Facial Neurons Research Articles

Partial restoration of blink reflex function after spinal accessory-facial nerve anastomosis.

Functional motor control requires perfect matching of the central connections of motoneurons with their peripheral inputs. It is not known, however, to what extent these central circuits are influenced by target muscles, either during development or after a lesion. Surgical interventions aimed at restoring function after peripheral nerve lesions provide an opportunity for studying this interaction in the mature human nervous system. A patient was studied in whom the spinal accessory nerve was anastomosed into a lesioned facial nerve, allowing voluntary contractions of the previously paralysed muscles. This procedure, in addition to replacing the facial neurons at peripheral synapses, allowed a new short latency trigeminospinal accessory reflex of the R1 blink reflex type to be demonstrated, implying that trigeminal neurons had sprouted towards spinal accessory motoneurons over a distance of at least 1 cm. These results show an unexpected influence of the periphery in remodelling central connectivity in humans. The motoneuronal excitability for this R1 reflex response was therefore studied to compare the convergent properties of facial motoneurons (normal side) with those of the spinal accessory motoneurons (operated side) using a classic double shock technique with variable interstimulus intervals (conditioning test stimulus). On the normal side, conditioning stimuli (to the ipsilateral or contralateral infraliminar supraorbital nerve) produced a clearcut facilitation of the R1 blink reflex when the interstimulus interval was 30-80 ms. By contrast, a similar procedure had no effect on the R1 blink reflex mediated via the trigeminal-spinal accessory reflex arc. These data indicate that despite the heterotopic sprouting of some axons from neurons in the XIth nucleus, motoneurons involved in the newly formed reflex arc remain totally inexcitable by other trigeminal afferents and seem unable to ensure a physiological functioning of the normal blink reflex. Thus the functional relevance of the recovered R1 blink response remains unclear.

PDGF and its receptors following facial nerve axotomy in rats: expression in neurons and surrounding glia.

We investigated the expression of platelet-derived growth factor (PDGF) and its receptors in rat facial nuclei following axotomy by in situ hybridization and immunohistochemistry. Facial nuclei were examined on days 3, 6, 12, 19 and 26 postoperatively (p.o.). Strong immunoreactivity for PDGF was found in facial neurons and surrounding astrocytes on the ipsilateral side of the brainstem already after 3 days p.o. and persisted at a high level until day 26 p.o. in rats with a facial nerve cut injury. After crushing of the facial nerve, a similar increase was seen in PDGF immunoreactivity which, however, decreased after day 19 p.o., when reinnervation had occurred. Reactive gliosis appeared on the operated side and was confirmed by an increase in intensity of GFAP staining. The kinetics of PDGF A-chain mRNA expression corresponded to the PDGF immunoreactivity, whereas the B-chain mRNA was present only in the neurons. The PDGF alpha-receptor immunoreactivity as well as the mRNA were detected in scattered glial cells. The density of the PDGF alpha-receptor mRNA expressing glial cells was higher on the injured side, but the intensity of the expression per cell did not change after axotomy. An increase in PDGF beta-receptor immunoreactivity was seen in the ipsilateral facial nuclei after 3-6 days p.o., however, the increase in the mRNA could not be detected. The staining persisted until day 26 p.o., when transected facial neurons showed heavier staining than those that had been crushed.(ABSTRACT TRUNCATED AT 250 WORDS)

Injection of doxorubicin into rabbit eyelid does not result in loss of facial motor neurons

In both laboratory and clinical studies, injection of doxorubicin directly into the eyelid results in permanent muscle loss of the majority of fibers within treated eyelids. A first clinical trial of this technique in blepharospasm and hemifacial spasm patients has been performed. All patients who completed a full course of doxorubicin treatment showed a permanent decrease in eyelid strength, with over 50% of these patients requiring no further treatment. Doxorubicin is known to be carried by retrograde axonal transport to the brain and is a known neurotoxin. This raises the question of the effect of these treatments on the facial neurons which innervate the orbicularis oculi muscle in the eyelids. The effect on the number of facial nuerons present after injection of doxorubicin into the eyelid of rabbits was determined using both HRP and diI retrograde labeling techniques. Despite the extensive and permanent muscle loss caused by the doxorubicin treatments, there was no measurable loss of facial neurons on the doxorubicin treated sides. DiI was shown to be myotoxic at high concentrations and amplified the myotoxic effect of doxorubicin. Lack of neuronal loss may offer assurance of clinical safety to the facial motor neurons of muscles spasm patients who receive doxorubicin injections into their eyelids.

