Organization of trigeminocollicular connections and their relations to the sensory innervation of the eyelids in the rat

Blink perturbation effects on saccades evoked by microstimulation of the superior colliculus

Current knowledge of saccade-blink interactions suggests that blinks have paradoxical effects on saccade generation. Blinks suppress saccade generation by attenuating the oculomotor drive command in structures like the superior colliculus (SC), but they also disinhibit the saccadic system by removing the potent inhibition of pontine omnipause neurons (OPNs). To better characterize these effects, we evoked the trigeminal blink reflex by delivering an air puff to one eye as saccades were evoked by sub-optimal stimulation of the SC. For every stimulation site, the peak and average velocities of stimulation with blink movements (SwBMs) were lower than stimulation-only saccades (SoMs), supporting the notion that the oculomotor drive is weakened in the presence of a blink. In contrast, the duration of the SwBMs was longer, consistent with the hypothesis that the blink-induced inhibition of the OPNs could prolong the window of time available for oculomotor commands to drive an eye movement. The amplitude of the SwBM could also be larger than the SoM amplitude obtained from the same site, particularly for cases in which blink-associated eye movements exhibited the slowest kinematics. The results are interpreted in terms of neural signatures of saccade-blink interactions.

Segregated anatomical input to sub-regions of the rodent superior colliculus associated with approach and defense

The superior colliculus (SC) is responsible for sensorimotor transformations required to direct gaze toward or away from unexpected, biologically salient events. Significant changes in the external world are signaled to SC through primary multisensory afferents, spatially organized according to a retinotopic topography. For animals, where an unexpected event could indicate the presence of either predator or prey, early decisions to approach or avoid are particularly important. Rodents’ ecology dictates predators are most often detected initially as movements in upper visual field (mapped in medial SC), while appetitive stimuli are normally found in lower visual field (mapped in lateral SC). Our purpose was to exploit this functional segregation to reveal neural sites that can bias or modulate initial approach or avoidance responses. Small injections of Fluoro-Gold were made into medial or lateral sub-regions of intermediate and deep layers of SC (SCm/SCl). A remarkable segregation of input to these two functionally defined areas was found. (i) There were structures that projected only to SCm (e.g., specific cortical areas, lateral geniculate and suprageniculate thalamic nuclei, ventromedial and premammillary hypothalamic nuclei, and several brainstem areas) or SCl (e.g., primary somatosensory cortex representing upper body parts and vibrissae and parvicellular reticular nucleus in the brainstem). (ii) Other structures projected to both SCm and SCl but from topographically segregated populations of neurons (e.g., zona incerta and substantia nigra pars reticulata). (iii) There were a few brainstem areas in which retrogradely labeled neurons were spatially overlapping (e.g., pedunculopontine nucleus and locus coeruleus). These results indicate significantly more structures across the rat neuraxis are in a position to modulate defense responses evoked from SCm, and that neural mechanisms modulating SC-mediated defense or appetitive behavior are almost entirely segregated.

Normal and abnormal lid function

This chapter on lid function is comprised of two primary sections, the first on normal eyelid anatomy, neurological innervation, and physiology, and the second on abnormal eyelid function in disease states. The eyelids serve several important ocular functions, the primary objectives of which are protection of the anterior globe from injury and maintenance of the ocular tear film. Typical eyelid behaviors to perform these functions include blinking (voluntary, spontaneous, or reflexive), voluntary eye closure (gentle or forced), partial lid lowering during squinting, normal lid retraction during emotional states such as surprise or fear (startle reflex), and coordination of lid movements with vertical eye movements for maximal eye protection. Detailed description of the neurological innervation patterns and neurophysiology of each of these lid behaviors is provided. Abnormal lid function is divided by conditions resulting in excessive lid closure (cerebral ptosis, apraxia of lid opening, blepharospasm, oculomotor palsy, Horner's syndrome, myasthenia gravis, and mechanical) and those resulting in excessive lid opening (midbrain lid retraction, facial nerve palsy, and lid retraction due to orbital disease).

Are locus coeruleus neurons involved in blinking?

