Interocular timing differences in the horizontal components of human saccades.
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The occurrence of systematic interocular differences in the time of initiation of saccades in various directions was investigated in normal human subjects (n=4). Saccades were recorded binocularly with scleral sensor coils on each of the eyes with a temporal resolution of 0.1 ms (sampling frequency 10000 Hz). Analysis was done in the velocity domain after digital differentiation. It was found that, in the initial phase of horizontal saccades, the nasalward moving eye lagged the temporalward moving eye consistently by slightly less than 1 ms. No such difference was found in vertical (upward or downward) saccades. In oblique saccades, the systematic initial lag of the nasalward moving eye was similarly present in the horizontal component of the saccade, but absent in the vertical component. It is postulated that this interocular timing difference in horizontal saccades is due to the additional synaptic delay caused by the abducens internuclear neuron in the pathway to the medial rectus muscle of the eye. (+info)
Isolated abducens nerve paresis associated with incomplete Horner's syndrome caused by petrous apex fracture--case report and anatomical study.
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A 17-year-old male presented with a wound on the right temporal region, oozing hemorrhagic necrotic brain tissue and cerebrospinal fluid, following a fall. Computed tomography showed temporoparietal and petrous apex fractures on the right. Neurological examination revealed abducens nerve paresis, ptosis, and myosis on the right side. The patient was treated surgically for the removal of the free bony fragments at the fracture site and to close the dural tear. The abducens nerve paresis, ptosis, and myosis persisted at the 3rd monthly postoperative follow-up examination. The anatomy of the abducens nerve at the petroclival region was studied in four cadaveric heads. Two silicone-injected heads were used for microsurgical dissections and two for histological sections. The abducens nerve has three different angulations in the petroclival region, located at the dural entrance porus, the petrous apex, and the lateral wall of the cavernous segment of the internal carotid artery. The abducens nerve had fine anastomoses with the trigeminal nerve and the periarterial sympathetic plexus. There were fibrous connections extending inside the venous space of the petroclival area. The abducens nerve seems to be vulnerable to damage in the petroclival region, either directly by trauma to its dural porus and petrous apex or indirectly by stretching of the nerve through the nervous and/or fibrous connections. Concurrent functional loss of the abducens nerve and the periarterial sympathetic plexus clinically manifested as incomplete Horner's syndrome in our patient. (+info)
Nitric oxide facilitates GABAergic neurotransmission in the cat oculomotor system: a physiological mechanism in eye movement control.
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Nitric oxide (NO) synthesis by prepositus hypoglossi (PH) neurons is necessary for the normal performance of horizontal eye movements. We have previously shown that unilateral injections of NO synthase (NOS) inhibitors into the PH nucleus of alert cats produce velocity imbalance without alteration of the eye position control, both during spontaneous eye movements and the vestibulo-ocular reflex (VOR). This NO effect is exerted on the dorsal PH neuropil, whose fibres increase their cGMP content when stimulated by NO. In an attempt to determine whether NO acts by modulation of a specific neurotransmission system, we have now compared the oculomotor effects of NOS inhibition with those produced by local blockade of glutamatergic, GABAergic or glycinergic receptors in the PH nucleus of alert cats. Both glutamatergic antagonists used, 2-amino-5-phosphonovaleric acid (APV) and 2,3-dihydro-6-nitro-7-sulphamoyl-benzo quinoxaline (NBQX), induced a nystagmus contralateral to that observed upon NOS inhibition, and caused exponential eye position drift. In contrast, bicuculline and strychnine induced eye velocity alterations similar to those produced by NOS inhibitors, suggesting that NO oculomotor effects were due to facilitation of some inhibitory input to the PH nucleus. To investigate the anatomical location of the putative NO target neurons, the retrograde tracer Fast Blue was injected in one PH nucleus, and the brainstem sections containing Fast Blue-positive neurons were stained with double immunohistochemistry for NO-sensitive cGMP and glutamic acid decarboxylase. GABAergic neurons projecting to the PH nucleus and containing NO-sensitive cGMP were found almost exclusively in the ipsilateral medial vestibular nucleus and marginal zone. The results suggest that the nitrergic PH neurons control their own firing rate by a NO-mediated facilitation of GABAergic afferents from the ipsilateral medial vestibular nucleus. This self-control mechanism could play an important role in the maintenance of the vestibular balance necessary to generate a stable and adequate eye position signal. (+info)
Functional localization of brainstem and cervical spinal cord nuclei in humans with fMRI.
