Identification of motoneurons innervating the tensor tympani and tensor veli palatini muscles in the cat

The somatotopic arrangement of the motoneurons associated with the two non-masticatory muscles innervated by the trigeminal motor nerve, tensor tympani (TT) and tensor veli palatini (TVP), was determined in the cat using retrograde transport of horseradish peroxidase.

The motoneurons of the TT are distinct and separate, ventral and ventral-lateral to the rostral two-thirds of the trigeminal motor nucleus. The cells are smaller than those of the motor nucleus and constitute a parvocellular division. Based on functional and morphological criteria, TT motoneurons may be considered as an accessory trigeminal nucleus.

The somatotopic arrangement of TVP motoneurons has been described for the first time. These motoneurons are located in the rostral two-thirds of the ventromedial division of the cat trigeminal motor nucleus.

The location of motoneurons associated with TT and TVP does not fit the parcellation of the cat trigeminal motor nucleus as described by previous investigators. The motoneurons of these muscles can now be assigned to areas either within (TVP) or adjacent to (TT) the rostral two-thirds of the motor nucleus.     

Localization of brain stem motoneurons innervating the laryngeal muscles in the rufous horseshoe bat,rhinolophus rouxi

The motoneurons innervating the laryngeal muscles were localized in the rufous horseshoe bat,Rhinolophus rouxi, using the HRP method. HRP was applied to the cricothyroid muscle and to the cut end of the recurrent laryngeal nerve. Labeled motoneurons were found in two completely separated regions of the nucleus ambiguus. The motoneurons innervating the cricothyroid muscle via the superior laryngeal nerve (SLN) are located withinn the ventrolateral portion of the nucleus reaching the caudal pole of the motor nucleus of the facial nerve. The motoneurons innervating the other intrinsic laryngeal muscles via the recurrent laryngeal nerve (RLN) are situated in the caudal half of the nucleus ambiguus. The innervation is strictly homolateral.     

Topographic organization of facial motoneurons to individual pinna muscles in rat (Rattus rattus) and bat (Rousettus aegyptiacus)

The location and number of motoneurons to individual pinna muscles were determined by retrograde transport of horseradish peroxidase in rat and flying fox. The degree of ear mobility differs considerably between these species in that rats perform simpler ear movements while flying foxes move their pinnae in a sophisticated way. Five pinna muscles were investigated in each species. Motoneurons lay within the medial subdivision of the facial motor nucleus extending over its entire rostrocaudal length. They were topographically organized; however, a somatotopic order could not be observed. With one exception homologous pinna muscles were represented in corresponding areas in both species, supporting the idea of a common representation of ear muscles in mammals. In rat, motoneuron pools overlapped considerably, whereas in flying fox overlap was minute. A total of 1,110 and 1,646 motoneurons were labeled in rat and flying fox, respectively. We conclude that the higher number of pinna motoneurons in the latter species in addition to the more clear-cut topography provide the structural substrates that underlie differences in the quality of ear movements as seen in bats vis-a-vis other mammals.     

Shaking rat Kawasaki (SRK): a new neurological mutant rat in the Wistar strain

Summary Shaking rat Kawasaki (SRK), a newly discovered neurological mutant rat in the Wistar strain, is described. The abnormalities of SRK rats are transmitted as an autosomal recessive trait. The neurological signs are shaking of the body and an ataxic-paretic gait from day 10 postnatal. The affected rats survive for about 1 month. Macroscopically, the cerebellum is small and frequently the vermis and paraflocculus lacking. The most conspicuous histological finding in the central nervous system is malposition of the neurons in the cerebral cortex, hippocampus and cerebellum. Myelination and synapse formation are intact. Abnormal myelinated fibers are present in the molecular layer of the cerebral cortex and in the central gray matter of the spinal cord. These morphological abnormalities resemble those reported in the reeler mutant mouse. SRK rats are another good animal model of human congenital malformations with neuronal migration disorders.Supported in part by grants No. 86-11-03 and No. 86-05-41 from National Institute of Neuroscience (NCNP) of the Ministry of Health and Welfare, Japan     

