1Department of Physiology,
Yoshida, Kaoru,
Yoshiki Iwamoto,
Sohei Chimoto, and
Hiroshi Shimazu.
Saccade-Related Inhibitory Input to Pontine Omnipause
Neurons: An Intracellular Study in Alert Cats.
J. Neurophysiol. 82: 1198-1208, 1999.
Omnipause neurons (OPNs)
are midline pontine neurons that are thought to control a number of
oculomotor behaviors, especially saccades. Intracellular recordings
were made from OPNs in alert cats to elucidate saccade-associated
postsynaptic events in OPNs and thereby determine what patterns of
afferent discharge impinge on OPNs to cause their saccadic inhibition.
The membrane potential of impaled OPNs exhibited steep
hyperpolarization before each saccade that lasted for the whole period
of the saccade. The hyperpolarization was reversed to depolarization by
intracellular injection of Cl Saccades are rapid eye movements that shift the
line of sight. The brain stem neural circuit that generates saccadic
eye movements, the saccade generator, contains two essential classes of
neurons, medium-lead burst neurons (MLBNs) and omnipause neurons
(OPNs). MLBNs exhibit a high-frequency burst of spikes immediately
before and during saccades (Büttner et al. 1977 The synaptic events that cause the cessation of OPN activity during
saccadic eye movements are not known. In particular, whether their
pause in activity is caused by a removal of tonic excitatory input
(disfacilitation) or by an increase in inhibitory input (postsynaptic
inhibition) is still uncertain. Because visually evoked saccades are
abolished by combined ablation of the superior colliculus (SC) and the
frontal eye field (Schiller et al. 1980 Another, probably more important, question is what patterns of afferent
discharge cause a pause in OPN activity. The information will be
indispensable in understanding how the onset and the duration of the
OPN pause are controlled. Anatomic studies (Ito et al. 1984 The present study was undertaken to investigate membrane potential
changes that underlie the cessation of OPN activity by means of
intracellular recording techniques. OPN recordings in alert cats
allowed us to clarify the synaptic events associated with saccades and
to evaluate contributions of the above two possible mechanisms
(disfacilitation and active inhibition) to the production of a pause.
Moreover, our intracellular recordings allowed us to estimate the
temporal pattern of afferent discharge impinging on OPNs because the
changes of membrane potentials reflect total afferent inputs. Our
results have shown that a pause of OPN activity is caused by inhibitory
postsynaptic potentials (IPSPs). To determine the signal carried by
inhibitory afferents to OPNs, we quantitatively analyzed the time
course and the magnitude of saccade-related IPSPs and correlated them
with the concomitant eye movement parameters. On the basis of these
analyses, possible origins of inhibitory inputs to OPNs will be
discussed with reference to previously studied saccadic activity of SC
cells and brain stem burst neurons.
Preliminary reports of a part of this study appeared previously
(Iwamoto et al. 1997 Preparation
Experiments were performed with four adult cats. Each animal
underwent the following surgical procedures under pentobarbital sodium
anesthesia and aseptic conditions. A heating pad was placed under the
body to control the body temperature, and heart and respiratory rates
were monitored for the duration of surgery. A coil of Teflon-coated
stainless steel wire was implanted beneath the insertions of the four
recti of the right eye to measure eye movements. The tympanic bulla on
each side was opened, and a silver ball electrode was placed on the
round window to stimulate the vestibular nerve. Stainless steel tubes
were embedded in a block of dental acrylic attached securely to the
frontoparietal skull with bone screws for insertion of stereotaxic
bars, permitting painless immobilization of the animal's head during
the intracellular recording sessions. An opening (5-7 mm diam) was
made in the posterior part of the parietal bones overlying the
cerebellar vermis, and the dura was removed to allow
recording-electrode access through the cerebellum to the brain stem. A
cylindrical chamber, made from a plastic microcentrifuge tube, was
placed over the opening and fixed to the skull with dental acrylic. All
experimental protocols complied with the guidelines of the University
of Tsukuba policy on the humane care and use of laboratory animals.