To investigate the involvement of the noradrenergic locus coeruleus (LC) in the reflex blink circuit, c-Fos and neuronal tracer experiments were performed in the rat. LC neurons involved in reflex blink were localized by analyzing c-Fos protein expression after electrical stimulation of the supraorbital nerve. Subsequently, neuronal tracers were injected in two different nuclei which are part of the reflex blink circuit. Anterograde tracer experiments in the sensory trigeminal complex (STC) explored the trigemino-coerulear connection; retrograde tracer experiments in the latero-caudal portion of the superior colliculus (SC) established coerulear-collicular connections. The combination of retrograde tracer injections into the latero-caudal SC portion combined with electrical stimulation of the supraorbital nerve identified c-Fos positive LC neurons that project to the latero-caudal SC. Our results revealed the existence of a STC-LC-SC loop.

Organization and origin of the connection from the inferior to the superior colliculi in the rat

The inferior colliculus (IC) is the main ascending auditory relay station prior to the superior colliculus (SC). The morphology and origin of the connection from inferior to superior colliculus (I-SC) was analyzed both by anterograde and retrograde tracing. Irrespective of the subregion of the IC in which they originate, the terminal fields of these connections formed two main tiers in the SC. While the dorsal one primarily involved the stratum opticum and the stratum griseum intermediale, the ventral one innervated the deep strata, although some fibers did connect these tiers. While the dorsal tier occupied almost the whole extension of the SC, the ventral one was mostly confined to its caudomedial quadrant. The fiber density in these tiers decreased gradually in a rostral gradient and the terminal fields became denser as the anterograde tracer at the injection site was distributed more externally in the cortex of the IC. Retrograde tracing confirmed this result, although it did not reveal any topographic ordering for the I-SC pathway. Most presynaptic boutons of the I-SC terminal field were located either inside or close to the patches of acetylcholinesterase activity. Together with previous anatomical and physiological studies, our results indicate that the I-SC connection relays behaviorally relevant information for sensory-motor processing. Our observation that this pathway terminates in regions of the superior colliculus, where neurons involved in fear-like responses are located, reinforce previous suggestions of a role for the IC in generating motor stereotypes that occur during audiogenic seizures.

Afferent Connections of the Optic Tectum in Lampreys: An Experimental Study

Tectal afferents were studied in adult lampreys of three species (Ichthyomyzon unicuspis, Lampetra fluviatilis, and Petromyzon marinus) following unilateral BDA injections into the optic tectum (OT). In the secondary prosencephalon, neurons projecting to the OT were observed in the pallium, the subhipoccampal lobe, the striatum, the preoptic area and the hypothalamus. Following tectal injections, backfilled diencephalic cells were found bilaterally in: prethalamic eminence, ventral geniculate nucleus, periventricular prethalamic nucleus, periventricular pretectal nucleus, precommissural nucleus, magnocellular and parvocellular nuclei of the posterior commissure and pretectal nucleus; and ipsilaterally in: nucleus of Bellonci, periventricular thalamic nucleus, nucleus of the tuberculum posterior, and the subpretectal tegmentum, as well as in the pineal organ. At midbrain levels, retrogradely labeled cells were seen in the ipsilateral torus semicircularis, the contralateral OT, and bilaterally in the mesencephalic reticular formation and inside the limits of the retinopetal nuclei. In the hindbrain, tectal projecting cells were also bilaterally labeled in the dorsal and lateral isthmic nuclei, the octavolateral area, the sensory nucleus of the descending trigeminal tract, the dorsal column nucleus and the reticular formation. The rostral spinal cord also exhibited a few labeled cells. These results demonstrate a complex pattern of connections in the lamprey OT, most of which have been reported in other vertebrates. Hence, the lamprey OT receives a large number of nonvisual afferents from all major brain areas, and so is involved in information processing from different somatic sensory modalities.

Reticulo-collicular and spino-collicular projections involved in eye and eyelid movements during the blink reflex

Reflex blinking provides a useful experimental tool for various functional studies on the peripheral and central nervous system, yet the neuronal circuitry underlying this reflex is not precisely known. In the present study, we investigated as to whether neurons in the reticular formation and rostral cervical spinal cord (C1) may be involved in the blink reflex in rats. To this end we investigated c-Fos expression in these areas following supraorbital nerve stimulation combined with retrograde tracing of gold conjugated horse radish peroxidase (Gold-HRP) from the superior colliculus. We observed many double labeled neurons in the parvocellular reticular nucleus, medullary reticular formation, and laminae IV and V of C1. Thus, these brain regions contain neurons that may be involved in blink reflexes as well as eye movements, because they both can be activated following peri-orbital stimulation and project to the superior colliculus. Consequently, we suggest that the medullary reticular formation and C1 region play a central role in the coordination of eye and eyelid movements during reflex blinking.

