Acta Oto-Laryngologica
ISSN: 0001-6489 (Print) 1651-2251 (Online) Journal homepage: http://www.tandfonline.com/loi/ioto20
Altered Distribution of Motor Neurons in Experimental Facial Nerve Paralysis Yoshiaki Nakao, Koji Matsumoto & Hidehaku Kumagami To cite this article: Yoshiaki Nakao, Koji Matsumoto & Hidehaku Kumagami (1992) Altered Distribution of Motor Neurons in Experimental Facial Nerve Paralysis, Acta Oto-Laryngologica, 112:6, 998-1003, DOI: 10.3109/00016489209137501 To link to this article: http://dx.doi.org/10.3109/00016489209137501
Published online: 08 Jul 2009.
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Date: 25 March 2016, At: 23:27
Acta Otolaryngol (Stockh) 1992; 112: 998-1003
Altered Distribution of Motor Neurons in Experimental Facial Nerve Paralysis YOSHIAKI NAKAO, KOJI MATSUMOTO and HIDEHAKU KUMAGAMI
Downloaded by [University of California Santa Barbara] at 23:27 25 March 2016
From the Department of Otolaryngology, Nagasaki University. School of Medicine 7-1, Sakamoto Machi, Nagasaki, 852, Japan
Nakao Y. Matsumoto K, Kumagami H. Altered distribution of motor neurons in experimental facial nerve paralysis. Acta Otolaryngol (Stockh) 1992; 112 998-1003. The alteration of motor neurons in the brainstem after recovery from experimental facial nerve paralysis was examined by the retrograde horseradish peroxidase (HRP) technique in the rabbit. Six months after nerve crush injury at the center of the vertical portion, HRP was injected into the zygomatic muscle on the recovered side. The distribution of labelled neurons in the brainstem was compared with that in the normal rabbit. In control animals, motor neurons in the facial nucleus were somatotopically organized, and there were no labelled neurons in other nuclei in the brainstem. In recovered animals, on the contrary, the somatotopic organization of the facial nucleus was obscure and multipolar neurons of varying size were labelled bilaterally in the reticular formation from the pons to the medulla. Key worak: facia[ nucleus, facial nerve paralysis, horseradish peroxidase, rabbit.
INTRODUCTION In Bell's palsy and in Ramsay Hunt's syndrome, sequelae such as synkinesis, facial spasm or contracture occur during recovery from facial nerve palsy. The mechanism of these sequelae has not yet been sufficiently clarified. The misdirection of regenerating nerve fibers at the site of injury may result in synkinesis. However, other possible mechanisms including rearrangement of synaptic vesicles in the facial nucleus, have been suggested (1). Cases of spontaneous return of mimic muscle function after facial nerve excision during parotidectomy have also been reported (2, 3), indicating the re-establishment of motor pathways to the facial musculature by way of the trigeminal nerve. Moreover, connections between the trigeminal and facial nerve systems may occur centrally within the brainstem, as deep facial communications or as superficial communications (4). The aim of the present investigation was to study the distribution of motor neurons in the brainstem after recovery from experimental facial nerve paralysis by the use. of the horseradish peroxidase (HRP) technique and thereby obtain more knowledge on the possible mechanisms underlying the development of facial dyskinesis.
I. CENTRAL LOCALIZATION O F MOTOR NEURONS INNERVATING FACIAL MUSCLES AND MASSETER MUSCLE IN THE NORMAL RABBIT Material and methodr Ten rabbits weighing about 2 kg each were used. The rabbits were anesthetized with an intravenous injection of sodium pentobarbital (30 mg/kg body weight). A small incision was made over the muscle to be studied, i.e. the orbicularis oculi, orbicularis oris, zygomaticus and the masseter muscle. After exposure of the muscle under an operating microscope, 50% HRP (TOYOBO) solution was injected into the muscle belly using a microliter syringe. The amount of injected HRP varied from 6- 10 pl. Care was taken to avoid unintentional spilling
Motor neurons and nerve paralysis 999
Acta Otolaryngol (Stockh) 112
of the solution. Forty-eight hours after injection of HRP, the rabbits were anesthetized and fixed by intracardiac perfusion with 2 1 of saline solution, followed by 21 of 2.5% glutaraldehyde in 0.2 mol phosphate buffer (pH 7.2). The brainstems were then carefully removed and cut serially into 100 pm-thick slices on a transverse plane using a freezing microtome. Staining for peroxidase activity was carried out by the diaminobenzidine method.
