Sources of direct excitatory and inhibitory inputs from the medial rhombencephalic tegmentum to lateral and medial rectus motoneurons in the cat

The paramedian pontine and bulbar tegmentum was explored by microstimulation to outline the sites of origin of direct excitatory and inhibitory inputs to lateral rectus (LR) and medial rectus (MR) motoneurons (MNs). In order to avoid activation of fibers of passage and axon reflexes originating outside the stimulation sites, experiments were carried out 4--22 days after brain stem transections causing degeneration of vestibulo-ocular pathways. Additionally, in some experiments the paramedian tegmentum was isolated from the contralateral side by midline transections. Mapping of stimulus sites from which monosynaptic EPSPs and IPSPs were elicited brought out the following preoculomotor reticular regions: 1. LR-MNs received monosynaptic IPSPs from the contralateral reticular formation corresponding to Nucl. reticularis points caudalis (R.p.c.) and the rostral part of Nucl. reticularis gigantocellularis (R.gc.). 2. Monosynaptic inhibitory input to MR-MNs could only be demonstrated after degeneration of excitatory pathways ascending from the internuclear neurons of the VIth nucleus and from the ipsilateral vestibular nuclei. Monosynaptic IPSPs originated in the ipsilateral dorso-medial tegmentum through the entire extent of the Nucl. reticularis pontis oralis and rostral R.p.c. including the region of the ipsilateral VIth nucleus. 3. Monosynaptic excitation of LR-MNs was induced by stimulation of the ipsilateral R.p.c. and the rostral half of the paramedian bulbar tegmentum (R.gc.). 4. The sites from which monosynaptic EPSPs were evoked in MR-MNs were confined to the contralateral VIth nucleus and its immediate vicinity. No evidence could be obtained for direct excitatory inputs to MR-MNs from the ipsilateral paramedian tegmentum. It is concluded that the paramedian rhombencephalic reticular formation contains four pools of premotor neurons related to coordination of conjugate horizontal eye movements. Two of them are excitatory for LR- and MR-MNs with ipsilateral ON-directions, the other two mediate reciprocal inhibition of the antagonistic motor nuclei.     

Synaptic linkage in the vestibulo-ocular and cerebello-vestibular pathways to the VIth nucleus in the rabbit

Summary Intra- and extra-cellular responses were recorded with glass microelectrodes from motoneurons in the VIth cranial nuclei of anesthesized rabbits. VIth nucleus motoneurons were identified by their antidromic activation from the VIth nerve. In these motoneurons stimulation of the ipsilateral VIIIth nerve produced IPSPs with disynaptic latencies (mean and S.D., 1.08 ± 0.1 msec) while stimulation of the contralateral VIIIth nerve produced EPSPs with disynaptic latencies (mean and S.D., 1.20 ± 0.18 msec). Correspondingly, direct stimulation of the ipsilateral medial vestibular nucleus (MV), produced IPSPs with monosynaptic latencies (mean and S.D., 0.61±0.15 msec) while direct stimulation of the contralateral MV produced EPSPs with monosynaptic latencies (mean and S.D., 0.61±0.09 msec). Further, with the recording electrode placed within the VIth nucleus to observe the extracellular potentials corresponding to the intracellularly recorded IPSPs and EPSPs, the medulla was systematically tracked with a monopolar stimulating electrode. It was demonstrated that the inhibitory relay cells could be effectively stimulated in the rostral half of the ipsilateral MV and the excitatory relay cells in the rostral half of the contralateral MV.Pharmacological investigation suggested that the inhibitory transmitter involved in the vestibular inhibition is gamma amino-butyric acid or a related substance.Electric stimulation of the flocculus produced a prominant depression in the inhibitory vestibulo-ocular reflex pathway to the VIth nucleus, while the excitatory pathway was free of any similar flocculus inhibition.     

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

Summary This study investigated the nature of synaptic inputs from the Forel's 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.     