Recording conditions
During recording sessions, the animal was placed in the
stereotaxic apparatus mounted on a turntable that could be rotated about the earth's horizontal and vertical axes, together with the
magnetic field-generating coils. The head was fixed in a 26.5° nose-down position with the interauricular midpoint on the axes of
rotation and the right eye in the center of the magnetic field. The
body was restrained gently with a cloth bag. The animals were kept
quiet without any signs of distress or discomfort for the duration of
recording. If the animals appeared to be getting bored, experiments
were stopped, or a small piece of food or milk was given during a break
in the recording. This procedure was effective in keeping the animals
alert. Eye movements were measured by a magnetic search-coil system
with a resolution of <0.1° and a bandwidth ranging between DC and
300 Hz (Fuchs and Robinson 1966 Intracellular recording from OPNs was collected while the animal was
alert. Saccades were induced usually by a visual stimulus such as
experimenter's hand, food and novel objects. Glass micropipettes filled with 4 M NaCl, instead of KCl, solution were used for
intracellular recording because, if the tip of the micropipette is
broken in the course of insertion, leak of high concentrations of
potassium would exert noxious effects on brain tissues. These
micropipettes were adequate for recording postsynaptic potentials
(Eccles 1964 Data analysis
Data analysis was performed on a Macintosh computer with the
software Spike2 (CED, Cambridge, UK) and homemade programs. The membrane potential and eye position signals were digitized and sampled
every 1.0 ms. Instantaneous eye velocity signals were obtained by
calculating the slope of the line fit to position samples contained in
a 5-ms moving window centered on the current time point. The radial eye
velocity was computed from horizontal and vertical eye velocity by use
of the Pythagorean theorem. The onset of saccades was defined as the
time when the radial eye velocity reached 15°/s. The onset of the
saccade-related membrane potential change in OPNs was determined by a
visually manipulated cursor; initial change was so steep that there was
little variation of measurement among experimenters. The amplitude and
the time of peak of the potential change were determined on a smoothed trace that had been obtained using a five-point running average of the
data to filter synaptic noise.
Histological studies
A sharpened metal needle was chronically attached to the edge of
the opening of the skull at its midline with dental acrylic as the
reference of the insertion point of the glass micropipettes. The
location of the tip of the intracellular microelectrode was measured
with reference to the tip of the metal needle by the scale of a
micromanipulator attached to the stereotaxic frame. When all recording
sessions were completed, a thin tungsten microelectrode was inserted,
with the aid of the micromanipulator, into the approximate center of
the area where OPNs had been recorded. After confirming that
extracellular spikes of OPNs still could be recorded in this region, an
electrolytic lesion was made by passing cathodal currents (50 µA,
30 s) through the tungsten microelectrode. At the end of the
experiment, the animals were killed with an overdose of pentobarbital
sodium and perfused with saline followed by 3 L of fixative containing
2% paraformaldehyde and 1.6% glutaraldehyde in 0.1 M phosphate
buffer. The marked spots were histologically studied in Nissl-stained
serial sections (100 µm in thickness). Sites for intracellular
recordings were reconstructed with reference to the marked spots. They
were distributed in the region near the midline and rostral to the
abducens nucleus, similar to those reported in previous studies in cats
(Curthoys et al. 1981 Identification of impaled OPNs and general features of
saccade-related membrane potential changes
Neuronal activity was recorded near the midline of the pons at the
level immediately rostral to the abducens nucleus. The midline of the
brain stem was estimated by recording vestibular-induced monosynaptic
volleys in the medial longitudinal fasciculus (MLF) (cf. Iwamoto
et al. l990). The location of the abducens nucleus was determined by
recording negative and positive field potentials induced after
stimulation of the contralateral and ipsilateral vestibular nerve,
respectively, and by recording units exhibiting characteristic
discharge patterns associated with eye movements (Fuchs and
Luschei 1970
ABSTRACT
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
ions, indicating it
consisted of temporal summation of inhibitory postsynaptic potentials
(IPSPs). The duration of the saccade-related hyperpolarization was
almost equal to the duration of the concurrent saccades. The time
course of the hyperpolarization was similar to that of the radial eye
velocity except for the initial phase. During the falling phase of eye
velocity, the correlation between the instantaneous amplitude of
hyperpolarization and the instantaneous eye velocity was highly
significant. The amplitude of hyperpolarization at the eye velocity
peak was correlated significantly with the peak eye velocity. The time
integral of the hyperpolarization was correlated with the radial
amplitude of saccades. The initial phase disparity between the
hyperpolarization and eye velocity was due to the relative constancy of
peak time (~20 ms) of the initial steep hyperpolarization regardless
of the later potential profile that covaried with the eye velocity. The
initial steep hyperpolarization led the beginning of saccades by
15.9 ± 3.8 (SD) ms, which is longer than the lead time for
medium-lead burst neurons. These results demonstrate that the pause of
activity in OPNs is caused by IPSPs initiated by an abrupt, intense
input and maintained, for the whole duration of the saccade, by
afferents conveying eye velocity signals. We suggest that the initial
sudden inhibition originates from central structures such as the
superior colliculus and frontal eye fields and that the eye
velocity-related inhibition originates from the burst generator in the
brain stem.
INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
;
Cohen and Henn 1972
; Keller 1974
;
King and Fuchs 1979
; Luschei and Fuchs
1972
; Nakao et al. 1990
). They consist of
excitatory and inhibitory burst neurons (EBNs and IBNs). In the
horizontal burst generator, EBNs project to the ipsilateral abducens
nucleus to excite agonist motoneurons (Igusa et al.
1980
; Sasaki and Shimazu 1981
; Strassman et al. 1986a
), and IBNs project to the contralateral abducens nucleus to inhibit antagonist motoneurons (Hikosaka and Kawakami 1977
; Hikosaka et al. 1978
; Scudder et
al. 1988
; Strassman et al. 1986b
; Yoshida
et al. 1982
). OPNs exhibit steady discharge during fixation and
cease firing before and during saccades in all directions (Cohen
and Henn 1972
; Evinger et al. 1982
;
Keller 1974
; Luschei and Fuchs 1972
).
Stimulation of the OPN region interrupts ongoing saccades, suggesting
that OPNs tonically inhibit MLBNs during fixation and that a pause of
tonic discharge of OPNs removes this inhibition during saccades
(Keller 1974
). In agreement with this suggestion, OPNs
have been shown to project to the EBN and IBN regions and make direct
inhibitory connections with these burst neurons (Curthoys et al.
1984
; Nakao et al. 1980
; Strassman et al.
1987
). These findings suggest that a pause of OPN activity is
important for the generation of saccades. However, the mechanism of OPN
pause induction and how the onset and the duration of the pause are
controlled are still unknown. Robinson (1975)
proposed an attractive model that suggested an inhibitory trigger signal initiates a pause of tonic discharge of OPNs, and once OPNs are suppressed, EBNs and IBNs are allowed to begin discharging in response
to excitatory input. Assuming an inhibitory connection to OPNs from
IBNs (or from EBNs through inhibitory interneurons), the OPNs are kept
silenced as long as the burst neurons continue firing (cf.
Keller 1979
, 1980
; Scudder 1988
, for
review, Fuchs et al. 1985
).
), the input that
initiates an OPN pause probably originates from these structures. The
rostral pole of the SC contains fixation cells that tonically discharge
during fixation and cease firing before and during saccades in cats
(Munoz and Guitton 1989
, 1991
) and in monkeys
(Munoz and Wurtz 1992
, 1993
). Stimulation of the SC
(King et al. 1980
; Raybourn and Keller
1977
), especially at its rostral part (Paré and
Guitton 1994
), induces monosynaptic excitation of OPNs. These
findings may favor the possibility that a pause in OPN activity is
caused by disfacilitation. On the other hand, electrical stimulation of
the SC suppresses OPN spikes after initial excitation in monkeys
(Raybourn and Keller 1977
) and in cats (Kaneko
and Fuchs 1982
; King et al. 1980
). It is well
known that the SC contains many cells that exhibit burst discharge for saccades and project to the brain stem reticular formation
(Berthoz et al. 1986
; Moschovakis et al.
1988b
; Munoz and Guitton 1986
; Munoz et
al. 1991
; Scudder et al. 1996a
). These findings
imply that burst discharge of SC neurons may contribute to production of a pause in OPN activity by postsynaptic inhibition.
