Localization of motoneurons controlling the extraocular muscles of the rat

The localization of extraocular motoneurons in the rat was investigated by injecting horseradish peroxidase and [¹²⁵I]wheat germ agglutinin¹⁷ as retrogade tracer substances into individual eye muscles. The organization of subnuclei was found to be most similar to the rabbit. The subgroups representing the medial rectus and inferior rectus muscles are located in the rostral two thirds of the ipsilateral oculomotor nucleus (nIII) with some medial rectus motoneurons scattered laterally along the edge of the medial longitudinal fasciculus. The motor pool controlling the inferior oblique muscle is located in the middle third of the ipsilateral nIII. The motoneurons of the superior rectus muscles are in the caudal two-thirds of contralateral nIII while the levator palpebrae muscle has a bilateral innervation in the oculomotor nucleus. The motoneurons of the superior oblique are located in the contralateral trochlear nucleus although a few labeled neurons were scattered laterally in amongst the fibers of the medial longitudinal fasciculus. The cell bodies of lateral rectus motoneurons regional separation between the latter and internuclear neurons was found after injecting HRP into the oculomotor nucleus.     

Extra- and intracellular HRP analysis of the organization of extraocular motoneurons and internuclear neurons in the guinea pig and rabbit

The distribution of extraocular motoneurons and abducens and oculomotor internuclear neurons was determined in guinea pigs by injecting horseradish peroxidase (HRP) into individual extraocular muscles, the abducens nucleus, the oculomotor nucleus, and the cerebellum. Motoneurons in the oculomotor nucleus innervated the ipsilateral inferior rectus, inferior oblique, medial rectus, and the contralateral superior rectus and levator palpebrae muscles. Most motoneurons of the trochlear nucleus projected to the contralateral superior oblique muscle although a small number innervated the ipsilateral superior oblique. The abducens and accessory abducens nuclei innervated the ipsilateral rectus and retractor bulbi muscles, respectively. The somata of abducens internuclear neurons formed a cap around the lateral and ventral aspects of the abducens nucleus. The axons of these internuclear neurons terminated in the medial rectus subdivision of the contralateral oculomotor nucleus. At least two classes of guinea pig oculomotor internuclear interneurons exist. One group, located primarily ventral to the oculomotor nucleus, innervated the abducens nucleus and surrounding regions. The second group, lying mainly in the dorsal midline area of the oculomotor nucleus, projected to the cerebellum. Intracellular staining with HRP demonstrated similar soma-dendritic organization for oculomotor and trochlear motoneurons of both guinea pigs and rabbits. Dendrites of oculomotor motoneurons radiated symmetrically from the soma to cover approximately one-third of the entire nucleus, and each motoneuron sent at least one dendrite into the central gray overlying the oculomotor nucleus. In both species, a small percentage of oculomotor motoneurons possessed axon collaterals that terminated both within and outside of the nucleus. The dendrites of trochlear motoneurons extended into the medial longitudinal fasciculus and the reticular formation lateral to the nucleus. Our data on the topography of motoneurons and internuclear neurons in the guinea pig and soma-dendritic organization of motoneurons in the guinea pig and rabbit show that these species share common organizational and morphological features. In addition, comparison of these data with those from other mammals reveals that dendritic complexity (number of dendrites per motoneuron) of extraocular motoneurons exhibits a systematic increase with animal size.     

Peripheral and central oculomotor organization in the goldfish, Carassius auratus

