The facial nucleus of cat: antidromic and synaptic activation and peripheral nerve representation

Evidence for central antispastic effect of botulinum toxin type A

BACKGROUND AND PURPOSE

Botulinum toxin type A (BoNT/A) injections into hyperactive muscles provide effective treatment for spasticity and dystonias, presumably due to its local effects on extrafusal and intrafusal motor fibres. A recent discovery of toxin's retrograde axonal transport to CNS might suggest additional action sites. However, in comparison to cholinergic peripheral terminals, functional consequences of BoNT/A direct central action on abnormally increased muscle tone are presently unknown. To address this question, the central effects of BoNT/A were assessed in experimental local spastic paralysis.

EXPERIMENTAL APPROACH

Local spastic paralysis was induced by injection of tetanus toxin (1.5 ng) into rat gastrocnemius. Subsequently, BoNT/A (5 U·kg^-1 ) was applied i.m. into the spastic muscle or intraneurally (i.n.) into the sciatic nerve to mimic the action of axonally transported toxin. Functional role of BoNT/A transcytosis in spinal cord was evaluated by lumbar i.t. application of BoNT/A-neutralizing antitoxin. BoNT/A effects were studied by behavioural motor assessment and cleaved synaptosomal-associated protein 25 (SNAP-25) immunohistochemistry.

KEY RESULTS

Tetanus toxin evoked muscular spasm (sustained rigid hind paw extension and resistance to passive ankle flexion). Subsequent injections of BoNT/A, i.m. or i.n, reduced tetanus toxin-evoked spastic paralysis. Beneficial effects of i.n. BoNT/A and occurrence of cleaved SNAP-25 in ventral horn were prevented by i.t. antitoxin.

CONCLUSIONS AND IMPLICATIONS

Axonally transported BoNT/A relieves muscle hypertonia induced by tetanus toxin, following the trans-synaptic movement of BoNT/A in the CNS. These results suggest that such direct, centrally mediated reduction of abnormal muscle tone might contribute to the effectiveness of BoNT/A in spasticity and hyperkinetic movement disorders.

What range of stimulus intensities should we apply to elicit abnormal muscle response in microvascular decompression for hemifacial spasm?

BACKGROUND

Abnormal muscle response (AMR) has been considered as a predictor of the prognosis after microvascular decompression (MVD) for hemifacial spasm (HFS). However, its predictive value has not always been satisfactory. The objective of this work was to confirm an optimal range of stimulus intensities to elicit AMR in surgery.

METHODS

Seventy-two consecutive patients with primary HFS treated by MVD were retrospectively included in this study. A wide range of stimulus intensities from 1 to 100 mA was applied in AMR monitoring. The AMR-elicited threshold value was quantitatively traced throughout all surgical procedures. The relationship between clinical outcomes and electrophysiological findings was analyzed.

RESULTS

Of the 72 patients, 44 were immediately cured and 24 were delayed cured; the remaining 4 were proved not to be cured in their follow-up periods. The patterns of AMR-elicited threshold changes were categorized into five types, which could only be discriminated with a wide range of stimulus intensities. The constituent ratio of the patterns was significantly different (P < 0.001) among the clinical outcomes.

CONCLUSIONS

Some patterns of AMR changes might have been ignored if we had only applied a narrow range of stimulus intensities (1-30 mA) to judge whether AMR disappeared or not. Thus, a wide range of stimulus intensities (1-100 mA) to trace the AMR-elicited threshold values was proposed for a more precise prediction.

The facial motor system

Facial movements support a variety of functions in human behavior. They participate in automatic somatic and visceral motor programs, they are essential in producing communicative displays of affective states and they are also subject to voluntary control. The multiplicity of functions of facial muscles, compared to limb muscles, is reflected in the heterogeneity of their anatomical and histological characteristics that goes well beyond the conventional classification in single facial muscles. Such parcellation in different functional muscular units is maintained throughout the central representation of facial movements from the brainstem up to the neocortex. Facial movements peculiarly lack a conventional proprioceptive feedback system, which is only in part vicariated by cutaneous or auditory afferents. Facial motor activity is the main marker of endogenous affective states and of the affective valence of external stimuli. At the cortical level, a complex network of specialized motor areas supports voluntary facial movements and, differently from upper limb movements, in such network there does not seem to be a prime actor in the primary motor cortex.