Direct projections from vestibular nuclei to facial nucleus in cats

Postsynaptic potentials were recorded from motoneurons in the facial nucleus in response to stimulation of the vestibular and trigeminal nerves. The motoneurons were identified by antidromic activation from their peripheral axons. Disynaptic excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs) and mixed EPSP/IPSPs were recorded in response to vestibular nerve stimulation, ranging in latency from 0.9 to 2.1 ms, with most at 1.5 ms. Activity in secondary vestibular axons recorded within the facial nucleus occurred at a latency of 0.7-1.1 ms. The amplitudes of the vestibular postsynaptic potentials were small, generally less than a millivolt, but double shocks produced marked summation. The average time to peak of ipsilateral vestibular EPSPs, 1.1 ms, was faster than that of either ipsilateral IPSPs, 1.6 ms, or contralateral EPSPs, 1.4 ms. The double-spiked vestibular activity was detectable in double-peaked PSPs. Disynaptic EPSPs, ranging in latency from 2.0 to 3.0 ms, were recorded in response to trigeminal nerve stimulation. The average time to peak was 1.3 ms. The multiple-spiked activity of the trigeminal neurons was detectable in multipeaked EPSPs. Inhibitory ipsilateral effects (Vi IPSPs) were recorded twice as often as excitatory ipsilateral effects (Vi EPSPs), being found in 29% versus 15% of the motoneurons. Contralateral effects were found in 13% of the motoneurons studied, and almost all were excitatory. Analysis of synaptic potential shapes suggested that the excitatory and inhibitory vestibular synapses probably contact distal dendrites preferentially, with the excitatory connections being somewhat closer to the soma. The trigeminal inputs probably contact the facial motoneurons more extensively near the soma. Horseradish peroxidase was injected into the facial nucleus, and retrograde uptake by vestibular neurons was studied. The majority of filled vestibular neurons was ipsilateral to the injection site, especially in the medial vestibular nucleus, ventral y group, and supravestibular nucleus. On the contralateral side, filled vestibular cells were found almost exclusively in the medial nucleus. Filled cells were also noted in the trigeminal nucleus, predominantly ipsilaterally at all rostrocaudal levels. We have demonstrated monosynaptic projections to facial motoneurons from both vestibular and trigeminal nuclei. The trigeminal input is likely to be involved in facial reflexes, especially blinking and grimacing. The afferent vestibular population overlaps that going to the oculomotor and cervical motoneurons; these projections may be collaterals of single vestibular neurons.4+.     

Monosynaptic inhibition of neck motoneurons by the medial vestibular nucleus

Summary Stimulation of the brain stem in cats anesthetized with pentobarbital evoked short-latency IPSPs in many neck motoneurons. From the segmental delay of these IPSPs, and from comparison of their latencies with those of monosynaptic EPSPs evoked in the same motoneuron population by stimulation of the brain stem, it is concluded that the IPSPs are monosynaptic and are produced by descending inhibitory fibers.As many as thirteen electrodes were inserted into the medulla and pons to compare threshold stimuli required to evoke monosynaptic IPSPs from different locations. The points with the lowest threshold were in the medial vestibular nucleus and the medial longitudinal fasciculus. The IPSPs are apparently produced by fibers that originate in the medial vestibular nucleus and reach the upper cervical segments via the MLF.Electrical stimulation of the ipsilateral labyrinth often produces disynaptic IPSPs in neck motoneurons, very probably by means of a relay in the medial nucleus. This inhibitory pathway between labyrinth and neck motoneurons, together with the previously described excitatory pathway relaying in Deiters' nucleus, provides some of the pathways utilized by the labyrinth in regulation of head position.     

Synaptic organization of the vestibulo-trochlear pathway

Summary Field and intracellular potentials evoked in the trochlear nucleus (TN) of the cat following stimulation of the ipsi and contralateral vestibular nerves (V_i, V_c) and the vestibular nuclei (VN) were recorded with microelectrodes.Single shock stimulation of either V_c or V_i evokes in the TN the presynaptic potentials, n₁ and n₂, which are generated by the action currents of repetitively firing axons of vestibular neurons reaching the TN via the medial longitudinal fascicle (MLF). In the case of V_c stimulation a slow negative potential (n₃) follows the presynaptic components of the field complex while a slow positive potential (p-wave) is evoked by V_i stimuli. The n₃ wave is composed of the excitatory synaptic and action currents generated in trochlear motoneurons (TMns) while the p-wave is produced by the inhibitory synaptic current. Disynaptic EPSPs and IPSPs are recorded intracellularly in TMns following V_c and V_i stimulation, respectively. Each synaptic potential shows a biphasic rising phase due to the synchronous n₁ and n₂ presynaptic barrage.On stimulation of the ipsilateral superior and contralateral medial vestibular nuclei, the latencies of the IPSPs and EPSPs, respectively, are reduced to the monosynaptic range. Thus, it has been directly demonstrated that the VN are the mediating links for both the short latency excitatory and inhibitory vestibuloocular reflexes. The above data suggest that IPSPs are for the most part generated at or near the soma of the motoneurons. As for the site of generation of the EPSPs, a predominantly dendritic origin is suggested.The organization of the neuronal circuitry is discussed in relation to the vestibular induced eye movements.     