Downloaded by [University of California Santa Barbara] at 23:27 25 March 2016
RESULTS Table I summarizes the results in normal animals following injection into the facial muscles. In animals No. 1 and 2 receiving injections into the orbicularis oculi muscle, the labelled neurons were located in the dorsomedial portion of the facial nucleus. The number of labelled neurons was 23 and 24, respectively. In animals No. 3, 4 and 5 receiving injections into the zygomatic muscle, the labelled neurons were located in the ventromedial portion of the facial nucleus. The number of labelled neurons was 16, 25 and 35, respectively. In animals No. 6 and 7 receiving injections into the orbicularis oris muscle, the labelled neurons were located in the ventrolateral portion of the facial nucleus. The number of labelled neurons was 11 and 20, respectively. The motor neurons supplying the facial muscles in the normal rabbit were somatotopically organized, and there were no labelled neurons in other nuclei in the brainstem (Fig. I). In the other 3 animals, which received injections into the masseter muscle, labelled neurons were located in the midpontine level dorsal and rostra1 to the facial nucleus (Fig. 1). The number of labelled neurons was 56, 63 and 116. There was no uptake in the contralateral side and no labelled neurons were found in other nuclei.
Table I. Location of IabelIed neuronsfollowing injection of HRP into the facial muscles in the control and recovered animals Labelled cells
Number
Injection site
Control animal 1 orbicularis oculi 2 orbicularis oculi 3 zygomaticus 4 zygomaticus 5 zygomaticus 6 orbicularis oris 7 orbicularis oris
Recovered animal 1 zygomaticus 2 zygomaticus 3 zygomaticus 5 zygomaticus
Number
Location in facial nucleus
Other site labelled in brainstem
23 24 16 25 35 I1 20
dorsomedial dorsomedial ventromedial ventromedial ventromedial ventrolateral ventrolateral
none none none none none none none
20 26
medial medial medial medial
nuclei reticularis nuclei reticularis nuclei reticularis nuclei reticularis
29 30
Downloaded by [University of California Santa Barbara] at 23:27 25 March 2016
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Y. Nakao et al.
x
Marastar
A
Orbicularis Oculi Orbicularis Oris typomaticus
Acta Otolaryngol (Stockh) 112
Fig. I . Drawing of transverse sections showing
HRP labelled neurons innervating facial muscles and the masseter muscle.
11. OBSERVATION O F MOTOR NEURONS AFTER RECOVERY FROM EXPERIMENTAL FACIAL NERVE PARALYSIS
Material and metho& Five rabbits weighing about 1.5 kg each were used. Under general anesthesia, the intratemporal portion of the facial nerve ranging from the stapedial muscle to the stylomastoid foramen was exposed under an operating microscope and then crushed with a hemostat forceps for 20 seconds at the center of the vertical portion. Three months flfter the nerve injury, the return of motor function was evaluated by testing the corneal reflex and by the electrophysiological method. The evoked electromyography (EMG) was recorded from the orbicularis oris muscle with bipolar needle electrode by electrical stimulation of the facial nerve trunk at the stylomastoid foramen. EMG recordings of synkinesis were not made. Six months after the nerve injury, HRP was injected into the zygomatic muscle on the recovered side. The subsequent procedure was the same as that described in the control study. RESULTS The rabbits showed dropear immediately after the nerve damage. Touching the cornea caused no blink on the damaged side. Three months later, however, the animals were able to raise their auricles and to produce blinking. In addition, synkinetic movement of the auricle was observed only on the damaged side simultaneously with the eye blink. Evoked electromyography showed evoked potentials with no clear difference other than a prolongation of duration as compared to the control side. Table I shows the results in damaged animals following injection of HRP into the zygomatic muscle. After recovery from the facial nerve paralysis, labelled neurons innervating the zygomatic muscle were located not only in the ventromedial portion but also partially in the dorsomedial portion where there were labelled neurons innervating the orbicularis oculi muscle in the control animals (Fig. 2, 3). The number of labelled motor neurons was 27 Ifr 4.
