Synaptic organization of the tectal-facial pathways in the cat. I. Synaptic potentials following collicular stimulation

1. The synaptic pathways underlying tectal influence over pinna movements were studied using an acute electrophysiological approach. Under pentobarbital anesthesia, postsynaptic potentials were recorded intracellularly in antidromically identified, cat facial motoneurons following electrical stimulation of the superior colliculus. How collicular topography is reflected in these synaptic potentials was examined using multiple stimulation sites. The pathways responsible for tectally evoked synaptic potentials were studied by making acute brain stem lesions and by intra-axonal horseradish peroxidase (HRP) staining. 2. Monosynaptic excitatory potentials (EPSPs) with latencies ranging from 0.7 to 1.1 ms and amplitudes that were always less than 1 mV were recorded in motoneurons following stimulation of the contralateral superior colliculus. Larger disynaptic EPSPs ranging in latency from 1.2 to 2.0 ms were recorded both in isolation and in association with monosynaptic EPSPs. In addition, disynaptic inhibitory synaptic potentials (IPSPs) with latencies ranging from 1.5 to 2.5 ms were observed, often in combination with monosynaptic EPSPs. Both disynaptic EPSPs and IPSPs were graded, augmented by multiple stimuli and found in all categories of motoneurons. 3. Stimulation of the ipsilateral superior colliculus produced nearly the same spectrum of potentials and latencies as did contralateral tectal stimulation. Occlusion between ipsi- and contralaterally evoked IPSPs suggests there might be a common element in the inhibitory disynaptic pathways. 4. More discrete populations of facial motoneurons were investigated. Specifically, motoneurons innervating the platysma and orbicularis oculi muscles, the intrinsic ear muscles, and muscles that move the vibrissae all displayed tectally elicited mono- and di-synaptic potentials. Collicular input was not restricted to motoneurons involved in orienting the pinnae. 5. The presence, polarity, and amplitude of the synaptic potentials evoked in individual facial motoneurons exhibited variations that were related to the site of stimulation in either the ipsi- or contralateral colliculus. These variations are compatible with the idea that the collicular input to facial motoneurons is topographically organized. 6. Acute lesions at the level of the superior olive indicated that the pathway producing the contralateral monosynaptic EPSPs runs, near the midline, ipsilateral to the target facial nucleus, whereas the contralateral disynaptic and the ipsilateral mono- and disynaptic pathways lie further lateral.(ABSTRACT TRUNCATED AT 400 WORDS)     

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     

Synaptic organization of tectal-facial pathways in cat. II. Synaptic potentials following midbrain tegmentum stimulation.

1. The organization of the synaptic pathways underlying midbrain tegmentum influence over the facial musculature was studied with the use of an acute electrophysiological approach in the cat. Under pentobarbital sodium anesthesia, synaptic potentials were recorded intracellularly in antidromically identified facial motoneurons following electrical stimulation of the paralemniscal zone. The cells of origin and the pathways responsible for the potentials evoked from the paralemniscal zone were defined with the use of retrograde transport of horseradish peroxidase (HRP). The putative role of the paralemniscal zone with regard to the production of disynaptic, tectally evoked potentials in facial motoneurons was investigated both by inactivating this nucleus with injections of lidocaine and by making acute brain stem lesions to sever the paralemniscal-facial and other afferent pathways. 2. Following paralemniscal stimulation, monosynaptic, excitatory postsynaptic potentials (EPSPs) with latencies ranging from 0.6 to 0.9 ms, steep rising phases, and amplitudes in excess of 4.0 mV were recorded in motoneurons of the temporal and auriculoposterior subdivisions, which supply the pinna muscles. Smaller amplitude EPSPs (less than 1.0 mV) with monosynaptic latencies were observed in the zygomatic subdivision. Polysynaptic EPSPs with latencies ranging from 1.0 to 1.8 ms were also observed in all three of these subdivisions. However, only long-latency EPSPs, arriving at 2.0 ms or later, were present in ventral subdivision motoneurons. 3. Inhibitory postsynaptic potentials (IPSPs) were also frequently recorded in facial motoneurons after paralemniscal stimulation. Monosynaptic IPSPs with latencies ranging from 0.8 to 1.2 ms and amplitudes in excess of 4.0 mV were recorded in facial motoneurons of the temporozygomatic and auriculoposterior subdivisions, as were polysynaptic IPSPs with latencies ranging from 1.2 to 1.8 ms. IPSPs were sometimes observed in combination with a smaller, shorter latency EPSPs. Only long-latency IPSPs of greater than 2.0 ms were recorded in ventral subdivision motoneurons. In all cases, both the EPSPs and the IPSPs were graded in character and could be augmented by multiple stimuli. 4. The contralateral paralemniscal zone and the supraoculomotor area, bilaterally, represented the two most prominent afferent sources labeled after HRP injection of the facial nucleus. The superior colliculus and numerous reticular formation regions were also identified as facial nucleus afferents by the presence of retrogradely labeled cells. The retrogradely labeled cells in the paralemniscal zone exhibited heterogeneous soma size. HRP-labeled axons of the paralemniscal-facial pathway were observed to cross the midline by traveling ventral to the brachium conjunctivum in the caudal mesencephalon.(ABSTRACT TRUNCATED AT 400 WORDS)     