The organization of the vestibulo-oculomotor and trochlear reflex pathways in the rabbit

Summary Intracellular and extracellular responses were recorded with glass micro-electrodes from motoneurons in the IIIrd and IVth cranial nuclei of anesthesized rabbits. Five subgroups of neurons innervating the superior rectus (SR), inferior oblique (IO), inferior rectus (IR), medial rectus (MR), and superior oblique (IVth) extraocular muscles were identified by their antidromic activation from the branches of the IIIrd and IVth cranial nerves. The relative positions of the subgroups thus determined were consistent with the histological data on the rabbit. In the SR, IO, IR, and IVth subgroups the effects of ipsilateral VIIIth nerve stimulation were inhibitory, producing disynaptic IPSPs, while the effects of contralateral VIIIth nerve stimulation were excitatory, producing disynaptic EPSPs. In the MR subgroup, however, a mixture of EPSPs and IPSPs was produced by VIIIth nerve stimulation: this was particularly clear on the ipsilateral side. Sites relaying these VIIIth nerve effects to each of the five subgroups were explored by direct stimulation of various brain stem sites. Stimulation of the superior vestibular nucleus (SV) produced IPSPs monosynaptically in all five subgroups on the ipsilateral side as well as in the contralateral MR subgroup. Stimulation of the medial vestibular nucleus (MV) produced EPSPs monosynaptically in all of the five subgroups on the contralateral side as well as in the ipsilateral MR subgroup. Stimulation of the brachium conjunctivum (BC) also produced EPSPs monosynaptically in the contralateral SR, IO, and IR subgroups. Further, while the recording electrode was placed within each of the five subgroups to observe the extracellular potentials corresponding to the intracellularly recorded IPSPs and EPSPs, the medulla and cerebellum were systematically tracked with a monopolar stimulating electrode. It was thus confirmed that the SV is the sole inhibitory relay site, while excitation is relayed by both the MV and the BC. The origin of the BC pathway was traced to the Y-Group for the IO, to the lateral nucleus of the cerebellum (LN) for the IR, and to both the Y-Group and the LN for the SR subgroup.     

The vestibulo-ocular reflex arc in the newborn kitten

Summary Field potentials and postsynaptic potentials were recorded in the vestibular and abducens nuclei and neurons following vestibular nerve stimulation in anesthetized newborn kittens (within 72 h after birth). Stimulation of the ipsilateral vestibular nerve evoked an initial P wave and an N₁ field potential in the vestibular nuclei. No N₂ potential was evoked. Latencies of the peak of the P wave, the onset and the peak of the N₁ potential were 0.99±0.16 ms, 1.66±0.18 ms, and 2.51±0.23 ms, respectively. Ipsilateral vestibular nerve stimulation evoked monosynaptic excitatory postsynaptic potentials (EPSPs) and polysynaptic inhibitory postsynaptic potentials (IPSPs) in vestibular nuclear neurons. Stimulation of the contralateral vestibular nerve evoked polysynaptic IPSPs in vestibular nuclear neurons. In abducens motoneurons, ipsilateral vestibular nerve stimulation evoked monosynaptic EPSPs and disynaptic IPSPs; contralateral vestibular nerve stimulation produced disynaptic EPSPs. We conclude that short circuit pathways of the vestibul-ovestibular and vestibulo-ocular reflex arc are present in the kitten already at birth.Supported by the Japanese Ministry of Education, Science, and Culture Grants-in-Aid for Scientific Research nos. 572 140 30 and 575 700 53     

