Monosynaptic projections from the pontine reticular formation to the 3rd nucleus in the cat

Wall-Eyed Internuclear Ophthalmoplegia: History and Hypothesis

BACKGROUND

Most patients with internuclear ophthalmoplegia (INO) are orthotropic, although a subset is exotropic. When INO is bilateral, this is termed wall-eyed bilateral internuclear ophthalmoplegia (WEBINO). In 1979, Sharpe described his "first case" of wall-eyed monocular internuclear ophthalmoplegia (WEMINO) as "a unique clinical syndrome" characterized by unilateral INO and ipsilateral exotropia.

METHODS

WEMINO was clinically identified in seven patients, with oculographic correlation in six and neuropathological confirmation in one. Oculographic features of exotropic INO patients were compared with those of six orthotropic INO patients using magnetic search coil and infrared oculography.

RESULTS

All clinically defined WEMINO patients showed slowed, hypometric ipsilateral saccades by oculography. Six patients had ipsilateral exotropia, and three had ipsilateral hypertropia. Ipsilateral abducting saccades had faster peak velocities for smaller saccades, more so for orthotropic patients. Exotropic patients had normal sinusoidal mean vestibulo-ocular reflex (VOR) gains and phases; orthotropic patients had subnormal mean VOR gains and phase leads.

CONCLUSION

WEMINO is a clinical ocular motor syndrome characterized by unilateral slow, hypometric adducting saccades with exotropia and hypertropia of the ipsilateral eye. We propose that it results from discrete unilateral damage to burst-tonic fibers in the medial longitudinal fasciculus (MLF) with sparing of the adjacent extrafascicular pathways. Paradoxically, orthotropic INO results from more extensive damage to ascending pathways lateral, ventral and caudal to the MLF. Direct injury to the medial rectus subnucleus is not required. This manuscript was in preparation at the time of Dr Sharpe's death in 2013 and is an acknowledgement of his forward-thinking, as his hypotheses have stood the test of time.

A review on screening tests for vestibular disorders

Although many studies have reported on tests of the vestibular system a valid and reliable, evidence-based screening battery for easy clinical use remains elusive. Many screening tests attempt to assess the vestibulo-ocular reflex. Therefore, head shaking, the Dix-Hallpike maneuver, the supine roll test, and head impulse tests are discussed. Other tests address the spatial orientation functions of the vestibular system, such as the Bucket Test and the Fukuda Stepping test. Still, other tests are based on the known correlates with balance skills, both static and dynamic, such as tandem walking and the modern variation of the Romberg test, the modified Clinical Test of Sensory Interaction and Balance. This review provides a critical overview of the literature on some of these tests and their value for clinical use and in epidemiological studies.

Abduction paresis with rostral pontine and/or mesencephalic lesions: Pseudoabducens palsy and its relation to the so-called posterior internuclear ophthalmoplegia of Lutz

BACKGROUND

The existence of a prenuclear abduction paresis is still debated.

METHODS

In a retrospective design, we identified 22 patients with isolated unilateral (n = 20) or bilateral (n = 2) abduction paresis and electrophysiologic abnormalities indicating rostral pontine and/or mesencephalic lesions. Another 11 patients had unilateral abduction paresis with additional ocular motor abnormalities indicating midbrain dysfunction. Eight of these 11 patients also had electrophysiological abnormalities supporting this location. Electrophysiological examinations in all patients included masseter and blink reflexes (MassR, BlinkR), brainstem auditory evoked potentials (BAEP), and direct current elctro-oculography (EOG).

RESULTS

Unilateral MassR abnormalities in patients with unilateral abduction paresis were seen in 17 patients and were almost always (in 16 of 17 patients) on the side of the abduction paresis. Another 11 patients had bilateral MassR abnormalities. BlinkR was always normal. EOG disclosed slowed abduction saccades in the non-paretic eye in 6 patients and slowed saccades to the side opposite to the abduction paresis in another 5 patients. Re-examinations were done in 27 patients showing normalization or improvement of masseter reflex abnormalities in 18 of 20 patients and in all patients with EOG abnormalities. This was always associated with clinical improvement.