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.     

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.     

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     

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.     

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.     

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).     

Direct excitation of neck motoneurons by interstitiospinal fibers

1. Responses of neck motoneurons to stimulation of the interstitial nucleus of cajal (INC) were recorded intracellularly in cats under chloralose anesthesia. When stimuli were applied within or close to the INC, short latency, monosynaptic excitatory postsynaptic potentials (EPSPs) were evoked in many neck motoneurons. Such EPSPs were not evoked by stimulating mesencephalic regions outside the INC. 2. Stimulation of the ipsilateral INC produced monosynaptic EPSPs consistently in biventer cervicis-complexus (BCC) motoneurons, while such EPSPs were observed in about two thirds of the splenius (SP) motoneurons and half of the trapezius (TR) motoneurons tested. Stimulation of the contralateral INC produced weak monosynaptic EPSPs in about half the BCC motoneurons and in a few SP and TR motoneurons. All types of motoneurons also received longer latency, apparently polysynaptic, PSPs from both INCs. In BCC and TR motoneurons these were mainly EPSPs, in SP, mixed excitatory and inhibitory PSPs. 3. Monosynaptic EPSPs evoked by INC stimulation were not eliminated by acute and chronic parasagittal and transverse lesions placed to interrupt the bifurcating axons of all vestibulospinal and many reticulospinal neurons. No significant collision was observed between EPSPs evoked by INC and vestibular or reticular stimulation. The EPSPs evoked by stimulation of the INC therefore appear to have been produced by activation of interstitiospinal neurons rather than by an axon reflex mechanism. 4. The properties of a number of interstitiospinal neurons were observed while recording extracellularly from the mesencephalon to map the location of the INC. One third of the interstitiospinal neurons activated antidromically from the C4 segment could also be activated antidromically from L1. These lumbar-projecting neurons had conduction velocities ranging from 15--123 m/s. Several interstitiospinal neurons sending axons to the ventral horn of the neck segments were identified and two of these were found to be branching neurons that projected both to the neck and to lower levels of the spinal cord.     

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.     

Excitatory connections between neurons of the central cervical nucleus and vestibular neurons in the cat

 The central cervical nucleus (CCN) of the cat receives input from upper cervical muscle afferents, particularly primary spindle afferents. Its axons cross in the spinal cord, and while in the contralateral restiform body give off collaterals to the vestibular nuclei. In order to study the connections between CCN axons and vestibular neurons, we stimulated the area of the CCN in decerebrate cats while recording intra- or extracellularly from neurons in the contralateral vestibular nuclei. CCN stimulation evoked excitatory postsynaptic potentials (EPSPs) or extracellularly recorded firing in the lateral, medial and descending vestibular nuclei. The latency of EPSPs (mean 1.6 ms) was on average 0.4 ms longer than the latency of antidromic spikes evoked in the CCN by stimulation of the contralateral vestibular nuclei (mean 1.2 ms), demonstrating that the excitation was typically monosynaptic. The results provide further evidence that the CCN is an important excitatory relay between upper cervical muscle afferents and neurons in the contralateral vestibular nuclei. Received: 1 August 1996 / Accepted: 16 December 1996     

Functional organization of vestibulofastigial projection in the horizontal semicircular canal system in the cat

