Trigeminal and spinal inputs to the facial nucleus

Modulation of reflex responses of the anterior and posterior bellies of the digastric muscle in freely moving rats

BACKGROUND

Chewing and licking are primarily activated by central pattern generator (CPG) neuronal circuits in the brainstem and when activated trigger repetitive rhythmic orofacial movements such as chewing, licking and swallowing. These CPGs are reported to modulate orofacial reflex responses in functions such as chewing.

OBJECTIVE

This study explored the modulation of reflex responses in the anterior and posterior bellies (ant-Dig and post-Dig, respectively) of the digastric muscle evoked by low-intensity trigeminal stimulation in conscious rats.

METHODS

The ant-Dig and post-Dig reflexes were evoked by using low-intensity electrical stimulation applied to either the right or left inferior alveolar nerve. Peak-to-peak amplitudes and onset latencies were measured.

RESULTS

No difference was observed between threshold and onset latency for evoking ant-Dig and post-Dig reflexes, suggesting that the latter was also evoked disynaptically. The peak-to-peak amplitude of both reflexes was significantly reduced during chewing, licking and swallowing as compared to resting period and was lowest during the jaw-closing phase of chewing and licking. Onset latency was significantly largest during the jaw-closing phase. Inhibitory level was similar between the ant-Dig and post-Dig reflex responses and between the ipsilateral and contralateral sides.

CONCLUSION

These results suggest that both the ant-Dig and post-Dig reflex responses were significantly inhibited, probably due to CPG activation during feeding behaviours to maintain coordination of jaw and hyoid movements and hence ensure smooth feeding mechanics.

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.

Tactile dysgeusia: a new clinical observation of middle ear and skull base surgery

OBJECTIVE

To describe a new clinical observation of "tactile dysgeusia," a phenomenon associated with otologic and skull base surgery likely caused by injury to the chorda tympani nerve (CTN) or the nervus intermedius (NI) with subsequent aberrant cross-innervation with somatosensory fibers in the surgical field.

STUDY DESIGN

Descriptive case series.

SETTING

Tertiary university referral center.

METHOD

Eight patients described a sensation of touch-evoked dysgeusia after surgery. Seven patients underwent a variety of middle ear procedures including stapedotomies and mastoidectomies. One patient had a large cerebellopontine angle meningioma excised through a translabyrinthine approach. There were no preoperatively sensory symptoms of any kind.

RESULTS

In five of seven patients who had an otologic procedure, the CTN nerve was transected, whereas in the remaining two, the nerve was preserved but stretched. In the meningioma patient, the facial nerve was preserved without clear identification of the NI; the facial nerve function was normal postoperatively. All of the otologic patients experienced symptoms of dysgeusia and sensory alteration triggered by touching various parts of the outer ear. One also has a secondary trigger in the V2 dermatome. In the meningioma patient, taste and sensory alteration was induced by stimulating the ipsilateral V2 and V3 dermatome. In all patients, the symptom was located in the lateral aspect of the ipsilateral hemitongue.

CONCLUSION

Tactile dysgeusia is an unusual complication after otologic and neurotologic procedures. It is likely related to aberrant re-innervation of the special sensory fibers within the CTN nerve or the NI with somatosensory fibers. Postoperative "tactile dysgeusia" may not be so rare; its true incidence could only be ascertained through a more rigorous postoperative assessment.

Does the chorda tympani nerve confer general sensation from the tongue?

OBJECTIVE

Patients often complain of numbness of the tongue after otologic procedures; this symptom may occur with or without taste alterations. The purpose of this study was to objectively assess possible changes in the general sensation of the tongue in patients undergoing middle ear surgery.

STUDY DESIGN AND SETTING

Thirty-three individuals were included in this prospective controlled clinical study conducted at a tertiary referral center. Fifteen patients underwent middle ear surgery and 18 volunteers served as a control group. Subjects underwent objective tests to assess general sensation of the tongue before and twice after surgery.

