case, however, that a test stimulus which is supramaximal prior to the tetanus is submaximal after the tetanus due to the large decrease in excitability in the optic nerve (see later). The duration of the period of potentiation is rather longer than that following a IS-set tetanus, the average value being about 2 min. As in the case of
FIG.
(5Wsec)
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Graph showing the at submaximal strength
changes in of stimulus.
t,
and
rI
following
a lo-min
tetanus
the 15set tetanus, the increase in t1 is exclusively in the negative phase of the individual response, the positive phase being usually depressed in amplitude (Fig. lob, d) . Both positive and negative phases are increased in duration but the change is again much more marked in the negative phase (Fig. lob, d). The rl response shows a two-stage recovery which is quite characteristic. The second peak of recovery is usually 10 to 20 per cent higher than the first peak and may exceed the pretetanic value. The response finally returns to its normal level 5 or 6 min from the end of the tetanus. With submaximal testing stimuli, post-tetanic potentiation has never exceeded 2 per cent (Fig. 5) and is usually absent. This is probably due to the large decrease in excitability in the optic nerve (see later). With supramaximal testing, the first peak averaged 95 per cent, the second 106 per cent. There is no delayed depression following a lo-min tetanus. EBect of Frequency and Duration of Tetanus. In most of these experiments the frequency of stimulation during the tetanus was kept at 500 to 700 per second.Earlier experiments showedthat in the fresh prep-
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aration, frequencies of 180 per second or less did not produce appreciable effects either of potentiation or depression (excluding changes occurring within 5 set from the end of the tetanus). Tetani at frequencies of about 360 per second seemed to be almost as effective as those at higher frequencies. Lloyd (28) found a similar limiting frequency in the spinal cord monosynaptic pathway. Frequencies of about 500 per second can be sustained by the optic nerve for 1.5 set and longer, in several cases for 10 min, although more usually at this time, the t1 response is depressed or alternately large and small, indicating that many fibers can no longer follow the rate of stimulation. In regard to duration, earlier work by Evarts and colleagues (12, 13, 23) and by ourselves had shown that delayed depression was maximal following a tetanus of about IS-set duration, whereas if the duration was more than 6 min there was no depression. Consequently, in the present study, the standard tetani were of either 15 set or 10 min duration. Relief of Post-Tetanic Delayed Depression by Further Tetarti. In agreement with Evarts and Hughes (12, 13), we have found that the prolonged depression which follows a IS-set tetanus (SOO/sec) can be relieved by a further tetanus. This second 15set tetanus relieves the block for about 5 min before the former level of block or an even greater degree of depression is re-established; during the post-tetanic potentiation the response may reach and exceed the normal level. A IO-min tetanus produces a permanent relief of the depression; the response post-tetanically is not different in waveform from the response following a lo-min tetanus in the normal preparation. A lo-n-tin tetanus will also reduce the degree and duration of the post-tetanic delayed depression due to a subsequent 15set tetanus. This effect is, of course, maximal immediately following the lomin tetanus and declines over about 10 min (13). Regarding the frequency of stimulation, the minimal frequency effective in producing some relief of block was about 100 per second (1.5 set). However, the effect could also be demonstrated following a tetanus at 180 per second for 5 sec. Relief of Post-Tetanic Delayed Depression dwing a Short Train of Stimuli. If a train of shocks at about 500 per second is’applied to the optic nerve in the normal preparation, the rl response is greatly depressed after a few shocks (Fig. 6a; cf. Ref. 6). Usually there is a progressive reduction in the rl response but often the depression does not commence until the third response, especially if the frequency of stimulation is a little higher as in Fig. 6c. The exact pattern is also dependent on the
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depth of anesthesia. On the other hand, during post-tetanic delayed depression, a similar train of shocks produces a potentiation of the r1 response maximal at about the third shock, following which the response declines again (Fig. 6b).
FIG. 6. Effect of train of five or six stimuli at about 400 per second on response of lateral geniculate nucleus; a, normal; b, same preparation during post-tetanic delayed depression; c, another preparation, normal. Upper time calibration applies to a, b; lower to c.
Effect of Anesthetic. The experiments described so far were all done under barbiturate anesthesia. Several experiments were also performed on the rerveau isole’ preparation. The response to a single stimulus was not appreciably different to that in the Dial-anesthetized animal. As far as the post-tetanic changes were concerned, no difference could be discerned in the tI response. The rl response following a lo-min tetanus recovered more slowly (Fig. 7) but usually in two stages as in the anesthetized preparation. Contrary to the experience of Evarts and Hughes (13), we have always found a delayed depression following a 15set tetanus (five experiments) not significantly different in degree to that in the anesthetized preparation.
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Excitability Changes in the ,Optic Nerve and Nerve Endings. The excitability of optic tract endings in the lateral geniculate nucleus was tested by stimulating before and after tetanic stimulation of the optic nerve. The test stimuli were delivered via a pair of electrodes, the tips of which were about 1 mm apart, the cathode lying either in the lateral geniculate nucleus or just above it.
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FIG. 7. Graph similar to Fig. 5 but in cerveau isole’ preparation. Tetanus frequency 540 per second. Upper curve and points as in Fig. 3, from series partly illustrated in Fig. 8, c-h.
With the stimulating electrodes immediately above cellular layer A, the strength of the shock was gradually increased until the spread of the stimulating current just involved the optic nerve endings on the cells in layer A. The antidromic responsewas recorded from the contralateral optic nerve, monophasic records being obtained by crushing the nerve at the electrode distal to the brain (5). Figure 8 illustrates the results of two experiments. The responseconsistsof two waves due to the two groups of fibers in the nerve, one rapidly and the other more slowly conducting (tl and tz respectively). Because of the distribution of these fibers in the lateral geniculate nucleus, it is not possible to stimulate one group selectively by varying the strength of stimulus. The stimulus was always submaximal, hence presumably some fibers were being stimulated critically. As indicated above it was probable that the stimulus was being
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delivered at or very close to the nerve endings in the nucleus. Following an orthodromic tetanus in the tl fibers, the antidromic response of only these fibers was reduced, whereas the t2 response remained unaffected. Figure 8 illustrates the effects of a 15set tetanus (ad) and (in another preparation) of a 10-min tetanus (e-h). Results from an experiment sim-
FIG. 8. Effect of tetanic stimulation of optic nerve on excitability of tract nerve endings; a-d, responses in optic nerve to stimulation of tract nerve endings; tl,a,, response of fast-conducting optic nerve fibers; tzCn), response of slomconducting fibers; a, before 15-set tetanus (SOO/sec) ; b, 10 set; c, 20 set; d, 40 set after end of tetanus; time-base, i. e-h, another preparation; e, before lo-min tetanus (540/set) ; f, 10 set; g, 70 set; h, 140 set after end of tetanus; time-base, j. In each case the tetanus was applied to t, fibers only.
ilar to that in Fig. 8 (a-d) are graphed in Fig. 3 (tr,,,); results from the experiment illustrated in Fig. 8 (e-h) are graphed in Fig. 7 (tr,,,). Provided that the orthodromic test stimulus is supramaximal (i.e., ensuring that all tl fibers are being stimulated), the return of the presynaptic (tl) response to normal amplitude always parallels the return of excitability of these fibers as tested by antidromic stimulation (e.g., Fig. 3). If the orthodromic test stimulus is submaximal, the tr response is decreased for a considerable time after the tetanus (Figs. 5, 7), but there is always an early period of increased tr response which again parallels the return to normal excitability of the tract fibers (e.g., Fig. 7). This initial increase in t1 response occurs solely in the negative phase; the positive phase is always reduced. After a lo-min tetanus, from about the second minute there is also a slight depression of the negative phase. The positive and negative phases then increase pari passu until complete recovery. (These changes occur also after a 15-set tetanus but are much smaller and difficult to de-
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tect.) This suggests that the tI waveform is affected by two distinct events: a change in the nerve endings associated with decreased excitability lasting 1 or 2 min and attributable to hyperpolarization; and a decrease in the number of optic fibers being stimulated during a period of 10 min or more, responsible for the prolonged tI depression. If the test stimulus is supramaximal for tI fibers, the prolonged depression is absent. The latter event is presumably the result of decreased excitability at the site of stimulation in the optic nerve and might be due to the same factor as that which operates at the nerve endings, viz., a hyperpolarization following the tetanus. To check this point electrodes were introduced into the optic tract about 1 or 2 mm behind the chiasma and used for both stimulation and recording. The effect of tetanic stimulation of the optic nerve on the orthodromic response of the optic tract fibers in this situation was recorded and their excitability tested by direct stimulation (Fig. 9). The excitability changes were qualitatively the same as those described above in the case of the tract endings in the lateral geniculate nucleus. Quantitatively, the decrease in excitability was much greater and its duration rather longer (Fig. 9B, C). Another surprising feature was that the orthodromic response was almost unaffected (Fig. 9A, B) although in keeping with this finding is the fact that the prodromal positivity of the tl response recorded in the lateral geniculate nucleus is also almost unaffected following a 15sec tetanus (Fig. 10a). Potential Changes in the Optic Tract Nerve Endings. In view of the above findings on the excitability changes in the optic nerve, the changes in waveform of the presynaptic (tl) response were examined in more detail. In Fig. lOa, b, d, taken respectively from three experiments, are shown superimposed tI responses before and after a tetanus. The test stimulus in a and b was supramaximal for tl fibers, submaximal in d. During the tetanus (Fig. 10~)) the latency and duration of both positive and negative phases increases progressively. There is a decrease in the amplitude of the positive phase; the amplitude of the negative phase first increases, then decreases. Following the tetanus (records 2 in Fig. lOa, b, d) there is an increase in the amplitude of the negative phase, maximal at 5 or 10 set from the end of the tetanus; latency is still longer than normal. The positive phase, on the other hand, shows very little change. These changes disappear, each at the same rate, over about 1 min following a 15set tetanus (Figs. 3, 4).
