Saccade-Related, Long-Lead Burst Neurons in the Monkey Rostral Pons

Abstract

The paramedian pontine reticular formation contains the premotoneuronal cell groups that constitute the saccadic burst generator and control saccadic eye movements. Despite years of study and numerous investigations, the rostral portion of this area has received comparatively little attention, particularly the cell type known as long-lead burst neurons (LLBNs). Several hypotheses about the functional role of LLBNs in saccade generation have been proposed, although there is little information with which to assess them. To address this issue, I mapped and recorded LLBNs in the rostral pons to measure their discharge characteristics and correlate those characteristics with the metrics of the concurrent saccades. On the basis of their discharge and location, I identified three types of LLBNs in the rostral pons: excitatory (eLLBN), dorsal (dLLBN), and nucleus reticularis tegmenti pontis (nrtp) LLBNs. The eLLBNs, encountered throughout the pons, discharge for ipsilateral saccades in proportion to saccade amplitude, velocity, and duration. The dLLBNs, found at the pontomesencephalic junction, discharge maximally for ipsilateral saccades of a particular amplitude, usually <10°, and are not associated with a particular anatomical nucleus. The nrtp LLBNs, previously described as vector LLBNs, discharge for saccades of a particular direction and sometimes a particular amplitude. The discharge of the eLLBNs suggests they drive motor neurons. The anatomical projections of the nrtp LLBNs suggest that their involvement in saccade production is less direct. The discharge of dLLBNs is consistent with a role in providing the "trigger" signal that initiates saccades.

Dynamic Coding of Vertical Facilitated Vergence by Premotor Saccadic Burst Neurons

“…In particular, approximately half of the neurons in our sample were broadly tuned and were not completely silent during vertical saccades. This result is in agreement with many previous studies that have also reported SBNs with broad tuning curves (Cullen and Guitton 1997;Hepp and Henn 1983;Kaneko 2006;Scudder et al 1988;Strassman et al 1986a,b). Although a metric-based analysis preformed by Scudder et al (1988) did report that the number of spikes generally increased for larger vertical movements this is the first study to describe the dynamic relationship between SBN discharge and eye velocity during vertical and/or oblique saccades.…”

“…Consistent with previous studies (Cullen and Guitton 1997;Hepp and Henn 1983;Kaneko 2006;Scudder et al 1988;Strassman et al 1986a,b), approximately half of the SBNs in our population were not completely silent during vertical saccades made between two far targets but instead produced a small discharge of action potentials. For these neurons (n ϭ 22/38) we also characterized the firing rates during vertical saccades using a vertical-eye velocity-based version of Eq.…”

Matching the Oculomotor Drive During Head-Restrained and Head-Unrestrained Gaze Shifts in Monkey

“…They are believed to provide a potent input to the putative BNs but not to the motoneurons directly (see review by Scudder et al 2002). The LLBNs exhibit a low frequency prelude that precedes the burst, and many are active for only a limited range of saccade amplitudes (Hepp and Henn 1983;Kaneko 2006;Keller et al 2000). Every neuron in our dataset discharged for all ipsiversive HR and HU movements, and none showed a long lead response prior to the movement.…”

“…The so-called long-lead burst neurons (LLBNs) can be intermingled with the BNs (Cullen and Guitton 1997a;Keller et al 2000) and are also located more rostral in the PPRF (Kaneko 2006). They are believed to provide a potent input to the putative BNs but not to the motoneurons directly (see review by Scudder et al 2002).…”

Visual-Vestibular Interaction Hypothesis for the Control of Orienting Gaze Shifts by Brain Stem Omnipause Neurons

“…LLBNs are monosynaptically activated from the SC (Raybourn and Keller 1977), and their stimulation produces short-latency inhibition of OPNs (Kamogawa et al 1996). A recent study by Kaneko (2006) furthermore states that discharge characteristics of a particular group of LLBNs (termed dorsal LLBNs) is well suited to subserve uniquely the "trigger" function and rules out their involvement in other saccadic functions that are traditionally attributed to these neurons. Similarly, as the fast phase is terminated and slow phase initiated, the end of the OPN pause is not believed to simply be triggered by the inhibitory release as burst activity ends.…”