RESULTS

Forty-seven percent of the patients complained of numbness or tingling of the tongue shortly after surgery. In these patients, a significant reduction in sensitivities to light touch and 2-point discrimination on the operated side were noted at that time (P < 0.01; P < 0.009 respectively). Values returned to baseline on subsequent evaluations. No sensitivity change was noted for the contralateral hemi-tongue.

CONCLUSION

Objective changes in the general sensory function of the tongue correlated with post-operative clinical symptoms experienced by patients. The results of this study support the notion that the chorda tympani nerve confers general sensation from the tongue.

EBM RATING

A-1b.

The Effects of Intensive Voice Treatment on Facial Expressiveness in Parkinson Disease

OBJECTIVE

The purpose of the present retrospective study was to examine the effects of intensive voice therapy on facial expression in Parkinson disease.

BACKGROUND

Parkinson disease (PD) often presents with symptoms that reduce communicative effectiveness on multiple levels, including decreased vocal loudness and reduced facial mobility. Recent advances in voice treatment have provided the first short- and long-term efficacy data indicating improvements in voice and speech following intensive voice therapy (Lee Silverman Voice Treatment [LSVT]). Anecdotal reports from both clinicians and patients indicate that the LSVT also has a positive impact on facial expression. These observations suggest a need to investigate more directly the effects of voice therapy on facial movement and expressiveness in PD.

METHOD

Forty-four individuals with idiopathic PD participated in this study. Video data were taken from recordings of individuals with PD who had received either one month of phonation-based treatment (LSVT) or respiratory treatment (RT) as part of a large treatment efficacy study designed to examine the effects of different types of therapy on speech and voice in PD. Twenty-second video samples of all subjects taken before and after treatment were paired and played at random without sound to trained raters, who judged each pair of video clips for facial mobility and engagement. All recordings were made while subjects were engaged in conversational speech.

RESULTS

Inter-rater reliability was extremely high (0.90) for both the rating of facial mobility and engagement. Overall, members of the LSVT group received more ratings of increased facial mobility (P = 0.036) and engagement (P = 0.056) following treatment relative to members of the RT group. In addition, the extent of change for facial mobility after treatment was perceived as greater (P = 0.05) for the LSVT group than for the RT group.

CONCLUSIONS

These results indicate that intensive voice therapy may have a positive effect on facial expressivity in PD. Such findings lend support to contemporary theories relating multiple expressive modalities (e.g., voice, face, and gesture) and suggest that targeting voice may be an effective and efficient way to influence expressive output in general.

Somatosensory evoked blink response: findings in patients with Miller Fisher syndrome and in normal subjects

Reflex blinking was elicited by the electrical stimulation of peripheral nerves and various parts of the body of seven of 11 patients with Miller Fisher syndrome. This reflex blinking disappeared during recovery. Reflex blinking was elicited in normal subjects only with specific stimulation of the peripheral nerves of their upper extremities. This response may be due to a release phenomenon transmitted via the brainstem reticular formation and may be useful in detecting latent CNS involvement in patients with Miller Fisher syndrome.

Orbicularis oculi responses to stimulation of nerve afferents from upper and lower limbs in normal humans

A brief mechanical or electrical stimulus to peripheral nerve afferents from the upper and lower limbs elicited a small and inconsistent EMG response of the orbicularis oculi muscles. This response was facilitated when the stimuli were delivered at fixed leading time intervals, of 45-300 ms, with respect to a supraorbital nerve electrical stimulus. Also, the peripheral nerve stimulus modified the conventional blink reflex responses, inducing facilitation of R1 and inhibition of R2. These results suggest a complex processing of sensory inputs from the face and the limbs at the brainstem, where they are probably integrated in a network of interneurons influencing the excitability of facial motoneurons.