Downloaded by [University of California Santa Barbara] at 23:27 25 March 2016
Acta Otolaryngol (Stockh) 112
A
0
Orbicularis Oculi Orbicularis O r i s Zy~omaticur Zyuomaticus (recovered)
Glycocalyx in normal and hvdrouic cochleas 1001
Fig. 2. Drawing of transverse section showing HRF' labelled facial neurons in the control and
recovered animals.
Fig. 3. Photomicrographs of labelled facial neurons following injection of HRP into the zygomatic muscle in the control (A) and recovered (B) animals. x 16.
Furthermore, multipolar neurons of varying size were labelled bilaterally in the reticular formation from the pons to the medulla (Fig. 4, 5). These neurons contained granules of HRP which were brown in color but paler than those in the facial nucleus (Fig. 6).
DISCUSSION Horseradish peroxidase (HRP) injected into the muscle is taken up at the neuromuscular junction and carried by retrograde axonal transport to the motor neurons. Our finding in rabbits that labelled motor neurons in the facial nucleus of control animals are somatotopically organized and not found in the other brainstem nuclei is similar to that reported previously in the cat (5). As in the study by Thomander (6), the somatotopic organization of the facial nucleus was obscure in recovered animals. This is thought to be due to the fact that regenerating axom are misdirected at the site of injury. The brainstem afferent fiber system to the facial nucleus was examined in the cat following the injection of cholera toxin as a retrograde tracer into the facial nucleus (7). Labelled neurons were observed bilaterally in the nuclei reticularis of both pons and medulla.
Downloaded by [University of California Santa Barbara] at 23:27 25 March 2016
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Y. Nakao
et
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Acta Otolaryngol (Stockh) I I2
Fig. 4. Drawing of transverse sections through trigeminal motor nucleus (A), superior olivary nucleus (B),and facial nucleus (C) levels following injection of HRP into the zygomatic muscle on the recovered side. Each dot represents one labelled neuron. R motor trigeminal nucleus; 7: labelled neurons in the facial nucleus.
Fig. 5. Photomicrograph of HRP labelled neurons in the reticular formation of the pons. Arrow indicates the raphe. x 16.
Fig. 6. photomicrograph of HRP labelled neurons in the retilcular formation of the pons. The HRP granules are visible in the qtoplasm as well as the dendrite. x 200.
Downloaded by [University of California Santa Barbara] at 23:27 25 March 2016
Acta
Otolaryngol (Stockh) 112
Glycocalyx in normal and hydropic cochleas
Therefore, the labelled neurons found bilaterally from the pons to the medulla in the present study are suggested to be premotor neurons to the facial nucleus. It is not known whether the HRP reaction product, transported retrogradely to a cell soma or its dendrites, is transported transsynaptically to terminals of other cells establishing synapses with labelled cells (8). Electronmicroscopic investigations have revealed that after axotomy, microglial cells intrude into the synaptic clefts, thereby removing the terminals from the soma (9). Although the mechanism of labelled neurons observed bilaterally in the brainstem reticular formation remains to be elucidated, the interneuronal transport of HRP may occur after recovery from facial nerve paralysis. Furthermore, the lower brainstem reticular formation can control a variety of spontaneous and reflex facial movements (10). The pontine reticular formation area described as a pontine blink premotor area plays a role in blink reflex ( 1 I). It seems that the premotor neurons labelled in this study also may have a role in muscle movements after recovery from facial nerve palsy.
ACKNOWLEDGEMENTS The authors are grateful to Dr. lwahori for helpful comments, and to Dr. Ito for technical guidance.
