Synaptic connections between primary trigeminal afferents and accessory abducens motoneurons in the monitor lizard, Varanus exanthematicus.

We studied the anatomical pathway underlying the nictitating reflex in the monitor lizard Varanus exanthematicus by the anterograde degeneration technique combined with retrograde transport of horseradish peroxidase (HRP) and electron microscopy. After application of HRP to the abducens nerve, retrogradely labeled neurons were observed in the ipsilateral principal and accessory abducens motor nuclei. The transection, in the same experiments, of the root of the trigeminal nerve resulted in massive degeneration of myelinated fibers in the descending trigeminal tract. In the ipsilateral accessory abducens nucleus, we observed electron-dense degenerating axon terminals that formed asymmetric synaptic contacts with the primary and secondary dendrites of large neurons retrogradely labeled with HRP. A few of the degenerating terminals could be traced in serial sections to myelinated axons. No terminal degeneration was found in the contralateral accessory abducens nucleus or in the ipsilateral and contralateral principal abducens nuclei. The present results are complementary with the findings of previous light microscopic experimental tracing studies (Barbas-Henry, H.A., and A.H.M. Lohman, J. Comp. Neurol. 1986, 254:314-329; see also J. Comp. Neurol. 1988, 267:370-386), and strongly suggest the existence in Varanus of a monosynaptic, unilateral reflex pathway in which trigeminal fibers, presumably originating from the cornea, synapse with motoneurons of the bursalis and retractor bulbi muscles, which are located in the accessory abducens nucleus. This monosynaptic pathway may mediate a rapid unilateral eyeball retraction and nictitating membrane extension.

Development of myelinated nerve fibers in the sixth cranial nerve of the rat: a quantitative electron microscope study.

Myelination was studied quantitatively in the sixth cranial nerves of rats by counting and measuring all myelinated fibers during the first three postnatal weeks. In transverse semithin and thin sections cut serially at a well-defined anatomical site in the midsphenoid region, only a few axons (mean 12) were myelinated at birth. On days 2, 4, and 8, counts of myelinated fibers were respectively 5 times (mean 57), 20 times (mean 230), and 24 times (mean 273) the number seen at birth. During the second postnatal week, the number of myelinated fibers remained constant, whereas growth of axons and their myelin sheaths continued. By 15 days these fibers were large and relatively uniform in size; they had compact, circular myelin sheaths. During the third postnatal week, myelination of previously unmyelinated, smaller axons began. The number of myelinated fibers increased again and the size distribution of myelinated fibers became bimodal. Axon diameters, fiber diameters, and myelin sheath dimensions for all fibers were calculated from measurements made on electron micrographs. The transverse length of the myelin membrane increased exponentially with time. The growth increased rapidly during the formation of the first 20 spiral layers and remained relatively constant during the subsequent enlargement of the compact sheath. The association of axon diameter and myelin sheath thickness was poor at young ages, but it improved progressively with maturation of the sheath. The results show that myelination begins around axons that have a wide range of diameters. Also, the first axons to be myelinated become the large myelinated fibers of the sixth nerve. The small myelinated fibers originate from axons that do not become myelinated until the third postnatal week. Myelination, though differing in onset by 2 weeks, appeared to be similar in both populations as judged by similarity of sheath morphology and growth rates. It is of interest that at the level studied, the sixth nerve also contains a fascicle of unmyelinated cranial sympathetic fibers.

Innervation and structure of extraocular muscles in the monkey in comparison to those of the cat.

