Shozo Nakao

Direct inhibitory projection of pause neurons to nystagmus-related pontomedullary reticular burst neurons in the cat

SummaryBrainstem pause neurons (PNs) exhibit a tonic discharge during the slow phase of horizontal vestibular nystagmus and pause prior to and during the quick phase in both directions. One type of pontomedullary burst neurons, burst inhibitory neurons (BINs), show a high frequency burst of spikes before and during the quick phase to the ipsilateral side and this burst directly inhibits contralateral abducens motoneurons, terminating the slow phase firing of these motoneurons. The present study focused on synaptic relations between PNs and BINs.The following data supported the conclusion that PNs probably make direct inhibitory connections with BINs and produce IPSPs in BINs during the slow phase of horizontal vestibular nystagmus: (a) there were positive field potentials in the BIN area during the slow phase; (b) PNs were antidromically activated from BIN areas bilaterally; (c) systematic microstimulation of the BIN area revealed a pattern consistent with axonal branching in the BIN area; (d) repetitive microstimulation of the PN area induced a positive shift in the field potential in the BIN area and suppressed both the characteristic bursts of BINs and nystagmic activity of the contralateral abducens nerve; (e) microstimulation of the PN area during intracellular recording of BINs induced monosynaptic latency hyperpolarizing potentials which could be reversed by Cl− injection; (f) during intracellular recording from BINs during vestibular nystagmus in either direction, the membrane potential during the slow phases had a tonic hyperpolarization which was shown to be due to IPSPs by means of Cl− injection.This study suggests that burst activity of BINs during the quick phase is caused by abrupt release from PN IPSPs (disinhibition), besides some excitatory inputs from other sources.

Cat medial pontine reticular neurons related to vestibular nystagmus: Firing pattern, location and projection

In alert cats, extracellular spikes of neurons in the medial pontine tegmentum were recorded simultaneously with whole nerve discharges of the abducens and medial rectus nerves during horizontal vestibular nystagmus. Nystagmus-related neurons were classified by their firing patterns in relation to the abrupt cessation of the slow phase nerve activity of abducens or medial rectus nerves. The ipsilateral abducens nucleus was electrically stimulated to examine the axonal projections of physiologically identified examples of each category of neurons. Anatomically, pause units clustered near the midline at the rostral pole of the abducens nucleus. Long- and medium-lead burst units were 1-4 mm rostral to the area for pause units. Most burst-tonic units, clearly distinguished from nearby axons of passage, were found close to the MLF. Physiologically, it was concluded that: (1) some long-lead burst units terminate in the abducens nucleus and may excite motoneurons and/or internuclear neurons; (2) pause units directly inhibit burst inhibitory neurons which terminate slow phase activities of contralateral abducens motoneurons; (3) burst-tonic units fire in a manner very similar to contralateral abducens motoneurons; and (4) some medium-lead burst, long-lead burst and burst-tonic neurons (but not pause neurons) project to the cerebellar flocculus.

Eye movement related neurons in the cat pontine reticular formation: projection to the flocculus.

This study examines projection to the cerebellar flocculus of eye movement-related neurons in the median and paramedian part of the cat pontine tegmentum between the trochlear and the abducens nucleus. They were identified by rhythmic activity related to horizontal vestibular nystagmus induced by sinusoidal rotation. These neurons were classified into several groups by their discharge patterns during nystagmus, using criteria of earlier studies on saccadic eye movements and vestibular nystagmus in the monkey. Electrical stimulation of the ipsilateral flocculus elicited antidromic spike responses in a number of burst-tonic neurons and long-lead and medium-lead burst neurons. These neurons were located in and around the medial longitudinal fasciculus, the nucleus raphe pontis and the nucleus reticularis tegmenti pontis. A few neurons tested were also activated antidromically by stimulation of the contralateral flocculus. In contrast, no pauser neurons were activated from the ipsilateral flocculus. It is concluded that eye movement-related neurons in the medial pontine tegmentum, except for pauser neurons, directly project to the flocculus and may convey information about eye movements of visual and vestibular origins to the flocculus.

Direct inhibitory projection of pontine omnipause neurons to burst neurons in the Forel's field H controlling vertical eye movement-related motoneurons in the cat

SummaryThis study examines the nature of the efferent projection of omnipause neurons (OPNs) in the midline pontine tegmentum to medium-lead burst neurons (BNs) in the Forels field H (FFH), both of which exhibit activities related to vertical eye movements, using chronically prepared alert cats. Antidromic spikes of the BNs evoked by oculomotor nucleus stimulation were suppressed by shortly preceding (less than 5 ms) microstimulation within the OPN area including actual recording sites of OPNs. Many OPNs were antidromically activated by microstimulation at recording sites of the BNs. Furthermore, systematic tracking in and around the FFH with the stimulating microelectrode substantiated that the OPNs issued axonal branches within the BN area. These results suggest direct inhibitory projection of OPNs to the BNs.