Axonal misdirection as contributing factor to aberrant reinnervation of muscles after facial nerve suture in cats

Abstract Whereas basic features of post-axotomy muscle reinnervation have been extensively studied in rats, little is known about axonal regrowth and pathfinding in cats. To address the question, adult cats were subjected to facial-facial anastomosis (FFA). First group served to establish optimal parameters for labeling of the zygomatic and buccal facial branches with 1,1'dioctadecyl-3,3,3,'3'-tetramethylindo-carbocyanine perchlorate (DiI) and Fast Blue (FB) placed onto respective transected nerves. The second group of animals underwent identical bilateral labeling 3 months after transection and suture of the right facial nerve. This group served to establish the number of motoneurons, which had branched after surgery and projected into both facial branches. On control side, DiI application onto zygomatico-orbital branch labeled 3883 +/- 598 (mean +/- S.D.) perikarya were confined to the dorsal and intermediate facial subnuclei, meanwhile an application of FB onto the buccal branch labeled 1617 +/- 552 perikarya in the lateral and ventrolateral subnuclei. There were no double-labeled cells. Three months after FFA all retrogradely labeled motoneurons were scattered throughout the entire facial nucleus. To establish the proportion of perikarya, that re-grew multiple axonal branches into both nerves, double-labeled (FB + DiI) motoneurons were counted from digital images. The zygomatico-orbital nerve contained 3311 +/- 430 DiI-labeled whereas the buccal nerve 1500 +/- 442 FB-labeled motoneurons. The occurrence of 311 +/- 103 double-labeled perikarya (DiI+FB) suggested that approximately 6% of all retrogradely labeled motoneurons branched axons into both nerves. I conclude that malfunctioning axonal pathfinding rather than deviant reinnervation contributed to poor recovery of function after FFA in the cat.

Facial nerve repair accomplished by the interposition of a collagen nerve guide

OBJECT

Facial nerve paralysis due to a surgical procedure or trauma is a frequently observed complication. The authors evaluated facial nerve repair achieved by the interposition of a collagen nerve guide.

METHODS

Ten cats were divided into three groups. Group 1 consisted of six animals in which a 5-mm facial nerve segment on one side was resected and replaced by a collagen tube that was sutured to bridge both nerve stumps. On the opposite side a 5-mm segment of facial nerve was resected, reversed 180 degrees, and sutured to the stumps as an autograft nerve. Group 2 consisted of two cats in which the collagen nerve guide was interposed on one side and the nerve on the other side was left intact. Group 3 consisted of two cats in which a reversed autograft nerve was placed on one side and the nerve on the other side was left intact. Histological, electrophysiological, and horseradish peroxidase labeling examinations were performed starting 3 weeks after surgery. Light and electron microscopic examinations of collagen tube-implanted specimens revealed a well-vascularized regenerated nerve. The electrophysiological study confirmed the recovery of electrical activity in regenerated axons. Horseradish peroxidase labeling also confirmed restoration of the whole facial nerve tract.

CONCLUSIONS

The collagen nerve guide shows great promise as a nerve conduit.

Long-term observation on the changes of somatotopy in the facial nucleus after nerve suture in the cat: morphological studies using retrograde labeling

To examine the time course of plasticity of the cranial nucleus during axonal regeneration, we followed the topographical reorganization of the cat facial nucleus (FN) up to 24 months after facio-facial nerve suture using retrograde labeling methods. The trunk of the temporal-zygomatico-orbital and both superior and inferior buccolabial branches (defined as main branch) of the facial nerve was cut and sutured again under ketamine hydrochloride anesthesia. At 11-722 days after nerve suture, Fast Blue (FB) and 1,1'-dioctadecyl-3, 3, 3', 3'-tetramethylindocarbocyanine perchlorate (Dil) or horseradish peroxidase (HRP) were injected into the distal part of the sutured main branch and the unoperated posterior auricular branch, respectively. Until about 3 months after suture, the topographical pattern in FN was similar to that observed in normal cats. At about 4 months after suture, FB-labeled motoneurons were distributed not only in the lateral part (including intermediate, dorsal and ventrolateral divisions) but also in the medial subdivision of FN. After a survival period of 18-24 months, FB-labeled neurons were found all over the FN, and their number increased significantly. Interestingly, in the longer survival cases, we noticed that the Dil- or HRP-labeled posterior auricular branch motoneurons also showed a tendency to distribute outside the medial region. The present study showed that somatotopic disorganization starts at around 4 months after suture, which seems to be somewhat slower than that in rats, and continues until a much later postoperative period. Furthermore, we suggested a possibility that the regeneration of one branch may affect the somatotopy of the unoperated nerve branch. These phenomena may contribute to aberrant facial nerve functions such as abnormal associated movement and facial spasm observed after nerve injury.

Discharge profiles of abducens, accessory abducens, and orbicularis oculi motoneurons during reflex and conditioned blinks in alert cats

