Orexin-A inputs onto visuomotor cell groups in the monkey brainstem.

Orexin-A, synthesized by neurons of the lateral hypothalamus helps to maintain wakefulness through excitatory projections to nuclei involved in arousal. Obvious changes in eye movements, eyelid position and pupil reactions seen in the transition to sleep led to the investigation of orexin-A projections to visuomotor cell groups to determine whether direct pathways exist that may modify visuomotor behaviors during the sleep-wake cycle. Histological markers were used to define these specific visuomotor cell groups in monkey brainstem sections and combined with orexin-A immunostaining. The dense supply by orexin-A boutons around adjacent neurons in the dorsal raphe nucleus served as a control standard for a strong orexin-A input. The quantitative analysis assessing various functional cell groups of the oculomotor system revealed that almost no input from orexin-A terminals reached motoneurons supplying the singly-innervated muscle fibers of the extraocular muscles in the oculomotor nucleus, the omnipause neurons in the nucleus raphe interpositus and the premotor neurons in the rostral interstitial nucleus of the medial longitudinal fasciculus. In contrast, the motoneurons supplying the multiply-innervated muscle fibers of the extraocular muscles, the motoneurons of the levator palpebrae muscle in the central caudal nucleus, and especially the preganglionic neurons supplying the ciliary ganglion received a strong orexin input. We interpret these results as evidence that orexin-A does modulate pupil size, lid position, and possibly convergence and eye alignment via the motoneurons of multiply-innervated muscle fibres. However orexin-A does not directly modulate premotor pathways for saccades or the singly-innervated muscle fibre motoneurons.

Histochemical characterisation of trigeminal neurons that innervate monkey extraocular muscles.

Sensory trigeminal innervation is a consistent feature of extraocular muscles across species, in spite of a variable occurrence of muscle spindles. We studied the histochemical properties of trigeminal ganglion (TG) cells projecting to the extraocular eye muscles to obtain more information about their function. In monkey TG neurons were retrogradely filled by tracer injections (cholera toxin subunit B; wheat-germ agglutinin) into the belly or myotendinous junction of eye muscles; one conjunctival injection served as a control. Retrogradely labelled TG neurons were processed for the presence of parvalbumin (PV), substance P (SP), or nitric oxide synthase (NOS) by double-immunofluorescence. The results indicate that approximately 10% of trigeminal afferents to all parts of the eye muscle are PV-positive, whereas around 20% are SP-positive. Twice as many SP-positive TG projection neurons were counted after a conjunctival tracer injection, presumably relaying nociceptive signals. A surprisingly large population of NOS-positive TG cells (30%) was found only after distal tracer injections. Up to now none of these TG cell groups could be related to the palisade endings located at the myotendinous junction.

The neuroanatomical basis of slow saccades in spinocerebellar ataxia type 2 (Wadia-subtype).

In a case of spinocerebellar ataxia type 2, Wadia-subtype (SCA2), with slow horizontal saccades, we used parvalbumin immunohistochemistry to identify the omnipause (OPNs) excitatory (EBNs), and inhibitory burst neurons (IBNs) of the saccade generator. Nissl sections was used to measure neuronal diameters, and synaptophysin staining to estimate of synaptic density on the cell somata. Morphometric and synaptic density measurements of the abducens motoneurons were identical in SCA2 and the control. A significant cell loss and reduced synaptic density on somata was found only in the EBN area. We conclude that degeneration of the EBNs is the most likely cause for the slowing of horizontal saccades.

Present concepts of oculomotor organization.

This chapter gives an introduction to the oculomotor system, thus providing a framework for the subsequent chapters. This chapter describes the characteristics, and outlines the structures involved, of the five basic types of eye movements, for gaze holding ("neural integrator") and eye movements in three dimensions (Listing's law, pulleys).

Sensory control of extraocular muscles.