Motoneurones that innervate the medial rectus, lateral rectus, and accessory lateral rectus muscles in the monkey have been identified and localized by retrograde transport of horseradish peroxidase. Medial rectus motoneurones were located within both dorsal and ventral regions of the oculomotor nucleus, with a differential distribution along the rostral-caudal axis of the nucleus. Lateral rectus motoneurones were located predominantly within the abducens nucleus, and were distributed throughout the rostral-caudal extent of the nucleus. Motoneurons that innervate the accessory lateral rectus muscle comprised a group of large cells located approximately 0.5 mm ventral to the rostral portion of the abducens nucleus, corresponding to the ventral abducens nucleus of Tsuchida ('06). The ventral subgroup of abducens motoneurones, which innervate both the lateral rectus and accessory lateral rectus muscles, thus do not occupy a motoneurones innervate the retractor bulbi muscle, to which the accessory lateral rectus muscle presumably is homologous. A few accessory lateral rectus motoneurones also were located within the abducens nucleus, overlapping the distribution of lateral rectus motoneurones. Electron microscope examination of the lateral rectus muscle revealed the presence of three morphological types of singly innervated muscle fibers and two morphological types of multiply innervated muscle fibers that exhibited a differential distribution within the orbital, intermediate, and global regions of the muscle. The accessory lateral rectus muscle resembled the global portion of the lateral rectus muscle in containing two morphological types of singly innervated fibers and one type of multiply innervated fiber. These findings indicate that the central differences in the brainstem locations of motoneurones that innervate the cat retractor bulbi and monkey accessory lateral rectus muscles are correlated with peripheral differences not only in the morphology, but also possibly in the mechanical roles, of the muscles they innervate. The accessory lateral rectus muscle thus appears to have evolved both structurally and functionally toward more of a role in patterned eye movement. Furthermore, with the phylogenetic regression of the retractor bulbi muscle, the various types of eye movement with which this muscle is associated in lower vertebrates may be assumed by the other extraocular muscles in higher mammals, including humans.

Axon-myelin relationships in rat cranial nerves III, IV, and VI: a morphometric study of large- and small-fibre classes.

The primary objectives of this study were to determine (1) if quantitative axon-myelin relationships are similar for large- and for small-fibre classes within individual nerves and (2) if the same axon-myelin relationships hold for equivalent fibre classes in closely similar nerves. The oculomotor, trochlear, and abducent nerves of the rat were examined since they each contain distinct large- and small-fibre classes and are similar in a wide range of anatomical and developmental respects. Accordingly, morphometric analyses of axon-myelin relationships were performed separately on large and small fibres of each of the three nerves. Within each nerve, the setting of the relationship between the two parameters was found to be different for the two fibre classes: Scatterplots relating sheath thickness to axon perimeter for large fibres were shifted upwards relative to those for small fibres. These differences were also reflected in the positions of the regression lines fitted to the plots and in the g-ratios. Significant differences were found between nerves in relation to their large fibres: Those of the abducent nerve had significantly thicker sheaths, those of the oculomotor nerve had significantly smaller axon perimeters, and the myelin sheath-axon perimeter relationship of the abducent nerve differed significantly from that of the other two. This study therefore shows that morphometric axon-myelin relationships may differ significantly between equivalent fibre classes of nerves that are closely similar in respect of morphological class, central origin, peripheral distribution, developmental environment, and function.

Morphology and distribution of serotoninergic and oculomotor internuclear neurons in the cat midbrain.