Peripheral and central oculomotor organization was studied in the goldfish. The sizes of the extraocular muscles were quantified by counting the fibers contained in a given muscle and by area measurements of the cross-sectional surfaces. All the muscles were of approximately similar size. Kinematics were determined by electrical stimulation of a given muscle. The macroscopic appearance and kinematics of the muscles had the characteristics of other lateral-eyed animals (e.g., rabbit). Locations of extraocular motor neurons were found by retrograde transport of horseradish peroxidase (HRP) following injections into individual extraocular muscles. The eye muscles were innervated by four ipsilateral (lateral rectus, medial rectus, inferior oblique, inferior rectus) and two contralateral (superior rectus, superior oblique) motor neuron pools. The oculomotor nucleus was found in the midbrain, at the level of the caudal zone of the inferior lobe of the hypothalamus. Inferior rectus motor neurons were located rostrally in the oculomotor nucleus, whereas medial rectus, superior rectus, and inferior oblique motor neurons were intermingled in its more caudal portions. All labelled cells were located dorsally and medially to the medial longitudinal fasciculus (MLF) in close proximity to either the floor of the ventricle or the midline region. Occasionally, motor neurons were interspersed within the fiber bundles of the MLF or the exiting fibers of the oculomotor nerve. The trochlear nucleus, containing superior oblique motor neurons, was found in the immediate lateral and caudal neighborhood of the oculomotor nucleus, where its rostral border overlapped with the caudal border of the latter. The abducens nucleus, containing lateral rectus motor neurons, was located in the posterior brainstem in the neighborhood of the vestibular nuclear complex. This nucleus was divided into a rostral and a caudal portion. The axons of ipsilaterally projecting motor neurons headed toward their respective nerve roots via the shortest possible route, as did the axons of superior rectus motor neurons, which crossed the midline without detour to enter the contralateral oculomotor nerve. In contrast, trochlear motor neuron axons arched around the dorsal aspect of the ventricle through the cerebellar commissure to reach the contralateral trochlear nerve. The morphology of individual motor neurons was visualized by intrasomatic injection of HRP. Cell somata had oblong shapes, and their large dendrites were oriented laterally and ventrally. The axons did not collateralize within the midbrain region or the oculomotor nerve as far as they could be traced.(ABSTRACT TRUNCATED AT 400 WORDS)     

Location and quantitative analysis of the motoneurons innervating the extraocular muscles of the guinea-pig, using horseradish peroxidase (HRP) and double or triple labelling with fluorescent substances

The location of the motoneurons innervating the extraocular muscles of the guinea-pig was investigated using horseradish peroxidase (HRP) and the fluorescent substances fast blue, propidium iodide and nuclear yellow as retrograde tracers. The innervation of the inferior rectus, medial rectus and inferior oblique muscles is exclusively ipsilateral, and these neurons form three well-defined and mutually separate subnuclei in the oculomotor nucleus. The subgroup innervating the medial rectus lies exclusively along the medial face of the oculomotor nucleus, with no aberrant neurons in the medial longitudinal fasciculus, as have been found in other mammals. The superior rectus and levator palpebrae are innervated almost entirely by contralateral motoneurons located both in the oculomotor nucleus and in a variety of extranuclear positions (in the periaqueductal grey, among the fibres of medial longitudinal fasciculus and ventral to this bundle). There is no anteroposterior separation between the oculomotor and trochlear nuclei, since superior rectus and levator palpebrae neurons are found flanking the latter laterally all along its anterior half. In the caudal two-thirds of the oculomotor nucleus the motoneurons innervating the superior rectus and levator palpebrae are partially intermingled with those corresponding to the ipsilaterally-innervated muscles, particularly those of the inferior rectus.     

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.     

Patterns of extraocular innervation by the oculomotor complex in the chick

The horseradish peroxidase retrograde tracer technique was used to map the projection pattern of the oculomotor nuclear complex to the extraocular muscles in the chick embryo. The following projection pattern was found: The dorsolateral oculomotor subnucleus innervates the ipsilateral inferior rectus muscle, the dorsomedial subnucleus innervates the ipsilateral medial rectus muscle, a lateral division of the ventromedial subnucleus innervates the ipsilateral inferior oblique muscle, and a medial division of the ventromedial subnucleus innervates the contralateral superior rectus muscle. The so-called central nucleus also innervates the contralateral superior rectus muscle. This pattern was extremely discrete, with virtually no overlapping representations. These results provide the first evidence for a functional medial-lateral subdivision of the ventromedial subnucleus. This pattern relates to the unusual development of this subnucleus and suggests that only part of the primordium for this cell group migrates across the midline during its ontogeny, rather than all of it, as was previously believed. The subnuclear organization of the avian oculomotor complex is also considered in comparison to such functional organization in other species.     