Facial nerve neuroma: diagnosis and management

Facial neuroma is a condition of insidious onset. Prior to the introduction of modern imaging techniques (computerised tomography, magnetic resonance imaging) delays between presentation and diagnosis were common place. Atypical facial paralysis and hearing loss are the most common presenting features. This combination is very suggestive of a facial neuroma and is an indication to proceed to computed tomography. This investigation can reveal expansion of the Fallopian canal at any point through the temporal bone. A few facial neuromas occur in the cerebello-pontine angle alone or in the parotid. Resection and grafting are always worthwhile as facial neurones seem to persist in the tumour mass keeping muscle alive until the facial nerve graft becomes functional. The presentation, management and results of treatment of 15 such cases is presented.

Change in facial nerve innervation following hypoglossal-facial anastomosis. An animal study.

Hypoglossal-facial nerve anastomosis was carried out in 20 adult guinea pigs. Horseradish peroxidase (HRP) was applied to the buccal branch of the facial nerve at intervals of 2, 4 and 6 months after the anastomosis operation. HRP labeled neurons and the number of regenerated axons in the buccal branch were counted to determine postoperative nerve regeneration. The number of surviving neurons in the facial and hypoglossal nuclei was also counted to determine postoperative change of these nuclei. Following anastomosis, 97% of the hypoglossal neurons remained surviving, while the facial neurons underwent pronounced degeneration of 65% survival ratio at 2 months and 37% at 6 months after the anastomosis operation. In 80% of the animals, a new nerve bundle regenerated from the proximal stump of the facial nerve to the anastomotic trunk. A linear increase of HRP-labeled neurons in the facial and hypoglossal nuclei paralleled the increase of the axons in the buccal branch. The HRP-labeled neurons in the facial nucleus were demonstrated to have direct connection with the newly formed bundle and the facial mimetic muscles were dually innervated by both the hypoglossal and facial nerves. Although the present study design might not fully represent the clinical situation, possible advantages of hypoglossal-facial nerve anastomosis are discussed from the view of nerve regeneration.

Modulation by divalent cations of the current generated by vasopressin in facial motoneurons

Vasopressin generates a voltage-gated, sodium-dependent current in facial motoneurons in brainstem slices. Reducing the extracellular calcium concentration from 2 to 0.01 mM caused a 30 to 120% increase in the amplitude of this current. Lowering extracellular magnesium also enhanced it, but less efficiently. In the physiological solution, the response of facial neurons to vasopressin is thus partially blocked. Increasing extracellular calcium was without effect. Current-voltage curves indicate that the vasopressin current reversed at around 0 mV and suggest that the low-calcium-induced potentiation was due to an attenuation of the region of negative slope conductance.

Metabolic changes in axotomized fetal and early postnatal hamster facial motoneurons: An autoradiographic study

The developing facial neurons of a series of hamsters ranging in age from the 14-day fetus to the 9 day postnatal were axotomized. Postoperative times were graded for each age so that the retrograde response could be observed before any significant amount of cell degeneration or death occurred. The incorporation of tritiated uridine was followed by the autoradiographic procedure. Although grain counts, relative to control values, were significantly reduced only in the axotomized fetus and at 24 hours postoperatively in 4-day postnatal animals, there was also a repression of isotopic incorporation in all the other axotomized animals. These results support data obtained from previous work with the hamsters which indicate that it is not until after the nerve cell nucleolus reaches full cytomorphic maturity (between 15 and 20 days postnatal age in hamster facial neurons) that the axotomized neurons respond with significantly increased incorporation levels of isotope over that of control neurons.

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.

Redirection of the hypoglossal nerve to facial muscles alters central connectivity in human brainstem

Functional motor control requires perfect matching of central connectivity of motoneurones with their peripheral connections. However, it is not known to what extent central circuitry is influenced by target muscles, either during development or following a lesion. Surgical interventions aimed at restoring function following peripheral nerve lesions provide an opportunity for studying this interaction in the mature human nervous system. We have followed 8 patients in whom the hypoglossal nerve was anastomosed into a lesioned facial nerve, allowing voluntary contractions of the previously paralyzed muscles. We show that, in addition to replacing the facial neurons at peripheral synapses, a new short-latency trigemino-hypoglossal reflex, of the R1 blink reflex type, can be demonstrated in patients showing recovery, implying a sprouting of trigeminal neurons towards hypoglossal motoneurones, over a distance of at least 0.5 cm. These surprising results show an unexpected influence of the periphery in remodelling central connectivity in man.

