Posterior Canal Nerve Research Articles

Expansion of afferent vestibular signals after the section of one of the vestibular nerve branches.

The anterior branch of N. VIII was sectioned in adult frogs. Two months later the brain was isolated to record in vitro responses in the vestibular nuclei and from the abducens nerves following electric stimulation of the anterior branch of N. VIII or of the posterior canal nerve. Extra- and intracellularly recorded responses from the intact and operated side were compared with responses from controls. Major changes were detected on the operated side: the amplitudes of posterior canal nerve evoked field potentials were enlarged, the number of vestibular neurons with a monosynaptic input from the posterior canal nerve had increased, and posterior canal nerve stimulation recruited stronger abducens nerve responses on the intact side than vice versa. Changes in the convergence pattern of vestibular nerve afferent inputs on the operated side strongly suggest the expansion of posterior canal-related afferent inputs onto part of those vestibular neurons that were deprived of their afferent vestibular input. As a mechanism we suggest reactive synaptogenesis between intact posterior canal afferent fibers and vestibularly deprived second-order vestibular neurons.

Input patterns and pathways from the six semicircular canals to motoneurons of neck muscles. II. The longissimus and semispinalis muscle groups.

To reveal patterns of input from the six semicircular canals to motoneurons of various neck muscles and their relationship to the mechanical actions of individual neck muscles, patterns of input to neck motoneurons of the longissimus and the semispinalis muscle groups were investigated in the upper cervical spinal cord of anesthetized cats. Intracellular potentials were recorded from motoneurons of the longissimus muscle group (obliquus capitis superior muscle, OCS; splenius muscle, SPL; longissimus muscle, LONG) and the semispinalis muscle group (biventer cervicis muscle, BIV; complexus muscle, COMP), and effects of separate electrical stimulation of the six ampullary nerves on them were analyzed in each preparation. Neck motoneurons usually received convergent inputs from all of the six ampullary nerves, and motoneurons that supplied a particular muscle had a homogeneous pattern of input from the six ampullary nerves. Two different patterns of input were identified for motoneurons of these two muscle groups; one pattern for motoneurons of the longissimus muscle group and the other pattern for motoneurons of the semispinalis muscle group. Motoneurons of the OCS, the SPL, and the LONG muscles received excitation from the three contralateral ampullary nerves and inhibition from the three ipsilateral ampullary nerves. BIV and COMP motoneurons received excitation from the bilateral anterior canal nerves (ACNs) and the contralateral canal nerve (LCN) and inhibition from the bilateral posterior canal nerves (PCNs) and the ipsilateral LCN. Latencies of postsynaptic potentials (PSPs) evoked by stimulation of each of the six ampullary nerves indicated that the earliest component of excitatory PSPs (EPSPs) and inhibitory PSPs (IPSPs) was disynaptic in these motoneurons. However, trisynaptic IPSPs were evoked by stimulation of the contralateral PCN in a considerable number of BIV and COMP motoneurons. In OCS, SPL, and LONG motoneurons, all of the excitation from the contralateral and all of the inhibition from the ipsilateral ampullary nerves were mediated through the ipsilateral medial longitudinal fascicle (MLF). In BIV and COMP motoneurons, disynaptic excitation from the contralateral ACN and LCN and disynaptic inhibition from the ipsilateral LCN and bilateral PCNs were mediated through the ipsilateral MLF, whereas disynaptic excitation from the ipsilateral ACN was mediated through the ipsilateral lateral vestibulospinal tract. The patterns of semicircular canal input to neck motoneurons of these two muscle groups are related closely to the mechanical actions of the individual neck muscles and the optimal stimulus to the semicircular canals such that the connections will tend to stabilize head positions in response to head perturbations.

Trisynaptic inhibition from the contralateral vertical semicircular canal nerves to neck motoneurons mediated by spinal commissural neurons.

