Morphology and soma-dendritic distribution of synaptic endings from the rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF) on motoneurons in the oculomotor and trochlear nuclei in the cat

Brain Stem Neural Circuits of Horizontal and Vertical Saccade Systems and their Frame of Reference

Sensory signals for eye movements (visual and vestibular) are initially coded in different frames of reference but finally translated into common coordinates, and share the same final common pathway, namely the same population of extraocular motoneurons. From clinical studies in humans and lesion studies in animals, it is generally accepted that voluntary saccadic eye movements are organized in horizontal and vertical Cartesian coordinates. However, this issue is not settled yet, because neural circuits for vertical saccades remain unidentified. We recently determined brainstem neural circuits from the superior colliculus to ocular motoneurons for horizontal and vertical saccades with combined electrophysiological and neuroanatomical techniques. Comparing well-known vestibuloocular pathways with our findings of commissural excitation and inhibition between both superior colliculi, we proposed that the saccade system uses the same frame of reference as the vestibuloocular system, common semicircular canal coordinate. This proposal is mainly based on marked similarities (1) between output neural circuitry from one superior colliculus to extraocular motoneurons and that from a respective canal to its innervating extraocular motoneurons, (2) of patterns of commissural reciprocal inhibitions between upward saccade system on one side and downward system on the other, and between anterior canal system on one side and posterior canal system on the other, and (3) between the neural circuits of saccade and quick phase of vestibular nystagmus sharing brainstem burst neurons. In support of the proposal, commissural excitation of the superior colliculi may help to maintain Listing's law in saccades in spite of using semicircular canal coordinate.

Tensor tympani motoneurons receive mostly excitatory synaptic inputs

The tensor tympani is a middle ear muscle that contracts in two different situations: in response to sound or during voluntary movements. To gain insight into the inputs and neural regulation of the tensor tympani, we examined the ultrastructure of synaptic terminals on labeled tensor tympani motoneurons (TTMNs) using transmission electron microscopy. Our sample of six TTMNs received 79 synaptic terminals that formed 126 synpases. Two types of synapses are associated with round vesicles and form asymmetric junctions (excitatory morphology). One of these types has vesicles that are large and round (Lg Rnd) and the other has vesicles that are smaller and round (Sm Rnd) and also contains at least one dense core vesicle. A third synapse type has inhibitory morphology because it forms symmetric synapses with pleomorphic vesicles (Pleo). These synaptic terminals can be associated with TTMN spines. Two other types of synapse are found on TTMNs but they are uncommon. Synaptic terminals of all types form multiple synapses but those from a single terminal are always the same type. Terminals with Lg Rnd vesicles formed the most synpases per terminal (avg. 2.73). Together, the synaptic terminals with Lg Rnd and Sm Rnd vesicles account for 62% of the terminals on TTMNs, and they likely represent the pathways driving the contractions in response to sound or during voluntary movements. Having a high proportion of excitatory inputs, the TTMN innervation is like that of stapedius motoneurons but proportionately different from other types of motoneurons.

Neural Organization of the Pathways From the Superior Colliculus to Trochlear Motoneurons

The neural organization of the pathways from the superior colliculus (SC) to trochlear motoneurons was analyzed in anesthetized cats using intracellular recording and transneuronal labeling techniques. Stimulation of the ipsilateral or contralateral SC evoked excitation and inhibition in trochlear motoneurons with latencies of 1.1–2.3 and 1.1–3.8 ms, respectively, suggesting that the earliest components of excitation and inhibition were disynaptic. A midline section between the two SCs revealed that ipsi- and contralateral SC stimulation evoked disynaptic excitation and inhibition in trochlear motoneurons, respectively. Premotor neurons labeled transneuronally after application of wheat germ agglutinin-conjugated horseradish peroxidase into the trochlear nerve were mainly distributed ipsilaterally in the Forel's field H (FFH) and bilaterally in the interstitial nucleus of Cajal (INC). Consequently, we investigated these two likely intermediaries between the SC and trochlear nucleus electrophysiologically. Stimulation of the FFH evoked ipsilateral mono- and disynaptic excitation and contralateral disynaptic inhibition in trochlear motoneurons. Preconditioning stimulation of the ipsilateral SC facilitated FFH-evoked monosynaptic excitation. Stimulation of the INC evoked ipsilateral monosynaptic excitation and inhibition, and contralateral monosynaptic inhibition in trochlear motoneurons. Preconditioning stimulation of the contralateral SC facilitated contralateral INC-evoked monosynaptic inhibition. These results revealed a reciprocal input pattern from the SCs to vertical ocular motoneurons in the saccadic system; trochlear motoneurons received disynaptic excitation from the ipsilateral SC via ipsilateral FFH neurons and disynaptic inhibition from the contralateral SC via contralateral INC neurons. These inhibitory INC neurons were considered to be a counterpart of inhibitory burst neurons in the horizontal saccadic system.