Descriptive and functional neuroanatomy of locus coeruleus-noradrenaline-containing neurons involvement in bradykinin-induced antinociception on principal sensory trigeminal nucleus

The present study was carried out in Wistar rats, using the jaw-opening reflex and dental pulp stimulation, to investigate noradrenaline- and serotonin-mediated antinociceptive circuits. The effects of microinjections of bradykinin into the principal sensory trigeminal nucleus (PSTN) before and after neurochemical lesions of the locus coeruleus noradrenergic neurons were studied. Neuroanatomical experiments showed evidence for reciprocal neuronal pathways connecting the locus coeruleus (LC) to trigeminal sensory nuclei and linking monoaminergic nuclei of the pain inhibitory system to spinal trigeminal nucleus (STN). Fast blue (FB) injections in the locus coeruleus/subcoeruleus region retrogradely labeled neurons in the contralateral PSTN and LC. Microinjections of FB into the STN showed neurons labeled in both ipsilateral and contralateral LC, as well as in the ipsilateral Barrington's nucleus and subcoeruleus area. Retrograde tract-tracing with FB also showed that the mesencephalic trigeminal nucleus sends neural pathways towards the ipsilateral PSTN, with outputs from cranial and caudal aspects of the brainstem. In addition, neurons from the lateral and dorsolateral columns of periaqueductal gray matter also send outputs to the ipsilateral PSTN. Microinjections of FB in the interpolar and caudal divisions of the STN labeled neurons in the caudal subdivision of STN. Microinjections in the STN interpolar and caudal divisions also retrogradely labeled serotonin- and noradrenaline-containing nucleus of the brainstem pain inhibitory system. Finally, the gigantocellularis complex (nucleus reticularis gigantocellularis/paragigantocellularis), nucleus raphe magnus and nucleus raphe pallidus also projected to the caudal divisions of the STN. Microinjections of bradykinin in the PSTN caused a statistically significant long-lasting antinociception, antagonized by the damage of locus coeruleus-noradrenergic neuronal fibres with (N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine) (DSP4), a neurotoxin that specifically depleted noradrenaline from locus coeruleus terminal fields. These data suggest that serotonin- and noradrenaline-containing nuclei of the endogenous pain inhibitory system exert a key-role in the antinociceptive mechanisms of bradykinin and the locus coeruleus is crucially involved in this effect.

Projections of the sensory trigeminal nucleus in a percomorph teleost, tilapia (Oreochromis niloticus)

The sensory trigeminal nucleus of teleosts is the rostralmost nucleus among the trigeminal sensory nuclear group in the rhombencephalon. The sensory trigeminal nucleus is known to receive the somatosensory afferents of the ophthalmic, maxillar, and mandibular nerves. However, the central connections of the sensory trigeminal nucleus remain unclear. Efferents of the sensory trigeminal nucleus were examined by means of tract-tracing methods, in a percomorph teleost, tilapia. After tracer injections to the sensory trigeminal nucleus, labeled terminals were seen bilaterally in the ventromedial thalamic nucleus, periventricular pretectal nucleus, medial part of preglomerular nucleus, stratum album centrale of the optic tectum, ventrolateral nucleus of the semicircular torus, lateral valvular nucleus, prethalamic nucleus, tegmentoterminal nucleus, and superior and inferior reticular formation, with preference for the contralateral side. Labeled terminals were also found bilaterally in the oculomotor nucleus, trochlear nucleus, trigeminal motor nucleus, facial motor nucleus, facial lobe, descending trigeminal nucleus, medial funicular nucleus, and contralateral sensory trigeminal nucleus and inferior olive. Labeled terminals in the oculomotor nucleus and trochlear nucleus showed similar densities on both sides of the brain. However, labelings in the trigeminal motor nucleus, facial motor nucleus, facial lobe, descending trigeminal nucleus, and medial funicular nucleus showed a clear ipsilateral dominance. Reciprocal tracer injection experiments to the ventromedial thalamic nucleus, optic tectum, and semicircular torus resulted in labeled cell bodies in the sensory trigeminal nucleus, with a few also in the descending trigeminal nucleus.