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During a IO-min tetanus, the changes are similar but progress further than during a 15set tetanus; following a lo-min tetanus, therefore, these changes are more in evidence (Fig. lob, d) and the normal waveform is regained more slowly. After a tetanus, the most marked change is always in the negative phase, regarding both amplitude and duration; the positive
FIG. 9. A, graph of the amplitude of t, response recorded in the optic tract, before and following a IS-see tetanus at 500 per second applied to optic nerve. Both tetanic and test stimuli were supramaximal for t, fibers. C, graph of the amplitude of t, response recorded in the optic nerve to stimulation of optic tract, before and after a 15-set tetanus at 500 per second to the optic nerve. Tetanic stimulus was supramaximal for t, fibers but test stimulus (antidromic) was submaximal. Responses in A, C expressed as percentage of mean pretetanic value. B shows typical responses; a, b from A; a, before, b, 2 set after tetanus; c, d, from B; c, before, d, 1 set after tetanus; t,, response of fast-conducting group of optic nerve stimulated in tract, recorded in optic nerve; fibers; tIca), response of same group t aca,, response of slow-conducting group.
phase shows only a slight increase in duration, often a slight decrease in amplitude (always a decrease in amplitude if the test stimulus is submaximal for tI fibers; Fig. 10d). By contrast with these potential changes at the nerve endings, the tI orthodromic response recorded in the tract proper (Fig. 9) shows only very minor changes, in either positive or negative phases, following a
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FIG. 10. Effect of tetanus on presynaptic response (tl) in lateral genicuiate nucleus. Tracings of t, records superimposed so that initial parts of positive phase coincide. Test stimulus in a, b, c was supramaximal for t1 fibers, submaximal in d; a, c, effect of 1.5-set tetanus; b, d, effect of IO-min tetanus. In a, b, d, record 1 is before tetanus; record 2 is 5 set (a, b) or 10 set (d) after end of tetanus. In c, record 1 (which is the same as record 1 of a) is before tetanus; record 2 is 5 set and record 3, 15 set after start of tetanus (records i, j of Fig. 1). Vertical bars indicate commencement of stimulus artifact of corresponding tracing. Discussion
The results are in general agreement with those of Hughes, Evarts, and Marshall (23) and Evarts and Hughes (12, 13) ; some minor differences have been noted. The two main points of interest concern the phenomena of post-tetanic potentiation and delayed depression of the postsynaptic response,and the extent of the subliminal fringe. Post-tetanic potentiation of the postsynaptic response is usually considered to be due to presynaptic changes(22). Thus, at the neuromuscular
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junction it has been shown that the end plate is unchanged in its sensitivity to acetylcholine during the period of post-tetanic potentiation (26). Tn the absence of an appropriate excitor substance we cannot as yet make a similar test on the lateral geniculate nucleus. However, it has been shown that when the optic nerve is stimulated repetitively (not necessarily at a very high frequency) the geniculate cells cease to discharge after a few stimuli (6 ; cf. Fig. 6a, c) . Hence, the striking differences in effect produced by varying either frequency or duration of tetanus occur in the almost complete absence of postsynaptic activity during the tetanus. This suggests that presynaptic changes are responsible for both post-tetanic potentiation and delayed depression. The presynaptic response has been examined in some detail in order to determine its relation to the postsynaptic response. The various changes which occur in the t1 response following a tetanus all run the same time course. Thus, neglecting the 5 set immediately following the tetanus (which has not been studied), there is an increase in the amplitude and duration of the negative phase and an increase in latency. Normal values are restored in each case in about 1 min after a 15-set tetanus, in about 2 min after a IO-min tetanus. Similar potential changes have been recorded by Lloyd (28), Eccles and Rall (ll), and Wall and Johnson (35) in muscle group I afferent fibers synapsing on motoneurons in the spinal cord of the cat. These changes in the optic fibers are paralleled by changes in excitability of the nerve endings. Wall and Johnson (35) have likewise found a decreased excitability in group I afferent nerve endings following a tetanus. The method of testing excitability is not unequivocal. A constant stimulus is applied to the nerve endings and the response is measured in the optic nerve. Ideally, the point at which the response is recorded should be unchanged in its properties but, as we know, this is not so in the case of the optic nerve. However, the changes in excitability which occur are most easily explained on the basis of a hyperpolarization following the tetanus. It is well known that in peripheral nerve, both mammalian and amphibian, the positive after-potential is larger and more prolonged the higher the frequency and the longer the duration of the tetanus. Similar changes in the optic nerve would account for differences between the IS-set and lomin tetani. Probably all parts of the optic nerve, and not merely the nerve endings, undergo a hyperpolarization. This would also explain the depression of the t1 response following a
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tetanus when the orthodromic test stimulus is submaximal; hyperpolarization would reduce the excitability and therefore the number of fibers being stimulated. The long duration of this depression, 10 to 20 min, might be due to the fact that the optic nerve was exposed and therefore at a lower temperature than its central parts. Post-tetanic positive afterpotentials probably last longer at lower temperatures (14, 15). The fact that the decrease in excitability in the tract is so much greater than in the nerve endings (Figs. 3, 9) suggests that the hyperpolarization is greater in the tract than in the endings. Differing anatomical arrangement of the fibers with respect to the stimulating electrodes in the two situations may, however, play an important part. An increase in the amplitude of the presynaptic response following tetanic stimulation has been observed at the motoneuron synapse (11, 28) and has been regarded as due to an increased spike height resulting from an increase in the positive after-potential following a tetanus (28). Although a spike potential elicited during hyperpolarization of the membrane should be increased in amplitude (20), it is evident from Fig. 9 A, B that, in the tract, the change in spike height following a tetanus is not more than a few per cent, in spite of a large decrease in excitability in these fibers. This small change in spike height is in agreement with the results of Ritchie and Straub (32) on mammalian C fibers. It is possible that the degree of hyperpolarization required to cause the observed decrease in excitability is not great. Since the decrease in excitability of the tract nerve endings following a tetanus is less than in the tract proper, it is unlikely that the spike height in the nerve endings will be much increased. It is therefore necessary to look for some other explanation of the increased negative phase of the t1 response. Basically, this negativity means that following a tetanus there is a period during which a single impulse propagating into the nerve endings causes them to draw more current from the rest of the nerve than they did in a single impulse before the tetanus. Among the recognized after-effects of a tetanus in peripheral nerve are slowed conduction (16) and increased duration of action potential (33). Figure 10 shows that there is an increased latency of response in the lateral geniculate nucleus after a tetanus and that this is greater after the longer tetanus. Some of this increase may be due to the impulse being initiated closer to the cathode after the tetanus because of the decreased excitability (16). The decrease in conduction velocity in these experiments was about
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5 per cent measured 5 set after a 15set tetanus, about 17 per cent after a lo-min tetanus, estimated from the latency (artifact to crossover point of t1 response). A decrease in conduction velocity has the effect of reducing the wavelength of the action potential and hence of establishing a greater potential gradient between any two parts along the fiber. One might expect, therefore, an increase in amplitude of both positive and negative phases of the t1 response. In fact, there is never an increase in the positive phase; there is usually a slight decrease even with supramaximal stimuli (e.g., Fig. lob) which might be due to some temporal dispersion. To account for the observed changes in the negative phase, it would be necessary to suppose that there was a much greater decrease in conduction velocity in the nerve endings than in the tract. Since the nerve endings are unmyelinated it is possible that post-tetanic changes here are more marked than in the myelinated portion of the nerve. A considerable decrease in conduction velocity in the nerve endings could account for the increase both in amplitude and duration of the negative phase. An increase in duration of the membrane action potential (33) and hence of the volume conductor response probably occurs but would not necessarily lead to an increased amplitude of the latter response. The increased duration of the action potential may be greater in the nerve endings. It is worth remarking that when the electrode position is deep in the lateral geniculate nucleus such that the tl negativity is large, then the post-tetanic increase in amplitude or duration of this phase is not so great (Fig. 10d; compare with Fig. lob). This would be expected if the recording is not mainly from nerve endings. During the tetanus, the decrease in conduction velocity is even more marked but the negative phase, after first increasing, is then reduced in amplitude although it increases progressively in duration (Fig. 10~). This is a further indication that the increased negative phase does not indicate increased spike amplitude, since this usually declines progressively during a tetanus ( 19). The subsequent decline in the negative phase may indicate a failure of the impulse always to invade the terminal parts of the fiber. Lloyd (28) has claimed that post-tetanic potentiation of motoneuron response is correlated with presynaptic potential changes and has suggested that output of transmitter is closely related to the size of the action potential in the nerve endings. Eccles and Rall ( 11) have pointed out that the correlation of pre- and postsynaptic response is not perfect. The rl