Localization of motoneurons innervating deep and superficial facial muscles in the rat: a horseradish peroxidase and electrophysiologic study

In the rat, distribution of the motoneurons supplying the deep facial muscles (DFM)--the posterior belly of the digastric (VP) and the stylohyoid (SH) muscles--and the superficial facial muscles (SFM) was studied using the horseradish peroxidase (HRP) method and the antidromic field-potential method. The HRP was injected individually into the VP or SH or applied directly to the central end of the facial nerve cut immediately before it enters the parotid gland. Electrical stimulation was administered to the common stem of the branches innervating the VP or SH and to the facial nerve trunk just before entering the parotid gland. Both VP and SH motoneurons were found not in the main but in the accessory facial nucleus, within which the VP motoneurons were more numerous and more dorsorostrally extended than were the SH motoneurons. Motoneurons supplying the SFM were confined within the main facial nucleus. Evidence was found that the distribution of antidromic field potentials evoked by stimulation at the above sites coincided with the distribution of motoneurons supplying either the DFM or SFM obtained from the HRP experiment. In the rat, the accessory and main facial nuclei can be considered to be the mass of motoneurons exclusively innervating the DFM and SFM, respectively.     

Reflex connections of the facial and vagal gustatory systems in the brainstem of the bullhead catfish, Ictalurus nebulosus

The primary gustatory sensory nuclei in catfish are grossly divisible into a vagal lobe and a facial lobe. In this study, the reflex connections of each gustatory lobe were determined with horseradish peroxidase (HRP) tracing methods. In addition, in order to determine the loci and morphology of the other brainstem cranial nerve nuclei, HRP was applied to the trigeminal, facial, glossopharyngeal, or vagus nerve. The sensory fibers of the facial nerve terminate in the facial lobe. The facial lobe projects bilaterally to the posterior thalamic nucleus, superior secondary gustatory nucleus, and medial reticular formation of the rostral medulla. The facial lobe has reciprocal connections with the n. lobobulbaris, medial reticular formation of the rostral medulla, descending trigeminal nucleus, medial and lateral funicular nuclei, and the vagal lobe, ipsilaterally; and with the facial lobe contralaterally. In addition, the facial lobe receives inputs from the raphe nuclei, from a pretectal nucleus, and from perilemniscal neurons located immediately adjacent to the ascending gustatory lemniscal tract at the level of the trigeminal motor nucleus. The gustatory fibers of the vagus nerve terminate in the vagal lobe, while the general visceral sensory fibers terminate in a distinct general visceral nucleus. The vagal lobe projects ipsilaterally to the superior secondary gustatory nucleus, lateral reticular formation, and n. ambiguus; and bilaterally to the commissural nucleus of Cajal. The vagal lobe has reciprocal connections with the ipsilateral lobobulbar nucleus and facial lobe. In addition, the vagal lobe receives input from neurons of the medullary reticular formation and perilemniscal neurons of the pontine tegmentum. In summary, the facial gustatory system has connections consonant with its role as an exteroceptive system which works in correlation with trigeminal and spinal afferent systems. In contrast, the vagal gustatory system has connections (e.g., with the n. ambiguus) more appropriate to a system involved in control of swallowing. These differences in central connectivity mirror the reports on behavioral dissociation of the facial and vagal gustatory systems.     

Morphology of motoneurons in different subdivisions of the rat facial nucleus stained intracellularly with horseradish peroxidase