REFERENCES 1. Crumley RL. Mechanisms of synkinesis. Laryngoscope 1979; 8 9 1847-54. 2. Martin H, Helsper JT. Spontaneous return of function following surgical section or excision of the seventh cranial nerve in the surgery of parotid tumors. Ann Surg 1957; 146: 715-27. 3. Cerny LJ, Steidl LJ. Reinnervation after resection of the facial nerve. Acta Otolaryngol (Stockh) 1974; 77: 102-7. 4. Baumel JJ. Trigeminal-facial nerve communications: their function in facial muscle innervation and reinnervation. Arch Otolaryngol 1974; 9 9 33-44. 5. Radpour S. Organization of the facial nerve nucleus in the cat. Laryngoscope 1977; 87: 557-74. 6. Thomander L. Reorganization of the facial motor nucleus after peripheral nerve regeneration. Acta Otolaryngol (Stockh) 1984; 9 7 619-26. 7. Fort P, Sakai K, Luppi PH, Salvert D, Jouvet M. Monoaminergic, peptidergic, and cholinergic afferents to the cat facial nucleus as evidenced by double immunostaining method with unconjugated cholera toxin as a retrograde tracer. J Comp Neurol 1989; 283: 285-302. 8. Brodal A. Neurological anatomy; In relation to clinical medicine. 3rd ed. Oxford: University Press, 1981: 3-45. 9. Blinzinger K. Displacement of synaptic terminals from regenerating motoneurons by microglial cells. Z Zellforsch 1968; 85: 145-57. 10. Sherrington CS. Reflexes elicitable in the cat from pinna, vibrissae and jaws. J Physiol 1917; 51: 401-31.
11. Holstage G, Tan J, Han JV,Graveland A. Anatomical observations on the afferent projections to the retractor bulbi motoneuronal cell group and other pathways possibly related the blink reflex in the cat. Brain Res 1986: 374 321-34. Manuscript received February 4, I992 accepted June 11, 1992
Address for correspondence: Yoshiaki Nakao, Nagasaki University, School of Medicine. Department of Otolaryngology, 7-1, Sakamoto Machi, Nagasaki, 852, Japan
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ISSN: 0001-6489 (Print) 1651-2251 (Online) Journal homepage: http://www.tandfonline.com/loi/ioto20
Altered Distribution of Motor Neurons in Experimental Facial Nerve Paralysis Yoshiaki Nakao, Koji Matsumoto & Hidehaku Kumagami To cite this article: Yoshiaki Nakao, Koji Matsumoto & Hidehaku Kumagami (1992) Altered Distribution of Motor Neurons in Experimental Facial Nerve Paralysis, Acta Oto-Laryngologica, 112:6, 998-1003, DOI: 10.3109/00016489209137501 To link to this article: http://dx.doi.org/10.3109/00016489209137501
Published online: 08 Jul 2009.
Submit your article to this journal
Article views: 2
View related articles
Citing articles: 1 View citing articles
Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=ioto20 Download by: [University of California Santa Barbara]
Date: 25 March 2016, At: 23:27
Acta Otolaryngol (Stockh) 1992; 112: 998-1003
Altered Distribution of Motor Neurons in Experimental Facial Nerve Paralysis YOSHIAKI NAKAO, KOJI MATSUMOTO and HIDEHAKU KUMAGAMI
Downloaded by [University of California Santa Barbara] at 23:27 25 March 2016
From the Department of Otolaryngology, Nagasaki University. School of Medicine 7-1, Sakamoto Machi, Nagasaki, 852, Japan
Nakao Y. Matsumoto K, Kumagami H. Altered distribution of motor neurons in experimental facial nerve paralysis. Acta Otolaryngol (Stockh) 1992; 112 998-1003. The alteration of motor neurons in the brainstem after recovery from experimental facial nerve paralysis was examined by the retrograde horseradish peroxidase (HRP) technique in the rabbit. Six months after nerve crush injury at the center of the vertical portion, HRP was injected into the zygomatic muscle on the recovered side. The distribution of labelled neurons in the brainstem was compared with that in the normal rabbit. In control animals, motor neurons in the facial nucleus were somatotopically organized, and there were no labelled neurons in other nuclei in the brainstem. In recovered animals, on the contrary, the somatotopic organization of the facial nucleus was obscure and multipolar neurons of varying size were labelled bilaterally in the reticular formation from the pons to the medulla. Key worak: facia[ nucleus, facial nerve paralysis, horseradish peroxidase, rabbit.