The localization of the motor neurons innervating the extraocular muscles in the oculomotor nuclei of the cat and rabbit,using horseradish peroxidase

The localization of the motor neurons innervating the extraocular muscles in the oculomotor nuclei of adult cats and rabbits was investigated by means of retrograde labelling with horseradish peroxidase (HRP). The groups consisting of the motor neurons innervating an individual muscle lay in the nucleus as elongated columns extending in a longitudinal direction. The position of each group in the transverse section varied according to the rostro-caudal level of the nucleus. In the cat and rabbit, entire contralateral innervation of the superior rectus and entire ipsilateral innervation of three muscles of the inferior rectus, medial rectus and inferior oblique were similarly observed. However, the arrangement of individual motor groups differed considerably in both animals except for the group innervating the inferior rectus which was generally found in the ventral position running through the rostral two-thirds of the oculomotor nucleus. In the case of cats, the central caudal nucleus bilaterally innervated the levator palpebrae superioris. The motor neurons innervating this muscle in the rabbit (which lacks the central caudal nucleus) formed a rostro-caudal club-shaped column close to the group innervating the superior rectus. The aberrant cellular mass in the adjoining medial longitudinal fasciculus which belongs to the medial rectus appears to play an important role in the eye movement, because it commonly appears in various animals.     

Reinnervation of the extraocular muscles in goldfish is nonselective

The selectivity of axonal regeneration to the extraocular muscles in teleosts has been reinvestigated by mapping, with retrogradely transported HRP, the motor pools of the muscles innervated by the oculomotor nerve. In normal goldfish, the motoneurons of the superior rectus, inferior rectus, and inferior oblique muscles formed discrete, nonoverlapping motor pools; the motor pool of the medial rectus muscle overlapped with those of the inferior oblique and inferior rectus muscles. In fish whose oculomotor nerve had regenerated (after intracranial transection), in contrast, many motoneurons in other, inappropriate motor pools reinnervated the superior rectus and inferior oblique muscles (the only muscles examined in lesioned animals). Furthermore, these inappropriate motoneurons continued to project to these muscles for at least 1 year. The oculomotor nerve and its molecular branches were examined by light and electron microscopy to determine the pathway by which axons regenerated to their muscles. Axons regenerated within the basal laminae of Schwann cells, which persisted in the distal nerve-stump after a lesion. After labeling the inferior oblique nerve with HRP in regenerated nerves, there were labeled axons in all of the muscular branches; this indicates that regenerating axons branched, which was confirmed by finding an increased number of myelinated axons in other, regenerated inferior oblique nerves. Thus, different branches of the same axons sometimes reinnervated different muscles. These results demonstrate that regenerating axons in the oculomotor nerve are misdirected to inappropriate muscles, and do not selectively reinnervate individual muscles, as had been previously suggested (Sperry and Arora, 1965).     

Twitch and nontwitch motoneuron subgroups in the oculomotor nucleus of monkeys receive different afferent projections

Motoneurons in the primate oculomotor nucleus can be divided into two categories, those supplying twitch muscle fibers and those supplying nontwitch muscle fibers. Recent studies have shown that twitch motoneurons lie within the classical oculomotor nucleus (nIII), and nontwitch motoneurons lie around the borders. Nontwitch motoneurons of medial and inferior rectus are in the C group dorsomedial to nIII, whereas those of inferior oblique and superior rectus lie near the midline are in the S group. In this anatomical study, afferents to the twitch and nontwitch subgroups of nIII have been anterogradely labeled by injections of tritiated leucine into three areas and compared. 1) Abducens nucleus injections gave rise to silver grain deposits over all medial rectus subgroups, both twitch and nontwitch. 2) Laterally placed vestibular complex injections that included the central superior vestibular nucleus labeled projections only in twitch motoneuron subgroups. However, injections into the parvocellular medial vestibular nucleus (mvp), or Y group, resulted in labeled terminals over both twitch and nontwitch motoneurons. 3) Pretectal injections that included the nucleus of the optic tract (NOT), and the olivary pretectal nucleus (OLN), labeled terminals only over nontwitch motoneurons, in the contralateral C group and in the S group. Our study demonstrates that twitch and nontwitch motoneuron subgroups do not receive identical afferent inputs. They can be controlled either in parallel, or independently, suggesting that they have basically different functions. We propose that twitch motoneurons primarily drive eye movements and nontwitch motoneurons the tonic muscle activity, as in gaze holding and vergence, possibly involving a proprioceptive feedback system.     

