The intraspinal morphology of single lateral vestibulospinal tract (LVST) axons was investigated with the method of intra-axonal staining with horseradish peroxidase (HRP) and three-dimensional reconstruction of the axonal trajectory. Axons penetrated in the ventral funiculus at C5-C8 were identified as LVST axons by their monosynaptic responses to stimulation of the ipsilateral vestibular nerve and by their direct responses to stimulation of the ipsilateral Deiters' nucleus and LVST. Reconstructions were made from 34 well-stained LVST axons. Of these, 23 terminated in the brachial segments (C5-Th1) and the other 11 projected below Th2. These axons were traced over distances of 2.9-16.3 mm rostrocaudally. Within these lengths, one to seven axon collaterals (mean +/- S.D., 3.2 +/- 2.0, N = 19) were given off at right angles from the stem axons of LVST axons terminating in the brachial segments. The mean diameters of stem axons and primary collaterals were 4.5 microns and 1.6 micron, respectively. In the gray matter, collaterals ramified successively, pursued a delta-like path, and terminated mainly in laminae VII and VIII or lamina IX. The rostrocaudal extension of a single collateral was very restricted (mean +/- S.D., 760 +/- 220 microns, N = 16), in contrast to the extensive dorsoventral and mediolateral extent of the terminal arborization. There were usually gaps between adjacent collateral arborizations from the same stem axons, since the intercollateral distances ranged from 400 to 4,300 microns (mean = 1,490 microns). LVST axons terminating in brachial segments were divided into two groups--a medial group and a lateral group--on the basis of their projection sites in the transverse plane of the gray matter. The axons of the medial type had their main projection to laminae VII and VIII of Rexed, while those of the lateral type terminated in lamina IX. The terminal arborizations of the medial type LVST axons were mainly distributed over lamina VIII, where synaptic boutons appeared to make contact with proximal dendrites or somata of medium-sized and large neurons in the ventromedial nucleus and also in the medial portion of lamina VII adjacent to the central canal and dorsal to lamina VIII. Five out of 15 medial type axons had a bilateral projection. One or two collaterals of each of these axons crossed the midline through the anterior commissure and terminated in lamina VII or VIII. It was concluded that the contralateral projection was sparse.(ABSTRACT TRUNCATED AT 400 WORDS)

Pause neurons (PNs) are inhibitory neurons close to the midline at the pontomedullary junction that fire tonically and then cease firing just prior to quick eye movements of visual or vestibular origin. Previous physiological evidence has shown that these neurons have a role of central importance in the generation of rapid eye movements in any direction and all major models of ocular motor control incorporate PNs as major elements. In this study in cats, we injected horseradish peroxidase intracellularly into somata or axons of physiologically identified PNs. After appropriate tissue preparation, cell body and axonal reconstructions were performed, with the aid of a camera lucida-equipped microscope. Fifty-three PNs were stained and reconstructed. These consisted of 17 cell bodies and dendrites and 36 axons. Seven of these included both cell bodies and axons. PN somas lay close to the midline in the nucleus raphe pontis and centralis superior, had extensive dendritic arborizations tending to arise from either pole of the elongated soma, and had axons which typically crossed the midline and bifurcated into long branches which extended rostrally and caudally, inferior to the medial longitudinal fasciculus. There were major terminal arborizations and boutons in areas just rostral and caudal to the abducens nucleus in areas where two types of premotor neurons, excitatory and inhibitory burst neurons, are concentrated. Many axosomatic contacts were noted. Other terminal arborizations and boutons were found close to the midline in a region rostral to abducens nucleus containing other neurons known to burst prior to quick eye movements, and in the nucleus reticularis gigantocellularis. Rostral stem axons could be traced to the level of the trochlear nucleus and inferior to the medial longitudinal fasciculus. The caudal stem axons could be traced parallel to the midline and inferior to the medial longitudinal fasciculus and as far caudally as the hypoglossal nucleus.

The morphology of single medial vestibulospinal tract (MVST) axons was investigated by iontophoretic injection of horseradish peroxidase into single axons at the upper cervical cord in pentobarbital-anesthetized cats. MVST axons were identified by their monosynaptic responses to stimulation of the vestibular nerve and their direct responses to stimulation of the medial longitudinal fusciculus (MLF). Reconstructions of the axonal trajectory were made from 22 uncrossed and 19 crossed MVST axons at C1-C4. MVST axons ran in the ventral funiculus and gave rise to multiple axon collaterals to the upper cervical gray matter at different segments. These axons could be traced over the distance of 2.5-15.3 mm. Within these lengths, up to 9 axon collaterals were identified per axon (mean +/- s.d., 3.3 +/- 2.0, n = 41). Axon collaterals ramified in the gray matter several times and spread in a delta-like manner in both the transverse and horizontal planes. There were usually gaps free from terminal arborizations between adjacent axon collaterals, since the rostrocaudal extension of individual axon collaterals (mean = 820 microns) was very much limited in contrast to wide intercollateral intervals (mean = 1,510 microns). Axon terminals were distributed mainly in laminae IX, VIII, and VII, and sometimes in laminae VI-IV. Most abundant terminals were observed in lamina IX, including the ventromedial (VM), the spinal accessory (SA) nuclei and the nucleus dorsomedial to the VM nucleus (DM nucleus). A majority of individual axon collaterals provided some terminal branches to at least one of the above three motor nuclei. Axon collaterals projecting to laminae VIII-VI without terminals in the motor nuclei were rarely observed. Individual MVST axons had a preferential terminal distribution in each motor nucleus, but all three motor nuclei were covered by axon terminals of an ensemble of all MVST axons, indicating that all neck muscles innervated by these three motor nuclei are influenced by vestibular inputs through MVST axons. Most collaterals from a single axon produced circumscribed terminal arborizations in one or two common areas in the transverse plane (mainly in lamina IX) that were in line with one another in the longitudinal axis of the cord. This longitudinal arrangement of discontinuous terminal arborizations in lamina IX from a single axon may correspond to a continuous sagittal column of motoneurons for a particular muscle.(ABSTRACT TRUNCATED AT 400 WORDS)