INTRODUCTION In Bell's palsy and in Ramsay Hunt's syndrome, sequelae such as synkinesis, facial spasm or contracture occur during recovery from facial nerve palsy. The mechanism of these sequelae has not yet been sufficiently clarified. The misdirection of regenerating nerve fibers at the site of injury may result in synkinesis. However, other possible mechanisms including rearrangement of synaptic vesicles in the facial nucleus, have been suggested (1). Cases of spontaneous return of mimic muscle function after facial nerve excision during parotidectomy have also been reported (2, 3), indicating the re-establishment of motor pathways to the facial musculature by way of the trigeminal nerve. Moreover, connections between the trigeminal and facial nerve systems may occur centrally within the brainstem, as deep facial communications or as superficial communications (4). The aim of the present investigation was to study the distribution of motor neurons in the brainstem after recovery from experimental facial nerve paralysis by the use. of the horseradish peroxidase (HRP) technique and thereby obtain more knowledge on the possible mechanisms underlying the development of facial dyskinesis.
I. CENTRAL LOCALIZATION O F MOTOR NEURONS INNERVATING FACIAL MUSCLES AND MASSETER MUSCLE IN THE NORMAL RABBIT Material and methodr Ten rabbits weighing about 2 kg each were used. The rabbits were anesthetized with an intravenous injection of sodium pentobarbital (30 mg/kg body weight). A small incision was made over the muscle to be studied, i.e. the orbicularis oculi, orbicularis oris, zygomaticus and the masseter muscle. After exposure of the muscle under an operating microscope, 50% HRP (TOYOBO) solution was injected into the muscle belly using a microliter syringe. The amount of injected HRP varied from 6- 10 pl. Care was taken to avoid unintentional spilling
Motor neurons and nerve paralysis 999
Acta Otolaryngol (Stockh) 112
of the solution. Forty-eight hours after injection of HRP, the rabbits were anesthetized and fixed by intracardiac perfusion with 2 1 of saline solution, followed by 21 of 2.5% glutaraldehyde in 0.2 mol phosphate buffer (pH 7.2). The brainstems were then carefully removed and cut serially into 100 pm-thick slices on a transverse plane using a freezing microtome. Staining for peroxidase activity was carried out by the diaminobenzidine method.
Downloaded by [University of California Santa Barbara] at 23:27 25 March 2016
RESULTS Table I summarizes the results in normal animals following injection into the facial muscles. In animals No. 1 and 2 receiving injections into the orbicularis oculi muscle, the labelled neurons were located in the dorsomedial portion of the facial nucleus. The number of labelled neurons was 23 and 24, respectively. In animals No. 3, 4 and 5 receiving injections into the zygomatic muscle, the labelled neurons were located in the ventromedial portion of the facial nucleus. The number of labelled neurons was 16, 25 and 35, respectively. In animals No. 6 and 7 receiving injections into the orbicularis oris muscle, the labelled neurons were located in the ventrolateral portion of the facial nucleus. The number of labelled neurons was 11 and 20, respectively. The motor neurons supplying the facial muscles in the normal rabbit were somatotopically organized, and there were no labelled neurons in other nuclei in the brainstem (Fig. I). In the other 3 animals, which received injections into the masseter muscle, labelled neurons were located in the midpontine level dorsal and rostra1 to the facial nucleus (Fig. 1). The number of labelled neurons was 56, 63 and 116. There was no uptake in the contralateral side and no labelled neurons were found in other nuclei.