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response in the lateral geniculate nucleus following a tetanus undergoes a series of changes lasting for hours. It is clear that changes occurring after 1 or 2 min cannot be related to the size of the presynaptic potential which is normal at this time. Explanations must be sought elsewhere for such features of the post-tetanic rl response as delayed depression and oscillation after a IS-set tetanus, diphasic recovery after a IO-min tetanus, slower recovery in the cerveuu isolt preparation. It is also clear that the changes in presynaptic potential do not suffice to explain the postsynaptic response within 1 or 2 min after the tetanus, since there is very little correlation of the two potential changes. If precautions are taken to ensure that the number of tract fibers being stimulated is unchanged, there is an immediate (5 set) potentiation of the presynaptic response after the tetanus, whereas the postsynaptic response is depressed below normal for 5 to 40 set and may not reach its peak until 1 min or more, at a time when the presynaptic response is normal. This need not mean that the presynaptic action potential is without influence on the output of transmitter. However, it cannot be the only factor. In regard to other synapses, it is still not clear what features of the presynaptic potential are important in relation to output of transmitter-total voltage swing of membrane potential, amount of reversed potential during the peak of the action potential, duration of action potential including after-potentials. One observation in the lateral geniculate nucleus is perhaps relevant here. The time of arrival of the impulse at the nerve endings is usually taken as the time at which the potential at the focal electrode becomes negative. The peak of the membrane action potential cannot very well be later than the peak of the volume conductor t1 negativity. Postsynaptic activity under normal conditions does not commence until the t1 negativity has returned to the baseline (6). It is worth noting that this relationship is largely preserved after a tetanus (Fig. lOa, b, d) even when there is no depression of the postsynaptic responses (compare h and k in Fig. 1). This observation suggests that the release of transmitter is more closely related to the falling phase or to the after-potential of the action potential in the nerve endings than to the spike itself. The occurrence of postsynaptic potentiation is dependent on the presence of a fringe of subliminally excited cells. The results indicate the presence of such a fringe of rl cells in the lateral geniculate nucleus supporting the conclusions of Marshall (31) and Bishop and Evans (4). It is more difficult to assess the extent of this fringe. Thus, with a stimulus
552
BISHOP,
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supramaximal for tl fibers the amount of post-tetanic potentiation was negligible. With submaximal test stimuli (50 to 75 per cent maximal) the amount of potentiation averaged 12 per cent. Unfortunately, this method of testing involves a reduction in the number of fibers being stimulated after the tetanus. Hence the figure of 12 per cent is a minimal one. In cases where the t1 depression is not great. the rl potentiation is slightly greater (e.g., Fig. 2) but even with full compensation for tl depression, the rl response is unlikely to exceed 120 per cent. This effect is, therefore, small compared to the amount of potentiation obtainable at the motoneuron synapse after tetanic stimulation of group I afferent fibers. At this synapse the maximum potentiation may be as much as fourteen times the control value (28). The differences are doubtless related to the function which the two types of cell perform. Thus, the motoneuron is primarily an integrating mechanism for stimuli from a large number of sources, any one of which may be inadequate to fire the cell. On the other hand, the geniculate cell may be discharged by the action of very few optic nerve fibers, even by a single fiber ( 1). In agreement with Evarts and Hughes ( 12, 13), it was found that a tetanus during the depression following a 15set tetanus temporarily restored the response to normal. The most likely explanation of this result is that there is a decreased output of transmitter during the phase of depression and that a further tetanus increases the output to normal or above. It will be shown in another paper (2) that in synaptic depression due to lysergic acid diethylamide (LSD) a 15set tetanus produces negligible relief of the block (although a lo-min tetanus produces considerable relief). It is argued that LSD is acting postsynaptically, i.e., that the output of transmitter during LSD block is normal; and hence, that the output of transmitter after a 1.5set tetanus is at most only slightly above normal. ,4 reference should be made to the observation that post-tetanic delayed depression may be partially relieved by the second or third stimulus of a train of stimuli at about 500 per second (Fig. 6b). During a short train of stimuli in the normal preparation (Fig. 6a, c) the response declines for two reasons: the synaptic potential is progressively reduced in amplitude (1, 6) ; and the geniculate cells develop a subnormality associated with a positive after-potential (34). The subnormality following a single response does not commence until 6 or 7 msec later. In many cases the synaptic potential does not fall below threshold until the third stimulus. Hence the first two or even three responses of a series may be normal (Fig. 6~). In
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a preparation depressed by a previous tetanus there will be a summation of synaptic potentials such that, depending on the degree of depression, some cells will fire on the second stimulus, others on the third or fourth. At the same time, the factors mentioned above which operate in the normal preparation in this case also restore the block by the fifth or sixth shock. Post-tetanic depression has been described at the neuromuscular synapse (8, 24), at the Renshaw cell (9), in the amygdala ( 17), in the auditory system (21, 25) and at the spinal motoneuron (lo), although the duration of depression at these sites is much shorter than in the lateral geniculate nucleus. The phenomenon in the lateral geniculate nucleus may not be of physiological importance since the minimal tetanic conditions needed to produce delayed depression (a frequency in excess of 180 per second applied for several seconds) would seem to be more than the equivalent of the maximal physiological response (27). This is probably true also for post-tetanic potentiation although the amount of potentiation is so small anyway as to be of no consequence to the normal working of the lateral geniculate nucleus. These observations are nevertheless valuable in helping us to understand the factors involved in the normal behavior of geniculate synapses. References 1. BISHOP, P. O., W. BURKE, and R. DAVIS, Synapse discharged by single fibre in mammalian visual system. Nature, Lond. 182: 728-730, 1958. 2. BISHOP, P. O., W. BURKE, and W. R. HAYROW, Lysergic acid diethylamide block of lateral geniculate synapses and relief by repetitive stimulation. I%~. Neur. 1: 5.56-568, 1959. 3. BISHOP, P. O., and R. COLLIN, Steel microelectrodes. i. Physiol., Lond. 112: S-lop, 1950. 4. BISHOP, P. O., and W. A. EVANS, The refractory period of the sensory synapses of the lateral geniculate nucleus. J. Physiol., Lond. 194: 538-557, 1956. 5. BISHOP, P. O., D. JEREMY, and J. W. LANCE, The optic nerve. Properties of a central tract. J. Physiol., Lond. 121: 415-432, 1953. 6. BISHOP, P. O., and J. G. MCLEOD, Nature of potentials associated with synaptic transmission in lateral geniculate of cat. J. Neurophysiol. 17: 387-414, 1954. 7. BREMER, F., L’activitC c&brale au tours du scmmeil et de la narccse. Contribution B l’ktude du mecanisme du sommeil. Bull. Acad. Mtd. Belg., Series 6, 2: 68-86, 1937. 8. DESMEDT, J. E., Nature of the defect of neuromuscular transmission in myasthenic patients: ‘post-tetanic exhaustion.’ Nature, Lond. 179: 156-157, 1957. 9. ECCLES, J. C., P. FATT, and K. KOKETSU, Cholinergic and inhibitory synapses in a pathway from motor-axon collaterals to motoneGrones. 1. Physiol., Lond. 126: 524-562, 1954.
5.54 10.
11. 12. 13. 14. 15. 16. 17.
18.
19. 20.
21. 22. 23. 24. 25. 26. 27. 28. 29. 30.