Facial sensibility in patients with unilateral facial nerve paresis

This study evaluated facial sensibility in 29 patients with unilateral lower motoneuron facial nerve paresis using standard clinical tests of sensory evaluation used at other anatomic sites, most commonly the hand. Vibratory and cutaneous pressure thresholds and moving and static two-point discrimination were measured. Statistically significant differences were found between the affected and unaffected sides of the face, with vibration threshold, cutaneous pressure threshold, and static two-point discrimination being greater on the affected side. Vibration thresholds and two-point discrimination (moving and static) progressively decreased, moving down the face from the forehead to the cheek, chin, and then the lip. Sensibility thresholds are altered in patients with unilateral lower motor neuron facial nerve paresis. These findings document a relationship between sensory disturbance and lower motoneuron facial nerve paresis. The potential functional significance of this relationship has clinical significance for patients undergoing rehabilitation training.

Contralateral early blink reflex in patients with facial nerve palsy: indication for synaptic reorganization in the facial nucleus during regeneration

Fifty patients with Bell's palsy and 30 patients with etiologically different symptomatic peripheral facial nerve palsy were studied by means of electrically evoked blink reflexes 1-23 days after onset of paresis. Their results were compared with a normal control group of 30 healthy subjects. In a significant number of patients (64% in Bell's palsy and 53% in symptomatic facial nerve palsy) a contralateral early blink reflex response (R1) could be elicited upon stimulation of the normal side as compared to 13% in the control group. It is suggested that this result may be explained by synaptic reorganization of the facial nucleus leading to functional unmasking of pre-existing crossed trigemino-facial reflex pathways during regeneration. This view is in line with previous experimental data in animals on the time course of structural changes in the facial nucleus after lesioning of the ipsilateral facial nerve.

Descending brainstem projections of the pedunculopontine tegmental nucleus in the rat

Descending brainstem projections from the pedunculopontine tegmental nucleus (PPN) were studied in the rat by use of the anterograde tracer Phaseolus vulgaris-leucoagglutinin (PHA-L) and the retrograde tracer lectin-conjugated horseradish peroxidase (HRP-WGA). Results of these experiments demonstrated prominent bilateral projections to the pontomedullary reticular nuclei, but direct connections to the motor and sensory nuclei of the cranial nerves could not be ascertained. The PPN fibers terminated mainly in the pontine reticular nuclei oralis and caudalis and in ventromedial portions (pars alpha and pars ventralis) of the gigantocellular reticular nucleus. A smaller number of labeled fibers distributed to more dorsal regions of the gigantocellular nucleus, lateral para-gigantocellular, ventral reticular nucleus of the medulla and lateral reticular nucleus. Although a significant number of PHA-L labeled fibers was seen in two cases in the contralateral medial portion of the facial nucleus, and all cases exhibited a sparse predominantly ipsilateral projection to the lateral facial motor neurons, the retrograde tracing experiments have revealed that these facial afferents originated in the nuclei surrounding the PPN. The results are discussed in the context of PPN involvement in motor functions. It is suggested that the PPN may participate in a complex network involved in the orienting reflex.

The cranial nerve vascular compression syndrome: II. A review of pathophysiology

The various hypotheses regarding the pathophysiologies of trigeminal neuralgia and hemifacial spasm are reviewed, and the results of recent physiological studies on the pathogenesis of hemifacial spasm are discussed. Evidence is presented that strongly supports the hypothesis that the symptoms and signs of hemifacial spasm are caused by hyperactivity in the facial motonucleus. Some of the contradictions regarding the prevalence of vascular conflicts in the cerebellopontine angle and the symptoms of vascular compression are discussed, and a hypothesis is presented that assumes that a suitable substrate must be present, in addition to vascular compression of the respective cranial nerve root, for the symptoms and signs of a cranial nerve vascular compression disorder to develop. Finally, it is discussed how this hypothesis can explain some of the differences between the disorders that can be cured by microvascular decompression of respective cranial nerves.