CONCLUSIONS

Electrophysiologically documented or clinically evident rostral pontine and/or mesencephalic lesions in our patients exclude an infranuclear intrapontine 6th nerve lesion and indicate the existence of an abduction paresis of prenuclear origin. An increased tone of the antagonistic medial rectus muscle during lateral gaze either by abnormal convergence or impaired medial rectus inhibition seems most likely.

The effects of afferent stimulation on congenital nystagmus foveation periods

Visual acuity in congenital nystagmus (CN) patients is related primarily to the duration of "foveation periods", during which the image of the target is relatively stationary in the foveal area. Thirteen individuals with CN were studied to test the hypothesis that somatosensory stimulation (vibration or electrical) of either the forehead or the neck damps CN and improves visual acuity. We identified characteristics of the nystagmus waveform that were likely to be important in determining visual acuity and combined these measures into an "acuity function" (NAFP) that correlated well with visual acuity (r2 = 0.91). Statistically significant changes in NAFP were used to assess the effects of afferent stimulation; positive effects were found in nine subjects. Vibratory stimulation (especially on the neck) was found to be more effective than electrical stimulation. CN amplitude reduction alone was neither necessary nor sufficient to improve acuity. Foveation duration was the single most important factor determining acuity. Based on our findings, afferent stimulation should be considered as an alternative or additional treatment to improve visual acuity in CN patients.

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.

Abduction nystagmus in internuclear ophthalmoplegia

Direct current electro-oculography revealed abduction nystagmus with hypermetric abduction saccades in 35 of 64 patients with unilateral and 55 of 66 patients with bilateral internuclear ophthalmoplegia. Slowing of abduction saccades occurred in 27 unilateral cases, mainly ipsilateral to the paretic eye, and in 36 bilateral cases. Abduction nystagmus with hypermetric abduction saccades of normal velocity is explained by an increased phasic innervation adjusted to adduction paresis. Slowed abduction saccades are attributed to impaired inhibition of the medial rectus muscle. Superposition of impaired medial rectus inhibition and increased phasic innervation best explains abduction nystagmus with slowed hypermetric (6 unilateral and 23 bilateral cases) or normometric abduction saccades (9 unilateral and 5 bilateral cases).

William Harvey 1578-1657

Images

Internuclear ophthalmoplegia of abduction: clinical and electrophysiological data on the existence of an abduction paresis of prenuclear origin

Three patients showed unilateral and five bilateral abduction paresis. Five had associated adduction nystagmus of the contralateral eye. Electrophysiological testing of masseter and blink reflexes indicated an ipsilateral rostral pontine or mesencephalic lesion, and excluded a lesion of the infranuclear portion of the abducens nerve. Abduction paresis was attributed to impaired inhibition of the tonic resting activity of the antagonistic medial rectus muscle. The prenuclear origin of the disorder is based on morphological and neurophysiological evidence of an ipsilateral inhibitory connection between the paramedian pontine reticular formation and the oculomotor nucleus running close to but separated from the medial longitudinal fasciculus.

Electrical activity of rat ocular motoneurons recorded in vitro

The electrophysiological properties of rat oculomotor neurons were studied in an in vitro slice preparation. Motoneurons were identified by their antidromic response to third nerve rootlet stimulation, and by their orthodromic responses to medial longitudinal fasciculus and reticular stimulations. Passive membrane properties showed the existence of an inward rectification mechanism in all the recorded motoneurons. The action potential is comprised of several distinct components. The fast initial spike, composed of an initial segment spike and a somatodendritic spike, is followed by a delayed depolarization, an afterhyperpolarization and a late afterdepolarization. The afterhyperpolarization has a maximum duration of 55 ms. The late afterdepolarization is a voltage-dependent mechanism that produces an oscillatory behavior in depolarized cells. Two types of motoneurons were distinguished on the basis of their response to long-lasting depolarizing current pulses. The intensity-frequency curves show the existence of a primary and secondary range of discharge and the study of the interspike intervals points to specific properties of the conductance underlying the afterhyperpolarization. It is concluded that large, stellate motoneurons of the brainstem maintained in vitro retain specific electrophysiological properties, comparable to those described in vivo and which differentiate the ocular motoneurons from spinal motoneurons.