Horseradish peroxidase was injected into single facial motoneurons of the rat. Neurons were identified by antidromic stimulation of either the buccal or the marginal mandibular or the posterior auricular nerve branches. Motoneuronal cell bodies supplying the buccal branch were located in the lateral subdivision of the facial nucleus, those supplying the marginal mandibular branch were in the intermediate subdivision, and those supplying the posterior auricular branch were in the medial subdivision. Eleven motoneurons were reconstructed with a computer-assisted technique. Their soma diameters averaged 20 microns; the average number of primary dendrites was 7.9 and the combined lengths of the dendritic trees averaged 17,650 microns. There was no distinction between the three motoneuron groups in terms of these and other quantitative data. However, on the basis of reconstructed dendritic tree orientation (i.e., dendritic distribution), major differences were observed between motoneurons of the three groups. Dendrites from all groups extended beyond the boundaries of the facial nucleus into the reticular formation. The border between the intermediate and the lateral subdivision was crossed by some dendrites but the overlap was small. In contrast, no dendrite of a motoneuron in the medial subdivision entered the intermediate subdivision and vice versa. The dendritic extent was totally restricted by the borders between these two subdivisions. Outside the Nissl-defined nuclear border, however, dendrites from cells in adjacent subdivisions overlapped. It is concluded that the medial subdivision of the facial nucleus can be distinguished from the intermediate and lateral subdivisions not only by its sharp Nissl-defined border but also by the discrete organization of its dendritic field.     

Aberrant trajectory of entorhino-dentate axons in the mutant Shaking Rat Kawasaki: a Dil-labelling study

The Shaking Rat Kawasaki (SRK) is a neurological mutant that exhibits abnormalities of cell migration and lamination, with many similarities to the mouse reeler mutant. We recently used lamina-specific antibody staining to show that despite severe aberrations in the laminar organization of the SRK dentate gyrus, the entorhinal terminal field in the outer dentate molecular layer appeared relatively normal (Woodhams & Terashima, 1999, J. Comp. Neurol. 409 p57). However, neurofilament immunostaining suggested that entorhino-dentate afferents take an abnormal trajectory in reaching their appropriate targets, the granule cells dendrites. In the present study, anterograde tracing with the carbocyanine dye 1, 1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) has been used to delineate directly the path that entorhinal axons take to the dentate gyrus, confirming that in SRK entorhinal axons do indeed reach their appropriate terminal fields in the molecular layer, with laminar segregation between projections from the lateral and medial entorhinal cortices. However, these fibres fail to cross the hippocampal fissure between the subiculum and the dentate gyrus, coursing instead parallel to it until they curve round the deepest point of the fissure in field CA3. Similar findings were seen in the murine reeler mutant. Insertion of DiI crystals into the entorhinal cortex of neonatal rats also retrogradely labelled the developmentally transient Cajal-Retzius cells at the hippocampal fissure; these survive for longer in SRK than in normal littermates. The presence of a marked astrogliosis at the SRK hippocampal fissure may play a part in determining the abnormal trajectory taken by entorhino-dentate afferents in this mutant.     

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.     

Trigeminal projections to hypoglossal and facial motor nuclei in the rat

This study was undertaken to identify the trigeminal nuclear regions connected to the hypoglossal (XII) and facial (VII) motor nuclei in rats. Anterogradely transported tracers (biotinylated dextran amine, biocytin) were injected into the various subdivisions of the sensory trigeminal complex, and labeled fibers and terminals were searched for in the XII and VII. In a second series of experiments, injections of retrogradely transported tracers (biotinylated dextran amine, gold-horseradish peroxidase complex, fluoro-red, fluoro-green) were made into the XII and the VII, and labeled cells were searched for in the principal sensory trigeminal nucleus, and in the pars oralis, interpolaris, and caudalis of the spinal trigeminal nucleus. Trigeminohypoglossal projections were distributed throughout the ventral and dorsal region of the XII. Neurons projecting to the XII were found in all subdivisions of the sensory trigeminal complex with the greatest concentration in the dorsal part of each spinal subnucleus and exclusively in the dorsal part of the principal nucleus. Trigeminofacial projections reached all subdivisions of the VII, with a gradual decreasing density from lateral to medial cell groups. They mainly originated from the ventral part of the principal nucleus. In the spinal nucleus, most of the neurons projecting to the VII were in the dorsal part of the nucleus, but some were also found in its central and ventral parts. By using retrograde double labeling after injections of different tracers in the XII and VII on the same side, we examined whether neurons in the trigeminal complex project to both motor nuclei. These experiments demonstrate that in the spinal trigeminal nucleus, neurons located in the pars caudalis and pars interpolaris project by axon collaterals to XII and VII. J. Comp. Neurol. 415:91–104, 1999. © 1999 Wiley-Liss, Inc.     