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BACKGROUND AND PURPOSE: To our knowledge, no published functional map of the human lower brainstem exists. Our purpose was to use 1.5-T functional MR imaging (fMRI) to visualize the location of cranial nerve (CN) nuclei and other pontine, bulbar, and cervical spinal cord nuclei by using specific sensory stimulation or motor performance. METHODS: We localized nuclei by using cross-correlation analysis of regional blood oxygen level-dependent (BOLD) signal intensity during specific motor and sensory procedures based on known functions of specific nuclei. Statistical parametric mapping (SPM) analysis was used for comparison. Head, cardiac, and respiratory motion artifact correction was applied. Histologic atlases aided localization. RESULTS: We obtained evidence of localization of the following nuclei by using tests, as follows: main trigeminal sensory (CN V), brushing the face; abducens (CN VI), left-right eye movement; facial (CN VII), smiling and lip puckering; hypoglossal (CN XII), pushing the tongue against the hard palate; nucleus ambiguus, swallowing; nucleus tractus solitarii (NTS), tasting a sweet-sour-salty-bitter mixture; nucleus cuneatus, finger tapping; and cervical spinal cord levels C1-C3, tongue movement to activate the strap muscles. Activation of cortical motor and sensory areas and somatosensory thalamus corresponded with the tasks and sites of brainstem activation. Head movement was minimal, typically less than 1 mm in all three axes. CONCLUSION: With 1.5-T fMRI, the CN nuclei of the pons and medulla, and other nuclei of the lower brainstem and cervical spinal cord, can be localized in awake humans with specific sensory stimulation or motor performance. (+info)
Influence of afferent synaptic innervation on the discharge variability of cat abducens motoneurones.
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The discharge variability of abducens motoneurones was studied after blocking inhibitory synaptic inputs or both excitatory and inhibitory inputs by means of an intramuscular (lateral rectus) injection of either a low (0.5 ng kg(-1)) or a high dose (5 ng kg(-1)) of tetanus neurotoxin (TeNT), respectively. Motoneuronal firing increased after low-dose TeNT. High-dose treatment, however, produced a firing depression, and in some cells, a total lack of modulation in relation to eye movements. Firing became increasingly more regular with larger TeNT doses as shown by significant reductions in the coefficient of variation after low- and high-dose treatments. Similarly, autocorrelation histograms of interspike intervals increased the number of resolvable peaks twofold in low-dose-treated motoneurones and sevenfold in high-dose-treated motoneurones. The plots of standard deviation versus the mean instantaneous firing frequency showed an upward deflexion with low firing frequencies. The upward deflexion occurred in controls at 39.9 +/- 4.9 ms, an interval similar to the mean afterhyperpolarisation (AHP) duration (48.4 +/- 8.8 ms). Low-dose TeNT treatment shifted the deflexion point to 20.9 +/- 3.9 ms, whereas the high dose increased it to 60.7 +/- 6.1 ms, in spite of the fact that no differences in AHP parameters between groups were found. The density of synaptophysin-immunoreactive boutons decreased by 14 % after the low-dose treatment and 40.5 % after the high-dose treatment, indicating that protracted synaptic blockade produces elimination of synaptic boutons. It is concluded that abducens motoneurone spike variability during spontaneous ocular fixations depends largely on the balance between inhibitory and excitatory synaptic innervation. (+info)
Dynamics of abducens nucleus neuron discharges during disjunctive saccades.
