ライフサイエンスコーパス: climbing fiber

1 activated by neurotransmitter released from climbing fibers ().

2 lutamate transporter 2 (VGluT2, to visualize climbing fibers).

3 llar cortex each receive input from a single climbing fiber.

4 l day (P) 21, most are contacted by a single climbing fiber.

5 inje cells which receive input from the same climbing fiber.

6 modulation of CSs and SSs depends on intact climbing fibers.

7 l Purkinje cells receive input from multiple climbing fibers.

8 grade tracers in the inferior olive to label climbing fibers.

9 ber filopodia and synaptic competition among climbing fibers.

10 spines in synaptic contact with parallel or climbing fibers.

11 is largely determined by the firing level of climbing fibers.

12 class of precerebellar neurons that project climbing fibers.

13 in the context of the background activity of climbing fibers.

14 innervation of Purkinje cells by cerebellar climbing fibers.

15 ns of variability driven by the parallel and climbing fibers.

16 ibers to Purkinje cells with the help of the climbing fibers.

17 structural plasticity by using, as a model, climbing fibers.

18 o visualize instructional signals carried by climbing fibers across the lateral cerebellum in mice be

19 glutamate molecules in response to a single climbing fiber action potential.

20 The region of climbing fibers activated by a localized peripheral stim

21 In addition, we observed that climbing fibers activated by periocular airpuffs also re

22 Here, we show that glutamate released from climbing fibers activates ionotropic and metabotropic re

23 Climbing fiber activation appears to reduce PKC expressi

24 tivity and changes in cerebellar output, and climbing fiber activation did not induce VOR-decrease le

25 hat result in synchronous (to within 1 msec) climbing fiber activation of Purkinje cells (complex spi

26 nely tuned timing between parallel fiber and climbing fiber activation.

27 re was no trial-by-trial correlation between climbing fiber activity and changes in cerebellar output

28 Cerebellar climbing fiber activity encodes performance errors durin

29 es the timing of both evoked and spontaneous climbing fiber activity in these cerebellar regions with

30 Climbing fiber activity increased when the monkeys were

31 e cell dendrites faithfully transformed this climbing fiber activity into dendrite-wide Ca(2+) signal

32 This pattern of climbing fiber activity is markedly similar to the respo

33 During VOR-increase training, climbing fiber activity on one trial predicted changes i

34 The effect of visual stimulation on the climbing fiber activity was studied in 19 floccular Purk

35 sy fiber synapse is controlled by nucleus or climbing fiber activity, the circuit is unable to retain

36 n of the cerebellar cortex (particularly the climbing fiber afferents and their sole source, the infe

37 ng electrical stimulation of mossy fiber and climbing fiber afferents as CS and US, while alternating

38 three zones receiving their olivo-cerebellar climbing fiber afferents from parts of, respectively, th

39 e postsynaptic response to the activation of climbing fiber afferents, believed to be fundamental to

40 kinje cells may phasically control their own climbing fiber afferents.

41 Activation of climbing fibers alone failed to induce the long-term dec

42 At both climbing fiber and inhibitory synapses onto PCs, we foun

43 Conjunctive stimulation of climbing fiber and parallel fiber inputs results in long

44 nje cell (PC) synapse induced by conjunctive climbing fiber and PF stimulation in vivo.

45 Comparison between climbing fiber and simple spike activity indicated diffe

46 IO neurons excite Purkinje cells via climbing fibers and depress their parallel fiber inputs.

47 Multiple innervation by climbing fibers and enhanced parallel fiber synaptic cur

48 or anatomical evidence for synapses between climbing fibers and Golgi cells.

49 uces the magnitude of the teaching signal in climbing fibers and limits learning.

50 and the cellular regions of termination for climbing fibers and parallel fibers are well separated.

51 tiple climbing fiber innervation, we labeled climbing fibers and performed reconstructions of immunof

52 al control over the induction of learning by climbing fibers and Purkinje cells can expand the learni

53 g training, joint control of learning by the climbing fibers and Purkinje cells may expand the learni

54 The interactions between the climbing fibers and the Purkinje cells were examined on

55 uantify the interactions between the olivary climbing fibers and the Purkinje cells when the cerebell

56 sion patterns that suggest a contribution of climbing fibers and their collaterals.

