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The Journal of Neurophysiology Vol. 84 No. 3 September 2000, pp. 1303-1313
Copyright ©2000 by the American Physiological Society
Smith-Kettlewell Eye Research Institute, San Francisco, California 94115
Keller, E. L.,
R. M. McPeek, and
T. Salz.
Evidence Against Direct Connections to PPRF EBNs From SC in
the Monkey. J. Neurophysiol. 84: 1303-1313, 2000. Direct projections from the superior colliculus (SC) to the
paramedian pontine reticular formation (PPRF) have been demonstrated anatomically. The PPRF contains cells called excitatory burst neurons
(EBNs) that execute the final premotoneuronal processing for saccadic
eye movements, as well as other burst cells called long-lead burst
neurons (LLBNs). Previous electrophysiological tests in monkey have
failed to find evidence for monosynaptic connections from the SC to
EBNs, but have shown that direct projections to LLBNs exist. The
validity of these results has been questioned because EBNs are
known to be inhibited during periods of fixation by cells called
omnipause neurons (OPNs). In later experiments in cat, the stimulus in
the SC was triggered during saccades (when OPNs are off) and direct
connections to EBNs were found. The present experiments were conducted
to determine whether direct connections from the SC to EBNs could be
demonstrated in monkey. LLBNs located near EBNs were also recorded.
Single-pulse stimuli were delivered at sites in the SC at current
levels well above those required to evoke saccades with pulse train
stimuli. The stimuli were triggered shortly after the onset of
ipsilateral or contralateral saccades and also slightly after the end
of saccades. A sample of 21 EBNs was recorded and none were activated
by postsaccadic stimulation or during contralateral saccades. The high
spontaneous discharge rates of EBNs during ipsilateral saccades made
activation of these cells more difficult to detect; however, when the
results were quantified by peristimulus time histograms aligned on
stimulus onset, only 1/21 EBNs showed evidence of activation in the
monosynaptic range of latencies (<1.6 ms), 13 EBNs were activated at
di- or polysynaptic latencies, and 7 were not activated. In contrast, 15/21 LLBNs were activated with latencies in the monosynaptic range.
Further evidence supporting the absence of direct connections to EBNs
was obtained by realigning the peristimulus time histograms for a
subset of EBNs with similar firing rates on the time of occurrence of
the last spike before stimulus onset. A subset of EBNs was also studied
during drowsy ipsilaterally directed eye drifts, during which these
cells were firing at low spontaneous rates and OPNs were off. No
evidence for direct connections to EBNs was found in this behavioral
state. The variance in results obtained for cat and monkey may be due
to a species difference that reflects the more complex signal
processing required in the monkey's saccadic system.
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