Interaction between the blink reflex and the abnormal muscle response in patients with hemifacial spasm: results of intraoperative recordings

Patients with hemifacial spasm (HFS) have an abnormal muscle response (AMR) that can be elicited by stimulating one branch of the facial nerve and recording electromyographically from muscles innervated by other branches of the facial nerve. In addition, the R1 component of the blink reflex can be elicited from the affected side in patients with HFS who are undergoing microvascular decompression (MVD) operations under inhalation anesthesia. A synkinetic component of the blink reflex response that corresponds to the R1 component can be recorded from the mentalis muscle. In the present study we show that the blink reflex elicited by electrical stimulation of the supraorbital nerve can suppress the AMR elicited by electrical stimulation of the temporal branch of the facial nerve in patients with HFS when the interval between stimulation of the supraorbital nerve and stimulation of the temporal branch of the facial nerve (interstimulus interval, ISI) is such that the blink reflex response would appear later than the AMR if they had been elicited independently. Within a short range of ISIs the two responses suppress each other partially or totally. We find evidence that the suppression of the AMR is the result of an interaction in the facial motonucleus. We believe that the results of the present study support the hypothesis that the facial motonucleus is hyperactive in patients with HFS, and we suggest that the AMR is a result of backfiring from the facial motonucleus and that it may thus be an exaggerated F-response.

Electrical properties of facial motoneurons in brainstem slices from guinea pig

Electrical properties of guinea pig facial motoneurons (FMNs) were studied in a brainstem slice preparation. FMNs were identified histologically and by antidromic activation. They displayed time-varying responses and inward rectification during both subthreshold depolarization and hyperpolarization. The depolarizing rectification was caused by a persistent Na+ current (INaP); the Cs+-sensitive hyperpolarizing response had a different mechanism. Hyperpolarizing prepulses caused a 4-aminopyridine-sensitive delay of spike initiation. An evoked spike was followed by a fast- and a medium-duration hyperpolarization (the fAHP and mAHP, respectively). Blockade of Ca2+ influx abolished the mAHP without affecting spike duration, whereas spikes were prolonged and the fAHP was abolished by TEA or 4-AP. Adequate depolarization evoked tonic repetitive firing characterized by a steep F-I slope and fast adaptation. Abolition of the mAHP was associated with reduced fast adaptation and increased F-I slope, whereas the mAHP was enhanced and firing rate was slowed after TEA application. Three outward ionic currents were identified during voltage clamp: a rapidly inactivating current, a slowly inactivating current and a slow persistent Ca2+-mediated current (IK(Ca]. We conclude that spike repolarization and the fAHP are governed mainly by fast voltage-dependent currents, whereas progressive activation of IK(Ca) causes fast adaptation and, together with INaP, regulates firing rate.

Physiologic and anatomic basis for contralateral R1 in blink reflex

We studied the rate of appearance and mechanism of contralateral R1 responses in normal subjects. Contralateral R1 could be produced by facilitating maneuvers such as a gentle contraction of the orbicularis oculi and conditioning stimulus of the median nerve. In addition, changing the position of the stimulating anode to the midline evoked these responses that were abolished by blocking the contralateral supraorbital nerve, confirming its peripheral origin. We conclude that crossed trigeminofacial pathways probably exist in normal subjects, but in some instances contralateral peripheral trigeminal ophthalmic sensory fibers may be stimulated, giving rise to a contralateral R1 response.

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.

