Motor responses of the facial muscles to local stimulation of the motor cortex and facial nerve nucleus in the white mouse

The character of motor responses of the facial muscles evoked by stimulation of various regions of the frontal neocortex and of the nucleus of the facial nerve was studied in outbred mice. Motor responses of the vibrissae, of the upper lip and the jaw to monopolar microstimulation in the frontal cortical areas in 55 per cent of the cases had the latencies from 5 to 15 ms. The latencies of the responses to the facial nucleus stimulation ranged from 3 to 12 ms with maximal expressed interval of 4-6 ms. Excitation conduction velocities of the facial nerve estimated on the basis of latencies measurements, were from 1.5 to 12 m/s.     

Craniofacial sequelae of lesions to facial and trigeminal motor nuclei in growing rats

Unilateral electrolytic lesions were made in the left-side facial motor nucleus (FMNu) of six Sprague-Dawley rats at 35 days of age in order to correlate craniofacial sequelae with changed motoneuron function. Experimental and control rats were killed at 22, 32, 42, and 52 days postoperatively to provide muscle weight, brain histology, and dry skull preparations for analyses. Dissection, muscle weight, motoneuron count, and osteometric data revealed that lesion-side facial and masticatory muscles were affected by the lesions. Paired t-tests indicated that significant differences existed between weights of experimental lesion- and nonlesion-side anterior digastric, temporalis, masseteric complex, and medial pterygoid muscles, numbers of facial and trigeminal motoneurons, and several skeletal dimensions of the skull. Basi-cranial dimensions of experimental animals were least affected by the lesion, whereas zygomatic arch, dorsal facial region, and mandibular condyle dimensions were most affected. Statistical analyses also detected significant differences between experimental and control groups for several skeletal dimensions of the skull. Data indicated that damage to the trigeminal motor nucleus (TMNu) was secondary to the primary lesion in the FMNu. Motoneurons within the facial and trigeminal neuromuscular complexes (FNC and TNC) play an important role in craniofacial growth and development.     

Cells of origin of the branches of the facial nerve: a retrograde HRP study in the rabbit

The origin of different branches of the facial nerve in the rabbit was determined by using retrograde transport of HRP. Either the proximal stump of specific nerves was exposed to HRP after transection, or an injection of the tracer was made into particular muscles innervated by a branch of the facial nerve. A clear somatotopic pattern was observed. Those branches which innervate the rostral facial musculature arise from cells located in the lateral and intermediate portions of the nuclear complex. Orbital musculature is supplied by neurons in the dorsal portion of the complex, with the more rostral orbital muscles receiving input from more laterally located cells while the caudal orbital region receives innervation from more medial regions of the dorsal facial nucleus. The rostral portion of the ear also receives innervation from cells located in the dorsomedial part of the nucleus, but the caudal aspect of the ear is supplied exclusively by cells located in medial regions. The cervical platysma, the platysma of the lower jaw, and the deep muscles (i.e., digastric and stylohyoid) receive input from cells topographically arranged in the middle and ventral portions of the nuclear complex. It is proposed that the topographic relationship between the facial nucleus and branches of the facial nerve reflects the embryological derivation of the facial muscles. Those muscles that develop from the embryonic sphincter colli profundus layer are innervated by lateral and dorsomedial portions of the nuclear complex. The muscles derived from the embryonic platysma layer, including the deep musculature, receive their input from mid to ventral regions of the nuclear complex.     

Face to face with the social brain

Recent comparative evidence suggests that anthropoid primates are the only vertebrates to exhibit a quantitative relationship between relative brain size and social group size. In this paper, I attempt to explain this pattern with regard to facial expressivity and social bonding. I hypothesize that facial motor control increases as a secondary consequence of neocortical expansion owing to cortical innervation of the facial motor nucleus. This is supported by new analyses demonstrating correlated evolution between relative neocortex size and relative facial nucleus size. I also hypothesize that increased facial motor control correlates with enhanced emotional expressivity, which provides the opportunity for individuals to better gauge the trustworthiness of group members. This is supported by previous evidence from human psychology, as well as new analyses demonstrating a positive relationship between allogrooming and facial nucleus volume. I suggest new approaches to the study of primate facial expressivity in light of these hypotheses.     