Testosterone effects on ribosomal RNA levels in injured peripheral motor neurons: A preliminary report

We have previously demonstrated that administration of testosterone to hamsters during the early phases of axonal regeneration following facial nerve injury accelerates both the rate of regeneration of the fastest growing population of axons and the return of functional movement. We hypothesized from those studies that testosterone primes the neuronal cell body in such a way as to accelerate the "switch" from a normal to a reparative state. That hypothesis was tested in this study using ribosomal DNA (rDNA) probes in conjunction with in situ hybridization to map the molecular response of the polymerase I system to axotomy, with and without hormone exposure. Adult male hamsters were subjected to right facial nerve severance, with the left side serving as an internal control. Half the animals were administered testosterone propionate via subcutaneous implants. In situ hybridization using a genomic rDNA probe complementary to the 28S rRNA species was accomplished, and levels of rRNA in injured facial neurons assessed both qualitatively and quantitatively. Our initial findings indicate that testosterone markedly upregulates rRNA levels after injury, and support the hypothesis of an acceleration in the metabolic switch to a reparative state. This leads us, in turn, to suggest that this effect of testosterone on the ribosomal system is causally related to the increase in axonal regeneration rate and return of functional movement previously documented in this system.

Responses of single facial taste fibers in the sea catfish, Arius felis, to amino acids

1. Taste buds in catfish are found not only within the oropharyngeal cavity, as in mammals, but are also located along the external body surface of the animal from the barbels and lips to the caudal fin. Because these taste buds are innervated by the facial (cranial VII) nerve, the extraoral taste system of catfish is analogous to the mammalian taste system of the anterior two-thirds of the tongue, which contains taste buds innervated by the chorda tympani nerve, and of the soft palate and nasoincisor ducts, which contain taste buds innervated by the greater superficial petrosal nerve. 2. The majority of information concerning the specificity of individual taste fibers in vertebrates has been obtained primarily in mammals to stimuli representing the four basic human taste qualities (i.e., salty, sweet, sour, and bitter). In the present report, we examine the evidence for gustatory fiber types within the stimulus class of amino acids, compounds known to be especially relevant gustatory stimuli for catfish and other teleosts. 3. Action potentials were recorded from 60 individual facial taste neurons obtained from 28 sea catfish (Arius felis). Stimuli were 10(-4) M concentrations of L-alanine, D-alanine, glycine, L-proline, L-histidine, and L-arginine, compounds selected from an original stimulus list of 28 amino acids. Responses were quantified as the number of action potentials evoked at various time intervals from the first 0.5 s up to 10 s of response time. 4. The spontaneous activity of 42 fully characterized neurons was 0.8 +/- 2.1 SD spikes/3 s. The average rate of spike discharge increased 50-fold during stimulation with the most effective amino acid (42 +/- 31 spikes/3 s, mean +/- SD). The majority of the sampled neurons were not narrowly tuned to the amino acid stimulants tested (mean breadth of responsiveness, H = 0.60; range 0-0.95). 5. Hierarchical cluster analysis of the fully characterized neurons identified two large and two small groups of cells. The largest group (n = 22) of neurons was stimulated most by L-alanine and glycine; the other large group (n = 17) was stimulated most by D-alanine. For this latter group, the response to glycine was relatively low, whereas the responses to L-alanine varied from 0 to nearly 100% of the D-alanine response.(ABSTRACT TRUNCATED AT 400 WORDS)

Ultrastructural and morphometric analysis of nuclear and nuclear changes during the early growth period in hamster facial neurons

In this study, progressive developmental changes in the nucleus and associated organelles, including the nucleolus, coiled bodies, nuclear envelope, and nucleoplasm, of hamster facial motor neurons were characterized by two parallel analyses: ultrastructural and morphometric. Golden hamsters (Mesocricetus auratus) used for this series were the 14-day fetus, newborn (less than 6 hr), and 1, 2, 3, 4, 5, 7, 9, 11 and 13 days postnatal ages, with 3 animals per group. Following anesthesia and perfusion fixation, facial nuclear groups were dissected and processed for electron microscopy. Electron micrographs and camera lucida tracings of nuclear profiles were collected and analyzed. The ultrastructural analysis revealed progressive changes in the nucleolus from a compact, segregated type to a reticulated form characteristic of actively protein-secreting cells. Nucleolar microbodies and fibrillar centers were seen at all ages; the latter structures appeared to decrease in size and increase with age in the series. The nucleolus-associated chromatin became less condensed, suggesting an increase in the incorporation of rDNA into the nucleolus proper. Coiled bodies, both free and attached to nucleoli, were found in varying frequencies. The nucleoplasm of neurons at the earliest stages contained large numbers of heterochromatin clumps, which decreased concomitantly with an increase in interchromatin granules and fibrils during the later stages. Nuclear envelope invaginations, polarized along one side of the nucleus, increased throughout the developmental period examined. These changes occurred in concert with a 61% increase in nuclear size and a 47% increase in the length of nuclear envelope. The sequence of nuclear changes observed during this early period of normal facial neuronal growth completes the study of a series of distinctly defined cytomorphic events in this cell type, the lability of which can be experimentally tested for their functional roles in neuronal development.

