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.     

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.     

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

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

Oculomotor system of the weakly electric fish Gnathonemus petersii

The peripheral and central aspects of the extraocular system were studied in the weakly electric fish Gnathonemus petersii. All six extraocular muscles show a similar composition of large and small fibers grouped characteristically in the proximal and distal regions respectively. The exit of the three extraocular nerves from the brain is similar to that in other vertebrates. However, the intracephalic and intracranial course of the trochlear nerve is unusual, partly because of the extraordinary hypertrophy of the cerebellum. The three nerves course rostrally on the ventral brain surface; the trochlear nerve penetrates the orbital cavity separately from the two other nerves. The fiber-diameter spectrum of each extraocular nerve is bimodal; unmyelinated fibers were not observed in any of the nerves. The location of the extraocular motor nuclei was established by retrograde axonal transport of HRP or cobaltic-lysine complex. The oculomotor nucleus is situated ventral to the posterior pole of the magnocellular mesencephalic nucleus and the trochlear nucleus is found caudal and dorsal to this. The abducens nucleus is situated at the level of the octavolateral efferent nucleus and consists of a single group of cells on each side of the ventral tegmentum. The oculomotor nucleus of G. petersii shows a somatotopic organization. The superior rectus muscle receives a contralateral innervation whereas the inferior rectus and oblique muscles and the internal rectus muscles receive an ipsilateral innervation. The superior oblique muscle is innervated by contralateral trochlear motoneurons and the external rectus by ipsilateral abducens motoneurons. The majority of extraocular motoneurons have piriform perikarya and long beaded dendrites that extend laterally in the oculomotor and abducens nuclei and rostrally in the trochlear nucleus. The terminal dendritic portions of trochlear motoneurons widely overlap with oculomotor dendrites and perikarya. In all three nuclei the axon originates opposite to the main dendrite. Collaterals of the hairpin-bend abducens axons could be identified in a few cases. The oculomotor system of G. petersii appears basically similar to that of other teleosts; the differences observed concern mainly the structure of the abducens nucleus, the intracranial and intracephalic course of the trochlear nerve, and the relatively small number of axons in each nerve.     

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.     

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)     

Accessory abducens nucleus and conditioned eye retraction/nictitating membrane extension in rabbit

The role of accessory abducens nucleus neurons in the conditioned eye retraction/nictitating membrane extension response was defined in the rabbit. Horseradish peroxidase injections into the retractor bulbi muscle showed that accessory abducens nucleus is the principal location of its motor-neurons. Single and multiple unit recording in accessory abducens indicated that these motor neurons show a marked responsiveness to corneal and periorbital stimulation and fire in close correlation with conditioned, unconditioned, or spontaneous eye retraction/nictitating membrane extension. Complete lesions of accessory abducens showed, at most, a partial reduction of the conditioned and unconditioned eye retraction response. Section of the extraocular muscles, other than retractor bulbi, also caused a partial reduction of the eye retraction response. Accessory abducens lesions, combined with extraocular muscle section, were necessary to dramatically reduce the eye retraction response permanently. These experiments demonstrated that accessory abducens is a primary controller of eye retraction through its axons to retractor bulbi. The other extraocular muscles act in concert with retractor bulbi to elicit conditioned and unconditioned eye retractions.     

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.     

Muscle units divided among retractor bulbi muscle slips and between the lateral rectus and retractor bulbi muscles in cat

Single muscle units of the retractor bulbi and lateral rectus muscles were activated with brief (0.5-ms) current pulses delivered through an intracellular micropipet penetrating single motoneuron cell bodies or axons in the principal abducens nucleus of the cat. Muscle unit mechanical characteristics were measured. A total of 40 retractor bulbi muscle units was studied. Thirty muscle units were contained in 2, 3, or all 4 retractor bulbi muscle slips. Three muscle units involved one retractor bulbi muscle slip and seven units were contained in the lateral rectus muscle plus one or both of the lateral retractor bulbi muscle slips. The largest percentage of muscle units (32.5%) was contained in the two inferior retractor bulbi muscle slips. The 33 retractor bulbi muscle units, having no muscle fibers in the lateral rectus muscle, caused the contraction of 76 retractor bulbi muscle slips. Twitch tension was measured in 58 slips and maximum tetanic tension in 27 slips. The average twitch tension per measured slip was 68.0 mg and the average maximum tetanic tension was 459.8 mg. The average retractor bulbi muscle unit contraction time was 10 ms and the lateral rectus average contraction time was 7.9 ms. We propose that peripheral branching of abducens nerve axons can account for the division of muscle unit components among the retractor bulbi muscle slips and between the lateral rectus and retractor bulbi muscles.     