; Langer and Kaneko 1984
, 1990
) have
revealed various origins of cells projecting to the OPN region,
including the SC and areas of saccade-related neurons in the midbrain
and pontomedullary reticular formation. However, little is known about
the discharge patterns of cells identified as directly connecting with OPNs.
; Yoshida et al.
1996
).
METHODS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
). Calibration of the eye
movement recording was made by assuming that the gain of the
vestibuloocular reflex (VOR) evoked by sinusoidal head rotation in the
light was equal to 1.0 (Iwamoto et al. 1990
).
). The microelectrodes had an electrical
resistance of ~10 M
. A conventional input stage (Nihon Kohden
MEZ-8300) was used for recording and passing current through the
microelectrode. Intracellular potentials (bandwidth DC to 5 kHz), and
eye position signals were monitored continuously on a computer screen
and oscilloscopes. The same signals were recorded on a magnetic tape
for later off-line analysis.
; Evinger et al.
1982
).
RESULTS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
). Extracellular spikes of OPNs were
identified by cessation of tonic discharge before and during saccades
in all directions (Fig. 1A) as
described previously (Cohen and Henn 1972
;
Evinger et al. 1982
; Keller 1974
;
Luschei and Fuchs 1972
). During advancement of the
microelectrode, the OPN spikes first recorded were usually negative in
polarity. Further advancement of the electrode tip on the cell revealed
an initial positivity of spikes. Then the electrode was advanced
slightly or positive currents were passed through the recording
microelectrode, thereby impaling the cell.
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Fig. 1.
Identification of an omnipause neuron (OPN). A:
discharge pattern of extracellular spikes associated with saccades
before impalement. B: intracellular recording of the
membrane hyperpolarization and a pause of spikes associated with a
saccade. C: saccade-related hyperpolarization after
inactivation of spike generation mechanism. In A-C, Hor
and Ver indicate horizontal and vertical eye positions, respectively.
The impaled cells were identified further as OPNs by their
characteristic changes in the membrane potential associated with saccades. A steep hyperpolarizing deflection of the membrane potential occurred before each saccade and completely suppressed ongoing spikes
(Fig. 1B). Hereafter, the membrane potential change in the
hyperpolarizing direction will be simply called the
"hyperpolarization" for the convenience of description. Figure
1B shows that the OPN resumed firing in midcourse of the
decay phase of the hyperpolarization when the membrane potential
reached the firing threshold. The membrane potential of OPNs during
intersaccadic intervals was about 40-50 mV for a period after
impalement. Usually, it gradually declined in the course of recording,
probably due to partial deterioration of the membrane, and was
maintained at a steady level of about
30 mV. Most measurements of the
amplitude of saccade-related hyperpolarization for quantitative
correlation analysis were taken at the steady level (about
30 mV) of
the membrane potential. Although spike generation mechanisms were
inactivated at this membrane potential level, a steep hyperpolarization
was induced for all saccades (Fig. 1C, see also Fig. 3).
Intracellular recordings were obtained from 23 OPNs. The maximum
duration of holding the cell was ~20 min in this study.
IPSPs in OPNs associated with saccades
Synaptic mechanisms of the saccade-related hyperpolarization in
OPNs were examined by Cl ion injection into the
cell by passing hyperpolarizing currents through the recording
microelectrode. Figure 2 shows examples of saccade-related membrane potential changes recorded in an OPN before
(Fig. 2A) and after injection of Cl
ions (Fig. 2B). Before injection, the membrane potential
showed a steep hyperpolarizing deflection followed by a relatively slow return to the previous steady potential level. After injection of
Cl
ions, the saccade-related hyperpolarization
was reversed to a depolarizing potential (Fig. 2B). Both the
hyperpolarization and depolarization preceded the onset of concurrent
saccades and lasted approximately the duration of the saccades. Figure
2C shows extracellular potentials recorded in the vicinity
of the cell as a control. These results indicated that the
saccade-related hyperpolarization consisted of temporal summation of
the IPSPs that were produced continuously before and during the whole
period of saccades. In agreement with these findings, we noted that
rapid fluctuation of the membrane potential increased during the period
of saccade-related hyperpolarization (Figs. 1C and
2A), indicating an increase in synaptic bombardment. The
hyperpolarizing IPSPs were in no case spontaneously reversed to a
depolarizing potential unless Cl
ions were
iontophoretically injected into the cell.
|
For some saccades, the hyperpolarization was followed by a slight depolarization and then returned to the presaccadic membrane potential level. The late depolarization, probably caused by the excitatory postsynaptic potentials, was generally small and will not be considered further.