57 A delayed arborization of mutant olivary climbing fibers and their defective translocation from t

58 ligin isoforms differentially contributed to climbing-fiber and basket/stellate-cell synapse function

59 Learning could be guided by comparison of climbing-fiber and vestibular signals at all stimulus fr

60 In contrast, climbing fibers are anticalretinin and anti-NR1 immunopo

61 We show that climbing fibers are distributed in narrow sagittal strip

62 rebellum at the time of, and at sites where, climbing fibers are eliminated.

63 lts indicate that instructive signals in the climbing fibers are not necessary for cerebellum-depende

64 y gated in vivo, even under conditions where climbing fibers are robustly activated by performance er

65 Climbing fibers are well suited to studies of recovery f

66 ve used the innervation of Purkinje cells by climbing fibers as a model system to explore potential f

67 ron, Hashimoto et al. show that, by the time climbing fibers ascend the dendrites, the winner and los

68 CFRs reflect the discharge of a single climbing fiber at multiple synaptic sites on the proxima

69 Purkinje cells are innervated by multiple climbing fibers at birth but undergo an activity-depende

70 In mammals, climbing fiber axons compete for sole innervation at eac

71 The inferior olive projects climbing fiber axons to cerebellar Purkinje neurons, whe

72 In contrast, CART was expressed in climbing fiber bands in all four transverse zones of the

73 We found that in mice CART was expressed in climbing fiber bands that generally corresponded to the

74 nje cells in anesthetized pigmented rabbits, climbing fiber burst patterns were investigated by deter

75 eform changes with the number of spikes in a climbing fiber burst, which depends on the phase of oliv

76 channel blockers and is mimicked by a brief climbing fiber burst.

77 sibility that signals are present within the climbing fiber bursts.

78 distal climbing-fiber synapses and weakened climbing-fiber but not parallel-fiber synapses, consiste

79 at did not elicit instructive signals in the climbing fibers, but nevertheless induced robust and con

80 al instructive signals carried by cerebellar climbing fibers, but with a stronger influence of the ba

81 Thus, climbing fibers can, in addition to modulation of their

82 ving excitatory synaptic input from a single climbing fiber (CF) and from approximately 200,000 paral

83 Here, we show that release of glutamate from climbing fiber (CF) axons produces AMPA receptor current

84 ns, but, regardless of activation frequency, climbing fiber (CF) coactivation provides an instructive

85 e transient depolarization associated with a climbing fiber (CF) EPSP activates voltage-gated Ca(2+)

86 eatures include morphological alterations in climbing fiber (CF) innervation of Purkinje cells (PCs).

87 mouse cerebellar Purkinje neurons (PNs), the climbing fiber (CF) input provides a signal to parallel

88 It is well established that the climbing fiber (CF) input to a cerebellar Purkinje cell

89 Climbing fiber (CF) input to the cerebellum is thought t

90 -term amplification of synaptic responses to climbing fiber (CF) or PF stimulation and enhance the am

91 of cerebellar function, coactivation of the climbing fiber (CF) synapse, which provides massive, inv

92 incident activity of parallel fiber (PF) and climbing fiber (CF) synapses causes a long-lasting decre

93 Climbing fiber (CF) synapses have a high probability of

94 sticity expressed at parallel fiber (PF) and climbing fiber (CF) synapses.

95 ays a predominant role in terminating DSE at climbing fiber (CF) to PC synapses, while both neuronal

96 We find that the powerful climbing fiber (CF) to Purkinje cell synapse regulates t

97 receptors are postsynaptically expressed at climbing fiber (CF) to Purkinje cell synapses in mice, r

98 Climbing fiber (CF)-evoked calcium transients play a key

99 rom acute cerebellar slices revealed altered climbing fiber (CF)-evoked complex spike generation, as

100 t glutamate transporter currents recorded at climbing fiber (CF)-PC synapses are absent in mice lacki

101 Purkinje cells (PCs), and prevented loss of climbing fiber (CF)-PC synapses in comparison to vehicle

102 ted the hypotheses that there are functional climbing fiber (CF)-Purkinje cell (PC) and parallel fibe

103 Powerful synapses between climbing fibers (CF) and Purkinje cells are crucial to c

104 GluR1 was expressed postsynaptically at the climbing fibers (CF) synapse at early ages during Purkin

105 Communication between climbing fibers (CFs) and molecular layer interneurons (

106 f the C1q family of proteins, is provided by climbing fibers (CFs) and serves as a crucial anterograd

107 not detectable: both were mono-innervated by climbing fibers (CFs) extending along their well-develop

108 rve competitive elimination of supernumerary climbing fibers (CFs) in the cerebellum of live mouse pu

109 Cerebellar climbing fibers (CFs) provide powerful excitatory input

110 quired for bursting, activation of AMPARs by climbing fibers (CFs) was sufficient to trigger bursts.

