Cells of origin of the spino-facial pathway in the cat: a horseradish peroxidase study

Macaque monkey trigeminal blink reflex circuits targeting orbicularis oculi motoneurons

The trigeminal blink reflex plays an important role in protecting the corneal surface from damage and preserving visual function in an unpredictable environment. The closing phase of the human reflex, produced by activation of the orbicularis oculi (ObOc) muscles, consists of an initial, small, ipsilateral R₁ component, followed by a larger, bilateral R₂ component. We investigated the circuitry that underlies this reflex in macaque (Macaca fascicularis and Macaca mulatta) monkeys by the use of single and dual tracer methods. Injection of retrograde tracer into the facial nucleus labeled neurons in the principal trigeminal nucleus, and in the spinal nucleus pars oralis and interpolaris, bilaterally, and in pars caudalis, ipsilaterally. Injection of anterograde tracer into the principal trigeminal nucleus labeled axons that directly terminated on ObOc motoneurons, with an ipsilateral predominance. Injection of anterograde tracer into pars caudalis of the spinal trigeminal nucleus labeled axons that directly terminated on ipsilateral ObOc motoneurons. The observed pattern of labeling indicates that the reticular formation ventromedial to the principal and spinal nuclei also contributes extensive bilateral input to ObOc motoneurons. Thus, much of the trigeminal sensory complex is in a position to supply a monosynaptic drive for lid closure, and the adjacent reticular formation can supply a disynaptic drive. These findings indicate that the assignment of the R₁ and R₂ components of the blink reflex to different parts of the trigeminal sensory complex cannot be exclusively based on subdivision connectional relationships with facial motoneurons. The characteristics of the R₂ component may be due, instead, to other circuit properties.

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.

Distribution of GABAergic and glycinergic premotor neurons projecting to the facial and hypoglossal nuclei in the rat

The distribution of inhibitory premotor neurons for the facial and hypoglossal nuclei was examined in the lower brainstem of the rat. A retrograde axonal tracing method with the fluorescent tracer, tetramethylrhodamine dextran amine (TMR-DA), was combined with immunofluorescence histochemistry for glutamic acid decarboxylase (GAD), i.e., the enzyme involved in gamma-aminobutyric acid synthesis, or glycine. In the rats injected with TMR-DA unilaterally into the facial or hypoglossal nucleus, the distribution of TMR-DA-labeled neurons showing GAD-like immunoreactivity (GAD/TMR-DA neurons) was essentially the same as that of TMR-DA-labeled neurons displaying glycine-like immunoreactivity (Gly/TMR-DA neurons). The distributions of GAD/TMR-DA and Gly/TMR-DA neurons in the rats injected with TMR-DA into the facial nucleus were also similar to those in the rats injected with TMR-DA into the hypoglossal nucleus. These neurons were seen most frequently in the lateral aspect of the pontine reticular formation, the supratrigeminal region, the dorsal aspect of the lateral reticular formation of the medulla oblongata, and the reticular regions around the raphe magnus nucleus and the gigantocellular reticular nucleus pars alpha, bilaterally with a slight dominance on the side ipsilateral to the injection site. A number of GAD/TMR-DA and Gly/TMR-DA neurons were also seen in the oral and interpolar subnuclei of the spinal trigeminal nucleus, bilaterally with a slight ipsilateral dominance. In the rats injected with TMR-DA into the facial nucleus, GAD/TMR-DA and Gly/TMR-DA neurons were also encountered in the paralemniscal zone of the midbrain tegmentum bilaterally with an apparent dominance on the side contralateral to the injection site. A large part of these inhibitory premotor neurons for the facial and hypoglossal nuclei and the excitatory ones may constitute premotor neuron pools common to the orofacial motor nuclei implicated in the control of integrated orofacial movements.