Table I. Location of IabelIed neuronsfollowing injection of HRP into the facial muscles in the control and recovered animals Labelled cells
Number
Injection site
Control animal 1 orbicularis oculi 2 orbicularis oculi 3 zygomaticus 4 zygomaticus 5 zygomaticus 6 orbicularis oris 7 orbicularis oris
Recovered animal 1 zygomaticus 2 zygomaticus 3 zygomaticus 5 zygomaticus
Number
Location in facial nucleus
Other site labelled in brainstem
23 24 16 25 35 I1 20
dorsomedial dorsomedial ventromedial ventromedial ventromedial ventrolateral ventrolateral
none none none none none none none
20 26
medial medial medial medial
nuclei reticularis nuclei reticularis nuclei reticularis nuclei reticularis
29 30
Downloaded by [University of California Santa Barbara] at 23:27 25 March 2016
lo00
Y. Nakao et al.
x
Marastar
A
Orbicularis Oculi Orbicularis Oris typomaticus
Acta Otolaryngol (Stockh) 112
Fig. I . Drawing of transverse sections showing
HRP labelled neurons innervating facial muscles and the masseter muscle.
11. OBSERVATION O F MOTOR NEURONS AFTER RECOVERY FROM EXPERIMENTAL FACIAL NERVE PARALYSIS
Material and metho& Five rabbits weighing about 1.5 kg each were used. Under general anesthesia, the intratemporal portion of the facial nerve ranging from the stapedial muscle to the stylomastoid foramen was exposed under an operating microscope and then crushed with a hemostat forceps for 20 seconds at the center of the vertical portion. Three months flfter the nerve injury, the return of motor function was evaluated by testing the corneal reflex and by the electrophysiological method. The evoked electromyography (EMG) was recorded from the orbicularis oris muscle with bipolar needle electrode by electrical stimulation of the facial nerve trunk at the stylomastoid foramen. EMG recordings of synkinesis were not made. Six months after the nerve injury, HRP was injected into the zygomatic muscle on the recovered side. The subsequent procedure was the same as that described in the control study. RESULTS The rabbits showed dropear immediately after the nerve damage. Touching the cornea caused no blink on the damaged side. Three months later, however, the animals were able to raise their auricles and to produce blinking. In addition, synkinetic movement of the auricle was observed only on the damaged side simultaneously with the eye blink. Evoked electromyography showed evoked potentials with no clear difference other than a prolongation of duration as compared to the control side. Table I shows the results in damaged animals following injection of HRP into the zygomatic muscle. After recovery from the facial nerve paralysis, labelled neurons innervating the zygomatic muscle were located not only in the ventromedial portion but also partially in the dorsomedial portion where there were labelled neurons innervating the orbicularis oculi muscle in the control animals (Fig. 2, 3). The number of labelled motor neurons was 27 Ifr 4.
Downloaded by [University of California Santa Barbara] at 23:27 25 March 2016
Acta Otolaryngol (Stockh) 112
A
0
Orbicularis Oculi Orbicularis O r i s Zy~omaticur Zyuomaticus (recovered)
Glycocalyx in normal and hvdrouic cochleas 1001
Fig. 2. Drawing of transverse section showing HRF' labelled facial neurons in the control and
recovered animals.
Fig. 3. Photomicrographs of labelled facial neurons following injection of HRP into the zygomatic muscle in the control (A) and recovered (B) animals. x 16.
Furthermore, multipolar neurons of varying size were labelled bilaterally in the reticular formation from the pons to the medulla (Fig. 4, 5). These neurons contained granules of HRP which were brown in color but paler than those in the facial nucleus (Fig. 6).
DISCUSSION Horseradish peroxidase (HRP) injected into the muscle is taken up at the neuromuscular junction and carried by retrograde axonal transport to the motor neurons. Our finding in rabbits that labelled motor neurons in the facial nucleus of control animals are somatotopically organized and not found in the other brainstem nuclei is similar to that reported previously in the cat (5). As in the study by Thomander (6), the somatotopic organization of the facial nucleus was obscure in recovered animals. This is thought to be due to the fact that regenerating axom are misdirected at the site of injury. The brainstem afferent fiber system to the facial nucleus was examined in the cat following the injection of cholera toxin as a retrograde tracer into the facial nucleus (7). Labelled neurons were observed bilaterally in the nuclei reticularis of both pons and medulla.
Downloaded by [University of California Santa Barbara] at 23:27 25 March 2016
1002
Y. Nakao
et
al.