The role of sensory receptors in eye muscles is not well understood, but there is physiological and clinical evidence for the presence of proprioceptive signals in many areas of the central nervous system. It is unclear which structures generate these sensory signals, and which central neural pathways are involved. Three different types of receptors are associated with eye muscles: (1) muscle spindles, (2) palisade endings, and (3) Golgi tendon organs, but their occurrence varies wildly between species. A review of their organization shows that each receptor is mainly confined to a morphologically separate layer of the eye muscle. The palisade endings - which are unique to eye muscles, are associated with the global layer; and they have been found in all mammals studied so far. Their function is unknown. The muscle spindles, if they are present in a species, lie in the orbital layer, or at its junction to the global layer. Golgi tendon organs appear to be unique to artiodactyls (i.e., sheep and goats, etc.); they lie in an outer distal marginal layer of the eye muscle, called the "peripheral patch layer" in sheep. The specific association between palisade endings and the multiply innervated type of muscle fibers of the global layer has led to the hypothesis that together they may act as a sensory receptor, and provide a source of central proprioceptive signals. But other interpretations of the morphological evidence do not support this role.

The extraocular motor nuclei: organization and functional neuroanatomy.

The organization of the motoneuron subgroups in the brainstem controlling each extraocular eye muscle is highly stable through the vertebrate species. The subgroups are topographically organized in the oculomotor nucleus (III) and are usually considered to form the final common pathway for eye muscle control. Eye muscles contain a unique type of slow non-twitch, fatigue-resistant muscle fiber, the multiply innervated muscle fibers (MIFs). The recent identification the MIF motoneurons shows that they too have topographic organization, but very different from the classical singly innervated muscle fiber (SIF) motoneurons. The MIF motoneurons lie around the periphery of the oculomotor nucleus (III), trochlear nucleus (IV), and abducens nucleus (VI), slightly separated from the SIF subgroups. The location of four different types of neurons in VI are described and illustrated: (1) SIF motoneurons, (2) MIF motoneurons, (3) internuclear neurons, and (4) the paramedian tract neurons which project to the flocculus. Afferents to the motoneurons arise from the vestibular nuclei, the oculomotor and abducens internuclear neurons, the mesencephalic and pontine burst neurons, the interstitial nucleus of Cajal, nucleus prepositus hypoglossi, the supraoculomotor area and the central mesencephalic reticular formation and the pretectum. The MIF and SIF motoneurons have different histochemical properties and different afferent inputs. The hypothesis that SIFs participate in moving the eye and MIFs determine the alignment seems possible but is not compatible with the concept of a final common pathway.

Identification of motoneurons supplying multiply- or singly-innervated extraocular muscle fibers in the rat.

In mammals, the extraocular muscle fibers can be categorized in singly-innervated and multiply-innervated muscle fibers. In the monkey oculomotor, trochlear and abducens nucleus the motoneurons of multiply-innervated muscle fibers lie separated from those innervating singly-innervated muscle fibers and show different histochemical properties. In order to discover, if this organization is a general feature of the oculomotor system, we investigated the location of singly-innervated muscle fiber and multiply-innervated muscle fiber motoneurons in the rat using combined tract-tracing and immunohistochemical techniques. The singly-innervated muscle fiber and multiply-innervated muscle fiber motoneurons of the medial and lateral rectus muscle were identified by retrograde tracer injections into the muscle belly or the distal myotendinous junction. The belly injections labeled the medial rectus muscle subgroup of the oculomotor nucleus or the greatest part of abducens nucleus, including some cells outside the medial border of abducens nucleus. In contrast, the distal injections labeled only a subset of the medial rectus muscle motoneurons and exclusively cells outside the medial border of abducens nucleus. The tracer detection was combined with immunolabeling using antibodies for perineuronal nets (chondroitin sulfate proteoglycan) and non-phosphorylated neurofilaments. In monkeys both antibodies permit a distinction between singly-innervated muscle fiber and multiply-innervated muscle fiber motoneurons. The experiments revealed that neurons labeled from a distal injection lack both markers and are assumed to represent multiply-innervated muscle fiber motoneurons, whereas those labeled from a belly injection are chondroitin sulfate proteoglycan- and non-phosphorylated neurofilament-immunopositive and assumed to represent singly-innervated muscle fiber motoneurons. The overall identification of multiply-innervated muscle fiber and singly-innervated muscle fiber motoneurons within the rat oculomotor nucleus, trochlear nucleus, and abducens nucleus revealed that the smaller multiply-innervated muscle fiber motoneurons tend to lie separate from the larger diameter singly-innervated muscle fiber motoneurons. Our data provide evidence that rat extraocular muscles are innervated by two sets of motoneurons that differ in their molecular, morphological, and anatomical properties.