The discharge profiles of identified abducens, accessory abducens, and orbicularis oculi motoneurons have been recorded extra- and intracellularly in alert behaving cats during spontaneous, reflexively evoked, and classically conditioned eyelid responses. The movement of the upper lid and the electromyographic activity of the orbicularis oculi muscle also were recorded. Animals were conditioned by short, weak air puffs or 350-ms tones as conditioned stimuli (CS) and long, strong air puffs as unconditioned stimulus (US) using both trace and delayed conditioning paradigms. Motoneurons were identified by antidromic activation from their respective cranial nerves. Orbicularis oculi and accessory abducens motoneurons fired an early, double burst of action potentials (at 4-6 and 10-16 ms) in response to air puffs or to the electrical stimulation of the supraorbital nerve. Orbicularis oculi, but not accessory abducens, motoneurons fired in response to flash and tone presentations. Only 10-15% of recorded abducens motoneurons fired a late, weak burst after air puff, supraorbital nerve, and flash stimulations. Spontaneous fasciculations of the orbicularis oculi muscle and the activity of single orbicularis oculi motoneurons that generated them also were recorded. The activation of orbicularis oculi motoneurons during the acquisition of classically conditioned eyelid responses happened in a gradual, sequential manner. Initially, some putative excitatory synaptic potentials were observed in the time window corresponding to the CS-US interval; by the second to the fourth conditioning session, some isolated action potentials appeared that increased in number until some small movements were noticed in eyelid position traces. No accessory abducens motoneuron fired and no abducens motoneuron modified their discharge rate for conditioned eyelid responses. The firing of orbicularis oculi motoneurons was related linearly to lid velocity during reflex blinks but to lid position during conditioned responses, a fact indicating the different neural origin and coding of both types of motor commands. The power spectra of both reflex and conditioned lid responses showed a dominant peak at approximately 20 Hz. The wavy appearance of both reflex and conditioned eyelid responses was clearly the result of the high phasic activity of orbicularis oculi motor units. Orbicularis oculi motoneuron membrane potentials oscillated at approximately 20 Hz after supraorbital nerve stimulation and during other reflex and conditioned eyelid movements. The oscillation seemed to be the result of both intrinsic (spike afterhyperpolarization lasting approximately 50 ms, and late depolarizations) and extrinsic properties of the motoneuronal pool and of the circuits involved in eye blinks.

The trigeminally evoked blink reflex. II. Mechanisms of paired-stimulus suppression

The paired-stimulus paradigm, presentation of a pair of identical reflex-eliciting stimuli to the supraorbital nerve (SO) with an interstimulus interval of less than 2 s, evokes a response to the second, test, stimulus which is less than that elicited by the first, conditioning, stimulus. In this study, we investigated the site of this suppression and its pharmacology in the alert guinea pig. Both the early (R1) and the late (R2) component of the SO-evoked blink reflex exhibited suppression in the paired-stimulus paradigm. Initiation of suppression appeared to be specific to the afferent limb of the reflex rather than the result of motor activity generated by the conditioning stimulus. Neither acoustic conditioning stimuli nor air puffs that elicited blinks via another branch of the trigeminal nerve suppressed the test response. Extremely weak SO shocks, however, that did not directly elicit a reflex, caused suppression of the response to subsequent SO stimuli of normal intensity. Paired stimulus suppression of the R1 component appeared to involve activation of GABAB receptors within the spinal trigeminal nucleus. Both systemic injections and microinjections of baclofen into the spinal trigeminal nucleus enhanced R1 suppression, whereas identical injections of CGP35348, a GABAB antagonist, diminished R1 suppression. Furthermore, single-unit recordings in alert animals revealed that spinal trigeminal neurons exhibited suppression in the paired-stimulus paradigm that resembled that of the R1 component of the blink reflex. These findings showed that sensory gating underlies paired-stimulus suppression of the SO-evoked blink reflex and that activation of GABAB receptors plays an important role in this process.

Relation of recurrent laryngeal nerve compound action potential to laryngeal biomechanics

This study was designed to investigate the compound action potential (CAP) of the recurrent laryngeal nerve (RLN) and to correlate this electrophysiologic signal to laryngeal biomechanics and phonatory function. Four adult mongrel canines were anesthetized. The RLN was isolated and stimulated, and recording electrodes were applied. The electromyographic (EMG) electrode was placed in the thyroarytenoid (TA) muscle. The RLN CAP and the EMG of the TA muscle were recorded and compared to the stimulation intensity, subglottic pressure (Psub), and each other. The CAP peak-to-peak and EMG peak-to-peak amplitudes demonstrated a sigmoidal relation to stimulus intensity and a linear relation to Psub and to each other. On the basis of these findings, the RLN CAP appears to be a reliable physiologic measure of laryngeal function.

Correlation of Compound action Potential and Electromyography with Facial Muscle Tension

Functional electric stimulation is a new method for dynamic rehabilitation of paralyzed muscles. The output of such prosthetic devices needs to be modulated by some index of the muscle movement. In facial paralysis a measure of the muscle contractions of the normal contralateral side seems to be an appropriate input. In the rabbit, we simultaneously measured the compound action potential of the buccal branch of the facial nerve, the electromyogram of the zygomaticus major muscle, and the muscle twitch tension through strain gauge. The compound action potential, electromyogram, and strain gauge each had a sigmoidal relationship to stimulus intensity. The compound action potential peak-to-peak amplitude was found to have a linear correlation to the peak twitch tension of the corresponding facial muscle. The electromyogram response, although more variable, also had a linear correlation with muscle contraction. The possibility of predicting the contraction of facial muscles before they actually occur is discussed in the context of available and future functional electric rehabilitation models.