Difference in projections to the lateral and medial facial nucleus: anatomically separate pathways for rhythmical vibrissa movement in rats

The present paper demonstrates that the lateral and medial subdivisions of the rat facial motor nucleus (NVII) receive differing mesencephalic and metencephalic projections. In order to study brain projections to facial nucleus, horseradish peroxidase (HRP) was injected iontophoretically into the entire facial nucleus or the following subdivisions: lateral, dorsolateral, medial, intermediate, and ventral. In the mesencephalic region, the retrorubral nucleus was found to project to the contralateral medial subdivision of NVII, while the red nucleus was found to project to the contralateral lateral subdivision of NVII. Other mesencephalic projections to the facial nucleus arose from the deep mesencephalic nucleus, oculomotor nucleus, central gray including interstitial nucleus of Cajal and nucleus Darkschewitsch, superior colliculus and substantia nigra (reticular). In the mesencephalic region, the Kölliker-Fuse nucleus, parabrachial nucleus, and the ventral nucleus of the lateral lemniscus projected mainly to the ipsilateral lateral subdivision of NVII. In addition, the trapezoid, pontine reticular, vestibular, and motor trigeminal nuclei were observed to have predominantly ipsilateral connections to the facial nucleus. In contrast, projections from the myelencephalic region were to both the lateral and medial subdivision of NVII. The medullary reticular nucleus, ambiguus nucleus, spinal trigeminal nucleus and parvocellular reticular nucleus projected to both lateral and medial subdivisions of NVII with an ipsilateral predominance. The gigantocellular and paragigantocellular reticular nuclei, raphe magnus, external cuneate nucleus and the nucleus of the solitary tract also projected to the facial motor nucleus. Surprisingly, no direct projections to the NVII were observed from diencephalic and telencephalic regions. Our findings that the lateral subdivision of NVII which innervates vibrissa-pad-muscles (Dom et al. 1973; Martin and Lodge 1977; Watson et al. 1982) receives different metencephalic and mesencephalic projections than medial subdivision which controls pinna movement (Henkel and Edwards 1978), suggest that the functional difference between these subdivisions is mediated by the anatomically separate pathways. We confirmed our anatomical findings by eliciting exclusively vibrissa responses by electrical stimulation of the nuclei which project to the lateral subdivision of NVII.

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.

The facial nucleus of the rat: representation of facial muscles revealed by retrograde transport of horseradish peroxidase

The representation of facial muscle groups in the facial nucleus of rat was examined by retrograde transport of HRP. Motoneurons supplying muscle groups are arranged in longitudinal columns. Those supplying nasolabial muscles are located in the lateral and ventral intermediate segments, posterior auricular muscles in a medial column, platysma in an intermediate column; the lower lip and ocular muscles are in the ventral and dorsal segments respectively of the intermediate column. The posterior belly of the digastric muscle is supplied by motoneurons extending from the dorsal aspect of the facial nucleus to the caudal pole of the trigeminal motor nucleus.

Electrophysiological evidence for crossed oligosynaptic trigemino-facial connections in normal man

A crossed short latency component (R1) of the human blink reflex could be elicited in orbicularis oculi muscles to stimulation of the contralateral supraorbital nerve, when infraliminal conditioning stimuli were applied to various cutaneous afferents of the body (facial, upper and lower limbs). The crossed R1 responses appeared when the time interval between the conditioning and the test stimuli was of 30 to 40 ms, 50 to 65 ms and 95 to 110 ms for facial, upper and lower limbs afferents respectively. For the same time intervals, these conditioning volleys also exerted a facilitatory effect on the ipsilateral R1 responses. Furthermore, crossed R1 responses were also obtained during supraspinal facilitation induced by a voluntary contraction of the eyelids. These data show that crossed oligosynaptic trigemino-facial reflex connections exist in normal subjects, which become functional when adequate conditioning stimuli are available.

Mechanisms regulating the activity of facial nucleus motoneurones--1. Antidromic activation

The antidromic activation of facial nucleus montoneurones has been studied in acute experiments on cats by means of extra- and intracellular recording techniques. Time and amplitude characteristics of separate components of the antidromic action potential and the after--potentials accompanying it have been analyzed. A correlation is found between the duration of the falling phase of the soma-dendritic component of the action potential, the duration of its after-hyperpolarization and impulse conduction time in the axon. The dendritic origin of after-depolarizing processes is shown. It is concluded that, since there is no recurrent collateral pathway in facial motoneurones, the modulating effect of spike after-potentials on the proper excitability of facial motoneurones acquires great functional significance.