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.     

Elasmobranch oculomotor organization: anatomical and theoretical aspects of the phylogenetic development of vestibulo-oculomotor connectivity

The oculomotor organization of two elasmobranch species, smooth dogfish (Mustelus canis) and little skate (Raja erinacea), was studied by investigating the extraocular muscle apparatus and the oculomotor motoneuron distribution. The macroscopic appearance of the eye muscles was similar to any lateral-eyed vertebrate species (e.g., goldfish, rabbit). The size of extraocular muscles was expressed by counting single muscle fibers and comparing cross-sectional areas of the extraocular muscles. There were significant differences in the number of fibers in the six extraocular muscles in dogfish, but not in skate. Fiber sizes varied considerably; thus, the number of fibers did not relate to cross-sectional areas. In the dogfish, no one pair of agonist-antagonist extraocular muscles was larger than the others, suggesting that there was no preference for eye movements in a particular plane of space. However, the lateral rectus was more than twice the size of most of the other muscles. In the skate, cross-sectional areas of the horizontal eye muscles were smaller than those of the vertical eye movers. This may indicate a reduced utilization of horizontal eye muscles, which may reflect the bottom-dwelling habitat and mode of locomotion of the skate. The distribution of the extraocular motoneurons was determined by injecting horseradish peroxidase (HRP) into single eye muscles. Medial rectus, superior rectus, and superior oblique motoneuron populations were located contralateral to their respective muscles. Lateral rectus, inferior rectus, and inferior oblique motoneurons were located ipsilateral to their muscles. This distribution is in contrast to almost all other vertebrates studied thus far, where medial rectus motoneurons are located ipsilateral to the muscle which they innervate. The oculomotor arrangement in elasmobranchs is likely to have consequences for the circuitry responsible for the production of conjugate compensatory eye movements in the horizontal plane. We hypothesize that, in contrast to other vertebrates, the basic elasmobranch vestibulo-ocular reflex pathway consists of three identically structured three-neuron-arcs connecting the three semicircular canals to their respective extraocular muscles. This innervation pattern may constitute a special feature of the elasmobranch brain or a phylogenetically older arrangement of eye movement pathways.     

The motor nuclei and sensory neurons of the IIIrd, IVth, and VIth cranial nerves in the monitor lizard, Varanus exanthematicus

The motor nuclei of the oculomotor, trochlear, and abducens nerves of the reptile Varanus exanthematicus and the neurons that subserve the sensory innervation of the extraocular muscles were identified and localized by retrograde and anterograde transport of horseradish peroxidase (HRP). The highly differentiated oculomotor nuclear complex, located dorsomedially in the tegmentum of the midbrain, consists of the accessory oculomotor nucleus and the dorsomedial, dorsolateral, intermediate, and ventral subnuclei. The accessory oculomotor nucleus projects ipsilaterally to the ciliary ganglion. The dorsomedial, dorsolateral, and intermediate subnuclei distribute their axons to the ipsilateral orbit, whereas the ventral subnucleus, which innervates the superior rectus muscle, has a bilateral, though predominantly contralateral projection. The trochlear nucleus, which rostrally overlaps the oculomotor nuclear complex, is for the greater part a comma-shaped cell group situated lateral, dorsal, and medial to the medial longitudinal fasciculus. Following HRP application to the trochlear nerve, almost all retrogradely labeled cells were found in the contralateral nucleus. The nuclear complex of the abducens nerve consists of the principal and accessory abducens nuclei, both of which project ipsilaterally. The principal abducens nucleus is located just beneath the fourth ventricle laterally adjacent to the medial longitudinal fasciculus and innervates the posterior rectus muscle. The accessory abducens nucleus has a ventrolateral position in the brainstem in close approximation to the ophthalmic fibers of the descending trigeminal tract. It innervates the retractor bulbi and bursalis muscles. The fibers arising in the accessory abducens muscles form a loop in or just beneath the principal abducens nucleus before they join the abducens nerve root. The afferent fibers conveying sensory information from the extraocular muscles course in the oculomotor nerve and have their perikarya in the ipsilateral trigeminal ganglion, almost exclusively in its ophthalmic portion.     