Temporal characteristics of the saccade-related hyperpolarization
Figure 3 illustrates an example of a continuous record of the membrane potential in an OPN (top) together with saccadic eye movements (bottom), showing that a clear hyperpolarization was associated with each saccade. The time course and amplitude of the hyperpolarization varied from saccade to saccade. Because the hyperpolarization occurred for saccades in all directions, the radial eye velocity (middle) is shown for comparison. There is a gross similarity between the profiles of the hyperpolarization and the radial eye velocity. Temporal characteristics of the hyperpolarization such as the lead time, the duration, and the time course were analyzed quantitatively in relation to those of the radial eye velocity.
|
LEAD TIME. The onset of hyperpolarization in OPNs was found to precede the onset of saccades. The interval from the onset of hyperpolarization to the onset of eye velocity change (lead time) showed some variability even in a single OPN, as exemplified in the histograms for three OPNs in Fig. 4, A-C. In these neurons, the mean lead time was 19.7 ± 3.4, 16.0 ± 4.3, and 16.3 ± 4.0 (SD) ms, respectively. The mean lead times for 23 OPNs (average number of saccades, 16.6) ranged from 7.5 to 23.9 ms, with an overall mean of 15.9 ± 3.8 ms (Fig. 4D).
|
DURATION.
As described in the preceding text, the duration of the
hyperpolarization appeared to be almost the same as the saccade
duration. To confirm this observation, we examined quantitatively the
relation between the two parameters. Because the end of the
hyperpolarization was usually very gradual and it was difficult to
determine the precise time of its termination, the interval measured at
the potential level of 20% of the maximum hyperpolarization was taken as an indicator of the duration. Correspondingly, the duration of
saccades was also measured at the velocity level of 20% of its peak.
Figure 5 shows an example of the
relationship between thus measured duration of the hyperpolarization
and saccade duration for an OPN. Regression analysis indicated that the
correlation between these two variables was highly significant
(r = 0.98, P < 0.001, n = 39). The regression line intersected the
y axis at 1.06 ms with its slope of 1.03. The correlation
coefficient for 17 OPNs (average number of saccades, 15.4), including
the cell shown in Fig. 5, ranged from 0.85 to 0.99 with a mean of 0.95 ± 0.04 (P < 0.001). The regression lines
intersected the y axis at 4.21 ± 14.51 ms with a mean
slope of 0.98 ± 0.15. In the remaining six OPNs, the number of
saccades was not sufficient for statistical analysis. However, when the
data for these six neurons were pooled, a significant correlation also
was found (r = 0.93, P < 0.001, n = 22). The regression line intersected the
y axis at 11.87 ms with its slope of 0.91. The results
indicate that the duration of hyperpolarization was almost equal to the duration of concomitant saccade.
|
TIME COURSE. Figure 6 shows three examples of saccade-related hyperpolarization (bottom) in an OPN exhibiting different time courses together with radial eye velocity profiles (top). The three records are arranged from short (A) to long duration (C) of saccades. Eye velocity traces are inverted to facilitate comparison with hyperpolarization traces. There is a close resemblance between the time course of the falling phase of eye velocity and the time course of the decay phase of the hyperpolarization. The similarity between the hyperpolarization and the eye velocity profiles was found for all saccades in all of 23 OPNs. However, there was a disparity between the hyperpolarization and the eye velocity profiles at their initial phase. The disparity was apparent especially when the velocity had a slow rise and prolonged time to peak. For example, in Fig. 6C, the slope of the initial phase of the hyperpolarization was much steeper than that of the eye velocity.