Motor and sensory innervation of extraocular eye muscles.

Eye muscles are unusual in several ways; one is that they have up to three different layers-the inner global layer, the outer orbital layer, and in some species an external marginal layer has been described. In sheep this is called the "peripheral patch layer." Three different types of proprioceptors are found in eye muscles-muscle spindles, Golgi tendon organs, and palisade endings. A survey of the organization of their location leads us to the hypothesis that each receptor is confined to a separate layer of the eye muscle. The palisade endings are associated with the global layer, the muscle spindles lie predominantly in the orbital layer, and the Golgi tendon organs are found only in the peripheral patch layer. This well-organized scheme may help us to understand the proprioceptive system in eye muscles.

Localizing value of torsional nystagmus in small midbrain lesions.

BACKGROUND: The topodiagnostic value and specificity of nystagmus in patients with mesencephalic lesions and its relation to tonic torsional deficits and vertical saccade deficits is controversial and anecdotal. METHODS: The authors examined 11 patients with vascular MRI-identified mesencephalic lesions and clinical evidence of vertical-torsional nystagmus on gaze straight ahead, focusing on the three-dimensional nystagmus components recorded with the three-dimensional search coil technique. RESULTS: Combined lesions of the rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF) and the interstitial nucleus of Cajal (iC) are much more frequent than riMLF and, in particular, iC lesions alone. Eight patients showed contralesional torsional nystagmus with a conjugate vertical component on gaze straight ahead and had anatomic (MRI) and clinical evidence (slowing of vertical saccades) for riMLF involvement. Tonic ocular torsion and the subjective visual vertical were shifted to the contralesional side (n = 7). Torsional nystagmus to the ipsilesional side was uncommon (n = 3) and found in patients with midbrain lesions involving the iC, all of whom also had decreased time constants of the slow phases of gaze-evoked nystagmus. CONCLUSIONS: Contrary to previous proposals, contralesional torsional nystagmus was the most frequent direction and is probably not compensatory for contralesional tonic ocular torsion. Small amplitude vertical saccades with normal velocities in association with ipsilesional torsional nystagmus may indicate isolated iC lesions. Torsional nystagmus following mesencephalic lesions may last for years and may help to distinguish rostral (riMLF) from caudal (iC) midbrain lesions.

Modern concepts of brainstem anatomy: from extraocular motoneurons to proprioceptive pathways.

The extraocular muscles, unlike the skeletal muscles, contain non-twitch muscle fibers. Recent experiments have located the non-twitch motoneurons. They lie around the periphery of the oculomotor, trochlear and abducens nuclei, separate from the more usual twitch motoneurons that cluster within the boundaries of the classical motor nuclei. The premotor inputs to non-twitch neurons were traced by the injection of rabies virus into the distal tip of the lateral rectus muscle. Retrogradely labeled cells were found in areas associated with the neural integrator, vergence and smooth pursuit premotor areas, but not the saccadic premotor burst neurons or the direct vestibulo-ocular pathways. The rabies tracing emphasizes for the first time that the central mesencephalic reticular formation (cMRF) and the supraoculomotor area exert direct premotor control over the non-twitch motoneurons. Because the two sets of motoneurons do not receive the same afferents, they must have different functions; these are not yet clarified. These results are not compatible with the concept of a single final common pathway from motoneurons to eye muscles. Putative sensory receptors, palisade endings, are located at the tips of non-twitch muscle fibers reminiscent of an inverted muscle spindle, which would make the non-twitch motoneurons, gamma-motoneurons. We propose that twitch motoneurons are the major source of tension used for eye movements, whereas non-twitch motoneurons are more important for fine alignment of the eyes. Furthermore, the non-twitch motoneurons could be controlled through sensory feedback networks (including perhaps proprioceptive signals from the palisade endings) that are relayed through the superior colliculus and via cMRF to the non-twitch motoneurons. The clinical repercussions of these hypotheses are discussed.