The role of the accessory abducens nucleus in the rabbit nictitating membrane response

Electrolytic and knife-cut lesions were employed in the rabbit to examine the role of the VIth cranial nerve, and of the motoneurons in the abducens (ABD) and accessory abducens (ACC) nuclei that supply the VIth nerve, in the reflex extension of the nictitating membrane. The nictitating membrane response (NMR) was elicited by tactual stimulation of the cornea with a puff of air or by electric shock delivered to the skin over the paraorbital region of the head. Total destruction of the VIth nerve or interruption of all ACC inputs to the VIth nerve (while leaving ABD inputs intact) produced a large and comparable reduction in the magnitude of the NMR elicited by air puff, although a small residual NMR of less than 1 mm could still be detected. In contrast, the magnitude of the NMR elicited by shock was not affected by ACC isolation and only reduced by 50% after VIth nerve lesions. Total isolation of ABD inputs to the VIth nerve (while leaving ACC inputs intact) had no effect on NMR magnitude elicited by either air puff or shock. The small residual NMRs to air puff and the larger NMRs to shock remaining after total destruction of the VIth nerve were not eliminated by the removal of all extraocular muscles (while leaving the retractor bulbi muscle intact). However, knife cut lesions that interrupted all ACC inputs to the VIth nerve and transected the VIIth (facial) nerve completely eliminated NMRs elicited by both air puff and shock. The results of this study indicate that NMRs elicited by tactual stimulation of the cornea are primarily mediated by retractor bulbi motoneurons in the ACC nucleus via the VIth nerve. In contrast, NMRs elicited by electric shock delivered to the skin over the paraorbital region of the head are produced by contraction of the retractor bulbi muscle via the VIth nerve and by contraction of the orbicularis oculi muscle via the VIIth nerve which then squeezes the nictitating membrane over the cornea.     

Retractor bulbi muscle responses to oculomotor nerve and nucleus stimulation in the cat

This study demonstratesthe presence of retractor bulbi motoneurons within the oculomotor nucleus which activate muscle units within all 4 slips of the cat retractor bulbi muscle. These muscle units are mechanically different and physiologically separate from retractor bulbi muscle units innervated by the abducens nerve. The retractor bulbi muscle, then, is innervated by two separate pools of motoneurons whose axons are carried in two different cranial nerves. These observations of mechanical properties of retractor bulbi muscle suggest that the oculomotor retractor bulbi motor units may be activated during patterned eye movements.     

The role of the extraocular muscles in the rabbit nictitating membrane response: a re-examination

Berthier and Moore showed that the rabbit nictitating membrane (NM) response principally results from contracting the retractor bulbi muscle which pulls the globe into the socket thereby passively effecting NM extension. They concluded that the remaining extraocular muscles can effect NM extension if the retractor bulbi is denervated. A re-examination of the role of the recti and oblique extraocular muscles in nictitating membrane extension was undertaken in the light of recent results of Marek et al., suggesting that the facial nerve, and not the extraocular muscles, participates in extension of the NM. In contrast to Marek et al., the present results indicated that section of the extraocular muscles was necessary to abolish eyeshock or tactilly elicited NM extension when the abducens and facial nerves were severed. It is therefore likely that extraocular (recti and oblique) muscles participate in globe retraction and NM extension, as originally noted by Lorente de No.     

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.     

Fatigue of lateral rectus and retractor bulbi motor units in cat

The fatigue properties of lateral rectus, retractor bulbi and split lateral rectus-retractor bulbi motor units were studied in the cat. Lateral rectus motor units showed a range in resistance to fatigue while retractor bulbi motor units were all fatigable. Within the abducens nucleus, axons of split lateral rectus-retractor bulbi motor units are found. These motor units are unique in that one motoneuron projects to two separate muscles. Split motor units were studied to determine if both the lateral rectus and retractor bulbi muscle fibers of split units would show uniform fatigue properties. The results showed that the fatigue resistance of the separate muscle components of these motor units are different, suggesting that the muscle fibers of a motor unit may be physiologically dissimilar.     

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.     

The innervation of the monkey accessory lateral rectus muscle

The source and pattern of innervation of the accessory lateral rectus muscle has been re-investigated in 3 Macaca fascicularis monkeys by means of a tracing method employing [125I] wheat germ agglutinin and a morphological analysis of the myo-neuronal junctions. The present findings suggest that this muscle is composed of exclusively singly innervated fibers and that its motoneurons are situated in the accessory abducens nucleus. This is in contrast to a previous study, where the monkey accessory lateral rectus was found to be composed of singly and multiply innervated fibers and to be innervated by a group of motoneurons lying within principle and accessory abducens nucleus. It is concluded that the monkey accessory lateral rectus reflects in principle the organization of the retractor bulbi of other vertebrates, although this muscle is gradually vanishing in primate evolution and remains vestigial in the macaque monkey. The absence of comparable motor units in man is more likely to mean an actual loss of structure and function than their integration into the lateral rectus system.     