Serotoninergic fibers have been reported in both the abducens and facial nuclei of the cat. Furthermore, serotoninergic dorsal raphe and oculomotor internuclear neurons occupy similar locations in the periaqueductal gray overlying the oculomotor and trochlear motor nuclei. To resolve the issue of whether these two populations of neurons overlap, serotoninergic fibers were assayed in the abducens and facial nucleus; then the morphologies and distributions of identified serotoninergic neurons and oculomotor internuclear neurons were determined. Both the abducens and facial nuclei contained varicosities labelled with antibody to serotonin, but a much higher density of immunoreactive fibers was present in the latter, especially in its medial aspect. Distinct synaptic profiles labelled with antibodies to serotonin were observed in both nuclei. In both cases, terminal profiles contained numerous small, predominantly spheroidal, synaptic vesicles as well as a few, large, dense-core vesicles. These profiles made synaptic contacts onto dendritic and, in the facial nucleus, somatic profiles that occasionally displayed asymmetric, postsynaptic, membrane densifications. Following injection of horseradish peroxidase into either the abducens or facial nuclei, double-label immunohistochemical techniques demonstrated that the serotoninergic and oculomotor internuclear neurons form two distinct cell populations. The immunoreactive serotoninergic cells were distributed within the dorsal raphe nucleus, predominantly caudal to the retrogradely labelled oculomotor internuclear neurons. The latter were located in the oculomotor nucleus along its dorsal border and in the adjacent supraoculomotor area. Intracellular injection of horseradish peroxidase revealed that oculomotor internuclear neurons have multipolar somata with up to ten long, tapering dendrites that bifurcate approximately five times. Their dendritic fields were generally contained within the nucleus and adjacent supraoculomotor area. In contrast, putative serotoninergic neurons were often spindle-shaped and exhibited far fewer primary dendrites. Many of these long, narrow, sparsely branched dendrites crossed the midline and extended to the surface of the cerebral aqueduct. In the vicinity of the aqueduct they branched repeatedly to form a dendritic thicket. The axons of the intracellularly stained serotoninergic neurons emerged either from the somata or the end of a process with dendritic morphology, and in some cases they produced axon collaterals within the periaqueductal gray. Thus the oculomotor internuclear and serotoninergic populations differ in both distribution and morphology.(ABSTRACT TRUNCATED AT 400 WORDS)

Synaptic structural modification following changes in activity induced by tetanus neurotoxin in cat abducens neurons.

A low or a high dose of tetanus neurotoxin (TeNT) injected in the lateral rectus muscle of the cat causes respectively, functional block of inhibitory synapses only or of both inhibitory and excitatory synapses simultaneously in abducens neurons (González-Forero et al. [2003] J. Neurophysiol. 89:1878-1890). As a consequence, neuronal firing activity increases (at low dose) or decreases (at high dose). We investigated possible structural modifications of inhibitory synapses in response to these activity alterations induced by TeNT. We used immunofluorescence against postsynaptic (gephyrin) and presynaptic (vesicular gamma-aminobutyric acid [GABA] transporter [VGAT]) markers of inhibitory synapses in combination with cell type markers for abducens motoneurons (calcitonin gene-related peptide or choline acetyltransferase) or internuclear neurons (calretinin). Seven days after high-dose treatment, the number of gephyrin-immunoreactive (IR) clusters per 100 microm of membrane perimeter was reduced on the soma of abducens motoneurons by 55.3% and by 60.1% on internuclear neurons. Proximal dendritic gephyrin-IR clusters were also significantly altered but to a lesser degree. Partial synaptic re-establishment was observed 15 days post injection, and complete recovery occurred after 42 days. Coverage by VGAT-IR terminals was reduced in parallel. In contrast, a low dose of TeNT caused no structural alterations. With electron microscopy we estimated that overall synaptic coverage was reduced by 40% in both types of neurons after a high dose of TeNT. However, F-type terminals with postsynaptic gephyrin were preferentially lost. Thus, the ratio between F and S terminals diminished from 1.28 to 0.39 on motoneurons and from 1.26 to 0.47 on internuclear neurons. These results suggest that the maintenance of proximal inhibitory synaptic organization on central neurons is best related to neuronal activity and not to the level of inhibitory synaptic function, which was equally blocked by the high or low dose of TeNT.

Abducens internuclear and ascending tract of deiters inputs to medial rectus motoneurons in the cat oculomotor nucleus: synaptic organization.