Reticulospinal excitation and inhibition of neck motoneurons

Summary Responses of neck motoneurons to electrical stimulation of the pontomedullary reticular formation were recorded intracellularly in cerebellectomized cats anesthetized with chloralose. Stimulation of nucleus reticularis (n.r.) ventralis and the dorsal part of n.r. gigantocellularis evoked short latency, monosynaptic inhibitory postsynaptic potentials (IPSPs) in the majority of motoneurons supplying the ipsilateral splenius, biventer cervicis and complexus muscles and in 25% of motoneurons projecting in the ipsilateral spinal accessory nerve. Monosynaptic IPSPs were also evoked by stimulating the medial longitudinal fasciculus (MLF) but lesion and collision experiments indicated that these IPSPs were independent of those evoked by reticular stimulation. Monosynaptic IPSPs were also occasionally observed following stimulation of the contralateral reticular formation, especially of the dorsal part of n.r. gigantocellularis.Monosynaptic excitatory postsynaptic potentials (EPSPs) were evoked in all classes of neck motoneurons studied by stimulation of n.r. pontis caudalis, gigantocellularis and ventralis. Each reticular nucleus appeared to contribute to this excitation. The excitation was bilateral but large monosynaptic EPSPs were most often seen in motoneurons ipsilateral to the stimulus site. Data indicated that pontine EPSPs were mediated by ventromedial reticulospinal fibers while medullary EPSPs were mediated by ventrolateral reticulospinal fibers. Neck motoneurons thus receive at least three distinct direct reticulospinal inputs, two excitatory and one inhibitory.Supported in part by grants NSF BMS 75-00487 and NIH NS 02619Recipient of N.I.H. Fellowship 1 F32 NS 05027     

Vestibulo-ocular reflex from the posterior canal nerve to extraocular motoneurons in the cat

Summary In the anesthetized cat, the posterior canal nerve (PCN) was stimulated by electric pulses and synaptic responses were recorded intracellularly in the three antagonistic pairs of extraocular motoneurons. Pure reciprocal effects were obtained in the motoneurons innervating the antagonistic pair of ipsilateral oblique muscles and the antagonistic pair of contralateral vertical rectus muscles. These responses consisted of low threshold disynaptic excitatory postsynaptic potentials (EPSPs) in either the contralateral superior oblique (c-SO) (trochlear) or contralateral inferior rectus (c-IR) motoneurons and of disynaptic inhibitory postsynaptic potentials (IPSPs) in either the ipsilateral inferior oblique (i-IO) or ipsilateral superior rectus (i-SR) motoneurons. In addition, disynaptic IPSPs were also found in (i-SO) motoneurons. Mixtures of low threshold (dior trisynaptic) EPSPs and IPSPs were found in all other extraocular motoneurons except for the contralateral lateral rectus (c-LR) motoneurons. These results may afford a basis for the characteristic eye movements induced by vertical canal nerve stimulation.     

Same spinal interneurons mediate reflex actions of group Ib and group II afferents and crossed reticulospinal actions

The aim of the study was to analyze interactions between neuronal networks mediating centrally initiated movements and reflex reactions evoked by peripheral afferents; specifically whether interneurons in pathways from group Ib afferents and from group II muscle afferents mediate actions of reticulospinal neurons on spinal motoneurons by contralaterally located commissural interneurons. To this end reticulospinal tract fibers were stimulated in the contralateral medial longitudinal fascicle (MLF) in chloralose-anesthetized cats in which the ipsilateral half of the spinal cord was transected rostral to the lumbosacral enlargement. In the majority of interneurons mediating reflex actions of group Ib and group II afferents, MLF stimuli evoked either excitatory or inhibitory postsynaptic potentials (EPSPs and IPSPs, respectively) or both EPSPs and IPSPs attributable to disynaptic actions by commissural interneurons. In addition, in some interneurons EPSPs were evoked at latencies compatible with monosynaptic actions of crossed axon collaterals of MLF fibers. Intracellular records from motoneurons demonstrated that both excitation and inhibition from group Ib and group II afferents are modulated by contralaterally descending reticulospinal neurons. The results lead to the conclusion that commissural interneurons activated by reticulospinal neurons affect motoneurons not only directly, but also by enhancing or weakening activation of premotor interneurons in pathways from group Ib and group II afferents. The results also show that both excitatory and inhibitory premotor interneurons are affected in this way and that commissural interneurons may assist in the selection of reflex actions of group Ib and group II afferents during centrally initiated movements.     