111 main input synapses from parallel (PFs) and climbing fibers (CFs).

112 s that receive input from distinct groups of climbing fibers (CFs); however, the physiological proper

113 med two-photon in vivo imaging of cerebellar climbing fibers (CFs; the terminal arbor of olivocerebel

114 atory response, demonstrating the ability of climbing fiber collaterals to significantly excite CN ne

115 of the cerebellum: the mossy fibers and the climbing fiber collaterals.

116 Our results challenge the view that the climbing fiber conveys an all-or-none signal to the cere

117 Although climbing fiber-dependent plasticity at pf*Pkj synapses a

118 rding to the initial membrane potential, the climbing fiber depolarizing transient activates two dist

119 Climbing fiber discharge [complex spikes (CSs)] increase

120 racking task in the monkey (Macaca mulatta), climbing fiber discharge dynamically controls the inform

121 ing in the granule cell population, allowing climbing-fiber-driven Purkinje cell learning at arbitrar

122 oded, we recorded the activity of individual climbing fibers during cerebellum-dependent eyeblink con

123 These results indicate that climbing fibers encode the expected reward size and sugg

124 ells was found to inhibit parallel fiber and climbing fiber EPSCs for tens of seconds.

125 r trains of parallel fiber stimuli and after climbing fiber evoked complex spikes.

126 mbing fiber synapse, which in turn decreases climbing fiber-evoked dendritic calcium signals.

127 results in long-term potentiation (LTP) of a climbing fiber-evoked glutamate transporter current reco

128 ng olivary microlesions reflects the loss of climbing fiber-evoked stellate cell discharge.

129 These results demonstrate that the size of climbing-fiber-evoked Ca(2+) signals in Purkinje cell de

130 ayer interneurons, these results reveal that climbing fibers exert control over inhibition at both th

131 zation and observed its effects on mossy and climbing fiber extension and growth cone size in vitro.

132 fibers to Purkinje cells' synapse guided by climbing fibers, feedforward inhibition of Purkinje cell

133 ltering the excitatory input provided by the climbing fiber from the inferior olive, which evokes a p

134 PCs integrate two excitatory inputs, climbing fibers from inferior olive and parallel fibers

135 ound the following topographic relationship: climbing fibers from the caudal lateral mcIO were locate

136 Climbing fibers from the inferior olive make strong exci

137 ssy fibers and a teaching signal through the climbing fibers from the inferior olive.

138 ated in lateral P2+ and P2- ZII stripes, and climbing fibers from the middle lateral mcIO were locate

139 e located in P1+ and medial P1- ZII stripes; climbing fibers from the rostral lateral mcIO were locat

140 rograde inhibition of synapses received from climbing fibers, granule cell parallel fibers, and inhib

141 Our findings identify the climbing fiber-->Purkinje neuron synapse as an important

142 th types of response were shown to be mainly climbing fiber in origin and therefore evoked by transmi

143 ng pathways, respectively, that terminate as climbing fibers in the "hindlimb-receiving" parts of the

144 s limits their ability to forward signals to climbing fibers in the cerebellar cortex.

145 activity-dependent pruning of supernumerary climbing fibers in the cerebellum.

146 ike the bank-gyrus subdivisions, most of the climbing fibers in the sulcus do not innervate the super

147 naptic activity of their axonal projections (climbing fibers) in the cerebellar cortex.

148 njunctive stimulation of parallel fibers and climbing fibers induced a long-term decrease (at least 1

149 Together, the results suggest that single climbing fiber innervation of Purkinje cells is critical

150 ms, including the neuromuscular junction and climbing fiber innervation of Purkinje cells, are models

151 viously, we demonstrated similar deficits in climbing fiber innervation when analyzed on PN14.