Abducens internuclear and ascending tract of Deiters (ATD) inputs to medial rectus motoneurons in the oculomotor nucleus are important for conjugate horizontal movements. In the present study, the organization of these separate populations of neurons and their synaptic connections with medial rectus motoneurons in the cat oculomotor nucleus have been examined by light and electron microscopy by using retrograde and anterograde axonal tracers. Consistent with the patterns of retrograde horseradish peroxidase labeling, the abducens internuclear projection is predominantly, if not exclusively, contralateral, whereas the ATD projection is exclusively ipsilateral, as demonstrated by anterograde autoradiographic and biocytin labeling. Both populations of synaptic endings contain spheroidal synaptic vesicles and establish synaptic contacts with modest postsynaptic densifications. In addition, ATD synaptic endings frequently are associated with subjunctional dense bodies and subsurface cisternae. The two populations of excitatory inputs differ, however, in their soma-dendritic distribution. The majority of abducens internuclear synaptic endings contact distal dendrites, whereas the majority of ATD synaptic endings contact proximal dendrites or somata. Abducens internuclear synaptic endings furthermore have a higher density of mitochondria than ATD synaptic endings. The more proximal location of ATD synaptic endings is consistent with the faster rise time and earlier reversal to polarizing currents of ATD excitatory postsynaptic potentials in comparison to those evoked by the abducens internuclear pathway as determined electrophysiologically. Given the differences in the physiologic signals conveyed by the abducens internuclear (eye velocity and eye position) and ATD (head velocity) pathways, the findings in this study suggest that the soma-dendritic stratification of the two inputs to medial rectus motoneurons may provide a means for the separate control of visuomotor and vestibular functions, respectively.

Metastatic infiltration of nerves in the cavernous sinus.

Painful dysesthesia in association with ocular motor palsies characterized intraneural carcinomatous invasion in a 48-year-old man. Transcranial biopsy of the cavernous sinus stopped the pain; the specimen confirmed the diagnosis and provided the rationale for radiation therapy. One year after treatment, the patient was healthy and pain-free, with improved ocular motor function. The source of the carcinoma remains unknown.

Role of GABA in the extraocular motor nuclei of the cat: a postembedding immunocytochemical study.

The GABAergic innervation of the extraocular motor nuclei in the cat was evaluated using postembedding immunocytochemical techniques. The characterization of GABA-immunoreactive terminals in the oculomotor nucleus was carried out at the light and electron microscopic levels. GABA-immunopositive puncta suggestive of boutons were abundant in semithin sections throughout the oculomotor nucleus, and were found in close apposition to somata and dendrites. Ultrathin sections revealed an extensive and dense distribution of GABA-immunoreactive synaptic endings that established contacts with the perikarya and proximal dendrites of motoneurons and were also abundant in the surrounding neuropil. GABAergic boutons were characterized by the presence of numerous mitochondria, pleiomorphic vesicles and multiple small symmetrical synaptic contacts. The trochlear nucleus exhibited the highest density of GABAergic terminations. In contrast, scarce GABA immunostaining was associated with the motoneurons and internuclear neurons of the abducens nucleus. In order to further elucidate the role of this neurotransmitter in the oculomotor system, retrograde tracing of horseradish peroxidase was used in combination with the GABA immunostaining. First, medial rectus motoneurons were identified following horseradish peroxidase injection into the corresponding muscle. This was carried out because of the peculiar afferent organization of medial rectus motoneurons that contrasts with the remaining extraocular motoneurons, especially their lack of direct vestibular inhibition. Semithin sections of the oculomotor nucleus containing retrogradely labeled medial rectus motoneurons and immunostained for GABA revealed numerous immunoreactive puncta in close apposition to horseradish peroxidase-labeled somata and in the surrounding neuropil. At the ultrastructural level, GABAergic terminals established synaptic contacts with the somata and proximal dendrites of medial rectus motoneurons. Their features and density were similar to those found in the remaining motoneuronal subgroups of the oculomotor nucleus. Second, oculomotor internuclear neurons were identified following the injection of horseradish peroxidase into the abducens nucleus to determine whether they could give rise to GABAergic terminations in the abducens nucleus. About 20% of the oculomotor internuclear neurons were doubly labeled by retrograde horseradish peroxidase and GABA immunostaining. A high percentage (80%) of the oculomotor internuclear neurons projecting to the abducens nucleus showed immunonegative perikarya. It was concluded that the oculomotor internuclear pathway to the abducens nucleus comprises both GABAergic and non-GABAergic neurons and, at least in part, the GABA input to the abducens nucleus originates from this source. It is suggested that this pathway might carry excitatory and inhibitory influences on abducens neurons arising bilaterally.