Location,morphology, and central projections of mesencephalic trigeminal neurons innervating rat masticatory muscles studied by axonal transport of choleragenoid-horseradish peroxidase

Retrograde and transganglionic transport of horseradish peroxidase conjugated to the B-fragment of cholera toxin (B-HRP) was used to study the location, morphology, and central projections of mesencephalic trigeminal (Me5) neurons innervating rat masticatory muscles. Labeled Me5 cell bodies were found throughout the Me5 nucleus from a level slightly caudal to the trigeminal motor nucleus to the level of the superior colliculus 5 mm further rostrally. Occasionally, labeled Me5 cells were observed in the anterior medullary velum, in the cerebellum, and in the brainstem contralateral to the B-HRP injection. The vast majority of the labeled Me5 cells were pseudounipolar, but multipolar cells were also found. Extensive central projections from labeled Me5 cells could be seen extending from the nucleus of Darkschewitsch rostrally to the C2 segment caudally. Small but consistent projections from Me5 neurons were observed in nuclear islands among the incoming Me5 root fibers. Trigeminal and hypoglossal motor nuclei received direct projections from Me5 cells, but not the facial motor nucleus. The most prominent Me5 projections appeared in the brainstem reticular formation, including the supratrigeminal nucleus. Smaller projections also extended into the main sensory trigeminal nucleus, trigeminal subnucleus oralis, and the nucleus of the solitary tract. © 1993 Wiley-Liss, Inc.     

Pathway of the blink reflex in the brainstem of the cat: Interneurons between the trigeminal nuclei and the facial nucleus

Blink reflex responses evoked by electrical stimulation of the supraorbital nerve were examined using cats and the pathway of the blink reflex in the brainstem was elucidated. Both early response (ER) and late response (LR) were mediated by the main sensory trigeminal nucleus and the spinal trigeminal nucleus. However, a lesion of the main sensory trigeminal nucleus had less effect on the blink reflex than a lesion of the spinal trigeminal nucleus. The ER was mediated not only by the shorter disynaptic pathway of 3 neurons through the trigeminal nerve, the trigeminal nuclei and the facial nucleus but also by a polysynaptic pathway of 4 neurons. The interneurons were located between the trigeminal nuclei and the facial nucleus. Some of these interneurons participated in the production of both ER and LR. The area of the brainstem responsible for ER and LR of the blink reflex was the reticular formation from the rostral part of the medulla to the pons except the medial area around the median sulcus. The LR interneurons were distributed more widely than the ER interneurons.     

Selective reinnervation of somatotopically appropriate muscles after facial nerve transection and regeneration in the neonatal rat

The organization of the facial motor nucleus (FMN) has been examined after transection and regeneration of the facial nerve (FN) in neonatal and adult rats. In one series of experiments, horseradish peroxidase (HRP) was applied bilaterally to the superior or inferior buccal ramus 5 months after neonatal FN transection. In another series of experiments, wheat germ agglutinin-horseradish peroxidase conjugate was injected in selected vibrissae follicular muscles on both sides in animals surviving 5 months after FN transection at the neonatal or adult stage. The number and distribution of HRP-labeled cell bodies in the FMN after regeneration was compared with the contralateral side. On the uninjured side, labeled neurons were somatotopically organized. Ipsilateral to nerve injury the number of labeled cells was markedly reduced after neonatal nerve transection, but somatotopy was preserved. However, after nerve lesion at the adult stage, no significant loss of motoneurons occurred, but motor nucleus somatotopy was not maintained. Two alternative principal explanations are proposed for the re-establishment of the normal somatotopy after neonatal injury: that regenerating axons grow in a random fashion but inappropriate connections are subsequently eliminated or that regenerating axons of surviving neurons immediately follow a pathway leading to the appropriate muscle.     