Localization of retractor bulbi motoneurons in the rabbit

Motoneurons innervating the rabbit retractor bulbi muscle have been identified by retrograde transport of horseradish peroxidase (HRP). Following injection of HRP into single slips or all 4 slips of the retractor bulbi muscle, labeled motoneurons were consistently observed in the abducens (ABD) nucleus and in the accessory abducens (ACC) nucleus located ventral, lateral and rostral to the ABD. Axons from the ACC motoneurons could be seen to enter the VIth nerve. Injection of HRP into the lateral rectus muscle produced consistent labeling of motoneurons in the ABD nucleus overlapping the distribution of retractor bulbi motoneurons, but labeling was never observed in the ACC nucleus. The number of labeled ABD neurons after lateral rectus injections was far less (36%) than after injection into all 4 slips of the retractor bulbi muscle (72%). Injection of HRP into the superior oblique, superior rectus or medial rectus muscle produced labeling of motoneurons in the corresponding subdivisions of the oculomotor nucleus or trochlear nucleus but no labeled motoneurons were observed in either the ABD or ACC nuclei. Some highly inconsistent labeling of oculomotor nucleus was observed after retractor bulbi or lateral rectus muscle injections and this was judged to be due to intraorbital diffusion of the HRP. It was concluded that the retractor bulbi muscle is innervated by motoneurons located in both the ABD and ACC nuclei.     

Organization of the six motor nuclei innervating the ocular muscles in lamprey

The topography of motoneurons supplying each of the six ocular muscles of the lamprey, Lampetra fluviatilis, was studied by selective application of HRP to the cut nerves of identified muscles. In addition, the distributions of motoneuron populations to both eyes were studied simultaneously with fluorescein and rhodamine coupled dextran-amines (FDA and RDA) applied to cut ocular muscle nerves of either side. The motoneuron pool of the caudal oblique muscle is represented bilaterally in the trochlear (N IV) motor nucleus. The dorsal rectus muscle is innervated from a contralateral group of oculomotor (N III) motoneurons and the remaining four muscles exclusively from the ipsilateral side (N III and N VI). The inferior and posterior rectus muscles are both innervated by the abducens nerve. In contrast to all jawed vertebrates, only three eye muscles (the dorsal rectus, rostral rectus, and rostral oblique) are innervated by the oculomotor nerve in lampreys (N III). Lampreys have a motor nucleus similar to the accessory abducens nucleus previously described only in tetrapods. They lack the muscle homologous to the nasal rectus muscle of elasmobranchs and the medial rectus muscle of osteognathostomes. The distribution of the dendrites of different groups of motoneurons was studied and is considered in relation to inputs from tectum and the different cranial nerves.     

Establishment of a beagle dog model of oculomotor nerve injury

The oculomotor nerves of beagle dogs received electrical stimulation at 0.3-2.0 V. After recording compound muscle action potentials of the inferior oblique muscle, the oculomotor nerve was quickly cut off and a direct end-to-end anastomosis was then performed. As a result, the stimulating elec-trode was smoothly inserted and placed, and ideal bioelectrical signals of the interior oblique muscle were acquired. After oculomotor nerve injury, compound muscle action potentials of the inferior oblique muscle were significantly decreased in beagle dogs. These findings suggest that an animal model of oculomotor nerve injury was successfully established for electrophysiological studies.     