Cat medial pontine reticular neurons related to vestibular nystagmus: firing pattern, location and projection

In alert cats, extracellular spikes of neurons in the medial pontine tegmentum were recorded simultaneously with whole nerve discharges of the abducens and medial rectus nerves during horizontal vestibular nystagmus. Nystagmus-related neurons were classified by their firing patterns in relation to the abrupt cessation of the slow phase nerve activity of abducens or medial rectus nerves. The ipsilateral abducens nucleus was electrically stimulated to examine the axonal projections of physiologically identified examples of each category of neurons. Anatomically, pause units clustered near the midline at the rostral pole of the abducens nucleus. Long- and medium-lead burst units were 1-4 mm rostral to the area for pause units. Most burst-tonic units, clearly distinguished from nearby axons of passage, were found close to the MLF. Physiologically, it was concluded that: (1) some long-lead burst units terminate in the abducens nucleus and may excite motoneurons and/or internuclear neurons; (2) pause units directly inhibit burst inhibitory neurons which terminate slow phase activities of contralateral abducens motoneurons; (3) burst-tonic units fire in a manner very similar to contralateral abducens motoneurons; and (4) some medium-lead burst, long-lead burst and burst-tonic neurons (but not pause neurons) project to the cerebellar flocculus.

Development of the brain stem in the rat. IV. Thymidine-radiographic study of the time of origin of neurons in the pontine region

Groups of pregnant rats were injected with two successive daily doses of 3H-thymidine from gestational day 12 and 13 (E12 + 13) until the day before parturition (E21 + 22) in order to label in their embryos the proliferating precursors of neurons. At 60 days of age the proportion of neurons generated (or no longer labeled) on specific embryonic days was determined quantitatively in 14 nuclei of the pontine region. Peak production time of neurons of the trigeminal mesencephalic nucleus was on day E11 or earlier, with a small proportion generated on day E12. Peak production time of the trigeminal motor neurons was on day E12, with a small proportion produced earlier. Neurons of the principal sensory nucleus were generated between days E13 and E16, with a peak on day E14; the late-produced neurons tended to belong to a class of intermediate and large cells. The bulk of the neurons of the supratrigeminal and infratrigeminal nuclei arose on days E15 and E16. Neurons of the locus coeruleus are produced mostly on day E12, with about 20% of the cells arising on day E13. The bulk of the neurons of the dorsal tegmental nucleus (Gudden's) are produced between days E13 and E15, whereas most of the neurons of the deep (ventral) tegmental nucleus are produced on day E15. A dorsal-to-caudal gradient was also obtained between the dorsal and ventral nuclei of the lateral lemniscus, the neurons of the former being generated between days E12 and E15; the latter, between days E13 and E17. The neurons of both the pars lateralis and the pars medialis of the parabrachial nucleus were produced simultaneously between days E13 and E15, with a peak on day E13. The heterogeneous collection of neurons of the pontine paramedial reticular formation was produced for day E11 (or earlier) until day E15. Finally, the neurons of the raphe pontis parvicellularis were generated at an even rate between days E13 and E15, whereas the bulk of the neurons of the raphe pontis magnocellularis were produced on days E15 and E16. On the basis of datings obtained for 9 subdivisions of the entire brain stem trigeminal complex, hypotheses were offered of the cytogenetic components of the system. The sequence of neuron production in the dorsal and deep tegmental nuclei was related to their connections with divisions of the mammillary and habenular nuclei on a "first come-first serve" basis.