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AND
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ECCLES, J. C., K. KRNJEVIC, and R. MILEDI, Delayed effects of peripheral severance of afferent nerve fibres on the efficacy of their central synapses. J. Physiol., Lond. 145: 204-220, 1959. ECCLES, J. C., and W. RALL, Effects induced in a monosynaptic reflex path by its activation. J. Neurophysiol. 14: 353-376, 1951. EVARTS, E. V., and J. R. HUGHES, Relation of post-tetanic potentiation to subnormality of lateral geniculate potentials. Am. J. Physiol. 188: 238-244, 1957. EVARTS, E. V., and J. R. HUGHES, Effects of prolonged optic nerve tetanization on lateral geniculate potentials. Am. J. Physiol. 188: 245-248, 1957. GASSER, H. S., and H. GRUNDFEST, Action and excitability in mammalian A fibres. Am. J. Physiol. 117: 113-133, 1936. GERARD, R. W., Delayed action potentials in nerve. Am. J. Physiol. 93: 337341, 1930. GERARD, R. W., and W. H. MARSHALL, Nerve conduction velocity and equilibration. Am. J. Physiol. 104: 575-585, 1933. GLOOR, P., Electrophysiological studies on the connections of the amygdaloid nucleus in the cat. Part II: The electrophysiological properties of the amygdaloid projection system. Electroencephalography, Montreal. 7: 243-264, 19.55. GRUND~EST, H., R. W. SENGSTAKEN, W. H. OETTINGER, and R. W. GURRY, Stainless steel microneedle electrodes made by electrolytic pointing. Rev. SC. Znstrum. 21: 360-361, 1950. HODGKIN, A. L., The subthreshold potentials in a crustacean nerve fibre. Proc. R. Sot., Ser. R, Biol. SC., Lond. 126: 76-121, 1938. HODGKIN, A. L., and A. F. HUXLEY, The dual effect of membrane potential on sodium conductance in the giant axon of Loligo. J. Physiol., Lond. 116: 497506, 1952. HUGHES, J. R., Auditory sensitization. J. Acoust. Sot. Am. 26: 1064-1070, 19.54. HUGHES, J. R., Post-tetanic potentiation. Physiol. Rev. 98: 91-113, 1958. HUGHES, J. R., E. V. EVARTS, and W. H. MARSHALL, Post-tetanic potentiaticn in the visual system of cats. Am. J. Physiot. 186: 483-487, 1956. HUGHES, J. R., and R. M. MORRELL, Post-tetanic changes in the human neuromuscular system. J. Appl. Physiol. 11: 51-57, 1957. HUGHES, J. R., and W. A. ROSENBLITH, Electrophysiological evidence for auditory sensitization. J. Acoust. Sot. Am. 29: 275-280, 1957. HUTTER, 0. F., Post-tetanic restoration of neuromuscular transmission blocked by d-tubocurarine. J. Physiol., Land. 118: 216-227, 1952. KUFFLER, S. W., Discharge patterns and functional organization of mammalian retina. J. Neurophysiol. 16: 37-68, 1953. LLOYD, D. P. C., Post-tetanic potentiation of response in monosynaptic reflex pathways of the spinal cord. J. Gen. Physiol. 88: 147-170, 1949. LLOYD, D. P. C., Early and late post-tetanic potentiation, and post-tetanic block in a monosynaptic reflex pathway. J. Gen. Physiol. 42: 475-488, 1959, MARSHALL, W. H., An application of the frozen sectioning technic for cutting serial sections thru the brain. Stain Techn. 15: 133-138, 1940.
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31. 32. 33.
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W. H., Excitability cycle and interaction in geniculate-striate system of cat. J. Neurophysiol. 12: 277-288, 1949. RITCHIE, J. M., and R. W. STRAUB, The after-effects of repetitive stimulation on mammalian non-medullated fibres. J. Physiol., Land. 134: 698-711, 1956. SPYROPOULOS, C. S., Changes in the duration of the electric response of single nerve fibers fcllowing repetitive stimulation. J. Gen. Pkysiol. 40: 19-25, MARSHALL,
34.
1956. VASTOLA,
35.
258-272, WALL,
E. F., After-pcsitivity
in lateral geniculate body. J. Neurophysiol.
22:
1959.
P. D., and A. R. JOHNSON, Changes associated with post-tetanic potentiation of a monosynaptic reflex. J. Neurophysiol. 21: 148-158, 1958.
NEUROLOGY
Repetitive
1,
534-555
(1959)
Stimulation of Optic Nerve Lateral Geniculate Synapses
P. 0. BISHOP,~. Brain Research UGversity
and
BURKE, AND W. R. HAYHOW' Unit, Department of Physiology, Sydney, Sydney, Australia
of
Received
August
19, 1959
The effects of repetitive stimulation of the optic nerve on the electrical response recorded in the dorsal nucleus of the lateral geniculate nucleus of the cat have been studied. Observations have been restricted to effects on the fast-conducting group of optic fibers. In the main, tetanic frequency has been 500 to 700 per second, and duration of tetanus either 15 set or 10 min. Tetanic stimulation results in post-tetanic potentiation and depression of both presynaptic and postsynaptic responses. The conditions determining which effect is obtained are described. The presynaptic response during and following the tetanus has been studied, and the excitability of the optic tract and tract nerve endings has been tested by antidromic stimulation. Decrease in excitability of the optic nerve after tetanization is explained on the hypothesis that a hyperpolarization in all parts of the optic fiber follows the tetanus. An increase in amplitude of the presynaptic response also follows the tetanus but can be due only in small degree to an increase in spike height. It is more probably due to slowed conduction and perhaps to a longer duration of action potential in the optic tract nerve endings. The relationship between presynaptic and postsynaptic potential changes after a tetanus is discussed; there is no evidence in these experiments that the size of the presynaptic action potential is the sole or even the main determining factor of the postsynaptic potential changes. Introduction
Repetitive stimulation of a synapse is often a useful way of examining the normal behavior of that synapse, even when the stimulation is at a 1 This study was aided by grants from the National Health and Medical Research Council of Australia, from the Ophthalmic Research Institute of Australia, and from the Consolidated Medical Research Funds of the University of Sydney, Sydney, Australia. We are grateful to Dr. J. Smith for his assistance with some of the early experiments and we wish to acknowledge the skilled help given by the technical staff of the Physiology Department, particularly Mr. J. Stephens, Mr. D. Larnach, and Mr. B. McGee. Our thanks are due to Miss J. Kearsey for considerable secretarial assistance. 534
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frequency and for a duration not experienced under physiological conditions. Hughes, Evarts, and Marshall (23) and Evarts and Hughes (12, 13) have studied the effects of tetanic stimulation of the optic nerve on the response of the cells of the lateral geniculate nucleus in the cat. This paper confirms and extends their observations and examines in more detail the aftereffects of a tetanus on the optic tract nerve endings and the relationship between presynaptic and postsynaptic potential changes. The observations have been restricted to the effects resulting from stimulation of the fast group (ti) of optic fibers (5). Furthermore, the test stimuli have been given at 5-set intervals; changes occurring within 5 set from the end of the tetanus have not been examined. Methods Adult cats were used in all experiments. They were anesthetized with intraperitoneal allobarbitone (Dial, Ciba, 0.5 ml/kg) plus pentobarbitone sodium (Sagatal, May and Baker, 0.1-0.3 ml/kg). The latter drug ensured a reasonably rapid induction; by the time the first records were obtained several hours later, its effect had probably worn off. In a few experiments the cerveuu isole preparation (7) was used, preliminary induction being with ether. The cat’s body temperature, read with a rectal thermometer, was controlled within normal limits (3%39°C) with the aid of an electric heating blanket. One eyeball was resected and the optic nerve prepared for stimulation or recording by being suspended free of orbital tissuee2 The response of the contralateral lateral geniculate nucleus was recorded by means of a stereotaxically-directed steel microelectrode inserted vertically through the intact cerebral cortex. The optic tract was stimulated by means of a pair of steel stimulating electrodes, mounted about 1 mm apart. The position of the tips of these electrodes was checked initially by recording from each separately, the waveform of the response serving as a guide to the position, and finally, by electrolytically depositing iron and staining subsequently by the Prussian blue reaction (30). The recording and stimulating electrodes were prepared by electrolytically pointing steel beading needles (3, 18) and insulating to the tip with Bakelite varnish. The stimuli were rectangular pulses 50 psec in duration, applied once every 5 set except during the period of repetitive stimulation. Repetitive 2 See Bishop,
Jeremy,
and Lance
(5)
for
a fuller
description
of the method.
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stimulation was usually at about 500 to 700 per second for periods of 15 set or 10 min (12, 13, 23). Results and Comments
Tetanus for 15 Sec. Figure 1 illustrates typical changes in electrical responseoccurring in the dorsal nucleus of the lateral geniculate nucleus during and following high-frequency tetanic stimulation of the contralateral optic nerve for 15 sec.
FIG. 1. Effect of 15-set tetanus (stimulus supramaximal for tl fibers) to optic nerve on lateral geniculate nucleus response; a-g, test stimulus submaximal for t, fibers; a, before tetanus (670/set) ; b, 5 set; c, 15 set; d, 35 set; e, 110 set; f, 6 min, 40 set; g, 129 min, after end of tetanus; time-base, o. h-n, supramaximal test stimulus in another preparation; a, before tetanus (SOO/sec) ; i, j, t, response during tetanus (i, fifth second; j, fifteenth second) ; k, 5 set; 1, 35 set; m, 16 min; n, 78 min, after end of tetanus; time-base, p. In this and all other figures, negative is upwards.