Excitatory input from interneurons in the abducens nucleus to medial rectus motoneurons mediating conjugate horizontal nystagmus in the cat

Summary A class of interneurons in the cat abducens nucleus was identified by its antidromic activation from the contralateral ascending MLF, disynaptic activation from the contralateral vestibular nerve and type II response to rotation of the turntable. They were also activated antidromically from the contralateral oculomotor nucleus, the region of medial rectus motoneurons. Extracellular spikes of single interneurons, spontaneous or glutamate-driven, were used as triggers for perior post-spike averaging of three kinds of potentials. (1) The average of the extracellular field potentials within the contralateral oculomotor nucleus consisted of an early positive or positive-negative spike and a late, slow negative wave. The early spike was an action current caused by impulses along the axon of the interneuron. The late potential was the extracellular counterpart of unitary EPSPs. (2) The averaged membrane potential of contralateral medial rectus motoneurons revealed unitary EPSPs with monosynaptic latencies, evidence that interneurons were excitatory in nature. (3) The average of compound potentials of the contralateral medial rectus nerve showed a monosynaptic excitatory effect relevant to unitary EPSPs. This effect was observed with nearly all interneurons. All interneurons thus identified exhibited discharge patterns closely correlated with the activity of medial rectus motoneurons in both slow and quick phases of vestibular nystagmus. It was concluded that these interneurons controlled activities of contralateral medial rectus motoneurons associated with conjugate horizontal eye movements by their monosynaptic excitatory connections.     

Vestibulospinal effects on hindlimb motoneurons of the frog

Summary Field and intracellular potentials were recorded in the lumbar spinal cord of the frog following stimulation of the anterior branch of the vestibular nerve and vestibular nucleus. The field potential recorded in the motoneuron pool after VIIIth nerve stimulation consisted of two presynaptic positive-negative potentials (latencies 1.7 and 2.6 msec) followed by a slow negative wave. The latency of the first presynaptic field potential was only 0.6 msec longer than the presynaptic field potential evoked by stimulation of the vestibular nucleus; it is suggested that electrotonic coupling in the vestibular nuclei is responsible for the fast vestibulospinal transmission.Whereas VIIIth nerve stimulation produced EPSPs in both flexor (peroneal) and extensor (tibial) motoneurons, IPSPs were found only in extensor motoneurons. The functional implication of these findings was discussed. Comparison of PSP latencies with the extracellular presynaptic field potentials generated by VIIIth nerve or nucleus stimulation indicated that EPSPs were produced by the excitatory action of vestibulospinal axons on motoneurons. The longer latencies of the vestibular induced IPSPs suggested that they were generated indirectly by inhibitory spinal interneurons. Preliminary experiments on the interaction of segmental and vestibular induced PSPs suggest that the latter are generated close to the soma of motoneurons.     

Vestibuloocular reflex of the adult flatfish. III. A species-specific reciprocal pattern of excitation and inhibition