Direct excitatory and inhibitory synaptic inputs from the medial mesodiencephalic junction to motoneurons innervating extraocular oblique muscles in the cat

SummaryThis study investigated the nature of synaptic inputs from the Forels field H (FFH) in the medial mesodiencephalic junction to inferior oblique (IO) motoneurons in the oculomotor nucleus and superior oblique (SO) motoneurons in the trochlear nucleus in anesthetized cats, using intracellular recording techniques. Stimulation of the FFH induced monosynaptic EPSPs in IO motoneurons on both sides. Paired stimulation of the ipsilateral FFH and contralateral vestibular nerve substantiated that the FFH-induced EPSPs were caused mainly by direct excitatory fibers from the FFH to IO motoneurons and partly by axon collaterals of excitatory neurons in the vestibular nuclei. Among parts of the FFH, the medial part was most effective for producing the EPSPs. Systematic tracking with the stimulating electrode in and around the FFH revealed that effective sites of stimulation inducing negative field potentials in the IO subdivision of the oculomotor nucleus, identified as extracellular counterparts of the EPSPs in IO motoneurons, were also located in the interstitial nucleus of Cajal, nearby reticular formation and posterior commissure, besides within and near the medial part of the FFH. Areas far rostral, dorsal and ventral to the FFH were ineffective. EPSP-IPSPs or EPSPs were mainly induced in SO motoneurons on both sides by FFH stimulation. Latencies of these EPSPs and IPSPs were close to those of the EPSPs in IO motoneurons, indicating their monosynaptic nature. Effective stimulation sites for inducing these synaptic potentials overlapped those for the EPSPs in IO motoneurons. Based on these results, it was suggested that excitatory and inhibitory premotor neurons directly controlling IO and SO motoneurons were located within and near the medial part of the FFH.

Mono- and disynaptic excitatory inputs from the superior colliculus to vertical saccade-related neurons in the cat Forel's field H.

SummaryExcitatory inputs to neurons in the Forels field H (FFH) related to visually induced vertical saccades from the ipsilateral superior colliculus (SC) were investigated in chronically prepared alert cats. By stimulation of the deep or intermediate layer of the SC, upward augmenting neurons (ANs) and one long-lead downward burst neuron (BN) were found to be activated monosynaptically, while medium-lead BNs were activated disynaptically. The monosynaptically activated neurons were not antidromically activated from the oculomotor nucleus, whereas disynaptically activated neurons were also activated antidromically from the inferior rectus subdivision of the nucleus. These results suggest that an excitatory input to the FFH from the SC for inducing vertical saccades of visual origin first reaches upward ANs and/or long-lead downward BNs in the FFH, which in turn drive medium-lead BNs in the same area synapsing with motoneurons related to vertical eye movements.

Ascending projections of posterior canal-activated excitatory and inhibitory secondary vestibular neurons to the mesodiencephalon in cats

The axonal projections of 62 posterior canal (PC)-activated excitatory and inhibitory secondary vestibular neurons were studied electrophysiologically in cats. PC-related neurons were identified by monosynaptic activation elicited by electrical stimulation of the vestibular nerve and activation following nose-up rotation of the animals head. Single excitatory and inhibitory neurons were identified by antidromic activation following electrical stimulation of the contralateral and ipsilateral medial longitudinal fasciculus, respectively. The oculomotor projections of identified neurons were confirmed with a spike-triggered averaging technique. The axonal projections of the identified neurons were then studied by systematic, antidromic stimulation of the mesodiencephalon. Excitatory neurons showed two main types of axonal projections. In one type, axonal branches were issued to the interstitial nucleus of Cajal, central gray, and thalamus including the ventral posterolateral, ventral posteromedial, ventral lateral, ventral medial, centromedian, central lateral, lateral posterior, and ventral lateral geniculate nuclei. The other type was more frequently observed, giving off axon collaterals to the above-mentioned regions and to Forels field H as well. Inhibitory neurons issued axonal branches to limited areas which included the central gray, interstitial nucleus of Cajal, its adjacent reticular formation and caudalmost part of Forels field H, but not the rostral part of the Forels field H and the thalamus. These results suggest that PC-related excitatory neurons participate in the genesis of vertical eye movements and in the perception of the vestibular sensation, and that PC-related inhibitory neurons seem to take part only in the genesis of vertical eye movements.

Excitatory and inhibitory synaptic inputs from the medial mesodiencephalic junction to vertical eye movement-related motoneurons in the cat oculomotor nucleus

The nature of synaptic inputs from the medial mesodiencephalic junction to motoneurons in the oculomotor nucleus innervating the superior and inferior rectus muscles was investigated in anesthetized cats, using intracellular recording techniques. Stimulation of the region produced mono- or disynaptic EPSPs and IPSPs in inferior and superior rectus motoneurons, respectively, on both sides. In a number of superior rectus motoneurons, EPSP-IPSPs or EPSPs were also induced in a similar latency range. The effective sites for inducing the monosynaptic PSPs were found within and near the dorsomedial part of the campus prerubralis.

Direct projection of cat mesodiencephalic neurons to the inferior rectus subdivision of the oculomotor nucleus

This study examines electrophysiologically the projection of neurons in the mesodiencephalic junction to the inferior rectus subdivision of the oculomotor nucleus, using cats anesthetized with pentobarbital sodium. Many of these neurons projected directly to the ipsilateral inferior rectus subdivision and some to both sides. The neurons appeared to distribute axon branches probably terminating within and near the subdivision. They were located in the medial region of the mesodiencephalic junction between A 6.5 and 8.0 histologically corresponding to the campus prerubralis.

An electrophysiologically defined trigemino-reticulo-facial pathway related to the blink reflex in the cat

This study examines the direct projection of neurons in the pontomedullary reticular formation (RF) to the dorsal division of the facial nucleus (FN), where the orbicularis oculi motoneuron pool is located, and the nature of synaptic inputs to the RF neurons from the supraorbital branch of the trigeminal nerve (SON), using electrophysiological techniques in anesthetized cats. A large number of the RF neurons directly projected to the ipsilateral FN dorsal division, and some of these neurons issued axon branches probably terminating on motoneurons in the division. Many of the RF neurons projecting to the dorsal division were synaptically activated by ipsilateral SON stimulation. Most latencies of their activation were between the latencies of the early and late blink reflex responses, and the remaining latencies were shorter than those of the early response. These results suggest that the RF neurons mediate both the early and the late blink reflex responses, but mainly the late response, as premotor relay neurons.