Figure la showsa typical responsein the normal preparation, resulting from a single stimulus to the fast group (tl) of optic fibers (5). The large positive-negative wave (tI) following the artifact represents activity in theseoptic tract fibers; the negative wave ( rl) is the responseof geniculate somata and radiation axons innervated by tl fibers. The method of measuring the amplitude of these responseswas the sameas that adopted by Bishop and Evans (4). Records a to g show the responsesto a test shock submaximal for tI fibers, obtained (a) before and (b-g) at various times after a high-frequency tetanus; records h to n are from another preparation in which the test shock was supramaximal. The tetanic stimuli were
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always at least maximal in strength in these and all other experiments. The complete sequence of changes over a period of several minutes has been plotted in Figs. 2 and 3, respectively, for the two experiments illustrated in Fig. 1.
1%; I 0
FIG. 2. sponses in which are dotted lines
-I8
0
I
2
3
rnihzs
5
b
7
8
9I
2’;;; 1
Effect of Ssec tetanus on presynaptic (tI) and postsynaptic (rl) relateral geniculate nucleus. Graph of amplitudes of responses, some of shown in Fig. 1, a-g. Vertical line marks end of tetanus. Horizontal indicate mean level of pretetanic response.
Provided that the test shock is supramaximal for tl fibers, the posttetanic response of these fibers is greater for about a minute before returning to normal (Figs. lk, 3, 4). The amplitude increased by an average value of 117 per cent (measured 5 set from the end of the tetanus; fifteen experiments, range 100-142 per cent). If the test shock is submaximal, the tl response shows post-tetanic depression (Figs. 1, 2). This depression is usually no greater than to 9Q per cent but may take 10 to 20 min to disappear. In the case of a submaximal test shock, or of one which is barely maximal, the post-tetanic response may show both effects, i.e., an early brief potentiation followed by a prolonged depression (Fig. 2 ; also Figs. 5, 7). The increase in amplitude in the tl response occurs solely in the negative portion of the waveform, and even when the total amplitude is decreased the negative phase is usually increased (Fig. lc). Although the positive
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phase increases slightly in duration, the negative phase increases markedly in this respect (Figs. lk, 10a). Following a 15set tetanus, the response of the geniculate ceils (rl) is depressed for a few seconds, may then be larger than normal (1 to 2 min), and finally is markedly reduced for a period lasting for hours (Figs. 2,
2
FIG. 3. Graph (t l(9) ), responses endings.
3
4
6
7
8
similar to Fig. 2 from results partly illustrated in Fig. 1, h-n; in the optic nerve to a submaximal stimulus applied to tract nerve
3).3 In no case has the recovery from this delayed depression been followed to completion. In one instance the rl response, which had been depressed to 24 per cent, recovered to 45 per cent in 5 hours. In Fig. 2 the recovery seems more rapid than this. The average increase in amplitude of the rl response in four experiments in which the test shock was submaximal (50 to 75 per cent maximal) was to 112 per cent of the pretetanic value. When the test shock was supramaximal (five experiments) there was a potentiation to an average value of 103 per cent. It is difficult to compare these two sets of results be3 This period of potentiation corresponds to the “late potentiation” of Lloyd (29). The depression has been termed “second subnormality” by Hughes, Evarts, and Marshall (23). We prefer to call this “delayed depression.” The term “late depression” has been used (29) to refer to depression preceding “late potentiation.”
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cause, as already mentioned, with submaximal testing the t1 response is reduced in amplitude for 10 to 20 min, whereas there is no depression with supramaximal testing. One other change in the response of geniculate cells following a 15set tetanus should be mentioned. There is frequently a considerable fluctuation in the response, even when the pretetanic response is very steady (Fig. 3). In several experiments this fluctuation has taken on a regular oscillatory character (Fig. 4). These changes are not associated with any
- 15 -set
_ tet. O
0
I 2
I 4
I
I
I
6
nlinsutes
I
I
I IO
I
t I2
FIG. 4. Graph similar to Fig. 2 from another experiment; ulus. Note oscillatory behavior of rl following tetanus (630/set).
I
I
I
14
supramaximal
I 16
stim-
change in the presynaptic response. In the experiment of Fig. 4, the period averaged 200 sec. In another experiment in which the oscillation was very marked, the period was about 500 set; tetanic stimulation was repeated four times in this preparation and on each occasion a similar periodicity was observed. In other cases (e.g., Fig. 3)) although some oscillation is evident, it is not possibleto be precise about the periodicity. This irregularity of responsehas not been seen after a IO-min tetanus (Figs. 5, 7). The effect has not been investigated further. Tetanus for 10 Min. The responsein the lateral geniculate nucleus following a lo-min tetanus has been graphed in Fig. 5. In this case, the test stimulus was submaximal for tl fibers and the t1 responseafter an early period of near-recovery was again depressedfor about 10 min. When the test stimulus is supramaximal there is a post-tetanic increase in tl amplitude, the mean value being 144 per cent (six experiments) at 5 set from
540
BISHOP,
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the end of the tetanus. It is often the case, however, that a test stimulus which is supramaximal prior to the tetanus is submaximal after the tetanus due to the large decrease in excitability in the optic nerve (see later). The duration of the period of potentiation is rather longer than that following a IS-set tetanus, the average value being about 2 min. As in the case of
FIG.
(5Wsec)
5.
Graph showing the at submaximal strength
changes in of stimulus.
t,
and
rI
following
a lo-min
tetanus
the 15set tetanus, the increase in t1 is exclusively in the negative phase of the individual response, the positive phase being usually depressed in amplitude (Fig. lob, d) . Both positive and negative phases are increased in duration but the change is again much more marked in the negative phase (Fig. lob, d). The rl response shows a two-stage recovery which is quite characteristic. The second peak of recovery is usually 10 to 20 per cent higher than the first peak and may exceed the pretetanic value. The response finally returns to its normal level 5 or 6 min from the end of the tetanus. With submaximal testing stimuli, post-tetanic potentiation has never exceeded 2 per cent (Fig. 5) and is usually absent. This is probably due to the large decrease in excitability in the optic nerve (see later). With supramaximal testing, the first peak averaged 95 per cent, the second 106 per cent. There is no delayed depression following a lo-min tetanus. EBect of Frequency and Duration of Tetanus. In most of these experiments the frequency of stimulation during the tetanus was kept at 500 to 700 per second.Earlier experiments showedthat in the fresh prep-
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aration, frequencies of 180 per second or less did not produce appreciable effects either of potentiation or depression (excluding changes occurring within 5 set from the end of the tetanus). Tetani at frequencies of about 360 per second seemed to be almost as effective as those at higher frequencies. Lloyd (28) found a similar limiting frequency in the spinal cord monosynaptic pathway. Frequencies of about 500 per second can be sustained by the optic nerve for 1.5 set and longer, in several cases for 10 min, although more usually at this time, the t1 response is depressed or alternately large and small, indicating that many fibers can no longer follow the rate of stimulation. In regard to duration, earlier work by Evarts and colleagues (12, 13, 23) and by ourselves had shown that delayed depression was maximal following a tetanus of about IS-set duration, whereas if the duration was more than 6 min there was no depression. Consequently, in the present study, the standard tetani were of either 15 set or 10 min duration. Relief of Post-Tetanic Delayed Depression by Further Tetarti. In agreement with Evarts and Hughes (12, 13), we have found that the prolonged depression which follows a IS-set tetanus (SOO/sec) can be relieved by a further tetanus. This second 15set tetanus relieves the block for about 5 min before the former level of block or an even greater degree of depression is re-established; during the post-tetanic potentiation the response may reach and exceed the normal level. A IO-min tetanus produces a permanent relief of the depression; the response post-tetanically is not different in waveform from the response following a lo-min tetanus in the normal preparation. A lo-n-tin tetanus will also reduce the degree and duration of the post-tetanic delayed depression due to a subsequent 15set tetanus. This effect is, of course, maximal immediately following the lomin tetanus and declines over about 10 min (13). Regarding the frequency of stimulation, the minimal frequency effective in producing some relief of block was about 100 per second (1.5 set). However, the effect could also be demonstrated following a tetanus at 180 per second for 5 sec. Relief of Post-Tetanic Delayed Depression dwing a Short Train of Stimuli. If a train of shocks at about 500 per second is’applied to the optic nerve in the normal preparation, the rl response is greatly depressed after a few shocks (Fig. 6a; cf. Ref. 6). Usually there is a progressive reduction in the rl response but often the depression does not commence until the third response, especially if the frequency of stimulation is a little higher as in Fig. 6c. The exact pattern is also dependent on the
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depth of anesthesia. On the other hand, during post-tetanic delayed depression, a similar train of shocks produces a potentiation of the r1 response maximal at about the third shock, following which the response declines again (Fig. 6b).