大鼠咬肌神经切断后三叉神经运动神经元高亲和性神经营养因子受体表达的变化

目的应用双重标记技术观察咬肌神经切断对支配咬肌的三叉神经运动神经元所含高亲和性神经营养物质受体———Trk受体,即TrkA、TrkB和TrkC表达的影响。方法切断大鼠咬肌神经7和14 d后,对脑切片进行免疫组织化学染色并观察荧光金(FG)标记的三叉神经运动核(Mo5)神经元表达的三种Trk受体。结果(1)荧光金标记神经元中TrkA免疫反应阳性神经元比例无显著性变化(P>0.05);(2)神经切断后7和14 d均观察到TrkB表达上调(P<0.05);(3)神经切断后14 d观察到TrkC表达下调(P<0.05)。结论咬肌神经切断后,三叉神经运动核神经元所含不同Trk受体的表达存在差异性调节。     

Central distribution and peripheral functional properties of afferent and efferent components of the superior laryngeal nerve: Morphological and electrophysiological studies in the rat

The central distribution of the afferent and efferent components of the superior laryngeal nerve (SLN), which in the rat is ramified into the three branches of the rostral branch (R.Br), middle branch (M.Br), and caudal branch (C.Br), was examined after application of horseradish peroxidase conjugated with wheat germ agglutinin (HRP-WGA) to the proximal cut end of each branch. In addition, the afferent and efferent neural activities of each branch were recorded to investigate the functional properties. The present study provided several new findings as to the distribution of each branch and the functional properties of the SLN. The following conclusions were drawn: 1) the R.Br, containing only afferent fibers projecting to the ipsilateral lateral region of the nucleus of the solitary tract (NST), extends between slightly below the obex and the region approximately 0.6 mm rostral from the obex, and it corresponds to the interstitial subnucleus of the NST; 2) the M.Br, innervating the cricothyroid muscle, contains only efferent fibers originating ipsilaterally from the motoneurons localized within the ambiguus nucleus (Amb) and in the area ventrolateral to the Amb; and 3) the C.Br, which innervates the inferior pharyngeal constrictor muscle, contains both efferent and afferent fibers. HRP-WGA-labeled cells are distributed within both the Amb and the dorsal motor nucleus of the vagus nerve, ipsilateral to the injection site. Afferent proprioceptive fibers project to the ipsilateral interstitial subnucleus of the NST. The present results provide evidence that each branch of the SLN has distinctive functional properties and contributes to the laryngeal functions. © 1996 Wiley-Liss, Inc.     

Co-expression of calretinin and parvalbumin in the rat facial nucleus**★

BACKGROUND： Calretinin and parvalbumin are members of the intracellular calcium binding protein family, which transform Ca^2＋ bioinformation into regulation of neuronal and neural network activities. OBJECTIVE： To observe expression and co-expression of calretinin and parvalbumin in rat facial nucleus neurons . DESIGN, TIME AND SETTING： Neuronal morphology experiment was performed at the Research Laboratory of Applied Anatomy, Department Neurobiology and Anatomy, Xiangya Medical College of Central South University from August to October 2007. MATERIALS： Five healthy, adult Sprague Dawley rats were selected. Polyclonal rabbit-anti-parvalbumin and mouse-anti-calretinin were provided by Sigma, USA. METHODS： Rat brains were obtained and cut into coronal slices using a freezing microtome. Slices from the experimental group were immunofluorescent stained with polyclonal rabbit-anti-parvalbumin and mouse-anti-calretinin antibodies. The control group sections were stained with normal rabbit and mouse sera. MAIN OUTCOME MEASURES： Immunofluorescent double-staining was used to detect calretinin and parvalbumin expression. Nissl staining was utilized for facial nucleus localization and neuronal morphology analysis. RESULTS： The majority of facial motor neurons was polygon-shaped, and expressed calretinin and parvalbumin. The calretinin-immunopositive neurons also exhibited parvalbumin immunoreactivity, that is, calretinin and parvalbumin were co-expressed in the same neuron. CONCLUSION： Calretinin and parvalbumin were expressed in facial nucleus neurons, with varied distribution.     