The morphology of horizontal canal second-order type I neurons was investigated by intracellular staining with horseradish peroxidase (HRP) and three-dimensional reconstruction of the cell bodies and axons. Axons penetrated in and around the abducens nucleus were identified as originating from type I neurons by their characteristic firing pattern to horizontal rotation and by their monosynaptic response to stimulation of the ipsilateral vestibular nerve. A total of 47 type I neurons were stained. The cell bodies were located in the rostral portion of the medial vestibular nucleus (MVN) and were large or medium sized and had rather elongated shapes and rich dendritic arborizations. The neurons were divided into two groups: those which projected to the contralateral side of the brain stem (type Ic neurons) and those which projected to the ipsilateral side of the brainstem (type Ii neurons). All stem axons of type Ic neurons crossed the midline and bifurcated into rostral and caudal branches in the contralateral medial longitudinal fasciculus (MLF). Two or three collaterals arising close to this bifurcation distributed terminals in a relatively wide area in the contralateral abducens nucleus. Some of these collaterals projected further to the contralateral MVN and thus are vestibular commissural axons. Some of the rostral and caudal stem axons had collaterals which projected to the contralateral nucleus prepositus hypoglossi (PH), nucleus raphe pontis, or medullary reticular formation. There were at least six classes of type Ii neurons, most of which distributed to a relatively limited region in the ipsilateral abducens nucleus and they were categorized according to their future projections into the following categories: A) no further collaterals beyond the abducens nucleus; B) collaterals in the abducens nucleus and a branch descending and terminating in ipsilateral PH; C) projected to the abducens nucleus, PH, and an area rostral to the abducens nucleus; D) projected to the abducens nucleus and to ipsilateral reticular formation rostral and caudal to the abducens nucleus; E) collaterals in the abducens nucleus and a thick caudal stem axon entering and descending in ipsilateral MLF; F) a thick caudal stem axon entering and descending in ipsilateral MLF and no collaterals to the abducens nucleus. Some type Ii neurons also had recurrent collaterals which projected back to the ipsilateral MVN; these may inhibit type II neurons during ipsilateral rotation.

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Both anatomical and physiological studies have shown that pause neurons (PNs) in the medial pontine reticular formation project to two groups of burst neurons (BNs) involved in the genesis of horizontal saccadic eye movements: The excitatory burst neurons (EBNs), which lie rostral to the abducens nucleus, and the inhibitory burst neurons (IBNs), which lie caudal to the abducens. This study is concerned with the projection from PNs to a group of vertical BNs in the nucleus of the H field of Forel (H FF) in the caudomedial subthalamus. Three anatomical methods were used to demonstrate this connection. First, intra-axonal horseradish peroxidase (HRP) injection into physiologically identified PN axons demonstrated axonal branching and axonal terminations in and around the H FF. Second, micro-injection of the tracer Phaseolus vulgaris leucoagglutinin (PHA-L) into the pontine PN region labelled terminal axons and boutons in the nucleus of the H FF. Third, extracellular pressure injection of HRP into H FF yielded retrogradely labelled pontine PN neurons. These anatomical results confirmed the termination of PNs in areas controlling rapid vertical eye movements as physiologically demonstrated by Nakao et al.: Exp. Brain Res. 70:632-636, '88). This work points to the major role of pontine PNs in the synchronization of BN activity in rapid eye movements in all directions.

Schwannoma arising in the posterior pharyngeal wall is rare. We report on a 60-year-old man who complained of discomfort in his pharynx, from whom a tumour was excised via an intraoral approach. No recurrence was seen after an 11-year follow-up. The nerve origin of the tumour is most likely to be the peripharyngeal plexus. This is the third such case reported.

Audiometricsurvey and endoscopic study of the external auditory canal were performed on a group of 31 professional divers, all of whom had experienced frequent exposure to dysbaric conditions.The results are as follows.1) Over 40% had exostosis of the external auditory canal. There was no relationship between the incidence of the exostosis and the length of their occupational career as a diver.Many of the divers had hearing loss whether they had exostosis or not.2) Over 70% had sensorineural hearing loss, taking into account hearing loss due to aging.Most had no experience of inner ear barotrauma on descent, causing sudden a shift in hearing threshold. Deafness was related to the length of their occupational career as a diver.In conclusion, we speculate that repetitive small changes in barometric pressure on the outer ear influences the pressure on the middle ear and further on that of the perilymph, finally damaging the inner ear auditory system.