Observations on morphology and electrophysiological properties of the normal and axotomized facial motoneurons in the cat

The correlation between the morphology of facial motoneurons stained intracellularly with horseradish peroxidase and their physiological parameters was examined in cats following facial nerve section and in cats with intact facial nerve. A certain statistical relationship exists between cell size and excitability in normal neurons. After axotomy, facial neurons showed a slow conduction velocity and a low rheobasic current, but had a normal cell size. Physiological changes include repetitive firing in response to intracellular current injection, reflecting an increase in the excitability in the axotomized neurons.

Afferent projections in the spinal accessory nerve to the facial motoneurons of the cat

Stimulation of the accessory nerve evoked polysynaptic excitatory postsynaptic potentials (EPSPs) in the facial nucleus (FN) neurons of anesthetized cats. From the experiments with severance of C1-C3 dorsal roots, it is suggested that accessory afferents enter the brainstem through the accessory nerve. It was also found that stimulation of the solitary tract nucleus produced exclusively monosynaptic EPSPs in the FN neurons and the afferent volleys are most likely to be relayed at the solitary tract nucleus.

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.

Monoaminergic, peptidergic, and cholinergic afferents to the cat facial nucleus as evidenced by a double immunostaining method with unconjugated cholera toxin as a retrograde tracer

Using a sensitive double immunostaining technique with unconjugated cholera-toxin B subunit as a retrograde tracer, the authors determined the nuclei of origin of monoaminergic, peptidergic, and cholinergic afferent projections to the cat facial nucleus (FN). The FN as a whole receives substantial afferent projections, with relative subnuclear differences, from the following areas: 1) the perioculomotor areas, the contralateral paralemniscal region, and the mesencephalic reticular formation dorsal to the red nucleus; 2) the ipsilateral parabrachial region and the nucleus reticularis pontis, pars ventralis; and 3) the nuclei reticularis parvicellularis, magnocellularis, ventralis, and dorsalis of the medulla. In addition, the present study demonstrated that the lateral portion of the FN receives specific projections from the contralateral medial and olivary pretectal nuclei and the ipsilateral reticular formation of the pons. It was also found that the FN receives: 1) serotoninergic inputs mainly from the nuclei raphe obscurus, pallidus, magnus, and the caudal ventrolateral bulbar reticular formation; 2) catecholaminergic afferent projections from the A7 noradrenaline cell group located in the Kölliker-Fuse, parabrachialis lateralis, and locus subcoeruleus nuclei; 3) methionin-enkephalin-like inputs originating in the pretectal complex, the nucleus paragigantocellularis lateralis and the caudal raphe nuclei; 4) substance P-like afferent projections mainly from the Edinger-Westphal complex and the caudal raphe nuclei; and 5) cholinergic afferents from an area located ventral to the nucleus of the solitary tract at the level of the obex. In the light of these anatomical data, the present report discusses the physiological significance of FN inputs relevant to tonic and phasic events occurring at the level of the facial musculature during the period of paradoxical sleep in the cat.

An electrophysiologically defined trigemino-reticulo-facial pathway related to the blink reflex in the cat

This study examines the direct projection of neurons in the pontomedullary reticular formation (RF) to the dorsal division of the facial nucleus (FN), where the orbicularis oculi motoneuron pool is located, and the nature of synaptic inputs to the RF neurons from the supraorbital branch of the trigeminal nerve (SON), using electrophysiological techniques in anesthetized cats. A large number of the RF neurons directly projected to the ipsilateral FN dorsal division, and some of these neurons issued axon branches probably terminating on motoneurons in the division. Many of the RF neurons projecting to the dorsal division were synaptically activated by ipsilateral SON stimulation. Most latencies of their activation were between the latencies of the early and late blink reflex responses, and the remaining latencies were shorter than those of the early response. These results suggest that the RF neurons mediate both the early and the late blink reflex responses, but mainly the late response, as premotor relay neurons.

Lability of the postauricular and inion microreflexes, studied in the normal human subject

Experiments have been carried out to investigate the lability of 2 brain-stem microreflexes, viz., the postauricular and inion responses to auditory stimulation. The size of the postauricular response was reduced by conditioning auditory or median nerve stimulation. Both forms of conditioning stimulus also induced a reduction in ongoing EMG activity in postauricular muscles, which suggests that there may be a direct inhibitory effect on facial nerve motoneurones. The effectiveness of conditioning auditory stimulation in blocking the response to a subsequent auditory testing stimulus was related to the size of the response evoked by the conditioning stimulus, which suggests that part of the inhibitory effect may be due to afterhyperpolarization or a feedback type of inhibition which is dependent on motoneurone activation. By contrast, auditory and median nerve stimulation were not found to have an inhibitory effect on the inion response.

Hemifacial spasm: ephaptic transmission or hyperexcitability of the facial motor nucleus?