Mechanisms regulating the activity of facial nucleus motoneurones--2. Synaptic activation from the caudal trigeminal nucleus

Field and postsynaptic potentials of facial motoneurones evoked by stimulation of the caudal trigeminal nucleus were studied in cats by means of extra- and intracellular recording. Mono- and polysynaptic input onto facial motoneurones from the caudal trigeminal nucleus were shown. Four types of responses were distinguished: excitatory postsynaptic potentials generating a single action potential; a gradual shift of depolarization inducing multiple discharges; a rhythmic discharge of action potentials appearing at a low level of depolarization; excitatory postsynaptic potentials or a sequence of excitatory and inhibitory postsynaptic potentials. Multiple discharge was shown to appear as a result of effective summation of high frequency excitatory influences from efferent neurones of the caudal trigeminal nucleus projecting into the facial nucleus. Factors facilitating the development of gradual depolarization are: dendritic localization of synaptic terminals, dendritic origin of after-depolarizing processes and the high input resistance of the facial motoneurone membrane. It is thought that specific features of facial motoneurones and properties of afferent inputs are supposed to provide high sensitivity of neuronal organization of the facial nucleus to afferent signals as well as wide diversity in controlling its activity.

Convergence of trigeminal afferents on retractor bulbi motoneurones in the anaesthetized cat

Retractor bulbi motoneurones were identified by intracellular recording of their antidromic invasion following stimulation of the motor axons. Characteristics of excitatory post-synaptic potentials (e.p.s.p.s) evoked by electrical stimulation of long ciliary nerves (corneal afferents), the supraorbital nerve and the ipsilateral or contralateral vibrissae were analysed. Comparison of the orthodromic responses induced by supra-threshold stimulation of the four trigeminal inputs showed that the most powerful excitatory effect was due to corneal afferent stimulation. Excitatory synaptic potentials were followed in some cases by a period of hyperpolarization lasting 15-20 msec. It is suggested that this is an inhibitory potential of post-synaptic origin. Interaction between condition and test e.p.s.p.s evoked by long ciliary nerve and supraorbital nerve stimulation revealed a partial blocking of test e.p.s.p.s over a longer period (more than 30 msec), and it is suggested that inhibitory mechanisms within the trigeminal nucleus may be in part responsible for the absence of facilitation at the level of the motoneurone.

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.

Depressive effect of high frequency peripheral conditioning stimulation upon the nociceptive component of the human blink reflex. Lack of naloxone effect

High frequency (100 Hz) low intensity (2 mA) peripheral conditioning stimulation of either segmental or heterosegmental cutaneous nerves induced non-naloxone-reversible depression of the nociceptive component of the human blink reflex. Electrophysiological studies suggest that this depression is due to a local synaptic inhibitory mechanism involving convergence interactions between several cutaneous afferents onto a same internuncial pool in the trigemino-facial path.

Altered synaptic organization in facial nucleus following facial nerve regeneration: an electrophysiological study in man

Seven patients with postfacial palsy contracture and mass contractions were investigated electrophysiologically. In 3 patients the early blink reflex showed an unusually high amplitude, which can be attributed to enhanced excitability of facial motor neurons. In 5 patients the early blink reflex had acquired a crossed character. It is assumed that changes in organization of the facial nucleus contribute to the altered function of reinnervated facial muscles.