The trigeminally evoked blink reflex. I. Neuronal circuits

In this study, we characterized the pathways that generate the trigeminal blink reflex in the guinea pig. Blinks were evoked by stimulation of the supraorbital branch of the trigeminal nerve and measured by recording electromyographic activity in the lid-closing orbicularis oculi muscle (OOemg) and, in one case, lid position. Blinks evoked by stimulation of the supraorbital nerve consisted of two bursts of muscle activity ipsilateral to the side of stimulation. The first, R1, had a latency of 6.9 ms and the second, R2, had a latency of 17.25 ms. Increasing stimulus intensity to 3 times threshold for evoking an ipsilateral blink elicited an R1 and R2 response contralaterally, with latencies of 9.2 ms and 19.25 ms, respectively. We investigated the causes for this bipartite response that is seen in the guinea pig, as well as other mammals including humans. The two-component response could arise from different populations of afferents, or from different central circuits, or a combination of these two causes. Multiunit recording in the trigeminal ganglion and simultaneous measurement of the OOemg showed that activation of A beta afferents alone was sufficient to elicit both the R1 and the R2 responses, but that activation of A delta afferents could enhance both responses. Different neural circuits, however, produce the R1 and R2 responses. Transganglionic tracing with wheatgerm agglutin or choleragenoid subunit of cholera toxin bound to HRP revealed that primary afferents from the supraorbital branch of the trigeminal nerve terminated densely in the dorsal horn of spinal cord segment C1 and in the caudalis-interpolaris border region of the spinal trigeminal nucleus. Injections of HRP into the orbicularis oculi motoneuron region of the facial nucleus showed that both of these regions projected to the facial nucleus. Hemisections at the level of C1 eliminated the R2 blink response, but not the R1 response, evoked by stimulation of the supraorbital branch of the trigeminal nerve. Subsequent hemisections at the level of the obex eliminated the R1 response. Microinjections of the GABAB agonist baclofen into the spinal trigeminal nucleus at the level of the obex abolished the R1 but not the R2 response. Thus, the spinal trigeminal nucleus produces the R1 component, whereas the R2 component originates in the C1 region of the spinal cord.

Direct connections of primary afferent fibers with spinal neurons projecting to the facial nucleus in the cat

Spinal neurons projecting to the facial nucleus, located in the lateral portion of lamina V from the C1 to C4 segments, had long dendrites extending into lamina IV. The detailed morphology of these spinal neurons was confirmed using the Golgi impregnation method. Dorsal root fibers were distributed mainly to lamina IV at the C1 and C2 segments. These findings indicated the possibility of direct connections between the primary afferent fibers and spinal neurons projecting to the facial nucleus in lamina IV of the upper cervical cords. In order to confirm the connections, electron microscopic observation was carried out on the dorsal horn after injections of horseradish peroxidase into the facial nucleus and dorsal root fibers.

An autoradiographic study of the spinofacial projection in the monkey

The spinofacial projection was revealed using anterograde transport of radioactively labeled protein in the monkey. The projection arises from cells in the lateral part of the spinal dorsal horn (i.e. the lateral part of lamina V of Rexed) at the upper cervical cord, mainly C1 segment, ascends in the medial or ventromedial part of the anterior funiculus after crossing the anterior white commissure, then courses through the dorsolateral part of the inferior olivary complex. Finally, it terminates within the medial parts of the facial nuclei bilaterally, with the cells from the side ipsilateral to the injection contributing more heavily. Some fibers of this projection cross initially at spinal levels and recross again at levels through the rostral medulla and caudal pons.

Reticulo-facial enkephalinergic pathway in the rat: an experimental immunohistochemical study

Enkephalins have been detected in the cranial motor nuclei and may play a role in the motor system. The afferent source of leucine-enkephalin-like immunoreactive fibers in the facial nucleus was investigated using experimental immunohistochemistry. These leucine-enkephalin-containing fibers were markedly reduced on the operated side after the destruction of the ventrolateral part of the caudal medullary reticular formation, where a number of leucine-enkephalin-positive cell bodies were observed. This fact strongly suggests that these leucine-enkephalin-positive cells project ipsilaterally to the facial nucleus and so could have some effects on facial motoneurons.

A pretectofacial projection in the cat: a possible link in the visually-triggered blink reflex pathways

A direct projection from the pretectum to the facial motor nucleus was shown to exist in the cat by the anterograde and retrograde horseradish peroxidase (HRP) methods. Pretectofacial fibers arise from the olivary pretectal nucleus and end mainly in the dorsal division of the facial motor nucleus, bilaterally, with a contralateral predominance. It is known that the olivary pretectal nucleus receives retinal fibers, and that the dorsal division of the facial motor nucleus contains orbicularis oculi motoneurons. Thus, the pretectofacial fibers are assumed to cause protective lid closure with certain visual stimuli.