Hemifacial spasm is a rare disorder that is caused by a blood vessel cross-compressing the facial nerve near the nerve's entry into the brain stem. Electrical stimulation of one branch of the facial nerve on the affected side in patients elicits a response from muscles that are innervated by another branch. Intraoperative recording of this abnormal muscle response, together with measurements of conduction times in parts of the facial nerve, in earlier studies have shown evidence that the cross-transmission of antidromic activity that causes this response occurs central to the site of vascular compression of the facial nerve. In this study we show that in rare circumstances during microvascular decompression operations, a small EMG potential precedes the regular abnormal muscle response. This early response has a latency that is equal to the sum of the conduction times of the portions of the facial nerve that would have been involved if the cross-transmission had occurred at the site of vascular compression of the facial nerve, and it may thus be a result of ephaptic transmission in the nerve at that location. Thus, cross-transmission of antidromic activity can indeed occur in the facial nerve at the site of compression, although we have observed this early potential in only 3 of more than 50 patients who were operated upon for hemifacial spasm; further, in those three patients it occurred only for a short time and only during surgical manipulation of the facial nerve. In view of the fact that this early response occurs only after the nerve has been manipulated, and because it is of such small magnitude, it seems unlikely that such cross-transmission (ephaptic transmission) at the site of the vascular compression plays a direct role in generating the spasm and the antidromic activity that spreads from one branch of the facial nerve to another.

Mechanisms regulating the activity of facial nucleus motoneurons--III. Synaptic influences from the cerebral cortex and subcortical structures

Peculiarities of synaptic processes in facial motoneurons evoked by stimulation of various regions of the cerebral cortex and subcortical structures were studied in acute experiments on cats by intracellular recording technique. Stimulation of the motor cortex as well as gyrus proreus and pyramidal tract was shown to evoke polysynaptic excitatory and inhibitory postsynaptic potentials in facial motoneurons. Stimulation of the lateral hypothalamus produced exclusively excitatory polysynaptic effects. It was also found that stimulation of the head of nucleus caudatus and globus pallidus evokes a polysynaptic activation in facial motoneurons, while stimulation of nucleus amygdala centralis leads to mono- and polysynaptic excitation of these neurons. Convergence of the above effects on the same motoneurons is shown to exist. Possible pathways and mechanisms of descending influences on the activity of facial motoneurons is discussed.

Mechanisms regulating the activity of facial nucleus motoneurons--IV. Influences from the brainstem structures

Peculiarities of synaptic processes of facial motoneurons evoked by stimulation of brainstem structures were studied in acute experiments on anaesthetized and immobilized cats by intracellular recording technique. It was shown that stimulation of nucleus reticularis parvocellularis of the medulla oblongata as well as interstitial nucleus of Cajal, nucleus Darkschewitsch, periaqueductal gray and pretectal area evokes in facial motoneurons monosynaptic excitatory postsynaptic potentials accompanied by single action potentials. Somatic localization of synapses under study is supposed. Convergence of inputs is revealed. Functional role of the midbrain structures as intermediate relays transmitting descending signals to the facial nucleus is discussed.

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.

Long-term increases in excitability of facial motoneurons and other neurons in and near the facial nuclei after presentations of stimuli leading to acquisition of a Pavlovian conditioned facial movement

Levels of neuronal excitability to injected current were measured intracellularly in facial motoneurons and other neurons in and near the facial nuclei of three groups of awake cats: a "Conditioned" group consisting of animals that had previously received sufficient numbers of paired presentations of click CSs and glabella tap USs to produce eyeblink CRs; a "US-only" group that had received presentations of the USs only; and a "Naive" group that had received neither of these stimuli. Thresholds of intracellularly applied, depolarizing pulse currents required to elicit repeatable spike activity were significantly lower in the "Conditioned" and "US-only" groups than in the "Naive" group. The increased levels of neuronal excitability were correlated with increases in neuronal input resistance. Levels of neuronal excitability remained elevated when measured more than a month after presentations of both CSs and USs, whereas the increases in neuronal excitability decayed within a few weeks in animals given USs only. The increases in neuronal excitability and input resistance following repetitive presentations of glabella tap USs alone appeared to support a latent facilitation of motor performance reflected by an absence of a blink CR to click CS after such presentations but an increased rate of acquisition of subsequent eyeblink conditioning using paired click CS and tap US. The rate of eyeblink conditioning was found to be accelerated in a group of cats given repetitive presentations of tap USs seven days prior to conditioning with paired CSs and USs, compared to a group that was not given USs or CSs before similar conditioning. These findings provide direct, in vivo evidence that increases in the excitability and input resistance of neurons in and near the facial nucleus can occur in cats following presentations of the stimuli used for conditioning.