Acta Otolaryngol (Stockh) I I2
Fig. 4. Drawing of transverse sections through trigeminal motor nucleus (A), superior olivary nucleus (B),and facial nucleus (C) levels following injection of HRP into the zygomatic muscle on the recovered side. Each dot represents one labelled neuron. R motor trigeminal nucleus; 7: labelled neurons in the facial nucleus.
Fig. 5. Photomicrograph of HRP labelled neurons in the reticular formation of the pons. Arrow indicates the raphe. x 16.
Fig. 6. photomicrograph of HRP labelled neurons in the retilcular formation of the pons. The HRP granules are visible in the qtoplasm as well as the dendrite. x 200.
Downloaded by [University of California Santa Barbara] at 23:27 25 March 2016
Acta
Otolaryngol (Stockh) 112
Glycocalyx in normal and hydropic cochleas
Therefore, the labelled neurons found bilaterally from the pons to the medulla in the present study are suggested to be premotor neurons to the facial nucleus. It is not known whether the HRP reaction product, transported retrogradely to a cell soma or its dendrites, is transported transsynaptically to terminals of other cells establishing synapses with labelled cells (8). Electronmicroscopic investigations have revealed that after axotomy, microglial cells intrude into the synaptic clefts, thereby removing the terminals from the soma (9). Although the mechanism of labelled neurons observed bilaterally in the brainstem reticular formation remains to be elucidated, the interneuronal transport of HRP may occur after recovery from facial nerve paralysis. Furthermore, the lower brainstem reticular formation can control a variety of spontaneous and reflex facial movements (10). The pontine reticular formation area described as a pontine blink premotor area plays a role in blink reflex ( 1 I). It seems that the premotor neurons labelled in this study also may have a role in muscle movements after recovery from facial nerve palsy.
ACKNOWLEDGEMENTS The authors are grateful to Dr. lwahori for helpful comments, and to Dr. Ito for technical guidance.
REFERENCES 1. Crumley RL. Mechanisms of synkinesis. Laryngoscope 1979; 8 9 1847-54. 2. Martin H, Helsper JT. Spontaneous return of function following surgical section or excision of the seventh cranial nerve in the surgery of parotid tumors. Ann Surg 1957; 146: 715-27. 3. Cerny LJ, Steidl LJ. Reinnervation after resection of the facial nerve. Acta Otolaryngol (Stockh) 1974; 77: 102-7. 4. Baumel JJ. Trigeminal-facial nerve communications: their function in facial muscle innervation and reinnervation. Arch Otolaryngol 1974; 9 9 33-44. 5. Radpour S. Organization of the facial nerve nucleus in the cat. Laryngoscope 1977; 87: 557-74. 6. Thomander L. Reorganization of the facial motor nucleus after peripheral nerve regeneration. Acta Otolaryngol (Stockh) 1984; 9 7 619-26. 7. Fort P, Sakai K, Luppi PH, Salvert D, Jouvet M. Monoaminergic, peptidergic, and cholinergic afferents to the cat facial nucleus as evidenced by double immunostaining method with unconjugated cholera toxin as a retrograde tracer. J Comp Neurol 1989; 283: 285-302. 8. Brodal A. Neurological anatomy; In relation to clinical medicine. 3rd ed. Oxford: University Press, 1981: 3-45. 9. Blinzinger K. Displacement of synaptic terminals from regenerating motoneurons by microglial cells. Z Zellforsch 1968; 85: 145-57. 10. Sherrington CS. Reflexes elicitable in the cat from pinna, vibrissae and jaws. J Physiol 1917; 51: 401-31.
11. Holstage G, Tan J, Han JV,Graveland A. Anatomical observations on the afferent projections to the retractor bulbi motoneuronal cell group and other pathways possibly related the blink reflex in the cat. Brain Res 1986: 374 321-34. Manuscript received February 4, I992 accepted June 11, 1992
Address for correspondence: Yoshiaki Nakao, Nagasaki University, School of Medicine. Department of Otolaryngology, 7-1, Sakamoto Machi, Nagasaki, 852, Japan
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