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In this report, we provide the first characterization of abducens nucleus neuron (ABN) discharge dynamics during horizontal disjunctive saccades. These movements function to rapidly transfer the visual axes between targets located at different eccentricities and depths. Our primary objective was to determine whether the signals carried by ABNs during these movements are appropriate to drive the motion of the eye to which they project. We also asked whether ABNs encode eye movements similarly during disjunctive saccades and disjunctive fixation. To address the first objective we 1) assessed whether we could predict the discharge dynamics of individual neurons during disjunctive saccades based on their discharge properties during conjugate saccades and 2) directly estimated the sensitivity of individual neurons to either the ipsilateral/contralateral eye or the conjugate/vergence position and velocity using bootstrap statistics. Our main finding was that during disjunctive saccades in the direction ipsilateral to the recording site (ON-direction), the majority of ABNs preferentially encoded the velocity and the position of the ipsilateral eye. The remaining neurons predominantly encoded the conjugate motion of the eyes (i.e., were equally sensitive to the motion of both eyes). Generally, ipsilateral/contralateral eye based models better described neuronal discharges than conjugate/vergence based models, yet both model structures yielded similar conclusions. Moreover, the preferred eye of individual neurons based on their position and velocity sensitivities were generally well matched. We also found that for saccades in the OFF-direction, the pausing behavior of ABNs was similar during conjugate and disjunctive saccades, with the exception that for movements of small amplitudes, more ABNs paused during conjugate saccades. Finally, we found that putative motoneurons and internuclear neurons encoded ON- and OFF-direction disjunctive saccades in a similar manner. To address our second objective, we compared the discharge properties of individual ABNs during disjunctive saccades and disjunctive fixation. Good coherence was observed between the preferred eye of individual ABNs during the two behaviors. Taken together, our results indicate that although individual ABNs can encode the motion of both eyes to various degrees, the population drive of ABNs accounts for most of the movement of the ipsilateral eye during disjunctive saccades and disjunctive fixation. (+info)
A-, T-, and H-type currents shape intrinsic firing of developing rat abducens motoneurons.
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During postnatal development, profound changes take place in the excitability of nerve cells, including modification in the distribution and properties of receptor-operated channels and changes in the density and nature of voltage-gated channels. We studied here the firing properties of abducens motoneurons (aMns) in transverse brainstem slices from postnatal day (P) 1-13 rats. Recordings were made from aMNs in the whole-cell configuration of the patch-clamp technique. Two main types of aMn could be distinguished according to their firing profile during prolonged depolarizations. Both types were identified as aMns by their fluorescence following retrograde labelling with the lipophilic carbocyanine DiI in the rectus lateralis muscle. The first type (BaMns) exhibited a burst of action potentials (APs) followed by an adaptation of discharge and were encountered in approximately 70 % of aMns. Their discharge profile resembled that of adult aMns and was encountered in all aMns after P9. BaMns exhibited a hyperpolarization-induced rebound potential that was blocked by low concentrations of Ni2+ or by Ca2+-free external solution. This current had the properties of the T-type current. Action potentials of BaMns showed a complex afterhyperpolarization (AHP). An inward rectification was evidenced following hyperpolarization and was blocked by external application of caesium or ZD7288, indicating the presence of the hyperpolarization-activated cationic current (IH). Blocking the IH current almost doubled the input resistance of BaMns. The second class of aMns (DaMns) displayed a delayed excitation that was mediated by A-type K+ currents and was observed only between P4 and P9. DaMns exhibited immature characteristics: an action potential with a simple AHP, a linear current-voltage relation and a large input resistance. The number of aMns remained unchanged when both types were present (P5-P6) and later in development when only BaMns were encountered (P19), suggesting that DaMns mature into BaMns during postnatal development. We conclude that aMns display profound reorganization in their intrinsic excitability during postnatal development. (+info)
Functional alterations of cat abducens neurons after peripheral tetanus neurotoxin injection.
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Tetanus neurotoxin (TeNT) cleaves synaptobrevin, a protein involved in synaptic vesicle docking and fusion, thereby preventing neurotransmitter release and causing a functional deafferentation. We injected TeNT into the lateral rectus muscle of adult cats at 0.5 or 5 ng/kg (low and high dose, respectively). In the periphery, TeNT slightly slowed motor axon conduction velocity, and at high doses, partially blocked neuromuscular transmission. TeNT peripheral actions displayed time courses different to the more profound and longer-lasting central actions. Central effects were first observed 2 days postinjection and reversed after 1 mo. The low dose induce depression of inhibitory inputs, whereas the high dose produce depression of both inhibitory and excitatory inputs. Simultaneous recordings of eye movement and neuronal firing revealed that low-dose injections specifically reduced inhibition of firing during off-directed saccadic movements, while high-dose injections of TeNT affected both inhibitory and excitatory driven firing patterns. Motoneurons and abducens interneurons were both affected in a similar way. These alterations resulted in modifications in all discharge characteristic analyzed such as background firing, threshold for recruitment, and firing sensitivities to both eye position and velocity during spontaneous movements or vestibulo-ocular reflexes. Removal of inhibition after low-dose injections also altered firing patterns, and although firing activity increased, it did not result in muscle tetanic contractions. Removal of inhibition and excitation by high-dose injections resulted in a decrease in firing modulation with eye movements. Our findings suggest that the distinct behavior of oculomotor and spinal motor output following TeNT intoxication could be explained by their different interneuronal and proprioceptive control. (+info)