[Identification of the vestibular projections in the oculomotor nuclei in the cat by autoradiography and electron microscopy]

The projection of vestibular pathways to the oculomotor nuclei ws investigated by electron microscopic radioautography. Unilateral injection of tritiated amino acids into the rostral vestibular complex was used in order to characterize the location and to identify the different types of labeled synaptic terminals involved in these pathways. In the normal oculomotor nuclei, 4 types of synaptic boutons were identified. Following the labeling of the vestibular synapses, in the ipsilateral oculomotor nucleus, types I and II boutons are the most prominent group and make up 75% of the synaptic vesicles, they are distributed on the cellular soma and the large dendrites of the oculomotor neurons. In contrast, in the contralateral oculomotor nucleus, type III boutons which are smaller and have larger diameter synaptic vesicles were predominant; they are prevalent on the distal part of the dendritic tree. From the results obtained, a relationship between the present anatomical findings and previously published physiological studies is established. The following conclusion is suggested: the inhibitory vestibular inputs probably terminate on the oculomotor neurons by these large types I and II boutons and the excitatory vestibular inputs by the smaller type III boutons. Also discussed is the complexity of the pattern of afferentation and the functional arrangement of the oculomotor nuclei.

Cell bodies of origin of reticular projections from the superior colliculus in the cat: an experimental study with the use of horseradish peroxidase as a tracer

By use of the retrograde axonal transport of horseradish peroxidase (HRP), the projection from the superior colliculus (SC) to the brain stem reticular formation (RF) was investigated in the cat. A 0.2-0.5 microliter of a 50% suspension of Sigma VI HRP was injected stereotactically in various portions of the pontomedullary RF, and, as a control to the injection to the RF, in the inferior olive or in the spinal cord. Labeled cells were found within and deep to the intermediate gray layer of the SC in the cats which survived for two or three days after HRP injection. The number of the labeled cells varied, according to the difference in the site of injection and the amount of injected HRP. About 400 labeled cells in twenty 50-micron sections, taken every fifth of the SC, occurred throughout its rostrocaudal extent, particularly in the case where the medial portion of the border zone of the nucleus reticularis pontis oralis and caudalis (R.p.o.-R.p.c. zone) or the border zone of the nucleus reticularis pontis caudalis and the gigantocellularis (R.p.c.-R.gc. zone) was heavily stained after three days of survival period. From 10 to 15% of these labeled cells were large in size (more than 40 micron in diameter), 20-30% were medium sized and the rest (60-70%) were small (10-25 micron). On the other hand, when HRP was placed in the inferior olive only eight cells were labeled in the SC, seven of which were small and medium-sized. When HRP was injected in the gray matter of C1-C3 level of the spinal cord, a total of 70 tectal cells (14, 42, and 14 were large, medium, and small cells, respectively) were observed to be labeled. The findings of the tectoreticular neurons are discussed and compared with those of the tectoolivary and the tectospinal neurons. Thus the three kinds of tectal neurons are located within and deep to the intermediate gray layer. The number of the labeled cells and the percentages of the collicular neurons of different sizes are obviously different among the three different projections. Topographic correlations between the SC and the RF could not be discerned in the present materials. These results were discussed in relation to possible influences of the tectoreticular neurons upon the extraocular and the spinal motoneurons.

The course of direct projections from the abducens nucleus to the contralateral medial rectus subdivision of the oculomotor nucleus in the cat

We have used autoradiography (tritiated leucine) to investigate the projections of a number of nuclear groups of the cat pons. Some cells of the abducens nucleus have axons that cross the midline, ascend in the opposite median longitudinal fasciculus (MLF) and synapse on the cells of the oculomotor complex which have been identified by others as those innervating the medial rectus muscle.