FIG. 6. Effect of train of five or six stimuli at about 400 per second on response of lateral geniculate nucleus; a, normal; b, same preparation during post-tetanic delayed depression; c, another preparation, normal. Upper time calibration applies to a, b; lower to c.
Effect of Anesthetic. The experiments described so far were all done under barbiturate anesthesia. Several experiments were also performed on the rerveau isole’ preparation. The response to a single stimulus was not appreciably different to that in the Dial-anesthetized animal. As far as the post-tetanic changes were concerned, no difference could be discerned in the tI response. The rl response following a lo-min tetanus recovered more slowly (Fig. 7) but usually in two stages as in the anesthetized preparation. Contrary to the experience of Evarts and Hughes (13), we have always found a delayed depression following a 15set tetanus (five experiments) not significantly different in degree to that in the anesthetized preparation.
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Excitability Changes in the ,Optic Nerve and Nerve Endings. The excitability of optic tract endings in the lateral geniculate nucleus was tested by stimulating before and after tetanic stimulation of the optic nerve. The test stimuli were delivered via a pair of electrodes, the tips of which were about 1 mm apart, the cathode lying either in the lateral geniculate nucleus or just above it.
6
7
8
9
FIG. 7. Graph similar to Fig. 5 but in cerveau isole’ preparation. Tetanus frequency 540 per second. Upper curve and points as in Fig. 3, from series partly illustrated in Fig. 8, c-h.
With the stimulating electrodes immediately above cellular layer A, the strength of the shock was gradually increased until the spread of the stimulating current just involved the optic nerve endings on the cells in layer A. The antidromic responsewas recorded from the contralateral optic nerve, monophasic records being obtained by crushing the nerve at the electrode distal to the brain (5). Figure 8 illustrates the results of two experiments. The responseconsistsof two waves due to the two groups of fibers in the nerve, one rapidly and the other more slowly conducting (tl and tz respectively). Because of the distribution of these fibers in the lateral geniculate nucleus, it is not possible to stimulate one group selectively by varying the strength of stimulus. The stimulus was always submaximal, hence presumably some fibers were being stimulated critically. As indicated above it was probable that the stimulus was being
544
BISHOP,
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HAYHOW
delivered at or very close to the nerve endings in the nucleus. Following an orthodromic tetanus in the tl fibers, the antidromic response of only these fibers was reduced, whereas the t2 response remained unaffected. Figure 8 illustrates the effects of a 15set tetanus (ad) and (in another preparation) of a 10-min tetanus (e-h). Results from an experiment sim-
FIG. 8. Effect of tetanic stimulation of optic nerve on excitability of tract nerve endings; a-d, responses in optic nerve to stimulation of tract nerve endings; tl,a,, response of fast-conducting optic nerve fibers; tzCn), response of slomconducting fibers; a, before 15-set tetanus (SOO/sec) ; b, 10 set; c, 20 set; d, 40 set after end of tetanus; time-base, i. e-h, another preparation; e, before lo-min tetanus (540/set) ; f, 10 set; g, 70 set; h, 140 set after end of tetanus; time-base, j. In each case the tetanus was applied to t, fibers only.
ilar to that in Fig. 8 (a-d) are graphed in Fig. 3 (tr,,,); results from the experiment illustrated in Fig. 8 (e-h) are graphed in Fig. 7 (tr,,,). Provided that the orthodromic test stimulus is supramaximal (i.e., ensuring that all tl fibers are being stimulated), the return of the presynaptic (tl) response to normal amplitude always parallels the return of excitability of these fibers as tested by antidromic stimulation (e.g., Fig. 3). If the orthodromic test stimulus is submaximal, the tr response is decreased for a considerable time after the tetanus (Figs. 5, 7), but there is always an early period of increased tr response which again parallels the return to normal excitability of the tract fibers (e.g., Fig. 7). This initial increase in t1 response occurs solely in the negative phase; the positive phase is always reduced. After a lo-min tetanus, from about the second minute there is also a slight depression of the negative phase. The positive and negative phases then increase pari passu until complete recovery. (These changes occur also after a 15-set tetanus but are much smaller and difficult to de-
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tect.) This suggests that the tI waveform is affected by two distinct events: a change in the nerve endings associated with decreased excitability lasting 1 or 2 min and attributable to hyperpolarization; and a decrease in the number of optic fibers being stimulated during a period of 10 min or more, responsible for the prolonged tI depression. If the test stimulus is supramaximal for tI fibers, the prolonged depression is absent. The latter event is presumably the result of decreased excitability at the site of stimulation in the optic nerve and might be due to the same factor as that which operates at the nerve endings, viz., a hyperpolarization following the tetanus. To check this point electrodes were introduced into the optic tract about 1 or 2 mm behind the chiasma and used for both stimulation and recording. The effect of tetanic stimulation of the optic nerve on the orthodromic response of the optic tract fibers in this situation was recorded and their excitability tested by direct stimulation (Fig. 9). The excitability changes were qualitatively the same as those described above in the case of the tract endings in the lateral geniculate nucleus. Quantitatively, the decrease in excitability was much greater and its duration rather longer (Fig. 9B, C). Another surprising feature was that the orthodromic response was almost unaffected (Fig. 9A, B) although in keeping with this finding is the fact that the prodromal positivity of the tl response recorded in the lateral geniculate nucleus is also almost unaffected following a 15sec tetanus (Fig. 10a). Potential Changes in the Optic Tract Nerve Endings. In view of the above findings on the excitability changes in the optic nerve, the changes in waveform of the presynaptic (tl) response were examined in more detail. In Fig. lOa, b, d, taken respectively from three experiments, are shown superimposed tI responses before and after a tetanus. The test stimulus in a and b was supramaximal for tl fibers, submaximal in d. During the tetanus (Fig. 10~)) the latency and duration of both positive and negative phases increases progressively. There is a decrease in the amplitude of the positive phase; the amplitude of the negative phase first increases, then decreases. Following the tetanus (records 2 in Fig. lOa, b, d) there is an increase in the amplitude of the negative phase, maximal at 5 or 10 set from the end of the tetanus; latency is still longer than normal. The positive phase, on the other hand, shows very little change. These changes disappear, each at the same rate, over about 1 min following a 15set tetanus (Figs. 3, 4).
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During a IO-min tetanus, the changes are similar but progress further than during a 15set tetanus; following a lo-min tetanus, therefore, these changes are more in evidence (Fig. lob, d) and the normal waveform is regained more slowly. After a tetanus, the most marked change is always in the negative phase, regarding both amplitude and duration; the positive
FIG. 9. A, graph of the amplitude of t, response recorded in the optic tract, before and following a IS-see tetanus at 500 per second applied to optic nerve. Both tetanic and test stimuli were supramaximal for t, fibers. C, graph of the amplitude of t, response recorded in the optic nerve to stimulation of optic tract, before and after a 15-set tetanus at 500 per second to the optic nerve. Tetanic stimulus was supramaximal for t, fibers but test stimulus (antidromic) was submaximal. Responses in A, C expressed as percentage of mean pretetanic value. B shows typical responses; a, b from A; a, before, b, 2 set after tetanus; c, d, from B; c, before, d, 1 set after tetanus; t,, response of fast-conducting group of optic nerve stimulated in tract, recorded in optic nerve; fibers; tIca), response of same group t aca,, response of slow-conducting group.