The organization of the extraocular motor nuclei in the atlantic stingray,Dasyatis sabina

Retrograde transport of HRP was used to determine the location and organization of the motor nuclei innervating the extrinsic eye muscles of the stingray, an elasmobranch fish. Oculomotor neurons are located both medial to and immediately ventrolateral to the MLF in the rostral midbrain. A ventral oculomotor nucleus was found among the IIIrd nerve rootlets close to the base of the midbrain. The dendrites of cells in the dorsal nucleus appear to be preferentially oriented in the transverse plane penetrating the MLF. Motoneuron pools innervating individual muscles are incompletely segregated in the dorsal group. However, the ventral nucleus innervates only the inferior oblique muscle. Dorsally, motoneurons innervating a single muscle are found on both sides of the MLF. In the caudal midbrain, the majority of trochlear motoneurons are located immediately ventrolateral to the MLF. Abducens motoneurons are scattered in the medulla from a ventrolateral position resembling the location of the nucleus in teleost fish to a dorsomedial position close to the MLF as in most other vertebrates. In contrast to other vertebrates, the medial rectus muscle is innervated by the contralateral oculomotor nucleus. Motoneurons innervating the other muscles have the same laterality as found in other vertebrates.     

Extraocular motoneuron pools develop along a dorsoventral axis in zebrafish,Danio rerio

Both spatial and temporal cues determine the fate of immature neurons. A major challenge at the interface of developmental and systems neuroscience is to relate this spatiotemporal trajectory of maturation to circuit‐level functional organization. This study examined the development of two extraocular motor nuclei (nIII and nIV), structures in which a motoneuron's identity, or choice of muscle partner, defines its behavioral role. We used retro‐orbital dye fills, in combination with fluorescent markers for motoneuron location and birthdate, to probe spatial and temporal organization of the oculomotor (nIII) and trochlear (nIV) nuclei in the larval zebrafish. We describe a dorsoventral organization of the four nIII motoneuron pools, in which inferior and medial rectus motoneurons occupy dorsal nIII, while inferior oblique and superior rectus motoneurons occupy distinct divisions of ventral nIII. Dorsal nIII motoneurons are, moreover, born before motoneurons of ventral nIII and nIV. The order of neurogenesis can therefore account for the dorsoventral organization of nIII and may play a primary role in determining motoneuron identity. We propose that the temporal development of extraocular motoneurons plays a key role in assembling a functional oculomotor circuit. J. Comp. Neurol. 525:65–78, 2017. © 2016 The Authors The Journal of Comparative Neurology Published by Wiley Periodicals, Inc.     

Bilateral ptosis with pupil sparing because of a discrete midbrain lesion: magnetic resonance imaging evidence of topographic arrangement within the oculomotor nerve.

The topographic arrangement within the midbrain oculomotor nerve is not adequately elucidated in humans. Two patients with a partial oculomotor palsy because of a localized infarction or hematoma were treated. Both patients had bilateral ptosis, impaired adduction, and supraduction. One patient had impaired infraduction and pupillary involvement on one side. Results of computed tomography and magnetic resonance imaging revealed discrete lesions at the dorsal midbrain tegmentum that spared the rostral midbrain. The authors' cases elucidate that pupillary components take the most rostral course. This report provides indirect magnetic resonance imaging evidence to prove the course of pupillary fibers. Based on the different neuro-ophthalmologic findings in the authors' cases (sparing or affecting pupillary component and infraduction), the nerves of the inferior rectus and inferior oblique for infraduction pass more rostrally than those of medial rectus, superior rectus, and levator palpebrae. The nuclear and fascicular arrangement within the midbrain oculomotor nerve is speculated to be pupillary, extraocular, and eyelid elevation in the rostro-caudal order, based on the neuro-ophthalmologic impairment and magnetic resonance imaging findings in the authors' patients and in previous animal experiments. Knowing the fascicular and nuclear arrangement within the midbrain in detail will offer diagnostic clues for differentiation of causes for partial oculomotor palsy.