Pathophysiology of slow vertical saccades in progressive supranuclear palsy.

OBJECTIVES: To investigate the relative roles of burst neurons (which generate the saccadic command) and omnipause neurons (which gate the activity of burst neurons) in the pathogenesis of slow saccades in progressive supranuclear palsy (PSP). BACKGROUND: Experimental inactivation of mesencephalic burst neurons impairs vertical but not horizontal saccades. Experimental inactivation of omnipause neurons causes slowing of both horizontal and vertical saccades. Combining saccadic with vergence movements in healthy subjects induces small, high-frequency, conjugate oscillations, which indicate that omnipause neurons are inhibited. METHODS: The authors studied seven patients with PSP, six patients with other parkinsonian syndromes, and seven age-matched control subjects. They compared vertical saccades of similar sizes made with or without associated vergence movements. They compared the speed of vertical and horizontal saccades. RESULTS: Five patients with PSP and the six patients with other parkinsonian made vertical saccades in combination with horizontal vergence; all showed conjugate horizontal oscillations (29 to 41 Hz) during 27% to 93% of saccade-vergence trials. Vertical saccades made in conjunction with vergence movements were not speeded up or increased in size compared with saccades made between equidistant targets for the PSP or parkinsonian groups. Vertical saccades were slowed more than horizontal saccades in the PSP group (p < 0.005) but not in the parkinsonian group. CONCLUSIONS: Dysfunction of omnipause neurons ("gate dysfunction") is unlikely to be the primary cause of slow vertical saccades in progressive supranuclear palsy. Deficient generation of the motor command by midbrain burst neurons is the more likely cause.

Motoneurons of twitch and nontwitch extraocular muscle fibers in the abducens, trochlear, and oculomotor nuclei of monkeys.

Eye muscle fibers can be divided into two categories: nontwitch, multiply innervated muscle fibers (MIFs), and twitch, singly innervated muscle fibers (SIFs). We investigated the location of motoneurons supplying SIFs and MIFs in the six extraocular muscles of monkeys. Injections of retrograde tracers into eye muscles were placed either centrally, within the central SIF endplate zone; in an intermediate zone, outside the SIF endplate zone, targeting MIF endplates along the length of muscle fiber; or distally, into the myotendinous junction containing palisade endings. Central injections labeled large motoneurons within the abducens, trochlear or oculomotor nucleus, and smaller motoneurons lying mainly around the periphery of the motor nuclei. Intermediate injections labeled some large motoneurons within the motor nuclei but also labeled many peripheral motoneurons. Distal injections labeled small and medium-large peripheral neurons strongly and almost exclusively. The peripheral neurons labeled from the lateral rectus muscle surround the medial half of the abducens nucleus: from superior oblique, they form a cap over the dorsal trochlear nucleus; from inferior oblique and superior rectus, they are scattered bilaterally around the midline, between the oculomotor nucleus; from both medial and inferior rectus, they lie mainly in the C-group, on the dorsomedial border of oculomotor nucleus. In the medial rectus distal injections, a "C-group extension" extended up to the Edinger-Westphal nucleus and labeled dendrites within the supraoculomotor area. We conclude that large motoneurons within the motor nuclei innervate twitch fibers, whereas smaller motoneurons around the periphery innervate nontwitch, MIF fibers. The peripheral subgroups also contain medium-large neurons which may be associated with the palisade endings of global MIFs. The role of MIFs in eye movements is unclear, but the concept of a final common pathway must now be reconsidered.

The premotor region essential for rapid vertical eye movements shows early involvement in Alzheimer's disease-related cytoskeletal pathology.