Trigemino-solitarii-facial pathway in rats

This study was undertaken to identify premotor neurons in the nucleus tractus solitarii (NTS) serving as relay neurons between the sensory trigeminal complex (STC) and the facial motor nucleus in rats. Trigemino-solitarii connections were first investigated following injections of anterograde and/or retrograde (biotinylated dextran amine, biocytin, or gold-HRP) tracers in STC or NTS. Trigemino-solitarii neurons were abundant in the ventral and dorsal parts of the STC and of moderate density in its intermediate part. They project throughout the entire rostrocaudal extent of the NTS with a strong lateral preponderance. Solitarii-trigeminal neurons were observed mostly in the rostral and rostrolateral NTS. They mainly project to the ventral and dorsal parts of the spinal trigeminal nucleus but not to the principal nucleus. Additional neurons located in the middle NTS were found to project exclusively to the spinal trigeminal nucleus pars caudalis. No solitarii-trigeminal cells were observed in the caudal NTS. In addition, evidence was obtained of NTS retrogradely labeled neurons contacted by anterogradely labeled trigeminal terminals. Second, solitarii-facial projections were analyzed following injections of anterograde and retrograde tracers into the NTS and the facial nucleus, respectively. NTS neurons, except those of the rostrolateral part, reached the dorsal aspect of the facial nucleus. Finally, simultaneous injections of anterograde tracer in the STC and retrograde tracer in the facial nucleus gave retrogradely labeled neurons in the NTS receiving contacts from anterogradely labeled trigeminal boutons. Thus, the present data demonstrate for the first time the existence of a trigemino-solitarii-facial pathway. This could account for the involvement of the NTS in the control of orofacial motor behaviors.

Connections between the lacrimal gland and sensory trigeminal neurons: a WGA/HRP study in the cynomolgous monkey

The sensory innervation of the lacrimal gland (LG) in the cynomolgous monkey was studied using the retrograde wheat germ agglutinin/horsereadish peroxidase (WGA/HRP) tracer technique. A small solidified piece of WGA/HRP was implanted in the LG. Labelled sensory first-order neurons were found in the ipsilateral trigeminal ganglion (TG) and in the ipsilateral mesencephalic trigeminal nucleus (MTN). The distribution of labelled TG neurons was restricted to ophthalmic and maxillary ganglionic parts. Sensory innervation of LG by primary afferents is not only restricted to TG; an MTN involvement has also been found. This may imply that there is a central sensory role in the production and release of tears.

Reticulo-collicular projections: a neuronal tracing study in the rat

Neuroanatomical tract-tracing methods were used to study the topography of the reticulocollicular projections. Injections of gold-HRP or BDA tracers into the medial and/or central portions of the superior colliculus resulted in labelled neurones mainly in the medial reticular formation, whereas injections into the lateral portion of the superior colliculus showed labelling in the medial and lateral reticular formation. When tracer was injected into the lateral portion of the caudal superior colliculus, extensive lateral labelling was observed in the contralateral parvocellular reticular nucleus and the contralateral dorsal medullary reticular nucleus, two areas involved in reflex blinking. The present study shows that these reticular areas project to the lateral superior colliculus, which is known to be involved in the coordination of eye and eyelid movements.

Projections from the superior colliculus to the trigeminal system and facial nucleus in the rat

To determine the influence of the superior colliculus (SC) in orienting behaviors, we examined SC projections to the sensory trigeminal complex, the juxtatrigeminal region, and the facial motor nucleus in rats. Anterograde tracer experiments in the SC demonstrated predominantly contralateral colliculotrigeminal projections. Microinjections in the deep layers of the lateral portion showed labeled nerve fibers and terminals in the ventromedial parts of the caudal principal nucleus and of the rostral oral subnucleus and in the medial part of the interpolar subnucleus. Some terminals were also observed in the juxtatrigeminal region and in the dorsolateral part of the facial motor nucleus contralaterally, overlying the orbicularis oculi motoneuronal region. Verification by retrograde tracer injections into the trigeminal target regions showed labeled SC neurons mostly in lateral portions of layers 4-7. When the juxtatrigeminal region was involved, a remarkable increase of labeled neurons was observed, having a patch-like arrangement with a decreasing gradient from lateral to medial SC portions. Retrograde tracer injections in the dorsolateral VII nucleus showed bilateral labeled neurons mainly in the deep lateral SC portion. Retrograde BDA microinjections into the same trigeminal or juxtatrigeminal regions, followed by gold-HRP into the dorsolateral VII nucleus, demonstrated a significant number of SC neurons in deep layers 6-7 projecting to both structures by axon collaterals. These neurons are mediolaterally grouped in patches along the rostrocaudal SC extent; a subset of them are immunoreactive for glutamic acid decarboxylase (GAD). They could be involved in the coordination of facial movements. Simultaneous anterograde and retrograde tracer injections into the lateral SC portion and the VII nucleus respectively localized trigeminofacial neurons receiving collicular input in the trigeminal principal nucleus and pars oralis. Therefore the SC should play a crucial role in regulating motor programs of both eye and eyelid movements.