phase shows only a slight increase in duration, often a slight decrease in amplitude (always a decrease in amplitude if the test stimulus is submaximal for tI fibers; Fig. 10d). By contrast with these potential changes at the nerve endings, the tI orthodromic response recorded in the tract proper (Fig. 9) shows only very minor changes, in either positive or negative phases, following a
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undergo a marked
decrease in ex-
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FIG. 10. Effect of tetanus on presynaptic response (tl) in lateral genicuiate nucleus. Tracings of t, records superimposed so that initial parts of positive phase coincide. Test stimulus in a, b, c was supramaximal for t1 fibers, submaximal in d; a, c, effect of 1.5-set tetanus; b, d, effect of IO-min tetanus. In a, b, d, record 1 is before tetanus; record 2 is 5 set (a, b) or 10 set (d) after end of tetanus. In c, record 1 (which is the same as record 1 of a) is before tetanus; record 2 is 5 set and record 3, 15 set after start of tetanus (records i, j of Fig. 1). Vertical bars indicate commencement of stimulus artifact of corresponding tracing. Discussion
The results are in general agreement with those of Hughes, Evarts, and Marshall (23) and Evarts and Hughes (12, 13) ; some minor differences have been noted. The two main points of interest concern the phenomena of post-tetanic potentiation and delayed depression of the postsynaptic response,and the extent of the subliminal fringe. Post-tetanic potentiation of the postsynaptic response is usually considered to be due to presynaptic changes(22). Thus, at the neuromuscular
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junction it has been shown that the end plate is unchanged in its sensitivity to acetylcholine during the period of post-tetanic potentiation (26). Tn the absence of an appropriate excitor substance we cannot as yet make a similar test on the lateral geniculate nucleus. However, it has been shown that when the optic nerve is stimulated repetitively (not necessarily at a very high frequency) the geniculate cells cease to discharge after a few stimuli (6 ; cf. Fig. 6a, c) . Hence, the striking differences in effect produced by varying either frequency or duration of tetanus occur in the almost complete absence of postsynaptic activity during the tetanus. This suggests that presynaptic changes are responsible for both post-tetanic potentiation and delayed depression. The presynaptic response has been examined in some detail in order to determine its relation to the postsynaptic response. The various changes which occur in the t1 response following a tetanus all run the same time course. Thus, neglecting the 5 set immediately following the tetanus (which has not been studied), there is an increase in the amplitude and duration of the negative phase and an increase in latency. Normal values are restored in each case in about 1 min after a 15-set tetanus, in about 2 min after a IO-min tetanus. Similar potential changes have been recorded by Lloyd (28), Eccles and Rall (ll), and Wall and Johnson (35) in muscle group I afferent fibers synapsing on motoneurons in the spinal cord of the cat. These changes in the optic fibers are paralleled by changes in excitability of the nerve endings. Wall and Johnson (35) have likewise found a decreased excitability in group I afferent nerve endings following a tetanus. The method of testing excitability is not unequivocal. A constant stimulus is applied to the nerve endings and the response is measured in the optic nerve. Ideally, the point at which the response is recorded should be unchanged in its properties but, as we know, this is not so in the case of the optic nerve. However, the changes in excitability which occur are most easily explained on the basis of a hyperpolarization following the tetanus. It is well known that in peripheral nerve, both mammalian and amphibian, the positive after-potential is larger and more prolonged the higher the frequency and the longer the duration of the tetanus. Similar changes in the optic nerve would account for differences between the IS-set and lomin tetani. Probably all parts of the optic nerve, and not merely the nerve endings, undergo a hyperpolarization. This would also explain the depression of the t1 response following a
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tetanus when the orthodromic test stimulus is submaximal; hyperpolarization would reduce the excitability and therefore the number of fibers being stimulated. The long duration of this depression, 10 to 20 min, might be due to the fact that the optic nerve was exposed and therefore at a lower temperature than its central parts. Post-tetanic positive afterpotentials probably last longer at lower temperatures (14, 15). The fact that the decrease in excitability in the tract is so much greater than in the nerve endings (Figs. 3, 9) suggests that the hyperpolarization is greater in the tract than in the endings. Differing anatomical arrangement of the fibers with respect to the stimulating electrodes in the two situations may, however, play an important part. An increase in the amplitude of the presynaptic response following tetanic stimulation has been observed at the motoneuron synapse (11, 28) and has been regarded as due to an increased spike height resulting from an increase in the positive after-potential following a tetanus (28). Although a spike potential elicited during hyperpolarization of the membrane should be increased in amplitude (20), it is evident from Fig. 9 A, B that, in the tract, the change in spike height following a tetanus is not more than a few per cent, in spite of a large decrease in excitability in these fibers. This small change in spike height is in agreement with the results of Ritchie and Straub (32) on mammalian C fibers. It is possible that the degree of hyperpolarization required to cause the observed decrease in excitability is not great. Since the decrease in excitability of the tract nerve endings following a tetanus is less than in the tract proper, it is unlikely that the spike height in the nerve endings will be much increased. It is therefore necessary to look for some other explanation of the increased negative phase of the t1 response. Basically, this negativity means that following a tetanus there is a period during which a single impulse propagating into the nerve endings causes them to draw more current from the rest of the nerve than they did in a single impulse before the tetanus. Among the recognized after-effects of a tetanus in peripheral nerve are slowed conduction (16) and increased duration of action potential (33). Figure 10 shows that there is an increased latency of response in the lateral geniculate nucleus after a tetanus and that this is greater after the longer tetanus. Some of this increase may be due to the impulse being initiated closer to the cathode after the tetanus because of the decreased excitability (16). The decrease in conduction velocity in these experiments was about
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5 per cent measured 5 set after a 15set tetanus, about 17 per cent after a lo-min tetanus, estimated from the latency (artifact to crossover point of t1 response). A decrease in conduction velocity has the effect of reducing the wavelength of the action potential and hence of establishing a greater potential gradient between any two parts along the fiber. One might expect, therefore, an increase in amplitude of both positive and negative phases of the t1 response. In fact, there is never an increase in the positive phase; there is usually a slight decrease even with supramaximal stimuli (e.g., Fig. lob) which might be due to some temporal dispersion. To account for the observed changes in the negative phase, it would be necessary to suppose that there was a much greater decrease in conduction velocity in the nerve endings than in the tract. Since the nerve endings are unmyelinated it is possible that post-tetanic changes here are more marked than in the myelinated portion of the nerve. A considerable decrease in conduction velocity in the nerve endings could account for the increase both in amplitude and duration of the negative phase. An increase in duration of the membrane action potential (33) and hence of the volume conductor response probably occurs but would not necessarily lead to an increased amplitude of the latter response. The increased duration of the action potential may be greater in the nerve endings. It is worth remarking that when the electrode position is deep in the lateral geniculate nucleus such that the tl negativity is large, then the post-tetanic increase in amplitude or duration of this phase is not so great (Fig. 10d; compare with Fig. lob). This would be expected if the recording is not mainly from nerve endings. During the tetanus, the decrease in conduction velocity is even more marked but the negative phase, after first increasing, is then reduced in amplitude although it increases progressively in duration (Fig. 10~). This is a further indication that the increased negative phase does not indicate increased spike amplitude, since this usually declines progressively during a tetanus ( 19). The subsequent decline in the negative phase may indicate a failure of the impulse always to invade the terminal parts of the fiber. Lloyd (28) has claimed that post-tetanic potentiation of motoneuron response is correlated with presynaptic potential changes and has suggested that output of transmitter is closely related to the size of the action potential in the nerve endings. Eccles and Rall ( 11) have pointed out that the correlation of pre- and postsynaptic response is not perfect. The rl