In juvenile flatfish the vestibuloocular reflex (VOR) circuitry that underlies compensatory eye movements adapts to a 90 degrees relative displacement of vestibular and oculomotor reference frames during metamorphosis. VOR pathways are rearranged to allow horizontal canal-activated second-order vestibular neurons in adult flatfish to control extraocular motoneurons innervating vertical eye muscles. This study describes the anatomy and physiology of identified flatfish-specific excitatory and inhibitory vestibular pathways. In antidromically identified oculomotor and trochlear motoneurons, excitatory postsynaptic potentials (EPSPs) were elicited after electrical stimulation of the horizontal canal nerve expected to provide excitatory input. Electrotonic depolarizations (0.8-0.9 ms) preceded small amplitude (<0.5 mV) chemical EPSPs at 1.2-1.6 ms with much larger EPSPs (>1 mV) recorded around 2.5 ms. Stimulation of the opposite horizontal canal nerve produced inhibitory postsynaptic potentials (IPSPs) at a disynaptic latency of 1.6-1.8 ms that were depolarizing at membrane resting potentials around -60 mV. Injection of chloride ions increased IPSP amplitude, and current-clamp analysis showed the IPSP equilibrium potential to be near the membrane resting potential. Repeated electrical stimulation of either the excitatory or inhibitory horizontal canal vestibular nerve greatly increased the amplitude of the respective synaptic responses. These observations suggest that the large terminal arborizations of each VOR neuron imposes an electrotonic load requiring multiple action potentials to maximize synaptic efficacy. GABA antibodies labeled axons in the medial longitudinal fasciculus (MLF) some of which were hypothesized to originate from horizontal canal-activated inhibitory vestibular neurons. GABAergic terminal arborizations were distributed largely on the somata and proximal dendrites of oculomotor and trochlear motoneurons. These findings suggest that the species-specific horizontal canal inhibitory pathway exhibits similar electrophysiological and synaptic transmitter profiles as the anterior and posterior canal inhibitory projections to oculomotor and trochlear motoneurons. Electron microscopy showed axosomatic and axodendritic synaptic endings containing spheroidal synaptic vesicles to establish chemical excitatory synaptic contacts characterized by asymmetrical pre/postsynaptic membrane specializations as well as gap junctional contacts consistent with electrotonic coupling. Another type of axosomatic synaptic ending contained pleiomorphic synaptic vesicles forming chemical, presumed inhibitory, synaptic contacts on motoneurons that never included gap junctions. Altogether these data provide electrophysiological, immunohistochemical, and ultrastructural evidence for reciprocal excitatory/inhibitory organization of the novel vestibulooculomotor projections in adult flatfish. The appearance of unique second-order vestibular neurons linking the horizontal canal to vertical oculomotor neurons suggests that reciprocal excitation and inhibition are a fundamental, developmentally linked trait of compensatory eye movement circuits in vertebrates.     

Supraspinal monosynaptic excitation and inhibition of thoracic back motoneurons

Summary The effects of brain stem stimulation on thoracic back motoneurons were studied in cats anesthetized with pentobarbital. The population sampled consisted of the extensors interspinales (IS), longissimus dorsi (LD) and spinalis dorsi (SD), and of unidentified (UIC) motoneurons. The location of the motoneurons, between Th 1 and Th 10, at widely varying distances from the stimulating electrode permitted linear regression analysis of the descending neural influences.EPSPs evoked by MLF stimulation in all types of motoneurons were produced by a pathway with an average conduction velocity in the thoracic cord of 127 m/sec, and were monosynaptic. IPSPs were also produced by MLF stimulation. The IPSPs in IS and UIC motoneurons were monosynaptic and were produced by a pathway with an average conduction velocity of 69 m/sec.Stimulation of Deiters' nucleus evoked short latency EPSPs in many motoneurons. EPSPs in LD and UIC motoneurons were shown to be monosynaptic, although latency scatter and sample size made accurate determination of vestibulospinal conduction velocity impossible.Stimulation of the labyrinth evoked disynaptic EPSPs and IPSPs in many cells, as previously observed in neck motoneurons. IPSPs were frequently produced by stimulation of the contralateral labyrinth, probably by a pathway with a relay in the contralateral medial vestibular nucleus. Ipsilateral stimulation usually produced EPSPs. The excitatory pathway relays in Deiters' nucleus and, we suggest, in the descending vestibular nucleus.Supported in part by a research grant from the Public Health Service (NSO2619).     

Mechanism for activation of locomotor centers in the spinal cord by stimulation of the mesencephalic locomotor region

The synaptic pathways of mesencephalic locomotor region (MLR)-evoked excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs) recorded from lumbar motoneurons of unanesthetized decerebrate cats during fictive locomotion were analyzed prior to, during, and after cold block of the medial reticular formation (MedRF) or the low thoracic ventral funiculus (VF). As others have shown, electrical stimulation of the MLR typically evoked short-latency excitatory or mixed excitatory/inhibitory PSPs in flexor and extensor motoneurons. The bulbospinal conduction velocities averaged approximately 88 m/s (range: 62-145 m/s) and segmental latencies for EPSPs ranged from 1.2 to 10.9 ms. The histogram of segmental latencies showed three peaks, suggesting di-, tri-, and polysynaptic linkages. Segmental latencies for IPSPs suggested trisynaptic or polysynaptic transmission. Most EPSPs (69/77) were significantly larger during the depolarized phase of the intracellular locomotor drive potential (LDP), and most IPSPs (35/46) were larger during the corresponding hyperpolarized phase. Bilateral cooling of the MedRF reversibly abolished locomotion of both hindlimbs as measured from the electroneurogram (ENG) activity of muscle nerves and simultaneously abolished or diminished the motoneuron PSPs and LDPs. Unilateral cooling of the VF blocked locomotion ipsilaterally and diminished it contralaterally with concomitant loss or decrease the motoneuron PSPs and LDPs. Relative to the side of motoneuron recording, cooling of the ipsilateral VF sometimes uncovered longer-latency EPSPs, whereas cooling of the contralateral VF abolished longer-latency EPSPs. It is concluded that MLR stimulation activates a pathway that relays in the MedRF and descends bilaterally in the VF to contact spinal interneurons that project to motoneurons. Local segmental pathways that activate or inhibit motoneurons during MLR-evoked fictive locomotion appear to be both ipsilateral and contralateral.     