Localization of trigeminal, spinal, and reticular neurons involved in the rat blink reflex

Electrical stimulation of the supraorbital nerve (SO) induces eyelid closure by activation of orbicularis oculi muscle motoneurons located in the facial motor nucleus (VII). Neurons involved in brainstem central pathways implicated in rat blink reflex were localized by analyzing c-Fos protein expression after SO stimulation in conjunction with tracing experiments. A retrograde tracer (gold-horseradish peroxidase [HRP]) was injected into the VII. The distribution patterns of activated c-Fos-immunoreactive neurons and of neurons exhibiting both c-Fos immunoreactivity and gold-HRP labeling were determined in the sensory trigeminal complex (STC), the cervical spinal cord (C1), and the pontomedullary reticular formation. Within the STC, c-Fos immunoreactivity labeled neurons in the ipsilateral ventral part of the principal nucleus, the pars oralis and interpolaris, and bilaterally in the pars caudalis. Colocalization of gold-HRP and c-Fos immunoreactivity was observed in neurons of ventral pars caudalis layers I-IV and ventral pars interpolaris. In C1, SO stimulation revealed c-Fos neurons in laminae I-V. After additional injections in VII, the double-labeled c-Fos/gold-HRP neurons were concentrated in laminae IV and V. Although c-Fos neurons were found throughout the pontomedullary reticular formation, most appeared rostrally around the motor trigeminal nucleus and in the ventral parvocellular reticular nucleus medial to the fiber bundles of the seventh nerve. Caudally, c-Fos neurons were in the lateral portion of the dorsal medullary reticular field. In addition, these reticular areas contained double-labeled neurons in electrically stimulated rats that had received gold-HRP injections in the VII. The presence of double-labeled neurons in the STC, C1, and the reticular formation implies that these neurons receive sensory information from eyelids and project to the VII. These double-labeled neurons could then be involved in di- or trisynaptic pathways contributing to the blink reflex.

Eyelid movements: behavioral studies of blinking in humans under different stimulus conditions

The kinematics and neurophysiological aspects of eyelid movements were examined during spontaneous, voluntary, air puff, and electrically induced blinking in healthy human subjects, using the direct magnetic search coil technique simultaneously with electromyographic recording of the orbicularis oculi muscles (OO-EMG). For OO-EMG recordings, surface electrodes were attached to the lower eyelids. To measure the vertical lid displacement, a search coil with a diameter of 3 mm was placed 1 mm from the rim on the upper eyelid on a marked position. Blink registrations were performed from the zero position and from 28 randomly chosen positions. Blinks elicited by electrical stimulation of the supraorbital nerve had shortest duration and were least variable. In contrast, spontaneous blinks had longer duration and greater variability. Blinks induced by air puff had a slightly longer duration and similar variability as electrically induced blinks. There was a correlation between the maximal down phase amplitude and the integrated OO-EMG. Blink duration and maximal down phase amplitude were affected by eye position. Eyes positioned 30 degrees above horizontal displayed the shortest down phase duration and the largest maximal down phase amplitude and velocity. At 30 degrees below horizontal, blinks had the longest total duration, the longest down phase duration, and the lowest maximal down phase amplitude and velocity. The simultaneously recorded integrated OO-EMG was largest in the 30 degrees downward position. In four subjects, the average blinking data showed a linear relation between eye position and OO-EMG, maximal down phase amplitude, and maximal downward velocity.