Brainstem projections to the facial nucleus of the opossum. A study using axonal transport techniques

The horseradish peroxidase and autoradiographic techniques have been used to determine the origin and intranuclear termination of brainstem axons projecting to the facial nucleus of the opossum and to define networks which could be utilized in some oral-facial behaviors. Two regions of the midbrain have dense projections to the facial nucleus. One region is the ventral periaqueductal gray and adjacent interstitial nucleus of the medial longitudinal fasciculus which project bilaterally to those areas of the facial nucleus supplying auricular and cervical musculature. A second is the paralemniscal zone of the caudolateral midbrain which innervates the same areas of the contralateral facial nucleus. The red nucleus and/or the adjacent tegmentum send a less dense projection to those regions of the contralateral facial nucleus which innervate buccolabial and zygomatic muscles. The dorsolateral pons (the parabrachial complex, the nucleus locus coeruleus, pars alpha, and the nucleus sensorius n. trigemini, pars dorsalis) projects densely to those areas of the ipsilateral facial nucleus which innervate buccolabial and zygomatic musculature. In contrast, the nucleus reticularis pontis, pars ventralis, projects bilaterally to parts of the facial nucleus supplying auricular and cervical muscles. There was evidence of some rostral to caudal organization in the latter projection. Neurons in medial parts of the lateral reticular formation project bilaterally to the facial nucleus. Those within the nucleus reticularis parvocellularis and the rostral nucleus reticularis medullae oblongatae ventralis innervate areas supplying buccolabial and zygomatic muscles. Neurons in the nucleus reticularis medullae oblongatae ventralis located caudal to the obex favor regions of the facial nuclei which supply auricular and cervical muscles. Neurons in the nucleus reticularis medullae oblongatae dorsalis and lamina V of the medullary and spinal dorsal horns project ipsilaterally to the facial nucleus in a manner suggesting that information from specific cutaneous areas reaches neurons supplying the muscles deep to them. The brainstem-facial connections are discussed in relation to the functionally diverse roles served by the facial nucleus in oral-facial behavior.

Development of the brain stem in the rat. II. Thymidine-radiographic study of the time of origin of neurons of the upper medulla, excluding the vestibular and auditory nuclei

Groups of pregnant rats were injected with two successive daily doses of 3H-thymidine from gestational days 12 and 13 (E12 + 13) until the day before birth (E21 + 22). In radiographs from adult progeny of these rats the proportion of neurons generated on specific days was determined in the major nuclei of the upper medulla, with the exception of the vestibular and auditory nuclei. The neurons of the motor nuclei are generated over a brief period. Neurons of the retrofacial nucleus are produced first, with more than 60% of the cells arising on day E11 or earlier. Peak generation time of abducens neurons is day E12 and of the neurons of the facial nucleus is day E13. In contrast, the neurons of the superior salivatory nucleus are produced late, predominantly on day E15 and some on day E16. The generation of the (sensory relay) neurons of the nucleus oralis of the trigeminal complex takes place over an extended period between days E12 and E15; the last generated cells include the largest neurons of this nucleus. Neurons of the raphe magnus are produced between days E11 and E14, the neurons of the rostral medullary reticular formation between days E12 and E15. The latest generated neurons of the upper medulla (excluding the cochlear nuclei) belong to a structure identified as the granular layer of the raphe. Combining these results with those of the preceding paper (Altman and Bayer, '80a) and with additional data, it is postulated that the laterally and ventrally situated motor nucleus of the trigeminal, the facial nucleus, and the nucleus ambiguous form a single longitudinal zone of branchial motor neurons with a rostral-to-caudal cytogenetic gradient. In contrast, the medially and dorsally situated (juxtaventricular) hypoglossal nucleus and abducens nucleus (together with the other nuclei of the ocular muscles) form a longitudinal somatic motor zone with a caudal-to-rostral gradient. The dorsal nucleus of the vagus and the superior salivatory nucleus may constitute a preganglionic motor zone, also with a caudal-to-rostral cytogenetic gradient.