Spike potentials of fastigial nucleus neurons were recorded extracellularly in decerebrate, unanesthetized cats. The neurons responding to head rotation in the horizontal plane with a type I fashion were located mainly in the middle and caudal regions of the fastigial nucleus. Three fourth of these fastigial type I neurons were antidromically activated by stimulation of the contralateral vestibular nuclei. These neurons were excited transsynaptically from the ipsilateral vestibular nerve or nuclei. Intra cellular recordings were made from those neurons which were located in the caudal half of the fastigial nucleus and were activated antidromically from the contralateral vestibular nuclei. Stimulation of the ipsilateral vestibular nerve produced EPSPs in these neurons with latencies of 1.0-6.6 msec. The shortest conduction time along primary vestibular aggerents from the labyrinth to the ipsilateral fastigial nucleus was 0,7 msec. The EPSPs with the shortest latency of 1.0 msec were therefore postulated to be due to monosynaptic connections of primary vestibular afferents with fastigial neurons. Stimulation of ipsilateral vestibular nuclei also produced monosynaptic EPSPs in fastigial neurons. These EPSPs were facilitated by conditioning stimulation of the ipsilateral vestibular nerve, indicating the existence of polysynaptic activation of fastigial neurons from the ipsilateral vestibular nerve through the vestibular nuclei.     

Synaptic actions of cutaneous A delta and C fibers on primate hindlimb alpha-motoneurons

Postsynaptic potentials evoked in hindlimb alpha-motoneurons by stimulation of a cutaneous nerve (sural) with finely graded stimulus strengths were analyzed in the primate, monitoring the spinal cord potentials and afferent nerve volleys in the sural nerve. It was observed that activities in A alpha beta, A delta and C fibers of the cutaneous nerve elicited characteristic excitatory and/or inhibitory postsynaptic potentials (EPSPs and/or IPSPs) with different latencies and durations in extensor and flexor motoneurons. Volleys in A delta fibers of the cutaneous nerve produced EPSPs in 57% of flexor and 31% of extensor motoneurons tested, whereas IPSPs were produced by A delta volleys in 41% of flexor and 62% of extensor motoneurons. EPSPs with longer latencies and longer durations were evoked by cutaneous C fiber volleys in 55% of flexor and 34% of extensor motoneurons, whereas IPSPs due to C volleys were recorded in 9% of flexor and 14% of extensor motoneurons. A alpha beta and A delta volleys caused motoneurons to fire in several instances, and some motoneurons discharged repetitively during the depolarizations evoked by activities in C fibers of the nerve. Central latency for transmission in interneuronal chains in the spinal cord was estimated from the onset of the cord potential (N3 wave) to the onset of the postsynaptic potential evoked by A delta volleys. Ranges of central latencies of the EPSPs and IPSPs evoked by A delta volleys were 2.0-7.0 ms and 3.5-8.5 ms, respectively. It is postulated that there may be at least two interneurons interposed in the excitatory reflex pathway from A delta afferent fibers to motoneurons and the A delta inhibitory pathway may involve longer interneuronal chains. In a few motoneurons, however, sural volleys with strengths sufficient to activate A delta fibers produced EPSPs with a central latency of about 1 ms, suggesting activation of a disynaptic segmental pathway with one interposed interneuron. Stimulation of the sural nerve with strengths sufficient to activate cutaneous C fibers produced slow negative cord dorsum potentials with long latencies. It is proposed that primate motoneurons, which show characteristic postsynaptic potentials evoked by cutaneous A delta and C fiber volleys, may provide a suitable model for analyzing the role of high threshold cutaneous afferent fibers not only in the flexor withdrawal reflex but also in motor control functions.     

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.     

Responses of neurons of lizard's,lacerta viridis,vestibular nuclei to electrical stimulation of the ipsi- and contralateral VIIIth nerves

Summary Field and intracellular potentials were recorded in the vestibular nuclei of the lizard following stimulation of the ipsi-and contralateral vestibular nerves. The field potentials induced by ipsilateral VIIIth nerve stimulation consisted of an early negative or positive-negative wave (presynaptic component) followed by a slow negativity (transsynaptic component). The spatial distribution of the field potential complex closely paralleled the extension of the vestibular nuclei. Mono- and polysynaptic EPSPs were recorded from vestibular neurons after ipsilateral VIIIth nerve stimulation. In some neurons early depolarizations preceded the EPSPs. These potentials may be elicited by electrical transmission. Often spikelike partial responses were superimposed on the EPSPs. It is assumed that these potentials represent dendritic spikes.Contralateral VIIIth nerve stimulation generated disynaptic and polysynaptic IPSPs in some neurons and EPSPs in others. The possible role of commissural inhibition in phylogeny is discussed.In a group of vestibular neurons stimulation of the ipsilateral VIIIth nerve evoked full action potentials with latencies ranging from 0.25–1.1 msec. These potentials are caused by antidromic activation of neurons which send their axons to the labyrinth.     