The dendrites of single brain-stem motoneurons intracellularly labelled with horseradish peroxidase in the cat. Morphological and electrical differences.

The geometrical differences between individual dendrites of a given motoneuron were investigated in the cat. We chose two brain-stem motoneurons involved in different motor activities. One abducens and one laryngeal motoneuron were selected from two series of experiments which had combined intracellular recording and horseradish peroxidase staining. Three-dimensional reconstructions were made using a computer-aided microscope to obtain high-resolution measurements from serial histological sections. Each dendrite was characterized by computer dissection. Comparisons between dendrites were made on the basis of the following parameters: spatial projections, length, diameters, tapering, branching pattern, daughter--branch ratio and branching power. The present findings show that each dendrite projects to specific terminal fields for both motoneurons and are different in the complexity of their geometry and branching structure. The consequences of this complexity for the cable properties of the motoneurons were analysed. The dendrites of the two motoneurons were partitioned into a series of contiguous regions deemed short enough to be considered an isopotential cylinder and the steady-state properties were calculated for each segment. The properties of each segment were then combined for each dendrite for the following parameters: electronic distance, somatopetal and somatofugal voltage attenuation, input resistance and charge transfer effectiveness ratio. The present results show significant differences in the electrical behaviour of individual dendrites. Branch-to-branch computation reveals low attenuation pathways between branches suggesting the possibility of local influences within the distal branches of the dendritic arborization. It is proposed that the individual dendrites of the motoneuron function as distinct channels and/or integrators for afferent inputs.

Effects of botulinum neurotoxin type A on abducens motoneurons in the cat: ultrastructural and synaptic alterations.

The synaptic alterations induced in abducens motoneurons by the injection of 3 ng/kg of botulinum neurotoxin type A into the lateral rectus muscle were studied using ultrastructural and electrophysiological techniques. Motoneurons identified by the retrograde transport of horseradish peroxidase showed a progressive synaptic stripping already noticeable by four days post-injection which increased over the study period. By 35 days post-injection, the normal coverage of motoneurons by synaptic boutons (66.4 +/- 4.0%) significantly decreased to 27.2 +/- 4.0%. Synaptic boutons detached by a widening of the subsynaptic space but remained apposed by synaptic contacts and desmosomes to the motoneuron. Detachment did not affect equally flat and round vesicle-containing boutons. The control motoneuron had almost equal numbers of both types of boutons, but after 35 days post-injection the ratio of round to flat vesicle-containing boutons was 1.20 +/- 0.01. Synaptic boutons impinging on motoneurons showed signs of alterations in membrane turnover, as indicated by an increase in the number of synaptic vesicles and a decrease in the number of coated vesicles and synaptic vesicles near the active zone. Abducens motoneurons had a transient increase in soma size by 15 days that returned to normal at 35 days, but no signs of chromatolysis or organelle degeneration were seen. Accompanying the swelling of motoneurons, a 15-fold increase in the number of spines, very infrequent in controls, was observed. Spines located in the soma and proximal dendritic trunk received synaptic contacts from both flat and round vesicle-containing boutons that could be either partly detached or completely attached to the motoneuron. An increased turnover of the plasmatic membrane of the motoneuron was observed, as indicated by a four-fold increase in the number of somatic coated vesicles. Animals were implanted with bipolar electrodes in the ampulla of both horizontal semicircular canals for evoking contralateral excitatory and ipsilateral inhibitory postsynaptic potentials. Motoneurons were antidromically identified from the lateral rectus muscle. Synaptic potentials of vestibular origin were recorded in abducens motoneurons. In the period between two and six days post-injection, a complete abolition of inhibitory synaptic potentials was observed. By contrast, excitatory synaptic potentials remained, but were reduced by 82%. The imbalance between excitatory and inhibitory inputs to motoneurons induced a progressive increase of firing frequency within a few stimuli applied to the contralateral canal. Between 7 and 15 days post-injection, both excitatory and inhibitory postsynaptic potentials were virtually abolished and remained so up to the longest time checked (105 days). Some motoneurons recorded beyond 60 days post-injection showed signs of recovery of excitatory postsynaptic potentials. During the whole time-span studied, presynaptic wavelets were present, indicating no affecting of the conduction of afferent volleys to the abducens nucleus. Taken together, these data indicate that botulinum neurotoxin at high doses causes profound synaptic alterations in motoneurons responsible for the effects seen in the behavior of motoneurons recorded in alert animals.