1. Neck motoneurons usually receive disynaptic excitation and inhibition from individual semicircular canal nerves. However, in motoneurons of some neck muscles, trisynaptic inhibition is evoked by stimulation of the contralateral vertical canal nerves. The present study was performed to analyze this pathway and the location and properties of the last-order interneurons responsible for mediating this trisynaptic inhibition from the contralateral vertical canal nerves to neck motoneurons in anesthetized cats. 2. Bipolar stimulating electrodes were implanted on the contralateral anterior (ACN), lateral (LCN), and posterior canal nerve (PCN), and postsynaptic potentials (PSPs) evoked by electrical stimulation of individual canal nerves were intracellularly recorded from motoneurons of the obliquus capitis inferior (OCI), longus capitis (LC), and rectus capitis posterior (RCP) muscles. Stimulation of the contralateral ACN evoked trisynaptic inhibitory PSPs (IPSPs) in OCI and LC motoneurons and disynaptic excitatory PSPs (EPSPs) in RCP motoneurons. Stimulation of the contralateral PCN evoked di- and trisynaptic IPSPs in OCI and RCP motoneurons and disynaptic EPSPs in LC motoneurons. Stimulation of the contralateral LCN evoked disynaptic EPSPs in all of the motoneurons examined. 3. To determine the pathway that mediates these trisynaptic IPSPs from the vertical canal nerves to neck motoneurons, a lesion was made in the lower medulla, and the patterns of PSPs evoked by stimulation of the three contralateral canal nerves were compared before and after the lesion. Interruption of the ipsilateral medial longitudinal fascicle (MLF) abolished all disynaptic EPSPs and IPSPs from the three contralateral canal nerves in OCI, LC, and RCP motoneurons. In contrast, trisynaptic IPSPs evoked by stimulation of the contralateral ACN or PCN remained unaffected by sectioning the MLFs bilaterally. Sectioning of the contralateral lateral vestibulospinal tract (LVST) eliminated the trisynaptic IPSPs in OCI and LC motoneurons evoked by contralateral ACN stimulation and trisynaptic IPSPs in OCI and RCP motoneurons evoked by contralateral PCN stimulation but did not affect disynaptic EPSPs and IPSPs. 4. Stimulation of the contralateral LVST in the lower medulla after sectioning the bilateral MLFs evoked disynaptic IPSPs in OCI, LC, and RCP motoneurons. Because the LVST only projects ipsilaterally, this finding indicates that the last-order interneurons that mediate the trisynaptic inhibition through the LVST are most likely commissural neurons located in the spinal cord. 5. To determine the locations of last-order commissural neurons terminating on OCI motoneurons, wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) was injected into the OCI muscle nerve at C1.(ABSTRACT TRUNCATED AT 400 WORDS)

Input patterns and pathways from the six semicircular canals to motoneurons of neck muscles. I. The multifidus muscle group

1. The pattern of connections between the six semicircular canals and neck motoneurons of the multifidus muscle group was investigated by recording intracellular potentials from motoneurons in the upper cervical cord of anesthetized cats. 2. Synaptic potentials were recorded in motoneurons of the rectus capitis posterior (RCP) muscle at C1, the obliquus capitis inferior (OCI) muscle at C1 and C2, and the cervical multifidus muscle (Multi) at C4 in response to electrical stimulation of individual ampullary nerves of the six semicircular canals. Excitatory or inhibitory postsynaptic potentials (EPSPs or IPSPs, respectively) were evoked by separate stimulation of individual ampullary nerves in all of the neck motoneurons. Virtually all of the neck motoneurons received convergent inputs from the six ampullary nerves. 3. Motoneurons that supplied a single muscle had a homogeneous pattern of input from the six semicircular canals. There were two patterns of input from the six semicircular canals to motoneurons of the multifidus muscle group. RCP and Multi motoneurons were excited by stimulation of the bilateral anterior canal nerves (ACNs) and the contralateral lateral canal nerve (LCN) and inhibited by stimulation of the bilateral posterior canal nerves (PCNs) and the ipsilateral LCN. This input pattern is similar to that previously observed in other dorsal extensor muscles, whereas the other input pattern observed in OCI motoneurons is entirely new. OCI motoneurons at C1 and C2 were excited by stimulation of the ipsilateral ACN, PCN, and the contralateral LCN and inhibited by stimulation of the contralateral ACN, PCN, and the ipsilateral LCN. 4. Most postsynaptic potentials (PSPs) were disynaptic, but there were trisynaptic inhibitory connections between the contralateral ACN and PCN and OCI motoneurons, and between the contralateral PCN and RCP motoneurons. 5. The pathways for mediating these inputs from different semicircular canals to neck motoneurons were determined by making lesions in the lower medulla. Transection of the ipsilateral medial longitudinal fascicle (MLF) abolished the following potentials: all disynaptic PSPs in RCP motoneurons except the disynaptic EPSPs from the ipsilateral ACN, and in OCI motoneurons, disynaptic PSPs from the bilateral LCNs, and disynaptic IPSPs from the contralateral PCN. Complete bilateral section of the MLF did not affect the disynaptic EPSPs from the ipsilateral ACN in RCP motoneurons, the disynaptic EPSPs from the ipsilateral ACN and PCN in OCI motoneurons, nor the trisynaptic IPSPs from the contralateral ACN and PCN in COI motoneurons and from the contralateral PCN in RCP motoneurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Operational unit responsible for plane-specific control of eye movement by cerebellar flocculus in cat