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Relationship of the hyperpolarization to the peak eye velocity
The amplitude of the eye velocity-related hyperpolarization was
compared with the peak radial eye velocity for each saccade. The
amplitude of the hyperpolarization was measured at the time corresponding to the peak of eye velocity corrected for the lead time
(Fig. 11A, ). Because the
amplitude of IPSPs should be affected by the membrane potential level,
the correlation was calculated for the records in which the membrane
potential was maintained at a relatively constant level (range of
variation less than ±4 mV) during intersaccadic intervals. Figure
11B shows an example of the relationship between the peak
eye velocity and the amplitude of hyperpolarization. The amplitude of
the hyperpolarization increased with the amplitude of the peak eye
velocity. Linear regression analysis indicated that the correlation
between the two variables was statistically significant
(r = 0.73, P < 0.01). Similar
correlations were found for all of eight OPNs examined (average number
of saccades, 21.1; P < 0.01). The correlation
coefficients ranged from 0.63 to 0.85 with a mean of 0.75 ± 0.07. In the remaining OPNs, the number of saccades or the range of saccade
velocity at a constant level of the membrane potential was not
sufficient for this analysis.
|
If the OPN hyperpolarization is produced by a linear summation of IPSPs attributable to afferents which convey each of horizontal and vertical component velocity signals and converge on an OPN, one might argue that the amplitude of hyperpolarization should be compared with a simple sum of horizontal and vertical components of eye velocity. Therefore the correlation between the amplitude of hyperpolarization and the sum of the absolute values of horizontal and vertical component velocities was calculated. The correlation coefficients for the eight OPNs ranged from 0.66 to 0.84 with a mean of 0.75 ± 0.07 and were almost identical to the values for the correlation with the radial eye velocity. Thus these regression analyses did not distinguish whether the saccade-related hyperpolarization in OPNs better reflected the radial eye velocity or the sum of horizontal and vertical component velocities.
Relationship of the hyperpolarization to the amplitude of saccades
An attempt was made to quantify the total inhibitory afferent input impinging on an OPN during each saccade. The membrane potential level at the onset of each hyperpolarization was taken as a baseline, and the area enclosed by this baseline and the hyperpolarization curve was calculated. Because the precise termination of the hyperpolarization was difficult to determine, the hyperpolarization curve was integrated from its onset for the period equal to the saccade duration. Figure 12 shows an example of the relationship between the area of the hyperpolarization and the radial amplitude of saccades. Regression analysis indicated that the two parameters were significantly correlated (r = 0.73, P < 0.01). Significant correlations were found for the eight OPNs (P < 0.01) described in the preceding section. The correlation coefficients ranged from 0.64 to 0.89 with a mean of 0.78 ± 0.08.
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DISCUSSION |
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The present study showed that a saccade-related pause of OPN spikes was caused by a hyperpolarization consisting of IPSPs, the time course of the hyperpolarization was in parallel with that of instantaneous saccadic eye velocity except for the initial steep part of hyperpolarization, the initial steep hyperpolarization occurred ~16 ms before the beginning of saccades and attained its peak in ~20 ms regardless of eye velocity profile, and the total duration of the hyperpolarization was equal to the duration of the saccade. We think that the saccade-related hyperpolarization reflects the time course and the density of total afferent discharge because it consists of IPSPs induced by afferent impulses. Discharge patterns and possible origins of afferents producing the saccade-related IPSPs will be considered on the basis of the present findings. Before that, we will first compare the present results with previous extracellular studies on the discharge patterns of OPNs, thereby trying to explain their pause based on the membrane potential changes.
Comparison with extracellular studies on timing parameters of the pause
Evinger et al. (1982) reported that the mean lead
time of the OPN pause was 22.5 ms in cats. As pointed out by these
authors, these values tended to overestimate the actual lead time by
half the average interspike interval (~5 ms). The mean lead time of the steep hyperpolarization (15.9 ms) is in good agreement with that of
the pause. This indicates that the steep hyperpolarization exerts, at
its very onset, a powerful inhibitory effect on spike generation.