Grafting of a new target prevents synapse loss in abducens internuclear neurons induced by axotomy.

The loss of afferent synaptic boutons is a prominent alteration induced by axotomy on adult central neurons. In this work we attempted to prove whether synapse loss could be reverted by reconnection with a new target. We severed the medial longitudinal fascicle of adult cats and then transplanted embryonic cerebellar primordia at the lesion site immediately after lesion. As previously shown, the transected axons from abducens internuclear neurons penetrate and reinnervate the graft [J Comp Neurol 444 (2002) 324]. By immunocytochemistry and electron microscopy we studied the synaptology of abducens internuclear neurons under three conditions: control, axotomy and transplant (2 months of survival time). Semithin sections of the abducens nucleus were immunostained against calretinin, to identify abducens internuclear neurons, and either synaptophysin (SF), to label synaptic terminals, or glial fibrillary acidic protein (GFAP) to detect the astrocytic reaction. Optical and linear density of SF and GFAP immunostaining were measured. Data revealed a significant decrease in the density of SF-labeled terminals with a parallel increase in GFAP-immunoreactive elements after axotomy. On the contrary, in the transplant group, the density of SF-labeled terminals was found similar to control, and the astrocytic reaction induced by lesion was significantly reduced. At the ultrastructural level, synaptic coverage and linear density of boutons were measured around the somata of abducens internuclear neurons. Whereas a significant reduction in both parameters was found after axotomy, cells of the transplant group received a normal density of synaptic endings. The ratio between F- and S-type boutons was found similar in the three groups. Therefore, these findings indicate that the grafting of a new target can prevent the loss of afferent synaptic boutons produced by the axotomy.

Electron microscopic serial analysis of GABA presynaptic terminals on the axon hillock and initial segment of labeled abducens motoneurons in the rat.

The aim of the present study was to provide a quantitative analysis of the synapses made onto the axon hillock and initial segment of rat abducens motoneurons retrogradely or intracellularly stained with HRP. GABA-immunoreactive terminals contacting these axons were visualized using a postembedding procedure. The presynaptic terminals contained either spherical or pleomorphic vesicles. gamma-Aminobutyric acid (GABA)-immunoreactive axon terminals, which belonged to this last category, were distributed both onto axon hillocks and the proximal part of initial segments. The percentage of axonal membrane covered by synapses ranged from 44.1 to 68.2%. A quantitative analysis performed on a series of ultrathin sectioned terminals contacting the axon of an intracellularly labeled motoneuron revealed a significant correlation between the length of membrane apposition of the terminals and their perimeter or surface area, and also between the area of membrane apposition and terminal volume. GABA-immunoreactive terminals had a mean perimeter and volume that were larger than those of unlabeled axon terminals. The number of active zones was correlated with the area of apposition. Some hypotheses concerning the functional role of the GABAergic innervation of this particular part of the neuron are discussed.

Fibre composition of the feline trochlear and abducens nerves.

Cat trochlear and abducens nerves were studied by electron microscopy at two different levels. Five mm peripheral to the exit from the brainstem, the average number of myelinated axons is 965 in the trochlear nerve and 1901 in the abducens nerve. The size spectrum is unimodal and small myelinated axons predominate. Both nerves contain 16% unmyelinated axons at this level. At the PNS/CNS transition, the nerve fascicles contain few unmyelinated axons, but bundles of such axons are present in the adjacent pia mater. We suggest that the trochlear and abducens nerves may channel unmyelinated sensory and/or autonomic axons to the leptomeningeal blood vessels and the pia mater of the brainstem.

Motoneuron loss in the abducens nucleus of wobbler mice.