1. Main findings in our previous studies are as follows: 1) there are three Purkinje cell zones running perpendicular to the long axis of the folia in the cat flocculus, 2) the caudal zone controls activity of the superior rectus (SR) and inferior oblique (IO) extraocular muscles via the y-group and oculomotor nucleus (OMN) neurons, and 3) the middle zone controls activity of the lateral (LR) and medial rectus (MR) muscles via the medial vestibular (MV) and abducens nucleus (ABN) neurons. In the present study, the neuronal pathways from the remaining rostral zone were investigated in the anesthetized cat. 2. Target neurons of rostral zone inhibition in the superior vestibular nucleus (SV) were identified by observing cessation of spontaneous discharges after rostral zone stimulation. Efferent projections were studied by the use of systematic microstimulation techniques. Unitary responses to stimulation of the eighth nerves were also investigated. 3. There are two types of the target neurons: 1) those, being located in the central and dorsal parts of the SV, project to the trochlear and oculomotor nuclei innervating superior oblique and inferior rectus muscles via the ipsilateral medial longitudinal fasciculus (MLF); and 2) those, being located along the dorsal border of the SV, project to the contralateral oculomotor nucleus innervating superior rectus and inferior oblique muscles via the extra-MLF route. 4. Both types receive monosynaptic anterior canal nerve input but not posterior canal nerve input. Some neurons receive polysynaptic excitatory input from the contralateral eighth nerve, although commissural inhibition was never observed. 5. From neuronal connections of the rostral and caudal zones and action of the extraocular muscles, it was expected that 1) activity changes of Purkinje cells in the rostral and/or caudal zones on one side resulted in conjugate eye movement in the plane of the anterior canal on the side of the activity changes, 2) cooperative increased activity on both sides resulted in conjugate downward eye movement, and 3) increased activity on one side and decreased activity on the other side resulted in conjugate rotatory eye movement. The rostral and caudal zones may be responsible for eye-movement control in the sagittal plane by cooperative activity changes on both sides and in the transverse plane by reciprocal activity changes on both sides. For eye-movement control in the anterior canal plane, Purkinje cell activity on one side would be sufficient to produce the required movement. In a functional sense, we call the rostral and caudal zones, the vertical-plane zones.(ABSTRACT TRUNCATED AT 400 WORDS)

Local mechanisms in vestibular receptor control. Effects of curare on the EPSPs and spike discharge recorded from single afferent fibres of the posterior canal nerve of the frog.

In the frog, the afferent discharge of EPSPs and spikes has been recorded intracellularly from single fibres of the posterior ampullar nerve before and after administration of D-tubocurarine (5 X 10(-6) M) in the following experimental conditions: (a) in animals in which the labyrinth was merely exposed (intact frogs), (b) in isolated acute labyrinth preparations, (c) in isolated chronic labyrinth preparations after degeneration of the efferent fibres. In both (a) and (c), curare induced either facilitation or inhibition of the resting discharge in a large number of units. By contrast, after degeneration of the efferent fibres, the effects of the drug were almost completely suppressed. These results indicate the existence of peripheral mechanisms involving the efferent nerve terminals which can control the release of afferent transmitter even in the absence of any central influence.

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

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 (di or 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.

Labyrinthine influence on cat forelimb motoneurons.

1. Intracellular responses in forelimb motoneurons to electrical stimulation of the whole labyrinth and of individual semicircular canal nerves were studied in decerebrated, unanesthetized cats. 2. Stimulation of the whole labyrinth typically produced EPSPs, usually bilaterally, in forelimb extensor (LON, LAT, MED) and shoulder (SI) motoneurons and bilateral IPSPs in forelimb flexor (BIC) motoneurons. 3. Latencies of PSPs indicated that most of those in extensor motoneurons were trisynaptic and many seen in flexor motoneurons may involve four synapses. 4. In the cells sampled, stimulation of the anterior, horizontal or posterior canal nerves often evoked EPSPs in extensor and IPSPs in flexor motoneurons, both ipsi- and contralaterally. Responses to canal stimulation were weaker and more variable than those to stimulation of the whole nerve. 5. Transection of the MLF in the lower medulla had no effect on PSPs evoked in forelimb motoneurons. Lesions in the medulla in the area of the LVST greatly reduced the occurrence of contralateral EPSPs in extensor and IPSPs in flexor motoneurons. The pathway linking labyrinths to forelimb motoneurons therefore appears to include the LVST. Hemisection shows that the pathway to contralateral motoneurons descends in the cord on the side of the stimulated labyrinth before crossing to influence these cells. 6. Labyrinthine control of forelimb motoneurons is less direct than control of neck and back motoneurons. It is suggested that the interneuron in the pathway to forelimb motoneurons is the site of integration of labyrinthine with other reflexes.

Labyrinthine Convergence on Vestibular Nuclear Neurons Using Natural and Electrical Stimulation

Publisher Summary In this chapter, neurons in the vestibular nuclei are examined by extracellular recording in cats with cervical cord transections, and are physiologically identified by their response to angular acceleration in the plane of the horizontal canal. They are then tested with natural and/or electrical labyrinthine stimulation. On natural stimulation of otolith receptors and the anterior and posterior canals, approximately 40% of type I and II horizontal canal neurons show no convergence at all; about 35% have convergence only from otolith receptors; about 10% from one or both vertical canals and about 10% from the otolith plus one or two canals. Tonic neurons receive convergence information about twice as often as did kinetic neurons. Sections of the brain stem vestibular commissural fibers do not alter these figures significantly. Single shock electrical stimulation of individual horizontal and anterior canal nerves and, less frequently, posterior canal nerves is performed while recording from neurons naturally responsive to horizontal canal stimulation. Unit thresholds are compared to eye movement and N 1 field thresholds. Convergence, principally determined by differences in latency, from single vertical canals is shown to take place on a minority of tonic I and II neurons, thus confirming the natural stimulation findings. No electrical stimulation is performed on the otolith receptors, those receptors that show the most frequent convergence on natural stimulation.