The pause duration of OPNs is somewhat shorter than the saccade
duration in cats (Evinger et al. 1982). This finding is
well explained by the present intracellular data. After the
hyperpolarizing deflection reached its peak and completely suppressed
spikes, the membrane potential reached threshold for spike generation during the gradual return to the presaccadic level. The firing began
before the end of the saccade, and the rate gradually increased in
parallel with the membrane potential recovery (Fig. 1B). The gradual resumption of firing rate after a pause is seen in
extracellularly recorded OPN spikes in cats (cf. Fig. 1 of
Evinger et al. 1982
). In monkey OPNs, the duration of
the pause is almost equal to the duration of the saccade (Keller
1974
; Luschei and Fuchs 1972
), and only the
initial interspike interval after the pause is slightly longer than
succeeding intervals (Cohen and Henn 1972
; Fig. 10B of
Everling et al. 1998
). The difference between cat and
monkey OPNs in the relation of pause duration to saccade duration is presumably related to the difference in velocity profile of saccade. Because the postpeak fall of saccade velocity in monkeys is much steeper than that in cats, we assume that the velocity-related hyperpolarization may return more rapidly to the presaccadic level in
monkeys than in cats, making pause duration almost equal to duration of hyperpolarization.
Role of brain stem MLBNs in production of OPN pause
The present results show a close correlation between the duration
of OPN hyperpolarization and saccade duration, between the amplitude of
hyperpolarization and saccade velocity, and between the time integral
of hyperpolarization and saccade amplitude. These relationships between
the parameters of OPN hyperpolarization and saccadic eye movements are
very similar to the relationships between the parameters of MLBN firing
and saccadic eye movements: i.e., there is also a close correlation
between the burst duration and saccade duration (King and Fuchs
1979; Luschei and Fuchs 1972
), between the
intraburst firing rate and saccade velocity (Kaneko et al.
1981
; Keller 1974
; King and Fuchs
1979
; Yoshida et al. 1982
), and between the
number of spikes and saccade amplitude (Kaneko and Fuchs
1981
; Kaneko et al. 1981
; Keller
1974
; King and Fuchs 1979
; van Gisbergen
et al. 1981
; Yoshida et al. 1982
). The
similarities between patterns of MLBN firing and OPN IPSPs suggest that
MLBNs provide OPNs with eye velocity-related signals during saccades.
The time course of OPN hyperpolarization resembles that of MLBN firing
not only for the eye velocity-related component but also for the
initial steep changes. The instantaneous firing rate of MLBNs rises
more steeply than eye velocity does (Keller 1974), and
the trajectories of the relationship between these two parameters generally make a loop (van Gisbergen et al. 1981
;
Yoshida et al. 1982
), like the relationship between the
instantaneous amplitude of OPN hyperpolarization and the instantaneous
eye velocity (Fig. 7). This raises the question whether the abrupt
increase in MLBN firing influences the onset of OPN hyperpolarization.
To clarify this point, it is essential to know the temporal
relationship between the onset of OPN inhibition and the onset of MLBN
bursts. The mean lead time of MLBN bursts in previous studies ranged
from 7.1 to 12.7 ms (Hepp et al. 1989
for review). These
values are shorter than the mean lead time of the steep
hyperpolarization in OPNs (15.9 ms). The methods of measurement to
determine the onset time of saccadic eye movements might vary somewhat
with each laboratory. Therefore we reexamined the lead time of MLBN bursts with the same techniques used for determination of the lead time
of OPN hyperpolarization (see METHODS). The lead time of
bursts in horizontal MLBNs was found to be 8.3 ± 2.0 ms
(n = 10), which was within the range of the values
found in the previous studies. It follows that bursts of MLBNs are
induced after the onset of steep hyperpolarizations in OPNs by 7.6 ms
on the average. Thus the onset of steep hyperpolarization appears to be
determined by afferent discharge that precedes MLBN bursts. However,
because the time to peak of the initial steep hyperpolarization in OPNs is ~20 ms, its profile may be influenced by the MLBN discharge.
Because OPNs are inhibited for saccades in all directions, they would
have to receive converging inputs from MLBNs with different ON directions. It is possible that afferents from
horizontal and vertical IBNs may converge directly on OPNs.
Alternatively, it is also possible that afferents from horizontal and
vertical EBNs may converge on common inhibitory interneurons that
project to OPNs. Anatomic studies have shown that the IBN region
contains neurons that project to the OPN region (Langer and
Kaneko 1984, 1990
). Morphophysiological studies with
intraaxonal horseradish peroxidase staining of IBNs identified as
projecting to the contralalateral abducens nucleus could not reveal
their termination in the OPN area in cats (Yoshida et al.
1982
) and monkeys (Strassman et al. 1986b
).