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.     

Behavior of cat abducens motoneurons following the injection of toxic ricin into the lateral rectus muscle

The aim of this investigation was to study the behavior of identified abducens motoneurons in the chronic cat following a single injection of toxic ricin into the lateral rectus muscle. Lateral rectus electromyographic potentials induced by VIth nerve stimulation disappeared, and abducens antidromic field potentials decreased by 90% 3 days following ricin injection. Several abnormalities and a significant decrease in eye position and velocity sensitivities were observed in motoneuron activity up to 8-10 days following ricin injection. Contrary to a previous report for axotomized abducens motoneurons, no functional sign of recovery was observed. Histological analysis showed a survival of 10-15% of the abducens motoneuron population 10 days following ricin injection. From this time on, recorded motoneurons behaved like controls, but showed a specific retraction signal suggesting an exclusive projection onto the retractor bulbi muscle. Although intermingled in the nucleus with motoneurons, no recorded abducens internuclear interneuron was affected by the ricin during one month following the injection.     

Localization of parvalbumin,calretinin, and calbindin D-28k in identified extraocular motoneurons and internuclear neurons of the cat

Calcium-binding proteins have been shown to be excellent markers of specific neuronal populations. We aimed to characterize the expression of calcium-binding proteins in identified populations of the cat extraocular motor nuclei by means of immunohistochemistry against parvalbumin, calretinin, and calbindin D-28k. Abducens, medial rectus, and trochlear motoneurons were retrogradely labeled with horseradish peroxidase from their corresponding muscles. Oculomotor and abducens internuclear neurons were retrogradely labeled after horseradish peroxidase injection into either the abducens or the oculomotor nucleus, respectively. Parvalbumin staining produced the highest density of immunoreactive terminals in all extraocular motor nuclei and was distributed uniformly. Around 15–20% of the motoneurons were moderately stained with antibody against parvalbumin, but their axons were heavily stained, indicating an intracellular segregation of parvalbumin. Colchicine administration increased the number of parvalbumin-immunoreactive motoneurons to approximately 85%. Except for a few calbindin-immunoreactive trochlear motoneurons (1%), parvalbumin was the only marker of extraocular motoneurons. Oculomotor internuclear neurons identified from the abducens nucleus constituted a nonuniform population, because low percentages of the three types of immunostaining were observed, calbindin being the most abundant (28.5%). Other interneurons located within the boundaries of the oculomotor nucleus were mainly calbindin-immunoreactive. The medial longitudinal fascicle contained numerous parvalbumin- and calretinin-immunoreactive but few calbindin-immunoreactive axons. The majority of abducens internuclear neurons projecting to the oculomotor nucleus (80.7%) contained calretinin. Moreover, the distribution of calretinin-immunoreactive terminals in the oculomotor nucleus overlapped that of the medial rectus motoneurons and matched the anterogradely labeled terminal field of the abducens internuclear neurons. Parvalbumin immunostained 42% of the abducens internuclear neurons. Colocalization of parvalbumin and calretinin was demonstrated in adjacent semithin sections, although single-labeled neurons were also observed. Therefore, calretinin is proven to be a good marker of abducens internuclear neurons. From all of these data, it is concluded that parvalbumin, calretinin, and calbindin D-28k selectively delineate certain neuronal populations in the oculomotor system and constitute valuable tools for further analysis of oculomotor function under normal and experimental conditions. J. Comp. Neurol. 390:377–391, 1998. © 1998 Wiley-Liss, Inc.     

Development of oculomotor axon projections in the chick embryo

The pattern of innervation of the extraocular muscles is highly conserved across higher vertebrate species and mediates sophisticated visuomotor processes. Defects in oculomotor development often lead to strabismus, a misalignment of the eyes that can cause partial blindness. Although it has been intensively studied from a clinical perspective, relatively little is known about how the system develops embryonically. We have therefore mapped the development of the oculomotor nerve (OMN) in chick embryos by using confocal microscopy. We show that OMN development follows a series of stereotyped steps that are tightly regulated in space and time. The OMN initially grows past three of its targets to innervate its distal target, the ventral oblique muscle, only later forming branches to the more proximal muscles. We have also investigated spatiotemporal aspects of the unusual contralateral migration of a subpopulation of oculomotor neurons by using molecular markers and have found the semaphorin axon guidance molecules and their receptors, the neuropilins, to be expressed in discrete subnuclei during this migration. Finally, we have created an embryological model of Duane retraction syndrome (DRS), a form of strabismus in which the OMN is believed to innervate aberrantly the lateral rectus, the normal target of the abducens nerve. By ablating rhombomeres 5 and 6 and hence the abducens, we have mimicked a proposed oculomotor deficit occurring in DRS. We find that the absence of the abducens nerve is not sufficient to produce this inappropriate innervation, so other factors are required to explain DRS.