The somatotopy of the masticatory neurons in the rat trigeminal motor nucleus as revealed by HRP study

Young adult albino rats of Wistar strain were used for the present study. 0.5 to 15 microliters of 20-50% of horseradish peroxidase (HRP) were injected into each individual muscle of mastication to label neurons in the trigeminal motor nucleus (TMON) for light microscopic study. The results reveal that: (1) Many HRP-labeled, multipolar neurons are observed in the motor nucleus in each jaw-closing muscle (JCM) with less in each the jaw-opening muscle (JOM). (2) The motor neurons innervating each masticatory muscle in the motor nucleus show a somatotopic arrangement: (a) those innervating the temporalis muscle are located in the medial and dorsomedial parts; (b) those innervating the masseter muscle are located in the intermediate and lateral; (c) those innervating the medial and lateral pterygoid muscles are located in the lateral, ventrolateral and ventromedial parts, respectively; and (d) those innervating the mylohyoid and the anterior belly of the digastric muscles are located in the most ventromedial part of the caudal one-third of the nucleus. Axons of most masticatory motor neurons run ventrolaterally in between the motor and the chief sensory nuclei of the trigeminal nerve. However, those of the mylohyoid and anterior belly of the digastric muscles ascend dorsally to the dorsal aspect of the caudal nucleus and then turn ventrolaterally to join the motor root of the trigeminal nerve. Furthermore, the dendrites of the motor neuron of JCM converge dorsocaudally to the supratrigeminal region. The diameters of neurons of each JCM display a bimodal distribution. However, an unimodal distribution is present in the motor neurons from each JCM. It is suggested that the motor nucleus innervating the JCM is comprised of comprised of alpha- and gamma-motor neurons. It, thus, may provide a neural basis for the regulation of the muscle tone and biting force.     

Topographical representation of the jaw muscles within the trigeminal motor nucleus. An HRP study in the mallard, Anas platyrhynchos

The location of the trigeminal motoneurons of the jaw muscles has been determined in the brainstem of the mallard utilizing retrograde axonal transport of horseradish peroxidase (HRP). Injections with HRP into the jaw muscles or application of HRP to the mandibular nerve showed that the trigeminal motor nucleus can be subdivided into five subnuclei, mV1-mV5. Three functional groups of jaw muscles are represented in separate subnuclei. The most lateral subnucleus mV2 innervates all but one adductor muscles, the intermediate mV1 innervates the pterygoid muscles + one adductor and the medial mV4 the two protractor muscles. The most ventral subnucleus mV3 contains the neurons innervating two extrinsic tongue muscles as well as some perikarya of adductor muscles. Subnucleus mV5 lies dorsomedial to mV4 and contains the motoneurons of the depressor muscle of the lower eye lid. Elements of the proprioceptive system, viz. presumptive gamma-neurons and mesencephalic trigeminal nucleus cells, could also be visualized. The topological and functional aspects of the subdivision of the motor nucleus are discussed.     

Oro-facial muscles: internal structure,function and ageing

Structure and function are reviewed in the masticatory muscles and in the muscles of the lower face and tongue. The enormous strength of jaw closure is in large part due to the pinnated arrangement of the muscle fibres in the masseter. This muscle, like other masticatory muscles, is unusual in that the cell bodies of the muscle spindle afferents lie in the brain stem rather than in an external ganglion; spindles are absent in the lower facial muscles. Although few data are available, the numbers of motor units in the masticatory muscles, and probably in the lower facial muscles also, appear to he much greater than in limb muscles. The motor units in the facial and tongue muscles are largely composed of histochemical type II (‘fast-twitch’) fibres, but in the masticatory muscles there are substantial numbers of fibres intermediate between type I (‘slow twitch’) and type II, and fibre type grouping is present. In comparison with limb muscles, there is little information on ageing changes in oro-facial muscles. The masticatory muscles do, however, show some atrophy and loss of X-ray density, while motor unit twitches are prolonged. Strength is reduced in the tongue and masticatory muscles. It is known that limb muscle properties are largely governed by their innervation, both through the pattern and amount of impulse activity, and the delivery of trophic messengers; the situation for oro-facial muscles is unclear. The structural and functional differences between the two types of muscle indicate the need for conducting ageing studies on the oro-facial muscles, rather than relying on extrapolations from limb muscles.     

The facial motor nucleus of the dog. II. Morphometric analysis

In this study, an area of 953.2 micron 2, a diameter of 36.7 micron and a circularity factor of 0.74 have been established for the neurons of the facial motor nucleus of the dog. Significant differences (p less than 0.01) were observed by comparing the means of some of the parameters determined in the six cytoarchitectonic subdivisions of the facial motor nucleus described in a previous study. Moreover, the neurons tend to increase in size in the caudocranial direction.     