The 'wobbler' mutant mouse can be recognized at about 4 weeks of age by its tremor and atrophy of forelimb muscles. In addition to degeneration of spinal motoneurons, especially in cervical spinal cord, selected bulbar motoneurons have also been reported to degenerate in the mutant. We examined a cranial motor nucleus and found a 31% loss of abducens motoneurons in 4-5-week-old wobbler mice as compared to age-matched control mice.

Localization of axonally transported [125I]wheat germ agglutinin in rat abducens motoneuron axons and terminals after intracisternal injection.

Iodinated wheat germ agglutinin (WGA) was taken up and transported by rat abducens motoneurons to nerve terminals in the lateral rectus muscle. Five days after intracisternal injection of lectin, axon terminals wee found to be the most radioactive source, based on the density of labeling. In axons the radioactive label was concentrated in an annular region about 1.26 microns wide beneath the plasma membrane. We hypothesize that the WGA was associated with smooth vesicular and tubular structures in the axoplasm but not limited to organelles immediately beneath the axon plasma membrane. No evidence of intercellular transfer to muscle cells was found.

Fibroblastic tumor of the abducens nerve. Case report.

Tumors of the cranial nerves are uncommon, and are usually schwannomas or neurofibromas. The authors describe a case of a fibroblastic tumor involving the sixth cranial nerve. Based upon electron microscopy and immunohistochemistry, the tumor was not of nerve-sheath origin, but was comprised of fibroblasts. Clinical, radiographic, and pathological material are presented, and the literature is discussed. This represents the third case report of a tumor of the abducens nerve, and the first report of a fibroma of a cranial or peripheral nerve.

Quantitative ultrastructural study of cat abducens interneurons.

Abducens interneurons project to the medial rectus subdivision of the contralateral oculomotor nucleus and have a role in the mediation of horizontal conjugate gaze. Two types of interneurons have been reported based on shape (fusiform and circular) and differences in organelles. In this study, 41 abducens interneurons from three young adult cats were examined in the transmission electron microscope and were classified, on the basis of eccentricity (e), as circular (e less than 0.82; N = 18) or fusiform (e greater than or equal to 0.82; N = 23). The volume fraction of nucleus, nucleolus, mitochondria, rough endoplasmic reticulum, polyribosomes, and Golgi complex and the number of synaptic profiles per 100 microns of membrane were determined. Although there is a qualitative difference in these cell types, statistical analyses indicate no significant differences in organelle content, mean area, number of synaptic profiles per 100 microns of membrane, or diameter. While the differences in shape may possibly indicate different functional groups, this notion is not supported by the variables examined or by physiological studies.

[Recent findings on the external oculomotor nucleus: the center for horizontal ocular movement]. / Données récentes concernant le noyau oculomoteur externe: centre du mouvement oculaire horizontal.

Various neurobiological methods allowing the analysis of electrophysiological and morphological properties of a cell population within the central nervous system will be illustrated in the study of the abducens nucleus. Intracellular recordings provided the functional identification of the different types of cells within the abducens nucleus. Antidromic excitation, refractoriness, conduction velocity and frequency of discharges indicated the properties of the abducens motoneurones. Various types of afferent stimulations were applied to study the synaptic responses of the neurones. A dye was then electrophoretically injected and the labelled neurone observed with a light microscope. Full reconstruction of dendritic arborization and of axon trajectory was performed. The morphological study has shown the heterogeneity of the population of the motor nucleus, and the ultrastructural features indicated a complex synaptic organisation. The origin of some afferent terminals was determined.

Morphometric nerve fiber analysis and aging process of the human abducent nerve.

Myelinated nerve fibers of the human abducent nerve were analyzed with a new staining method that permits simultaneous observation of the axon and surrounding myelin sheath. The following equipment was employed for the measurements: an image-analyzing digitizer, a microscope equipped with a drawing tube (or camera lucida), and a computer for data storage and statistical analysis. The numbers, transverse areas, and circularity ratios of axons were measured in 10 human abducent nerves. The average number was 1,997 with a definite decrease with age, and the average area was 3.90 micron 2.