Effects of advancing age on the central response of rat facial neurons to axotomy: Light microscope morphometry

Following axotomy, the regrowth of peripheral axons takes longer in older individuals than in young ones. The present study compares central responses of facial motor neurons to a crush injury of the facial nerve in 3-month-old and 15-month-old male rats sampled through 28 days post-crush (dpc). Neuronal somata, nuclei, and nucleoli were measured in 30 microns brain stem sections within subdivisions of the facial nucleus that contain the cell bodies responsible for the movement of the vibrissae. The temporal patterns of change in the size of the three structures were interpreted with reference to the re-establishment of functional connections, i.e., the return of voluntary vibrissae activity, which is delayed by 4 days in the older animals relative to the younger ones. There was no age-related difference in the pattern of somal swelling and recovery, nor was there an age-related difference in the response of nuclei and nucleoli to axotomy through 4 dpc. Both nuclei and nucleoli increased in size in animals of both age groups, but after 4 dpc in the older animals nuclear enlargement was prolonged and the nucleolar increases were less robust compared to the younger animals. The greatest age difference appeared with the re-establishment of functional connections. In the 3-month-old animals, the resumption of whisker activity coincided with vigorous transient increases in the sizes of nuclei and nucleoli; in the 15-month-old animals, there was little nuclear response to functional recovery and a comparatively small increase in nuclear sizes.(ABSTRACT TRUNCATED AT 250 WORDS)

Testosterone-induced acceleration of recovery from facial paralysis following crush axotomy of the facial nerve in male hamsters

In this study, the effects of testosterone propionate (TP) on recovery from facial paralysis following crush axotomy of the facial nerve in male hamsters were examined. In the first experiment, TP (5 mg/ml sesame oil; 0.1 ml) was injected subcutaneously and on alternate days in one-half of the animals subjected to crush axotomies of the facial nerve, with the second half receiving vehicle alone. An accelerative effect of TP on recovery from facial paralysis was observed near the end of the first and beginning of the second week after crush axotomy. When the dosage and frequency were doubled in the next experiment, a greater accelerative effect of TP on recovery from facial paralysis was observed. In the last experiment, castrated animals were used in order to eliminate the endogenous source of the hormone and two different modes of hormone administration, TP implants vs TP injections, were compared. The results of that experiment indicate that continuous exposure to the hormone, in the form of subcutaneous implants of 100% crystalline TP, had the most pronounced effect on acceleration of recovery from facial paralysis. In addition, no differences in the responses of the castrated, axotomized animals and the normal, axotomized animals were found. This suggests that the presence of endogenous hormone contributes little to the acceleration of functional recovery observed with TP. Finally, the time course of the accelerative effect of TP suggests that the hormone is acting primarily at the level of the facial neuron, which contains androgen receptors, and perhaps secondarily at the level of the facial muscles, which are also known to contain androgen receptors.

Non-motoneurons in the facial and motor trigeminal nuclei projecting to the cerebellar flocculus in the cat

The cerebellar projection from the facial and motor trigeminal nuclei was studied in the cat by means of retrograde axonal transport of wheat germ agglutinin-horseradish peroxidase and fluorescent tracers. The feline facial nucleus was cytoarchitectonically subdivided into ventromedial, ventrolateral, lateral, dorsal, intermediate and medial divisions (see Papez 1927), and the motor trigeminal nucleus into medial, ventral, intermediate, lateral and dorsal divisions. The neurons in the facial and motor trigeminal nuclei were classified as small (ovoid to round cells with a maximum diameter of the cell body of about 20 microns) or large (polygonal to round cells with maximum diameter of about 40 microns). After floccular injections of the wheat germ agglutinin-horseradish peroxidase complex, retrogradely labelled cells were found throughout the facial nucleus, but especially in its medial and dorsal divisions. In the motor trigeminal nucleus, labelled neurons were found only in the ventral, intermediate and lateral divisions. Cases with tracer deposition (implants or injections) in other parts of the cerebellar cortex or nuclei were all negative. All facial and motor trigeminal neurons labelled after floccular injections were smaller than the neurons labelled after injections in the facial mimic and masticatory muscles, and only single-labelled neurons were found following floccular injections of Fluoro-Gold and muscular injections of rhodamine-B-isothiocyanate in the same animals. These observations strongly suggest that the neurons in the facial and motor trigeminal nuclei which project to flocculus are of the non-motoneuron type.