The lateral reticular nucleus in the cat

The afferent paths from the spinal cord and from trigeminal afferents to the lateral reticular nucleus (LRN) were investigated by intracellular recording from 204 LRN neurones in preparations with a spinal cord lesion at C3 that spared only the ipsilateral ventral quadrant. Stimulation of nerves in the limbs evoked EPSPs and JPSPs in 201 of 204 tested LRN neurones. The strongest input was from the ipsilateral forelimb (iF) which evoked EPSPs in 49% and IPSPs in 73% of the LRN neurones. Each of the other limbs evoked EPSPs in approximately 20% and IPSPs in approximately 25% of the neurones. Stimulation of the ipsilateral trigeminal nerve (iTrig) evoked EPSPs in 32% and IPSPs in 46% of the neurones. The shortest latencies of the EPSPs and IPSPs indicated a disynaptic connection between primary afferents in the iF and iTrig and the LRN. The most direct pathways for excitatory and inhibitory responses from the other limbs were trisynaptic. Stimulation of the ventral part of the ipsilateral funiculus (iVLF) at C3 (C3iVLF) evoked monosynaptic responses in 189 of 201 tested LRN neurones. Monosynaptic EPSPs were recorded in 104 neurones and monosynaptic IPSPs in 126 neurones. Monosynaptic EPSPs and IPSPs were encountered in all parts of the LRN. Stimulation of the iVLF at L1 (L1iVLF) evoked monosynaptic EPSPs and IPSPs in the ventrolateral part of the LRN. The termination areas of excitatory and inhibitory fibres appeared to be the same. LRN neurones without monosynaptic EPSPs or IPSPs from the L1iVLF were located mainly in the dorsal part of the magnocellular division. Stimulation of the dorsal funiculi (DF) at C2 and the ipsilateral trigeminal nerve (iTrig) evoked excitatory and inhibitory responses in the LRN. The shortest latencies of EPSPs and IPSPs indicated disynaptic connections.     

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.     

Vestibulo-reticular projections in adult lamprey: their role in locomotion

This study describes the anatomical projections from vestibular secondary neurons to reticulospinal neurons in the adult lamprey and the modulation of vestibular inputs during fictive locomotion. Anatomical tracers were applied in the posterior (PRRN) and middle rhombencephalic reticular nuclei as well as to the proximal stumps of cut vestibular nerve branches to identify the neurons projecting to the reticular nuclei that were in close proximity with vestibular primary afferents. Labeled neurons were found in the intermediate (ION) and posterior (PON) octavomotor nuclei, and were more numerous on the side of the injection (around 56-87 and 101-107 for the ION and the PON, respectively). Morphologies varied but cells were mostly round or oval. Axonal projections from the PON formed a dense bundle, whereas those from the ION were less densely packed. Based on their morphology and the distribution of their projections, most vestibulo-reticular neurons were presumed to be vestibulospinal cells. Reticulospinal cells from the PRRN were recorded intracellularly in the in vitro brainstem-spinal cord preparation and large excitatory post-synaptic potentials (EPSPs) were evoked following stimulation of the ipsilateral anterior and the contralateral posterior branches of the vestibular nerves, whereas inhibitory post-synaptic potentials (IPSPs) or smaller EPSPs were elicited by stimulation of the ipsilateral posterior or of the contralateral anterior branches. During fictive locomotion, both the excitatory and the inhibitory responses displayed phasic changes in amplitude such that the amplitude of the EPSPs was minimal when the spinal cord activity switched from the ipsilateral to the contralateral side of the recorded reticulospinal cell. The IPSPs were then of maximal amplitude. We propose that this modulation could serve to reduce the influence of vestibular inputs in response to head movements during locomotion.