The facial "motor" nerve of the rat: control of vibrissal movement and examination of motor and sensory components

Rhythmical whisking of the mystacial vibrissae at about 7 Hz during exploration is one of the most conspicuous behavioral patterns in the rat. To identify the final common pathway for vibrissal movement, individual motor branches of the facial nerve, including the posterior auricular, temporal, zygomatic, buccal, marginal mandibular, cervical, stylohyoid, and posterior digastric branches, were cut, either singly or in various combinations. We found that vibrissal movement could be abolished only by transection involving the buccal branch and the upper division of the marginal mandibular branch. To trace back the central origins of the buccal and marginal mandibular, as well as the other branches of the facial nerve, all distal to the stylomastoid foramen, horseradish peroxidase (HRP) was applied to the cut proximal ends of these individual branches. The retrograde HRP labelling in the facial motor nucleus revealed topographical representation of these branches in which the buccal and marginal mandibular branches were represented laterally. The stylohyoid and posterior digastric branches originated from cells in the suprafacial nucleus. Consistent with earlier observations with intramuscular HRP injections, the motoneuronal population devoted to vibrissal movement did not seem to be substantially larger than that for other facial movements. An additional examination was made of the labelled afferent component of the facial motor nerve. We confirmed and extended previous findings that none of the above facial motor nerve branches, except the posterior auricular branch, contained a significant number of afferent fibers originating from the geniculate ganglion, the sensory ganglion of the seventh nerve. In addition, no labelling was seen in the mesencephalic trigeminal nucleus or trigeminal ganglion. These findings, in combination, suggest that, with the exception of the posterior auricular branch, all the facial motor nerve branches, including those involved in vibrissal movement, are almost entirely efferent.

Correlation of the main peripheral branches of the facial nerve with the cytoarchitectonic subdivisions of the facial nucleus in the guinea pig

Correlation of the main peripheral branches of the facial nerve with morphological subdivisions of the facial nucleus was examined in the guinea pig by the retrograde horseradish peroxidase method. The facial nucleus of the guinea pig was divided cytoarchitectonically into the dorsolateral, lateral, intermediate, medio-intermediate, medial, and ventromedial divisions; the ventromedial division was further divided into the major, dorsal and lateral parts. Six main branches of the facial nerve were identified; the zygomatico-orbital, cervical, posterior auricular, anterior auricular, superior labial, and inferior labial branches. After applying HRP to the main branches of the facial nerve, the pattern of distribution of HRP-labelled neuronal cell bodies within the facial nucleus was examined: the dorsolateral division, dorsal part of the ventromedial division, major part of the ventromedial division, lateral part of the ventromedial division, or medial division contained the cell bodies of respectively the zygomatico-orbital, cervical, posterior auricular, anterior auricular, or superior labial branches, while each of the lateral, intermediate, and medio-intermediate divisions contained the cell bodies of both the superior labial and inferior labial branches.

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.

Antidromically evoked facial nerve response

Facial nerve electrodiagnostic tests that are currently available indirectly assess the severity of injury to the intratemporal facial nerve. Antidromic conduction testing is an alternate approach that, if feasible, could provide direct and immediate assessment of proximal facial nerve function. This possibility was tested in a guinea pig model, in which near-field (intracranial) and far-field (extradural) recording techniques were used to assess antidromically evoked facial nerve activity. Response characteristics, topographical distribution, and lesion effects suggest that the recorded potentials represent antidromic activation of the facial nerve. If response amplitude and/or latency can be correlated with the functional state of the nerve, antidromic testing may provide a useful means of assessing proximal facial nerve function in pathologic states.

Peptidergic and aminergic innervation of the facial nucleus of the rat with special reference to ontogenetic development

The distribution and ontogenetic development of several neuropeptides such as enkephalin, substance P, somatostatin, neuropeptide Y, and of monoamines such as serotonin and catecholamines in the facial nucleus of the rat were investigated with immunocytochemistry. The neuropeptides were concentrated in certain subnuclei. Enkephalin-immunoreactive fibers were distributed in the medial and dorsal subnuclei, substance P in the intermediate and dorsal subnuclei, somatostatin in the intermediate subnucleus, and neuropeptide Y in the dorsal subnucleus. The amines were distributed evenly throughout the nucleus. These distribution patterns suggest that peptidergic fibers are closely related to the functions of different subnuclei, while fibers containing monoamines are more basic--not specific to individual muscles. Few of these fibers were observed in the prenatal stage of the rat, but they increased markedly in number during the first postnatal week, and had established their innervation pattern by the tenth postnatal day, which coincides with the establishment of nerve-muscle innervation. The present study further showed that fibers containing serotonin are supplied mainly from the raphe nucleus, that catecholamine fibers are from neurons containing catecholamine surrounding the facial nucleus, and that fibers containing neuropeptide Y are from the lateral part of the caudal medullary reticular formation. These findings suggest that catecholamine and neuropeptide Y are not both present in the single neurons projecting to the facial nucleus.