Afferent connections of cat facial nucleus; a study using retrograde axonal transport of horseradish peroxidase

Neuronal populations in the brainstem and spinal cord — the sources of fiber pathways to the facial nucleus — were investigated in adult cats by microiontophoretically injecting horseradish peroxidase into restricted areas of the facial nucleus. Projections were identified from thenucleus nervi hypoglossi, nucleus praepositus hypoglossi, nucleus raphe pallidus, nucleus intercalatus, medial nucleus of the solitary tract, dorsal motor nucleus of the vagus, neurons of genu of the facial nerve, ipsilateral red nucleus, and reticular formation of the midbrain to the facial nucleus. Projections from a number of other brain structures to the facial nucleus also received confirmation. A topographic map was drawn up, showing how brainstem and spinal cord afferents are distributed in the facial nucleus.L. A. Orbeli Institute of Physiology, Academy of Sciences of the Armenian SSR, Erevan. Translated from Neirofiziologiya, Vol. 18, No. 1, pp. 35–45, January–February, 1986.     

Androgen receptors in cranial nerve motor nuclei of male and female rats

This study compared AR proteins in four cranial nerve motor nuclei among male and female rats that were intact, gonadectomized, or gonadectomized and given TP by immunohistochemistry. AR-immunoreactive (ir) neurons were found, in descending order of abundance, in the nucleus ambiguus, hypoglossal nucleus, and the facial and trigeminal motor nuclei of both males and females of intact and gonadectomized plus TP rats. Virtually every neuron of the nucleus ambiguus was AR-ir. In contrast, AR-ir neurons were either restricted to a specific area of the hypoglossal nucleus, or randomly distributed in the facial and trigeminal motor nuclei. The predominant AR-ir site shifted from cell nuclei to the cytoplasm, depending upon the presence or absence of ligand. Sex differences in the amount and staining intensity of AR-ir neurons were discernable in all four motor nuclei of intact rats, and these differences were maintained in gonadectomized plus TP rats, with the exception of the nucleus ambiguus. The immunostaining results were complemented by results from AR binding studies. Cytosolic AR binding values for the hypoglossal and facial motor nuclei of females were only approximately 50% of those of males despite the absence of a sex difference in neuron number. These results indicate that intrinsic sex differences in AR levels and androgenic regulation of AR exist in cranial nerve motor nuclei, and that there are differences in the abundance and distribution pattern of AR responsive neurons in cranial nerve motor nuclei. These results are consistent with the idea that sex differences in AR could account for sex differences observed in nerve regeneration and neuron loss following cranial nerve injury.     

Evolution of the brainstem orofacial motor system in primates: a comparative study of trigeminal, facial, and hypoglossal nuclei

The trigeminal motor (Vmo), facial (VII), and hypoglossal (XII) nuclei of the brainstem comprise the final common output for neural control of most orofacial muscles. Hence, these cranial motor nuclei are involved in the production of adaptive behaviors such as feeding, facial expression, and vocalization. We measured the volume and Grey Level Index (GLI) of Vmo, VII, and XII in 47 species of primates and examined these nuclei for scaling patterns and phylogenetic specializations. Allometric regression, using medulla volume as an independent variable, did not reveal a significant difference between strepsirrhines and haplorhines in the scaling of Vmo volume. In addition, correlation analysis using independent contrasts did not find a relationship between Vmo size or GLI and the percent of leaves in the diet. The scaling trajectory of VII volume, in contrast, differed significantly between suborders. Great ape and human VII volumes, furthermore, were significantly larger than predicted by the haplorhine regression. Enlargement of VII in these taxa may reflect increased differentiation of the facial muscles of expression and greater utilization of the visual channel in social communication. The independent contrasts of VII volume and GLI, however, were not correlated with social group size. To examine whether the human hypoglossal motor system is specialized to control the tongue for speech, we tested human XII volume and GLI for departures from nonhuman haplorhine prediction lines. Although human XII volumes were observed above the regression line, they did not exceed prediction intervals. Of note, orang-utan XII volumes had greater residuals than humans. Human XII GLI values also did not differ from allometric prediction. In sum, these findings indicate that the cranial orofacial motor nuclei evince a mosaic of phylogenetic specializations for innervation of the facial muscles of expression in the context of a generally conservative scaling relationship with respect to medulla size.     

Afferent and efferent components of the facial nerve in the bullfrog (Rana catesbeiana).