The facial motor nucleus of the opossum: synaptic endings on dendrites

The diameters of dendrites of large, medium and small neurons (Falls and King, '76) were measured from Golgi impregnations of the opossum facial motor nucleus in order to classify dendritic profiles sectioned in the transverse plane in electron micrographs. Three categories of dendrites are described: (1) proximal (4-7 mu in diameter); (2) intermediate (2-4 mu in diameter) and (3) distal (0.5-2 mu in diameter). The distribution of axodendritic synaptic endings was determined, recognizing that the neuronal source of individual dendritic profiles when seen in the transverse plane of section cannot be absolutely determined in view of the overlap in size of the dendrites issuing from the three types of neurons. Presynaptic terminals were categorized according to vesicle shape (spherical, pleomorphic or ellipsoidal), vesicle size, terminal size, junctional characteristics and post synaptic distribution. The vesicle size is expressed as a mean area (nm2) and was determined by using a cybergraphic tablet and a PDP-12 computer system. In any given plane of section, synaptic terminals cover most of the membrane of proximal dendrites and decrease in number as intermediate and distal dendrites are encountered. In Golgi impregnations four classes of afferent fibers which ramify among the dendrites of facial neurons can be distinguished. As yet, their sources have not been identified. Possible sites of origin for presynaptic profiles are discussed in the context of previous light microscopic findings.

The facial motor nucleus of the opossum: cytology and axosomatic synapses

The normal cytology and synaptic organization of the opossum facial motor nucleus was determined by use of Nissl preparations, Golgi impregnations, one-micron plastic sections and electron microscopy. Neurons from all regions of the nucleus can be classified into three categories based on the size and shape of their perikarya, the distribution of Nissl substance and extent of their dendritic arbors. Large neurons (30--50 mu) exhibit numerous Nissl granules and their dendrites often span more than one-half the width of the nucleus in the transverse plane. Their proximal dendrites measure 4--7 mu in diameter, taper to 0.5--2 mu and display few spines. Medium neurons (20--30 mu) can be distinguished from the large nerve cells by their size and their less extensive dendritic arbor. Proximal dendrites measure 4--7 mu in diameter, tapes to 0.5--2 mu and are relativel aspinous. Small neurons (10--20 mu) display a thin rim of cytoplasm which is either uniformly basophilic or achromatic. The dendrites of these neurons are relatively unbranched and taper from 4--5 mu proximally to 0.5--2 mu distally. Their axons give off one or two collaterals within the nuclear borders. More than one-half the perikaryal membrane of large and medium neurons is covered by presynaptic profiles which were grouped into three primary categories based upon differences in their vesicle shape (spherical, pleomorphic, or ellipsoidal). Only a few axosomatic contacts are present on small neurons. Further differentiation of axosomatic synaptic endings is suggested by differences in vesicle size, in pre- and postsynaptic membrane densities and in the size of the synaptic terminals. Vesicle size was determined by a cybergraphic tablet and a PDP-12 computer system and is expressed as a mean area in nm2.

The ventral spinothalamic tract and other ascending systems of the ventral funiculus of the spinal cord

The ascending degeneration resulting from experimental lesions of the ventral funiculus of the spinal cord of Macaca mulatta has been studied using the Nauta technique and its variants. The ventral spinothalamic tract is shown to be an independent entity with respect to the lateral spinothalamic tract; its fibers are widely distributed in the ventral funiculus and it establishes connections with the brain stem and thalamus which are analogous but not identical to those of the latter. Its role in the relay of nociceptive input is discussed in view of the similarity in hodology of the two systems and it is proposed that it may be responsible for the failure of anterolateral cordotomy to control pain on a long term basis. Other ascending systems in the ventral funiculus include the spino-olivary and spino-reticular tracts, as well as minor connections to the N. of Edinger-Westphal, the red nucleus and the superior colliculus. The projections from the ventral quadrant of the spinal cord to the brain stem are almost entirely ipsilateral until the rostral mesencephalon is reached, at which level the N. of Darkschewitz receives both ipsilateral and crossed input; the magnocellular nucleus of the medial geniculate body receives a small contribution which is mainly ipsilateral. In the thalamus the VPL receives predominantly ipsilateral projections while the input to the paralaminar nuclei is only slightly less pronounced contralaterally than ipsilaterally.