152 naptic Purkinje cell responses, monosynaptic climbing fiber innervation, and cerebellar-dependent beh

153 hological correlate of differential multiple climbing fiber innervation, we labeled climbing fibers a

154 firing provide for a new hypothesis in which climbing fiber input adjusts the encoding of SS informat

155 n not only during complex spikes elicited by climbing fiber input but also with direct electrical sti

156 ts suggest a novel function of CSs, in which climbing fiber input dynamically controls the state of P

157 Climbing fiber input produces complex spike synchrony ac

158 The climbing fiber input to Purkinje cells acts as a teachin

159 The climbing fiber input to the cerebellar cortex is thought

160 The climbing fiber input to the cerebellum from the inferior

161 Climbing fiber input to the cerebellum is believed to se

162 In the present study we examined the climbing fiber input to the medial half of folium IXcd,

163 neous dendritic calcium transients linked to climbing fiber input were observed in multiple neighbori

164 that shows paired pulse facilitation and to climbing fiber input with a large all-or-none AMPA-media

165 h the parallel fiber is coactivated with the climbing fiber input, but how order sensitivity is achie

166 , determined by coincident activation of the climbing fiber input, underlies cerebellar learning.

167 Purkinje cell synapses under control of the climbing fiber input, which provides an error signal as

168 esence of cerebellar microzones organized by climbing fiber input.

169 in simple spike firing can be independent of climbing fiber input.

170 trikingly, such stimulations trigger delayed climbing-fiber input signals in the stimulated Purkinje

171 Theories of cerebellar learning assert that climbing fiber inputs control plasticity at synapses ont

172 ng in advance of reward under supervision of climbing fiber inputs signaling reward delivery.

173 Climbing fiber inputs to Purkinje cells are thought to b

174 We probed the role of climbing fiber inputs to Purkinje cells in generating an

175 are: (1) early, fast, acquisition driven by climbing fiber inputs to the cerebellar cortex, with poo

176 Climbing fiber inputs to the cerebellum encode error sig

177 nsory (SI) cerebral cortex and the timing of climbing fiber inputs to the lateral hemispheres of the

178 Climbing-fiber inputs appear to play a fast and primary,

179 ely depressed in Purkinje cells that receive climbing-fiber inputs from the instruction.

180 tion of pursuit and therefore do not receive climbing-fiber inputs related to the instruction, simple

181 to a large calcium release transient if the climbing fiber is activated up to 100 ms before or up to

182 ndicates that PF-PC LTD under control of the climbing fibers is not required for general motor adapta

183 ly robust putative error signals in the same climbing fibers: learned increases and decreases in the

184 Moreover, ethanol inhibited climbing fiber long-term depression, a form of synaptic

185 ggest that interneurons driven by vestibular climbing fibers may determine SS modulation.

186 rning, MCS and ZCS cells developed increased climbing fiber (MCS) or parallel fiber (ZCS) input durin

187 dritic spines, we find that pairing IP3 with climbing fiber-mediated calcium entry leads to a large c

188 y learned movement will generate a series of climbing fiber-mediated corrections.

189 oriented modules made up of Purkinje cells, climbing fibers, molecular layer interneurons, and cereb

190 rally (mossy fiber neurons) or intramurally (climbing fiber neurons).

191 re misspecified to dI4 interneurons, and the climbing-fiber neurons (inferior olive nucleus) are comp

192 Vestibular climbing fibers not only evoke low-frequency CFRs, but a

193 , with accompanying later enlargement of the climbing fiber nucleus and reductions in mossy fiber nuc

194 ns caused by cerebellar afferents (e.g., the climbing fibers of the inferior olive) is a critical fac

195 g, we analyzed excitatory synapses formed by climbing fibers on Purkinje cells in cerebellum and inhi

196 kinje cell would typically be activated by a climbing fiber only once.

197 cal models of cerebellar learning posit that climbing fibers operate according to a supervised learni

198 Following glutamate spillover from climbing fibers or application of CNQX, evoked GABA rele

199 d, instructive signals carried by either the climbing fibers or Purkinje cell simple spikes may be su

200 vironment along translational axes and their climbing fibers originate in the lateral half of the med

201 nferior olivary nuclei from which vestibular climbing fibers originate; the beta-nucleus and dorsomed

202 and timing in the cerebellar cortex via the climbing fiber pathway, but direct characterization of t

203 calcium spikes, which was modulated by a non-climbing fiber pathway.

204 ebellar theories assume that mossy fiber and climbing fiber pathways carry information from different

205 ecording techniques that the mossy fiber and climbing fiber pathways converge in cerebellar cortex.

206 in access to the cerebellum is via ascending climbing fiber pathways.

207 ty of fluo-4 dextran by measuring Ca(res) at climbing fiber presynaptic terminals.