The afferent and efferent components of the facial nerve were traced within the brain stem of Rana catesbeiana, using three different neuroanatomical techniques. Primary afferent fibers could be traced to the spinal tract of trigeminal nerve and to fasciculus solitarius as far caudally as the first or second spinal segment, using silver degeneration methods. Cobalt filling of of the entire nerve showed the same distribution of afferent fibers, as well as the filling of the cells within the mesencephalic nucleus of trigeminal, indicating the origin of a proprioceptive component of the facial nerve. Cobalt iontophoresis and horseradish perioxidase experiments showed that the motor nucleus of the facial nerve was located just ventral to the fourth ventricle, and caudal to the motor nucleus of trigeminal. The distribution of afferent fibers to fasciculus solitarius and the spinal tract of trigeminal is similar in some respects to the distribution of afferent fibers from the trigeminal and vagal nerves in the bullfrog. The afferent fibers from the three cranial nerves are found as far caudally in the brain stem as the second spinal segment.     

Neuronal mechanisms of motor signal transmission in the thalamic ventrooral nucleus in spasmodic torticollis patients

A microelectrode technique was used to study the neuronal mechanisms of motor signal transmission in the ventrooral internus nucleus (Voi) of the motor thalamus during voluntary and involuntary pathological (dystonic) movements in patients with spasmodic torticollis. Voi cell elements proved highly reactive to various functional (mostly motor) tests. An activity analysis of 55 Voi neurons detected during nine stereotactic operations revealed, first, a difference in neuronal mechanisms of motor signal transmission for voluntary movements that do or do not involve the affected axial muscles of the neck and for passive and abnormal involuntary dystonic movements. Second, a sensory component was found to play a key role in the mechanisms of sensorimotor interactions during voluntary and involuntary dystonic head and neck movements activating the axial muscles of the neck. Third, rhythmic and synchronized activity of Voi neurons was shown to play an important role in motor signal transmission during voluntary and passive movements. The Voi nucleus was directly implicated in the mechanisms of involuntary head movements and tension of the neck muscles in spasmodic torticollis. The results can be used to identify the Voi nucleus of the thalamus during stereotactic neurosurgery in order to select the optimal destruction or stimulation target and to reduce the postoperative effects in spasmodic torticollis patients.     

Reciprocal connections between the red nucleus and the trigeminal nuclei: a retrograde and anterograde tracing study.

An anterograde biocytin and a retrograde WGA-colloidal gold study in the rat can provide information about reciprocal communication pathways between the red nucleus and the trigeminal sensory complex. No terminals were found within the trigeminal motor nucleus, in contrast with the facial motor nucleus. A dense terminal field was observed in the parvicellular reticular formation ventrally to the trigeminal motor nucleus. The parvicellular area may be important for the control of jaw movements by rubrotrigeminal inputs. On the other hand, the contralateral rostral parvicellular part of the red nucleus receives terminals from the same zone in the rostral part of the trigeminal sensory complex, where retrogradely labelled neurones were found after tracer injections into the red nucleus. Such relationships could be part of a control loop for somatosensory information from the orofacial area.     

Bilateral brainstem connections of the rat supratrigeminal region

Efferent and afferent connections of the supratrigeminal region were studied in the rat using iontophoretically delivered horseradish peroxidase and Phaseolus vulgaris leuco-agglutinin. Projections of supratrigeminal efferents were found to the contralateral supratrigeminal region, to the ipsi- and contralateral trigeminal motor nuclei and the medullary reticular formation, and to the ipsilateral facial and hypoglossal motor nuclei. Neurons projecting to the supratrigeminal region were located in the contralateral supratrigeminal nucleus, in the ipsilateral mesencephalic trigeminal nucleus and bilaterally in the medullary reticular formation. This organization is discussed with respect to bilateral oral motor control mechanisms.     

Effects of transections and electrical coagulations in the medulla oblongata upon the activities in the respiratory muscles of the crucian carp (author's transl)]

The following conclusions may be drawn from the results in this work. The respiratory cycles are formed by the neuronal machinery in the reticular formation under the posterior part of the vagal motor nucleus. The motor neurones or the neuronal networks composing the motor nucleus of the respiratory muscles tonically discharge the action potentials, when the neurones or the networks are released from the inhibitory influences of the interneurones connecting the neuronal machinery to the motor neurones. Furthermore, the interneurones probably generate the tonic discharges after removing the inhibitory influences of the other interneurones or the neuronal machinery on them. A reflex mouth closing is elicited by a mechanical stimulus applying on the upper lip. The motor neurones of the m. adductor mandibulae are activated via only one synapse in the reflex. The reflex action potentials recorded from the motor nerve reduce in amplitude at the resting phase of the nerve in the respiratory cycles. These results suggest that the respiratory motor neurones are by nature spontaneous generators of the tonic action potentials and, in the time of the normal breathing, the tonic activity is interrupted by an inhibitory influence of the neuronal machinery generating the respiratory cycles.     