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response in the lateral geniculate nucleus following a tetanus undergoes a series of changes lasting for hours. It is clear that changes occurring after 1 or 2 min cannot be related to the size of the presynaptic potential which is normal at this time. Explanations must be sought elsewhere for such features of the post-tetanic rl response as delayed depression and oscillation after a IS-set tetanus, diphasic recovery after a IO-min tetanus, slower recovery in the cerveuu isolt preparation. It is also clear that the changes in presynaptic potential do not suffice to explain the postsynaptic response within 1 or 2 min after the tetanus, since there is very little correlation of the two potential changes. If precautions are taken to ensure that the number of tract fibers being stimulated is unchanged, there is an immediate (5 set) potentiation of the presynaptic response after the tetanus, whereas the postsynaptic response is depressed below normal for 5 to 40 set and may not reach its peak until 1 min or more, at a time when the presynaptic response is normal. This need not mean that the presynaptic action potential is without influence on the output of transmitter. However, it cannot be the only factor. In regard to other synapses, it is still not clear what features of the presynaptic potential are important in relation to output of transmitter-total voltage swing of membrane potential, amount of reversed potential during the peak of the action potential, duration of action potential including after-potentials. One observation in the lateral geniculate nucleus is perhaps relevant here. The time of arrival of the impulse at the nerve endings is usually taken as the time at which the potential at the focal electrode becomes negative. The peak of the membrane action potential cannot very well be later than the peak of the volume conductor t1 negativity. Postsynaptic activity under normal conditions does not commence until the t1 negativity has returned to the baseline (6). It is worth noting that this relationship is largely preserved after a tetanus (Fig. lOa, b, d) even when there is no depression of the postsynaptic responses (compare h and k in Fig. 1). This observation suggests that the release of transmitter is more closely related to the falling phase or to the after-potential of the action potential in the nerve endings than to the spike itself. The occurrence of postsynaptic potentiation is dependent on the presence of a fringe of subliminally excited cells. The results indicate the presence of such a fringe of rl cells in the lateral geniculate nucleus supporting the conclusions of Marshall (31) and Bishop and Evans (4). It is more difficult to assess the extent of this fringe. Thus, with a stimulus
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supramaximal for tl fibers the amount of post-tetanic potentiation was negligible. With submaximal test stimuli (50 to 75 per cent maximal) the amount of potentiation averaged 12 per cent. Unfortunately, this method of testing involves a reduction in the number of fibers being stimulated after the tetanus. Hence the figure of 12 per cent is a minimal one. In cases where the t1 depression is not great. the rl potentiation is slightly greater (e.g., Fig. 2) but even with full compensation for tl depression, the rl response is unlikely to exceed 120 per cent. This effect is, therefore, small compared to the amount of potentiation obtainable at the motoneuron synapse after tetanic stimulation of group I afferent fibers. At this synapse the maximum potentiation may be as much as fourteen times the control value (28). The differences are doubtless related to the function which the two types of cell perform. Thus, the motoneuron is primarily an integrating mechanism for stimuli from a large number of sources, any one of which may be inadequate to fire the cell. On the other hand, the geniculate cell may be discharged by the action of very few optic nerve fibers, even by a single fiber ( 1). In agreement with Evarts and Hughes ( 12, 13), it was found that a tetanus during the depression following a 15set tetanus temporarily restored the response to normal. The most likely explanation of this result is that there is a decreased output of transmitter during the phase of depression and that a further tetanus increases the output to normal or above. It will be shown in another paper (2) that in synaptic depression due to lysergic acid diethylamide (LSD) a 15set tetanus produces negligible relief of the block (although a lo-min tetanus produces considerable relief). It is argued that LSD is acting postsynaptically, i.e., that the output of transmitter during LSD block is normal; and hence, that the output of transmitter after a 1.5set tetanus is at most only slightly above normal. ,4 reference should be made to the observation that post-tetanic delayed depression may be partially relieved by the second or third stimulus of a train of stimuli at about 500 per second (Fig. 6b). During a short train of stimuli in the normal preparation (Fig. 6a, c) the response declines for two reasons: the synaptic potential is progressively reduced in amplitude (1, 6) ; and the geniculate cells develop a subnormality associated with a positive after-potential (34). The subnormality following a single response does not commence until 6 or 7 msec later. In many cases the synaptic potential does not fall below threshold until the third stimulus. Hence the first two or even three responses of a series may be normal (Fig. 6~). In
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a preparation depressed by a previous tetanus there will be a summation of synaptic potentials such that, depending on the degree of depression, some cells will fire on the second stimulus, others on the third or fourth. At the same time, the factors mentioned above which operate in the normal preparation in this case also restore the block by the fifth or sixth shock. Post-tetanic depression has been described at the neuromuscular synapse (8, 24), at the Renshaw cell (9), in the amygdala ( 17), in the auditory system (21, 25) and at the spinal motoneuron (lo), although the duration of depression at these sites is much shorter than in the lateral geniculate nucleus. The phenomenon in the lateral geniculate nucleus may not be of physiological importance since the minimal tetanic conditions needed to produce delayed depression (a frequency in excess of 180 per second applied for several seconds) would seem to be more than the equivalent of the maximal physiological response (27). This is probably true also for post-tetanic potentiation although the amount of potentiation is so small anyway as to be of no consequence to the normal working of the lateral geniculate nucleus. These observations are nevertheless valuable in helping us to understand the factors involved in the normal behavior of geniculate synapses. References 1. BISHOP, P. O., W. BURKE, and R. DAVIS, Synapse discharged by single fibre in mammalian visual system. Nature, Lond. 182: 728-730, 1958. 2. BISHOP, P. O., W. BURKE, and W. R. HAYROW, Lysergic acid diethylamide block of lateral geniculate synapses and relief by repetitive stimulation. I%~. Neur. 1: 5.56-568, 1959. 3. BISHOP, P. O., and R. COLLIN, Steel microelectrodes. i. Physiol., Lond. 112: S-lop, 1950. 4. BISHOP, P. O., and W. A. EVANS, The refractory period of the sensory synapses of the lateral geniculate nucleus. J. Physiol., Lond. 194: 538-557, 1956. 5. BISHOP, P. O., D. JEREMY, and J. W. LANCE, The optic nerve. Properties of a central tract. J. Physiol., Lond. 121: 415-432, 1953. 6. BISHOP, P. O., and J. G. MCLEOD, Nature of potentials associated with synaptic transmission in lateral geniculate of cat. J. Neurophysiol. 17: 387-414, 1954. 7. BREMER, F., L’activitC c&brale au tours du scmmeil et de la narccse. Contribution B l’ktude du mecanisme du sommeil. Bull. Acad. Mtd. Belg., Series 6, 2: 68-86, 1937. 8. DESMEDT, J. E., Nature of the defect of neuromuscular transmission in myasthenic patients: ‘post-tetanic exhaustion.’ Nature, Lond. 179: 156-157, 1957. 9. ECCLES, J. C., P. FATT, and K. KOKETSU, Cholinergic and inhibitory synapses in a pathway from motor-axon collaterals to motoneGrones. 1. Physiol., Lond. 126: 524-562, 1954.
5.54 10.
11. 12. 13. 14. 15. 16. 17.
18.
19. 20.
21. 22. 23. 24. 25. 26. 27. 28. 29. 30.
BISHOP,
BURKE,
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HAYHOW
ECCLES, J. C., K. KRNJEVIC, and R. MILEDI, Delayed effects of peripheral severance of afferent nerve fibres on the efficacy of their central synapses. J. Physiol., Lond. 145: 204-220, 1959. ECCLES, J. C., and W. RALL, Effects induced in a monosynaptic reflex path by its activation. J. Neurophysiol. 14: 353-376, 1951. EVARTS, E. V., and J. R. HUGHES, Relation of post-tetanic potentiation to subnormality of lateral geniculate potentials. Am. J. Physiol. 188: 238-244, 1957. EVARTS, E. V., and J. R. HUGHES, Effects of prolonged optic nerve tetanization on lateral geniculate potentials. Am. J. Physiol. 188: 245-248, 1957. GASSER, H. S., and H. GRUNDFEST, Action and excitability in mammalian A fibres. Am. J. Physiol. 117: 113-133, 1936. GERARD, R. W., Delayed action potentials in nerve. Am. J. Physiol. 93: 337341, 1930. GERARD, R. W., and W. H. MARSHALL, Nerve conduction velocity and equilibration. Am. J. Physiol. 104: 575-585, 1933. GLOOR, P., Electrophysiological studies on the connections of the amygdaloid nucleus in the cat. Part II: The electrophysiological properties of the amygdaloid projection system. Electroencephalography, Montreal. 7: 243-264, 19.55. GRUND~EST, H., R. W. SENGSTAKEN, W. H. OETTINGER, and R. W. GURRY, Stainless steel microneedle electrodes made by electrolytic pointing. Rev. SC. Znstrum. 21: 360-361, 1950. HODGKIN, A. L., The subthreshold potentials in a crustacean nerve fibre. Proc. R. Sot., Ser. R, Biol. SC., Lond. 126: 76-121, 1938. HODGKIN, A. L., and A. F. HUXLEY, The dual effect of membrane potential on sodium conductance in the giant axon of Loligo. J. Physiol., Lond. 116: 497506, 1952. HUGHES, J. R., Auditory sensitization. J. Acoust. Sot. Am. 26: 1064-1070, 19.54. HUGHES, J. R., Post-tetanic potentiation. Physiol. Rev. 98: 91-113, 1958. HUGHES, J. R., E. V. EVARTS, and W. H. MARSHALL, Post-tetanic potentiaticn in the visual system of cats. Am. J. Physiot. 186: 483-487, 1956. HUGHES, J. R., and R. M. MORRELL, Post-tetanic changes in the human neuromuscular system. J. Appl. Physiol. 11: 51-57, 1957. HUGHES, J. R., and W. A. ROSENBLITH, Electrophysiological evidence for auditory sensitization. J. Acoust. Sot. Am. 29: 275-280, 1957. HUTTER, 0. F., Post-tetanic restoration of neuromuscular transmission blocked by d-tubocurarine. J. Physiol., Land. 118: 216-227, 1952. KUFFLER, S. W., Discharge patterns and functional organization of mammalian retina. J. Neurophysiol. 16: 37-68, 1953. LLOYD, D. P. C., Post-tetanic potentiation of response in monosynaptic reflex pathways of the spinal cord. J. Gen. Physiol. 88: 147-170, 1949. LLOYD, D. P. C., Early and late post-tetanic potentiation, and post-tetanic block in a monosynaptic reflex pathway. J. Gen. Physiol. 42: 475-488, 1959, MARSHALL, W. H., An application of the frozen sectioning technic for cutting serial sections thru the brain. Stain Techn. 15: 133-138, 1940.
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W. H., Excitability cycle and interaction in geniculate-striate system of cat. J. Neurophysiol. 12: 277-288, 1949. RITCHIE, J. M., and R. W. STRAUB, The after-effects of repetitive stimulation on mammalian non-medullated fibres. J. Physiol., Land. 134: 698-711, 1956. SPYROPOULOS, C. S., Changes in the duration of the electric response of single nerve fibers fcllowing repetitive stimulation. J. Gen. Pkysiol. 40: 19-25, MARSHALL,
34.
1956. VASTOLA,
35.
258-272, WALL,
E. F., After-pcsitivity
in lateral geniculate body. J. Neurophysiol.
22:
1959.
P. D., and A. R. JOHNSON, Changes associated with post-tetanic potentiation of a monosynaptic reflex. J. Neurophysiol. 21: 148-158, 1958.