Premotor circuits controlling eyelid movements in conjunction with vertical saccades in the cat: II. interstitial nucleus of Cajal

Vertical saccadic eye movements are accompanied by concurrent eyelid movements in the same direction. The interstitial nucleus of Cajal (InC) controls eye position for vertical eye movements and may also control saccade-related lid position as well. This study investigates whether the InC serves as a premotor center for eyelid saccades, by employing dual-tracer methods in cats to label both the projections of the InC and the motoneurons supplying the levator palpebrae superioris (LPS) muscle, which lie in the caudal central subdivision (CCS) of the oculomotor complex. Injections of biotinylated dextran amine (BDA) into the InC anterogradely labeled axons that terminated bilaterally throughout the CCS and in the oculomotor nuclei proper. Labeled terminals lay in close association with labeled LPS motoneurons, which were retrogradely labeled following injections of wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) into the muscle. Ultrastructural investigation revealed that most terminals contained spherical vesicles and formed asymmetric synaptic contacts with the labeled motoneurons. These results strongly suggest that the InC monosynaptically controls lid movements in conjunction with vertical eye movements, including saccades. To identify the neurons of origin for this pathway, WGA-HRP injections were centered in the CCS. These experiments indicate that lid and eye motoneurons may share a common source of bilateral InC input. Thus, a common vertical position signal may be employed to maintain the lid and eye at appropriate elevations during fixation, such that the lid sits just above the pupil, allowing unobstructed vision, but at the ready to protect the cornea.

The reticular formation

The reticular formation of the brainstem contains functional cell groups that are important for the control of eye, head, or lid movements. The mesencephalic reticular formation is primarily involved in the control of vertical gaze, the paramedian pontine reticular formation in horizontal gaze, and the medullary pontine reticular formation in head movements and gaze holding. In this chapter, the locations, connections, and histochemical properties of the functional cell groups are reviewed and correlated with specific subdivisions of the reticular formation.

Morphological study of the healthy human oculomotor nerve by atomic force microscopy

This study concerns an investigation of oculomotor nerves via imaging semithin sections with an atomic force microscope (AFM). The study used 10 oculomotor nerves removed from individuals aged between 40 and 60 years old at time of death. We adopted semithin sections for our AFM studies to extract topographical data of the fibers of this nerve. Both light microscope and electron microscope images of the same samples were employed to augment our capabilities for orientation in the AFM analyses. Transverse sections of the oculomotor nerve, imaged via AFM, show clearly identifiable ring-like nerve fibers with the myelinated axons. The samples were simultaneously characterized by error signal mode and lateral force microscopy. These AFM-related techniques revealed no information concerning friction force and elasticity due to the presence of the embedding material (epoxy resin), but showed the myelinated nerve fibers with ring-like structures corresponding to the myelin sheaths. These are the first AFM images of physiological human oculomotor nerves, containing clear three-dimensional representations of these samples.

Premotor circuits controlling eyelid movements in conjunction with vertical saccades in the cat: I. The rostral interstitial nucleus of the medial longitudinal fasciculus

Saccadic eye movements in the vertical plane are controlled by the rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF) and the interstitial nucleus of Cajal. Eye movements in the vertical direction are accompanied by concurrent upper eye lid movements. These gaze-related lid movements are produced by the levator palpebrae superioris muscle, whose motoneurons are located in the caudal central subdivision (CCS) of the oculomotor nucleus. The neural circuits that direct such gaze-related lid movements were examined by use of both conventional and dual neuronal tracing methods in the cat. Injections of wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) into the area of the CCS revealed a distinctive subset of retrogradely labeled neurons located in the caudomedial portion of the riMLF. This subset of riMLF neurons was not labeled when injections were localized within the oculomotor nucleus proper, without involving the CCS. Injections of biotinylated dextran amine (BDA) that included this caudomedial riMLF region anterogradely labeled axons that projected profusely throughout the CCS. Labeled terminals were seen in close association with retrogradely labeled levator palpebrae motoneurons, which were primarily found contralateral to WGA-HRP muscle injections. Ultrastructural examination revealed that most BDA-labeled terminals contained clear spherical vesicles and formed asymmetrical synaptic contacts, primarily on the proximal dendrites of WGA-HRP-labeled motoneurons. A few had pleiomorphic vesicles. In summary, these results strongly suggest that the caudomedial part of the cat riMLF is a premotor center that monosynaptically controls lid movements in conjunction with vertical saccades.

Anatomical substrates of oculomotor control

It is convenient to describe oculomotor neuroanatomy in terms of five to six different eye movement types, each with relatively independent neural circuitry: saccades, vestibulo-ocular reflex, optokinetic response, smooth pursuit, vergence and, most recently added to the list, gaze-holding. Current research indicates that many structures participate in several eye movement types, such as the nucleus reticularis tegmenti pontis, frontal eye fields and pretectum. However, the circuits appear to run in parallel rather than being integrated.