Intramedullary projections of the rostral nucleus of the solitary tract in the rat: gustatory influences on autonomic output

The efferent connections of the rostral nucleus of the solitary tract (NTS) in the rat were studied by anterograde transport of Phaseolus vulgaris leucoagglutinin. Rostral to the injection site, fibers travel through the rostral parvocellular reticular formation and deflect medially or laterally around the motor trigeminal nucleus, giving off few terminals in these nuclei and terminate in the parabrachial nucleus. Moderate projections to the peritrigeminal zone, including the intertrigeminal nucleus and the dorsal subcoeruleus nucleus, were observed. Caudally to the injection site, dense innervations from the rostral nucleus of the solitary tract were detected in the parvocellular reticular formation ventral and caudal to the injection site and in the intermediate and ventral medullary reticular formation. The rostral central and ventral subdivisions of the NTS up to the level where the nucleus of the solitary tract abuts the fourth ventricle and the hypoglossal nucleus, receive moderate input from the rostral nucleus of the solitary tract. In general, the projections from the rostral nucleus of the solitary tract were bilateral with an ipsilateral predominance. The caudal part of the nucleus of the solitary tract, the dorsal motor nucleus of the vagus and the facial nucleus were not labeled. It is concluded that medullary rNTS projections participate in oral motor behavior and autonomic control of abdominal organs.     

Estrogen receptor expression in the facial nucleus of adult hamsters: does axotomy recapitulate development?

Testosterone propionate (TP) augments hamster facial motoneuron regeneration following axonal injury by an androgen-mediated mechanism. Although many of the trophic properties of TP are androgenic, TP can be metabolized to estradiol (E). We have recently shown that E administered in supraphysiological doses can also enhance facial nerve regeneration. The mechanism by which E alters nerve regeneration is unknown. The recent discovery of transient estrogen receptor (ER) expression in the developing rat facial motor nucleus (FMN), coupled with the concept that regeneration may recapitulate development, has led to the hypothesis that facial nerve injury may transiently induce expression of ER in the adult hamster FMN or one of its chief afferents, the principal nucleus of the trigeminal nerve (Nu5). In the present study, this hypothesis was tested using steroid hormone autoradiographic procedures. The right facial nerve was injured in castrated or castrated plus TP adult hamsters. A gonadally intact, nonaxtomized group of hamsters was also included to examine constitutive expression of ER in the FMN or Nu5. The paraventricular nucleus of the hypothalamus (PVN; positive control), FMN, and Nu5, were qualitatively and quantitatively examined for the presence of ER. As expected, ER were present in the PVN-positive control in all groups. ER were neither present nor induced with facial nerve injury or TP administration in either the FMN or Nu5. Alternate mechanisms by which E enhancement of facial nerve regeneration without ER might be explained are discussed.     

Facial motor responses to microstimulation of the superior colliculi in the white mouse

Facial motor responses to microstimulation of different zones of the superior colliculi have been investigated in the albino mice craniotomized under thiopental anaesthesia. Local responses of the mystacial vibrissae, upper lip and eyelids were initiated by microstimulation of the rostral parts of the inner layers of the colliculus superior (high-frequency volleys of 5-7 pulses with a current limit of 35 microA). Sequential changes in the pattern of facial responses were observed within microelectrode traces indicating vertical orientation of facial motor representations in the superior colliculus. Some differences in the localization and pattern of facial responses in the right and left superior colliculi were revealed: 1) vibrissae and lip representations in the right superior colliculus occupy more extensive zone (vertical distribution from 300 to 2,300 microns) as compared to those in the left one (700-2,000 microns); 2) microstimulations of the right superior colliculus produce both uni- and bilateral vibrissal motor responses, whereas stimulation of the left superior colliculus evokes only unilateral responses. The duration of the latent period of the vibrissal and lip motor responses to stimulation of the right superior colliculus varied from 10 to 26 ms (16.1 +/- 2.4 ms; n = 199), to stimulation of the left one-from 10 to 18 ms (mean 14.9 +/- 1.8 ms; n = 55). It is suggested that polysynaptic motor responses to microstimulation of the superior colliculi are realized via the reticular and other premotor nuclei of the brain stem which have direct inputs from the superior colliculus and direct projections to the facial motor nucleus.