Branching pattern and properties of vertical- and horizontal-related excitatory vestibuloocular neurons in the cat

1. The axonal trajectories of excitatory vestibuloocular neurons and their synaptic contacts with extraocular motoneurons were studied by means of spike-triggered signal averaging and microstimulation techniques. A majority of the excitatory neurons related to the vertical semicircular canals were located in the border of the descending and medial nuclei and the rostral half of the descending nucleus. 2. Individual vestibuloocular neurons activated by stimulation of the ampullary nerve of the anterior semicircular canal excited motoneurons within both the contralateral inferior oblique and contralateral superior rectus motoneuron pools. 3. Individual vestibuloocular neurons receiving input from the ampullary nerve of the posterior semicircular canal excited motoneurons in both the contralateral trochlear nucleus and contralateral inferior rectus motoneuron pools. The branching pattern of single vestibuloocular neurons activated by the anterior and posterior canals probably underlies conjugate eye movement during vertical head rotation. 4. Time to peak and shape indices of unitary excitatory postsynaptic potentials (EPSPs) suggested that the location of the synaptic contact of vestibuloocular neurons was on the soma or proximal dendrites of the target extraocular motoneurons. 5. In contrast, we did not find conclusive evidence that single vestibuloocular neurons receiving input from the horizontal semicircular canal give off axon collaterals to motoneurons innervating both the contralateral lateral rectus and the ipsilateral medial rectus muscles. Projection of horizontal vestibuloocular neurons to motoneurons supplying individual muscles might be useful for convergence during horizontal head movement.     

Postsynaptic influences on the vestibular non-deiters nuclei from primary vestibular nerve

Summary Neurones in the descending, medial and superior vestibular nuclei of the cats were explored with intracellular microelectrodes. Cerebellar- and spinal-projecting neurones were identified by their antidromic invasion from the region of fastigial nuclei and from the second cervical segment, respectively, and the others by their location. The central actions of the primary vestibular impulses upon these non-Deiters vestibular nuclei neurones were investigated by using electric stimulation of the ipsilateral vestibular nerve. Many of these cells received excitatory postsynaptic potentials (EPSPs) monosynaptically, similar to those evoked in the ventral Deiters neurones, as described elsewhere, except that the unitary EPSPs are often larger. Some cells received only polysynaptic EPSPs or IPSPs and a few cells were not influenced at all.     

Vertical semicircular canal inputs to cat extraocular motoneurons

Summary Synaptic potentials were recorded in identified extraocular motoneurons in anesthetized cats, following stimulation of ampullary nerves of the anterior and posterior semicircular canals.Superior rectus motoneurons received disynaptic EPSPs and IPSPs following stimulation of the two ampullary nerves of the anterior and posterior semicircular canals, respectively. In the inferior rectus motoneurons, the effects of anterior and posterior semicircular canal stimulation were a mirror image of those on superior rectus motoneurons.Inferior oblique motoneurons developed disynaptic EPSPs and IPSPs following stimulation of the ampullary nerves of the contralateral anterior and ipsilateral posterior semicircular canals, respectively. In addition, some inferior oblique motoneurons displayed disynaptic IPSPs following stimulation of the contralateral ampullary nerve of the posterior semicircular canal. In the superior oblique (trochlear) motoneurons, disynaptic EPSPs and IPSPs were recorded after stimulation of the contralateral posterior and ipsilateral anterior semicircular canals, respectively.There was no significant connection between the ampullary nerves of the vertical semicircular canals and motoneurons innervating lateral and medial rectus muscles.Abbreviations i- Ipsilateral to the recorded motoneuron - c- Contralateral to the recorded motoneuron - ACN Ampullary nerve of the anterior semicircular canal - HCN Ampullary nerve of the horizontal semicircular canal - PCN Ampullary nerve of the posterior semicircular canal - IO Inferior oblique - IR Inferior rectus - LR Lateral rectus - MR Medial rectus - SO Superior oblique - SR Superior rectus - EPSP Excitatory postsynaptic potential - IPSP Inhibitory postsynaptic potential - PSP Postsynaptic potential - MLF Medial longitudinal fasciculus     