The motor innervation of the soft palate. An anatomical study in guinea pigs and monkeys.

In an effort to gain a more precise understanding of the motor innervation of the soft palate, a neuroanatomical tracer (horseradish peroxidase) was injected into the vela of nine guinea pigs and four monkeys. After 48 h the animals were sacrificed and frozen frontal sections were appropriately reacted with tetramethylbenzidine and counterstained with neutral red, and then evaluated by means of light microscopy. In all specimens retrogradely labeled neurons appeared in the brain stem. The distribution patterns of these labeled cells were to a large degree comparable in both the guinea pigs and the monkeys. The complex motor innervation of the soft palate was found to involve at least five different centers, with the strongest staining seen in the nucleus ambiguus and the motor nucleus of the trigeminal nerve. Additional cells of origin were discernible in the retrofacial nucleus. Facial neurons, however, were labeled only occasionally. Findings also showed that a previously undescribed component from the hypoglossal nerve appears to be essential for lateral soft palate movement in both animal models.

Direct axonal projection of neurons in pontomedullary reticular formation to facial nucleus neurons innervating orbicularis oculi muscle in cat

Direct axonal projection of neurons in pontomedullary reticular formation to facial nucleus neurons innervating orbicularis oculi muscle in cat

Differential effects of axotomy on immature and mature hamster facial neurons: a tritiated-uridine autoradiographic study.

In this study, tritiated-uridine incorporation was autoradiographically examined following axotomy of hamster facial motor neurons (HFMN) at the critical development age of 15 days postnatal and in the adult. The postoperative times selected were 0.5, 1, 2, and 4 days. In the 15-day operative series, no changes in incorporation were observed at any of the postoperative times, except at 4 days postoperative, when there was a decrease in tritiated-uridine incorporation in the axotomized neurons relative to the controls. In the adult operative series there were no changes in incorporation at 0.5 or 1 day postoperative, relative to the controls. At 2 days postoperative in the adult, there was a transient increase in tritiated-uridine incorporation that returned to control levels by 4 days postoperative. When axotomized and control cytoplasmic/nuclear grain densities were compared, no changes were found in either operative series. These results of the time course of axotomy-induced changes in RNA synthesis in HFMN corroborate our previous findings of an age-dependent reactive sequence in HFMN and lend support to the hypothesis that the young neurons are synthesizing at peak capacity related to final growth and cannot be stimulated further by axotomy. As discussed, the transient increase in RNA levels in the adult, the lack of any changes in the rate of transfer of RNA from the nucleus to the cytoplasm, and the decrease in RNA levels in the 15-day neurons may be related to the presence of an unusual intranucleolar body within the nucleolus of HFMN that contains ribosomal precursors.

Retrograde transport and differential accumulation of serum proteins in motor neurons

We found immunocytochemical evidence of serum albumin and immunoglobulin G in motor neurons of rats under physiologic conditions, but not when axoplasmic flow was abolished. The amount of serum proteins was highest in somatic motor neurons of the spinal anterior horn, nucleus ambiguous, and trigeminal motor nucleus; less in hypoglossal and facial neurons; and sparse in preganglionic autonomic, oculomotor, and hypothalamic neurons. Toxic or trophic substances, bound to serum proteins, could also be incorporated into motor neurons. Quantitative differences in accumulation paralleled the selective vulnerability of somatic motor neurons in motor neuron diseases.

Ultrastructural changes in the Nucleoplasm of Hamster Facial Neurons during a postnatal maturation period

Changes in the characteristics of the nucleoplasm were examined in young hamster facial motoneurons of 15 and 20 days postnatal age and in the adult (100 days postnatal age) at both the light and electron microscope levels. In toluidine-blue stained 1-μm thick sections, a progressive increase in basophilic islands within the nucleoplasm occurred during maturation. Ultrastructural changes that were observed during final development included a transition from a homogeneous, ‘filled-in’ appearing nucleoplasm to a clumped-appearing nucleoplasm. This process principally involved the formation of distinct clusters of interchromatin granules that was associated with a loss of fine fibrils, an increase in clear spaces between intervening fibrillar and granular material, and an increase in small scattered clumps of heterochromatin. These changes in both ribonucleoprotein (RNP)- and DNA-containing nuclear constituents (interchromatin granules and heterochromatin, respectively) occurred during a postnatal maturational period previously demonstrated to involve other alterations in nuclear structures. It is interpreted that these cytomorphic changes in the nucleoplasm reflect an underlying metabolic shift at the transcriptional level during the transition from an actively growing neuron to an adult functioning neuron.