The rostral interstitial nucleus of the medial longitudinal fascicle (riMLF) contains premotor neurons essential for the generation of rapid vertical eye movements. The Alzheimer's disease (AD)-related cytoskeletal changes and beta-amyloid deposits in this nucleus were examined in 30 autopsy cases and compared to the involvement of three associated nuclei - Edinger-Westphal nucleus, nucleus of Darkschewitsch and interstitial nucleus of Cajal. The riMLF displays slight cytoskeletal alterations already in the early stages in the development of the cortical cytoskeletal pathology (cortical NFT/NT-stages I-II, representing the preclinical phase of AD). In the cortical NFT/NT-stages III-IV (i.e. incipient phase of AD), the cytoskeletal pathology in the riMLF is pronounced and in stages V-VI (i.e. clinical phase of AD) it is severe. The progression of the cytoskeletal pathology in the riMLF correlates significantly with the cortical NFT/NT-stages I-VI that reflect the clinical course of AD. Isolated beta-amyloid deposits appear in the riMLF for the first time in the final beta-amyloid stage. In the Edinger-Westphal nucleus, in the nucleus of Darkschewitsch and most markedly in the interstitial nucleus of Cajal, the pathological changes were significantly less severe than those in the riMLF. In the event that the cytoskeletal pathology impairs the function of the premotor neurons of the riMLF, one would predict a progressive slowing of vertical saccades corresponding to the advancing cortical NFT/NT-stages.

Functions of the nucleus of the optic tract (NOT). I. Adaptation of the gain of the horizontal vestibulo-ocular reflex.

We studied the role of the nucleus of the optic tract (NOT) in adapting the gain of the angular vestibulo-ocular reflex (aVOR) in rhesus and cynomolgus monkeys using lesions and temporary inactivation with muscimol. The aVOR gain was adaptively reduced by forced sinusoidal rotation (0.25 Hz, 60 degrees/s) in a self-stationary visual surround, i.e., a visual surround that moved with the subject, or by wearing x0.5 reducing lenses during natural head movements. The aVOR gains dropped by 20-30% after 2 h and by about 30% after 4 h. Muscimol injections caused a loss of adaptation of contraversive-eye velocities induced by the aVOR, and their gains promptly returned to or above preadapted levels. The gains of the adapted ipsiversive and vertical eye velocities produced by the aVOR were unaffected by muscimol injections. Lesions of NOT significantly reduced or abolished the animals' ability to adapt the gain of contraversive aVOR-induced eye velocities, and the monkeys were unable to suppress these contraversive-eye velocities in a self-stationary surround. The lesions did not affect ipsiversive aVOR-induced eye velocities, and the animals were still able to suppress them. Lesions of NOT also affected the unadapted or "default" aVOR gains. After unilateral NOT lesions, gains of ipsiversive aVOR-induced eye velocity were reduced, while gains of contraversive aVOR-induced eye velocity were either unaffected or slightly increased. Consistent with this, muscimol injections into the NOT of unadapted monkeys slightly reduced the gains of ipsiversive and increased the gains of contraversive-eye velocities by about 8-10%. We conclude that each NOT processes ipsiversive retinal-slip information about visual surround movement relative to the head induced by the aVOR. In the presence of visual surround movement, the retinal-slip signal is suppressed, leading to adaptive changes in the gain of aVOR-induced contraversive horizontal eye velocities. NOT also has a role in controlling and maintaining the current state of the aVOR gains. Thus, it plays a unique role in producing and supporting adaptation of the gain of the horizontal aVOR that is likely to be important for stabilizing gaze during head movement. Pathways through the inferior olive are presumably important for this adaptation.

Functions of the nucleus of the optic tract (NOT). II. Control of ocular pursuit.