Morphology of motoneurons in a mixed motor pool of the cat facial nucleus that innervate orbicularis oculis and quadratus labii superioris, stained intracellularly with horseradish peroxidase

Retrograde tracing with horseradish peroxidase showed that motoneurons to two distinct muscles, the orbicularis oculis and quadratus labii superioris, are intermixed within the dorsolateral subnucleus of the cat facial nucleus. Intracellular electrodes were used to identify and fill the motoneurons of the dorsolateral subnucleus with horseradish peroxidase. Soma diameters averaged 55 micron. The average number of primary dendrites was 11.6. The area covered by the dendritic trees varied in shape according to the position of the soma within the subnucleus. Axon hillocks were seen arising in many orientations, bearing no apparent relation to subsequent axonal path, cell position within the nucleus or somatic geometry. Motoneurons to the two muscles appeared to be indistinguishable on the basis of morphology, even though they appear to be functionally independent. Their functional differences are not reflected in any measure of somadendritic shape studied here. Of further interest is the variability in shape associated with the neurons's position within the subnucleus. We conclude that many details of dendritic shape do not reflect specific physiological function.

Examination of geniculate ganglion cells contributing sensory fibers to the rat facial 'motor' nerve

Using a method to visualize HRP-containing cells in the geniculate ganglion (GG) in situ after decalcifying surrounding bone, we found that about 30% of the total (about 1000) GG cells contributed sensory fibers to the posterior auricular branch of the facial motor nerve. These cells are relatively large for GG cells in general. The remaining facial motor nerve branches, including those involved in vibrissal movement, contained few sensory afferent fibers originating from GG cells.

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.

Afterhyperpolarization in neurones of the red nucleus

Afterhyperpolarization (AHP) following single or short trains of spikes in rubrospinal neurones (RN neurones) of the cat has been studied with intracellular recording techniques. The AHP amplitude was potential dependent; it increased with depolarization and decreased with hyperpolarization and had an extrapolated reversal potential about 20 mV below resting membrane potential. The AHP was associated with an increase in the membrane conductance and it was concluded that the AHP is primarily caused by an increase in membrane conductance to potassium ions. The time course of the conductance change underlying the AHP was measured with short current pulses and calculated from the AHP voltage. The AHP following a single spike was conditioned at different interspike intervals by a preceding spike (or several spikes). In many RN neurones the AHP (conductance) following a spike added approximately linear to that generated by a preceding spike. In most cells, however, the AHP following a spike was instead depressed by a preceding spike. The summation of AHPs increased progressively, while the depression appeared to be already maximal with one preceding spike. The depression was then approximately constant for interspike intervals less than the AHP duration. It will be shown in a following paper that these properties of the AHP are reflected in the behaviour of the repetitive discharge evoked by constant current pulses in the same neurones.

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.

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.

Distribution of sensory ganglion cells innervating facial muscles in the cat. An anatomical study with the horseradish peroxidase technique

The question of a possible sensory component in branches of the facial nerve innervating facial mimetic muscles in the cat was examined by the technique of retrograde axonal transport of horseradish peroxidase (HRP). HRP was applied to the proximal cut end of facial nerve branches innervating different facial muscle groups. Following survival periods of 71-75 h the animals were fixed by perfusion. Certain craniospinal sensory ganglia and the brain stem were processed histochemically for demonstration of HRP. HRP-labelled cell bodies, structurally resembling sensory neurons, were consistently observed ipsilaterally in the geniculate and proximal vagal ganglia and under certain conditions in the trigeminal ganglion. Measurements of HRP-labelled neurons in the geniculate and proximal vagal ganglia showed a wide size range but a unimodal size distribution with peaks in the small size range. These findings support the view that facial nerve branches innervating the mimetic muscles contain different types of sensory fibers.

Serotonin-induced depolarization of rat facial motoneurons in vivo: comparison with amino acid transmitters

Intracellular recordings were obtained from facial motoneurons in anesthetized rats. The effects of iontophoretically applied serotonin were compared to those of the excitatory amino acids glutamate and DL-homocysteic acid (DLH), and the inhibitory amino acids, glycine, GABA and muscimol, under various conditions of membrane polarization and intracellular chloride concentration. Iontophortically applied serotonin caused a depolarization of facial motoneurons which was accompanied by increased input resistance and increased neuronal excitability. Experiments comparing the response to serotonin with those of glycine, GABA, and muscimol demonstrated that the serotonin effect does not involve changes in membrane conductance to chloride. Comparisons of serotonin with glutamate and DLH at varying levels of membrane hyperpolarization indicated that the serotonin-induced depolarization is not caused by increased conductance to sodium or calcium, and differs in its underlying ionic mechanism from depolarizations induced by glutamate and DLH. Results were consistent with the hypothesis that serotonin causes depolarization, increased input resistance, and increased excitability in rat facial motoneurons by decreasing resting membrane conductance to potassium ions. Such changes in motoneurons in the brain stem and spinal cord probably account for some of the physiological and behavioral effects observed during pharmacological activation of serotonin receptors.

Synaptic actions of vagal afferents on facial motoneurons in the cat

Synaptic potentials in facial motoneurons of cats were intracellularly recorded on stimulation of the vagal nerve, superior laryngeal nerve, solitary tract nucleus and spinal trigeminal tract nucleus. A possible disynaptic excitation was elicited in the facial motoneurons by stimulation of the vagal nerves and superior laryngeal nerves on both sides. Activation of the neurons in the solitary tract nucleus and/or trigeminal tract nucleus induced monosynaptic excitatory postsynaptic potentials (EPSPs) in the facial motoneurons.