Strassman et al. have argued that the inhibitory input to OPNs during
the saccade arises from a different population of burst neurons than
immediate premotor IBNs. Although the eye velocity-related inhibitory
signals to OPNs most likely originate from the burst generator in the
brain stem, the location of the inhibitory neurons that make direct
connection with OPNs remains to be studied.
Role of the SC in production of OPN pause
Likely candidates for the origin of the input that generates the earliest IPSPs in OPNs appear to be the SC and the frontal eye field. Because firing characteristics of SC cells have been studied extensively with reference to saccade metrics, we will discuss the relation between the OPN IPSPs and SC activity.
INHIBITORY INPUT FROM SC BURST CELLS.
Ablation of both the SC and the frontal eye field abolishes generation
of visually evoked saccades (Schiller et al. 1980), and
focal electrical stimulation of the intermediate and deep layers of the
SC induces saccades in monkeys (Robinson 1972
;
Schiller and Stryker 1972
) and cats (Straschill
and Rieger 1973
). Wurtz and Goldberg (1971
,
1972
) first described neurons in the SC that discharge prior to
saccades. Saccade-related bursts begin ~20 ms (ranging from 16.0 to
24.8 ms) before saccade onset in monkeys (Sparks 1978
).
Similar lead times (means ranged from 14 to 26 ms) have been found for
saccade-related burst cells in the cat SC that have spatial and
temporal properties similar to primate burst cells (Peck
1987
). Some of the SC cells that project to the brain stem have
been shown to exhibit saccade-related discharge (Berthoz et al.
1986
; Munoz et al. 1991
). Morphophysiological studies have demonstrated that saccadic signals are conveyed by a class
of SC cells, called T cells (Moschovakis et al.
1988a
,b
), which project to various structures including the OPN
region (Scudder et al. 1996a
). These findings agree with
the notion that the SC plays a crucial role in the initiation of
saccades (see Sparks 1986
; Sparks and Mays
1990
for review). Because stimulation of the SC produces
disynaptic IPSPs in OPNs (Iwamoto et al. 1997
; Yoshida et al. 1996
), it seems reasonable to suggest
that SC burst cells provide, via inhibitory interneurons, signals that
determine the onset of saccade-related IPSPs. It should be noted here
that the initial steep hyperpolarization does not necessarily represent the profile of the "trigger signal" per se (Robinson
1975
; van Gisbergen et al. 1981
), because it may
contain IPSPs caused by afferents from MLBNs as described in the
preceding text.
DISFACILITATION RELATED TO SC FIXATION CELLS.
A recent study (Everling et al. 1998) has shown that
fixation cells located in the rostral SC (Munoz and Guitton
1991
; Munoz and Wurtz 1992
, 1993
; Munoz
et al. 1991
; Paré and Guitton 1994
) pause
earlier than OPNs. On the basis of their presumed excitatory connections with OPNs, disfacilitation caused by the pause of fixation
cells may assist decreasing excitability of OPNs during saccades.
However, this disfacilitation effect seems to be too weak to act as a
trigger signal for the OPN pause. It has been shown that the onset of
pause in fixation cells is more gradual and less synchronized to
saccade onset than the onset of pause in OPNs (Everling et al.
1998
). Tonic activity of OPNs probably is maintained not only
by the SC but also other sources of excitatory input (Raybourn
and Keller 1977
). The present results indicate that an abrupt
onset of OPN pause is caused by the steep IPSPs. Because IPSPs are
accompanied with an increase in the membrane conductance (Eccles
1964
), they would exert a strong inhibitory action on spike
generation even if some tonic excitatory inputs are still existent.
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ACKNOWLEDGMENTS |
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The authors thank A. Ohgami for technical assistance.
This study was supported by Grants-in-Aid for Scientific Research (Grants 10164210 and 11145209 to K. Yoshida) from the Ministry of Education, Science, and Culture of Japan and by Core Research for Evolutional Science and Technology of Japan Science and Technology Corporation. S. Chimoto was supported by the Research Fellowship of the Japan Society for the Promotion of Science for Young Scientists.
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FOOTNOTES |
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Address reprint requests to K. Yoshida.
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Received 14 September 1998; accepted in final form 20 May 1999.
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REFERENCES |
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