Ocular pursuit in monkeys, elicited by sinusoidal and triangular (constant velocity) stimuli, was studied before and after lesions of the nucleus of the optic tract (NOT). Before NOT lesions, pursuit gains (eye velocity/target velocity) were close to unity for sinusoidal and constant-velocity stimuli at frequencies up to 1 Hz. In this range, retinal slip was less than 2 degrees. Electrode tracks made to identify the location of NOT caused deficits in ipsilateral pursuit, which later recovered. Small electrolytic lesions of NOT reduced ipsilateral pursuit gains to below 0.5 in all tested conditions. Pursuit was better, however, when the eyes moved from the contralateral side toward the center (centripetal pursuit) than from the center ipsilaterally (centrifugal pursuit), although the eyes remained in close proximity to the target with saccadic tracking. Effects of lesions on ipsilateral pursuit were not permanent, and pursuit gains had generally recovered to 60-80% of baseline after about 2 weeks. One animal had bilateral NOT lesions and lost pursuit for 4 days. Thereafter, it had a centrifugal pursuit deficit that lasted for more than 2 months. Vertical pursuit and visually guided saccades were not affected by the bilateral NOT lesions in this animal. We also compared effects of these and similar NOT lesions on optokinetic nystagmus (OKN) and optokinetic after-nystagmus (OKAN). Correlation of functional deficits with NOT lesions from this and previous studies showed that rostral lesions of NOT in and around the pretectal olivary nucleus, which interrupted cortical input through the brachium of the superior colliculus (BSC), affected both smooth pursuit and OKN. In two animals in which it was tested, NOT lesions that caused a deficit in pursuit also decreased the rapid and slow components of OKN slow-phase velocity and affected OKAN. It was previously shown that slightly more caudal NOT lesions were more effective in altering gain adaptation of the angular vestibulo-ocular reflex (aVOR). The present findings suggest that cortical pathways through rostral NOT play an important role in maintenance of ipsilateral ocular pursuit. Since lesions that affected ocular pursuit had similar effects on ipsilateral OKN, processing for these two functions is probably closely linked in NOT, as it is elsewhere.

Neuroanatomical identification of mesencephalic premotor neurons coordinating eyelid with upgaze in the monkey and man.

Except during blinks, movements of the upper eyelid are tightly coupled to vertical eye movements. The premotor source for the coordination of lid and eye movements is unknown. The present paper provides the anatomical identification of a new premotor cell group in the rostral mesencephalon of the monkey and human, which lies in close proximity to the premotor center for vertical saccades and is thought to participate in lid-eye coordination. After injections of a retrograde transsynaptic tracer (tetanus toxin fragment C or BII(b)) into the levator palpebrae (LP), the superior rectus (SR), or the inferior oblique (IO) muscle of macaque monkeys, a small circumscribed group of premotor neurons was labeled in the central gray of the rostral mesencephalon, but not after superior oblique or inferior rectus muscle injections. This group lies immediately rostral to the interstitial nucleus of Cajal and medial to the rostral interstitial nucleus of the medial longitudinal fasciculus, each of which contain premotor neurons for vertical saccades, and was termed the M-group. Injections of tritiated leucine into the M-group led to afferent labeling primarily over LP motoneurons. In addition, label was present over the SR- and IO-motoneuron subgroups in the oculomotor nucleus and frontalis muscle motoneurons in the facial nucleus. This projection pattern of the M-group suggests a role in the coordination of the upper eyelid and eyes during upgaze. Double-labeling experiments in macaque monkeys revealed that the M-group is strongly parvalbumin immunoreactive and contains high levels of cytochrome oxidase activity. With these two histochemical markers, the homologue of the M-group was identified in the human brain as well.

Projections from the superior colliculus motor map to omnipause neurons in monkey.

Descending projections from the superior colliculus (SC) motor map to the saccadic omnipause neurons (OPNs) were examined in monkeys by using anterograde transport of tritiated leucine. The SC was divided into three zones: the rostral pole of the motor map, a small horizontal saccade zone in central SC, and a large horizontal saccade zone in caudal SC. Tracer injections into the intermediate layers of the three zones led to different patterns of silver grain deposits in and around nucleus raphe interpositus (RIP), which contains the OPNs: 1) From the rostral pole of the motor map, coarse axon branches of the crossed predorsal bundle spread medially into the RIP, branched, and terminated predominantly unilaterally over cells on the same side. 2) From the small horizontal saccade zone, the axon branches were of a finer caliber and terminated diffusely in the RIP, mainly on the same side. 3) From the large horizontal saccade zone, no terminal labeling was found within the RIP. 4) From the rostral pole of the motor map and small horizontal saccade zone, fiber branches from the ipsilateral descending pathway terminated diffusely over RIP. 5) In addition, terminal labeling in reticulospinal areas of the pons and medulla increased in parallel with the size of the saccade according to the SC motor map. The results suggest that there are multiple projections directly onto OPNs from the rostral SC but not from the caudal SC associated with large gaze shifts. The efferents from the rostral pole of the motor map may subserve the suppression of saccades during visual fixation, and those from the small horizontal saccade zone could inhibit anatagonist premotor circuits.