Morphology of vertical canal related second order vestibular neurons in the cat

Summary The morphology of vertical canal related second order vestibular neurons in the cat was studied with the intracellular horseradish peroxidase method. Neurons were identified by their monosynaptic potentials following electrical stimulation via bipolar electrodes implanted into individual semicircular canal ampullae. Anterior and posterior canal neurons projected primarily to contralateral or ipsilateral motoneuron pools (excitatory and inhibitory pathways, respectively). The axons of contralaterally projecting neurons crossed the midline at the level of the abducens nucleus and bifurcated into an ascending and a descending main branch which travelled in the medial longitudinal fasciculus (MLF). Two types of anterior canal neurons were observed, one with unilateral and one with bilateral oculomotor projection sites. For both neuron classes, the major termination sites were in the. contralateral superior rectus and inferior oblique subdivisions of the oculomotor nucleus. In neurons which terminated bilaterally, major collaterals recrossed the midline within the oculomotor nucleus to reach the ipsilateral superior rectus motoneuron pool. Other, less extensive, termination sites of both neuron classes were in the contralateral vestibular nuclear complex, the facial nucleus, the medullary and pontine reticular formation, midline areas within and neighboring the raphé nuclei, and the trochlear nucleus. The ascending main axons continued further rostrally to reach the interstitial nucleus of Cajal and areas around the fasciculus retroflexus. The descending branches proceeded further caudal in the medial vestibulo-spinal tract but were not followed to their spinal target areas. In addition to two previously described posterior canal related neuron types (Graf et al. 1983), we found neurons with bilateral oculomotor terminals and a spinal collateral. Typical for posterior canal neurons, the major termination sites were in the trochlear nucleus (superior oblique motoneurons) and in the inferior rectus subdivision of the oculomotor nucleus. Axon collaterals recrossed the midline to reach ipsilateral inferior rectus motoneurons. The axons of ipsilaterally projecting neurons ascended through the reticular formation to join the MLF caudal to the trochlear nucleus. The main target sites of anterior canal related neurons were in the trochlear nucleus and the inferior rectus subdivision of the oculomotor nucleus. Minor collaterals reached the pontine reticular formation and areas in between the fiber bundles of the ipsilateral MLF. In some cases, small collaterals crossed the midline within the oculomotor nucleus to terminate in the inferior rectus subdivision on the contralateral side. The axon proceeded further rostral to project to the interstitial nucleus of Cajal and beyond. The main termination sites of posterior canal neurons were in the superior rectus and inferior oblique subdivisions of the oculomotor nucleus. Minor collaterals were also observed to reach the midline area within the oculomotor nucleus, however, prospective contralateral termination sites could not be identified. More rostral projections were found in the interstitial nucleus of Cajal. The described axonal arborization of second order vestibular neurons reflects the organization of intrinsic coordinate systems as exemplified by the geometry of the semicircular canal and the extraocular muscle planes. These neurons are interpreted to provide a matrix for coordinate system transformation, i.e. from vestibular into oculomotor reference frames, and to play a role in gaze control and related reflexes by distributing their signals to multiple termination sites.Abbreviations DV descending vestibular nucleus - INC interstitial nucleus of Cajal - INT nucleus intercalatus - IQ inferior oblique subdivision - LV lateral vestibular nucleus - MLF medial longitudinal fasciculus - MRF medullary reticular formation - MV medial vestibular nucleus - nVII facial nerve - PH nucleus praepositus hypoglossi - PRF pontine reticular formation - RO nucleus Roller - SR superior rectus subdivision - SV superior vestibular nucleus - III oculomotor nucleus - IV trochlear nucleus - VI abducens nucleus - VII facial nucleus - XII hypoglossal nucleus Supported by NIH grants EY04613 and NS02619