Differential maturation of motoneurons innervating ankle flexor and extensor muscles in the neonatal rat

The first postnatal week is a critical period for the development of posture in the rat. The use of ankle extensor muscles in postural reactions increases during this period. Changes in excitability of motoneurons are probably an important factor underlying this maturation. The aim of this study was to identify whether variations in the maturation exist between motor pools innervating antagonistic muscles. Intracellular recordings in the in vitro brain stem-spinal cord preparation of neonatal rats (from postnatal day 0-5) were used to examine the developmental changes in excitability of motoneurons innervating the ankle flexors (F-MNs) and the antigravity ankle extensors (E-MNs). No significant difference in resting potential, action potential threshold, input resistance or rheobase was observed at birth. The age-related increase in rheobase was more pronounced for F-MNs than for E-MNs. The development of discharge properties of E-MNs lagged behind that of F-MNs. More F-MNs than E-MNs were able to fire repetitively in response to current injection at birth. F-MNs discharged at a higher frequency than E-MNs at all ages. Differences in the duration of action potential afterhyperpolarization accounted, at least partly, for the differences in discharge frequency between E-MNs and F-MNs at birth, and for the age-related increase in firing rate. These results suggest that E-MNs are more immature at birth than F-MNs and that there is a differential development of motoneurons innervating antagonistic muscles. This may be a critical factor in the development of posture and locomotion.     

Co-expression of calretinin and parvalbumin in the rat facial nucleus

BACKGROUND: Calretinin and parvalbumin are members of the intracellular calcium binding protein family, which transform Ca2 bioinformation into regulation of neuronal and neural network activities. OBJECTIVE: To observe expression and co-expression of calretinin and parvalbumin in rat facial nucleus neurons. DESIGN, TIME AND SETTING: Neuronal morphology experiment was performed at the Research Laboratory of Applied Anatomy, Department Neurobiology and Anatomy, Xiangya Medical College of Central South University from August to October 2007. MATERIALS: Five healthy, adult Sprague Dawley rats were selected. Polyclonal rabbit-anti-parvalbumin and mouse-anti-calretinin were provided by Sigma, USA. METHODS: Rat brains were obtained and cut into coronal slices using a freezing microtome. Slices from the experimental group were immunofluorescent stained with polyclonal rabbit-anti-parvalbumin and mouse-anti-calretinin antibodies. The control group sections were stained with normal rabbit and mouse sera. MAIN OUTCOME MEASURES: lmmunofluorescent double-staining was used to detect calretinin and parvalbumin expression. Nissi staining was utilized for facial nucleus localization and neuronal morphology analysis. RESULTS: The majority of facial motor neurons was polygon-shaped, and expressed calretinin and parvalbumin. The calretinin-immunopositive neurons also exhibited parvalbumin immunoreactivity, that is, calretinin and parvalbumin were co-expressed in the same neuron. CONCLUSION: Calretinin and parvalbumin were expressed in facial nucleus neurons, with varied distribution.     

Amygdaloid and cortical facilitation or inhibition of trigeminal motoneurons in the rat

The effects of electrical stimulation of the rat's contralateral frontal cortex as well as bilateral central amygdaloid nuclei were examined on the masseteric and mylohyoid-anterior digastric motoneurons which innervate a jaw closing muscle and jaw opening ones, respectively.Thirty-six percent of masseteric motoneurons were inhibited and 14% were facilitated by stimulation of the cortex and the central amygdaloid nucleus. Fifteen percent were inhibited or facilitated by either stimulation, although 36% were unaffected. No masseteric motoneuron was facilitated by one, stimulation and inhibited by the other.A majority of mylohyoid-digastric motoneurons were facilitated by stimulation of the cortex and the central amygdaloid nucleus although 17% were facilitated by either stimulation and 17% were unaffected. No mylohyoid-digastric motoneuron was inhibited.Therefore, the frontal cortex and the central amygdaloid nucleus have convergent control of jaw movements primarily in the opening direction.