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

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

Intracellular studies showing modulation of facial motoneurone excitability by serotonin

The application of serotonin to certain myenteric plexus neurones in the guinea pig small intestine causes a slow depolarization of membrane potential, accompanied by increased neuronal excitability and input resistance. On the other hand, microiontophoretic application of large amounts of serotonin onto mammalian spinal motoneurones is reported to cause membrane hyperpolarization and decreased excitability. However, on the basis of recording spinal reflex activity, serotonin has been reported to enhance net motoneurone activity. Moreover, studies using extracellular single-cell recording techniques indicate that serotonin in small amounts facilitates synaptically or glutamate-induced excitation of mammalian motoneurones in the facial nucleus and spinal cord. It was suggested that these facilitatory actions were modulatory in nature, as serotonin did not induce motoneurone spiking in the absence of extrinsic excitatory input. The study reported here investigated the membrane mechanisms underlying these modulatory effects by obtaining intracellular recordings from rat facial motoneurones during extracellular microiontophoretic application of serotonin, methysergide (a serotonin antagonist) and noradrenaline. Serotonin caused a slow depolarization of membrane potential of about 5 mV which remained sub-threshold, accompanied by an increase in electrical excitability of the neurone, and an increase in input resistance. Noradrenaline caused the same changes. Methysergide antagonized the effects of serotonin, but not noradrenaline, indicating that these actions of serotonin are selective and receptor mediated.

Effect of membrane polarization and synaptic activity on the timing of antidromic invasion

The latency from the stimulus (S) to the IS and SD components of the antidromic spike was measured in motoneurones and spinocerebellar tract cells following displacement of the membrane potential either by current pulses or by synaptic potentials. Changes in the latency to the SD spike (S-SD delay) were mainly caused by changes in the IS-SD delay and varied from 10 to 100 musec per mV change in membrane potential, depending on the initial value of the IS-SD delay. Changes in the S-IS delay were also observed and these changes could, especially in spinocerebellar cells, give a significant contribution to the change in the total delay. EPSPs shortened the S-SD delay as efficiently as current-evoked depolarizations of similar magnitude while IPSPs were often more effective in prolonging the delay than current-evoked hyperpolarizations. This difference was related to the larger conductance increase during IPSPs than during IPSPs and to the longer IS-SD delays at hyperpolarized potentials. The presented data contribute to the understanding of the method which uses extracellular recording of antidromic latency changes as an indirect measure of intracellular membrane potential changes. Our results show that the recording of antidromic latency changes is a particularly sensitive method for detecting inhibition of neurones.

Hemifacial spasm and the facial nucleus

Spontaneous and associated hyperkinetic facial movements and contracture which follow injury to the seventh cranial nerve (postparalytic hemifacial spasm) or arise without known previous injury (cryptogenic hemifacial spasm) are pathological motor phenomena not found in the distribution of other cranial or somatic motor nerves. The commonly expressed hypotheses of pathogenesis--aberrant regeneration and fiber excitation by false synapse formation (ephapses) at the site of injury--cannot account for all aspects of these phenomena or for the uniqueness of such movements to the distribution of the seventh nerve. The suggestion is made that the existing diversity of facial motor behavior, which encompasses voluntary, emotional, and especially automatic, associated, and reflexive movements, is based on a unique central organization that sets it apart from other motor groups. I hypothesize that because of this organization, the changes following axonal injury--which include selective deafferentation, glial response, axonal sprouting, functional reconnection, and hyperexcitability from dendritic spike generation--can unmask and augment automatic, associated, and reflexive movements already present in the facial neuronal network to result in facial hyperkinesia.

Effect of repetitive activation on the afterhyperpolarization in dorsal spinocerebellar tract neurones

1. The changes in the afterhyperpolarization (a.h.p.) with repetitive activation have been studied in dorsal spinocerebellar tract cells of the cat using intracellular recording techniques. 2. The a.h.p. following a single spike was conditioned at different interspike intervals by a single preceding spike. In the majority of neurones the a.h.p. following a spike added approximately linearly with that generated by a preceding spike. 3. In other cells the a.h.p. following a spike was instead depressed by a preceding spike. THis depression was approximately constant at interspike intervals less than the a.h.p. duration (50-100 msec). Thereafter the a.h.p. slowly recovered during the next 100-300 msec. There was no associated decrease in the initial brief hyperpolarizing undershoot. 4. With shortlasting repetitive activation at high frequency (greater than 100 impulses/sec) the a.h.p, peak amplitude increased progressively with successive spikes (5-15 spikes). No change in the time constant of decay was observed. A good correspondence was found between the observed increase in peak amplitude of the a.h.p.s and that given by a theoretical linear superposition of the successive a.h.p.s. 5. Changes in the brief hyperpolarizing undershoot with repetitive activation is also described.

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.