The ultrastructure of the nucleus of the oculomotor nerve (somatic efferent portion) of the cat

The ultrastructure of the somatic efferent portion of the nucleus of the oculomotor nerve was studied in four adult cats. The neuronal population is composed of neurons of variable size. A continuous pattern of morphological aspects is evident between the large neurons, which show abundant cytoplasm with well developed organelles, and the small neurons which have a reduced amount of cytoplasm. The dendrites are generally smooth, with few short spines. Axo-dendritic synapses are numerous. Synaptic boutons are also present on the axon hillock. The neuropil is characterized by the occurrence of small groups of dendrites which may be in direct touch with their membranes. Direct membrane appositions may also occur between neighbouring neurons and between the cell somata and tangentially running dendrites. Generally beneath the site of apposition there is accumulation of mitochondria, multivesicular bodies, coated vesicles and moderately dense amorphous material. The morphological features suggest the possibility of cellular interchanges at the sites of direct membrane apposition. Five types of synaptic boutons were recognized on the basis of their vesicular content, the presence of abundant filaments in the pre-synaptic bag, the occurrence of post-synaptic specializations. The different synaptic types and their distribution are similar to those reported in the spinal motor nuclei. Many of the synapses make synaptic contacts with two or more post-synaptic elements. Axo-axonic synapses were also observed.

Effects of ponto-mesencephalic lesions and electrical stimulation upon PGO waves and EMPs in unanesthetized cats

1. Phasic waves similar to those observed in the visual system of the awake and sleeping cat ("EMP" and "PGO waves" respectively) were recorded in an area surrounding brachium conjunctivum of the mesencephalic reticular formation (MRF). 2. MRF waves during paradoxical sleep (PS) (MRF PGO wave) occurred 1--2 msec after the waves recorded in the abducens nucleus (N.VI PGO waves), while MRF waves in the alert cat (MRF EMPs) occurred 35--70 msec or more after EMPs in N.VI. 3. Both MRF EMPs in the dark and MRF PGO waves preceded EMPs and PGO waves recorded in the lateral geniculate body (LGB) by 20 msec. 4. There was a time lag of about 40 msec between right and left MRF PGO waves. 5. MRF stimulation elicited short latency responses (5--9 msec) in LGB during waking and sleeping states. During PS, the same stimuli further elicited PGO waves (latency greater than or equal to 40 msec) in the same structure. 6. Stimulation of LGB elicited short latency responses (5--9msec) in MRF. 7. Unilateral MRF lesions reduced by about 50% the number of LGB PGO waves without modifying other characteristics of the PGO wave activity. 8. Following unilateral MRF lesion, LGB EMPs were observed with each eye movement in the dark. However, LGB EMPs ipsilateral to the lesion were greatly reduced in amplitude and followed the EMPs in the opposite LGB by a time lag similar to that observed between bilateral LGB PGO waves. 9. Bilateral MRF lesions completely abolished PGO waves and EMPs observed with the animal in the dark. However, EMPs observed in the light remained unmodified. 10. From the present study it was concluded that LGB and visual cortex gross potential changes related to eye movements in the alert cat have both retinal and non-retinal components. Further, the significance of the MRF area in the mechanisms of EMP and PGO wave activities was discussed.

Coding of information about rapid eye movements in the pontine reticular formation of alert monkeys

Neurons in the rostral paramedian zone of the pontine reticular formation (PRF) have distinct frequency changes prior to and during quick eye movements, but generally little or no tonic activity associated with eye position. Evidence indicates that horizontal saccades and quick phases of nystagmus are generated in this region. Firing of units activated with eye movements (burst units) and of units which are inhibited (pause units) was analyzed. Eye movements were described by a vector having an amplitude (A) and an angle (a). These parameters were related to position changes in certain planes by the equation delta pos = A - cos alpha. In each of 80 cells in the PRF which were encountered, the activity could be related to some parameter of the above equation: change of position (delta pos), amplitude (A), or the cosine of the angle between the direction of movement and a reference direction (cos alpha). Units coding amplitude of eye movement or change in position in a particular plane conveyed the information by number of spikes. Units coding direction of movement did so by frequency. In many units, the information coding was precise so that the direction or amplitude of single eye movements could be predicted from the frequency changes of single units. In other units, this could be determined from averages of the frequency changes. Cells coding direction or change in position had frequency maxima only in planes corresponding to the pulling direction of the eye muscles. The results suggest that a vector description is not only a convenient mathematical tool, but is the way eye movements are coded in the PRF and possibly elsewhere in the central nervous system.