208 ral to Math1 yet within Wnt1(+) territory, a climbing fiber primordium dominated by Ngn1-expressing c

209 e involved in eye movement control via their climbing fiber projection to the cerebellar flocculus, P

210 s control limb and digit movements via their climbing fiber projection to the lateral cerebellar hemi

211 ial relationship between the mossy fiber and climbing fiber projections to crus IIa in the lateral he

212 g a crucial time in postnatal development of climbing fiber-Purkinje cell connectivity suggest a role

213 bdivisions; however, the basic properties of climbing fiber-Purkinje cell EPSCs such as kinetics, amp

214 , the establishment of the proper pattern of climbing fiber-Purkinje cell innervation, and induction

215 Here we report that LTD at the climbing fiber-Purkinje cell synapse in rat cerebellum w

216 eport that tetanic stimulation of cerebellar climbing fiber-Purkinje cell synapses results in long-te

217 the result of glutamate diffusing out of the climbing fiber-Purkinje cell synaptic clefts ().

218 At the same time, adult climbing fibers react by sprouting new branches through

219 eptor-mediated quantal events resulting from climbing fiber release are observed in Bergmann glial ce

220 t, we propose that exocytosis can occur from climbing fiber release sites located directly across fro

221 f cortical input results in a lengthening of climbing fiber response latency to peripheral stimuli.

222 stimulation of SI results in a shortening of climbing fiber response latency.

223 urkinje cells whose low-frequency interburst climbing fiber response was modulated by movement about

224 The earliest learning occurs when one climbing-fiber response to a learning instruction causes

225 Purkinje cells have two action potentials: Climbing fiber responses (CFRs) and simple spikes (SSs).

226 Climbing fiber responses in cerebellar Purkinje cells ar

227 We find distinct climbing fiber responses in three lateral cerebellar reg

228 all stimulus frequencies tested, but only if climbing fiber responses were compared with the vestibul

229 re activated by voltage waveforms taken from climbing fiber responses, suggesting that they help shap

230 inje cells showed characteristic all-or-none climbing fiber responses.

231 Repetitive stimulations of climbing fibers resulted in a NMDAR-dependent reduction

232 Climbing fibers run in the horizontal plane and terminat

233 nodulus, while leaving intact the vestibular climbing fiber signal from the contralateral inferior ol

234 postcomplex spike pause, a component of the climbing fiber signal in Purkinje neurons, and show that

235 us of the inferior olive reduced a modulated climbing fiber signal to the contralateral uvula-nodulus

236 ings of Purkinje cell activity, we show that climbing fibers signal reward expectation, delivery and

237 We found that climbing fibers signaled both the unexpected delivery an

238 This reduced vestibular climbing fiber signaling to the contralateral folia 8-10

239 transmission in these pathways controls when climbing fiber signals can modify cerebellar activity.

240 cy of vestibularly modulated mossy fiber and climbing fiber signals in evoking CFRs and SSs in Purkin

241 It is suggested that the climbing fiber signals may act as a molecular 'filter' o

242 icated that the comparison of vestibular and climbing-fiber signals across the 100 msec delay must be

243 igh-frequency burst (approximately 500/s) of climbing fiber spikes.

244 Complex spikes were elicited by climbing fiber stimulation, and their somatic waveforms

245 otentials remained just above two spikes per climbing fiber stimulation, but the instantaneous freque

246 own that Golgi cell activity is regulated by climbing fiber stimulation, yet there is little function

247 f the Bergmann glial AMPA response evoked by climbing fiber stimulation.

248 f excitation and inhibition following single climbing fiber stimulation.

249 neurons depends on coincidence detection of climbing fiber stimulus evoking extracellular calcium fl

250 Depression dominated at the climbing fiber synapse, facilitation was prominent at th

251 decreasing the probability of release at the climbing fiber synapse, which in turn decreases climbing

252 nexpectedly essential for maintaining normal climbing-fiber synapse numbers.

253 2 occurs at both parallel fiber synapses and climbing fiber synapses early in development but is rest

254 synapses; however, PSD-93 also is present at climbing fiber synapses of the adult rat, where delta2 i

255 phorabphilin is specifically enriched in the climbing fiber synapses of the cerebellar cortex.

256 ted the glutamate concentration transient at climbing fiber synapses on Purkinje cells by measuring t

257 and AMPA glutamate receptors at parallel and climbing fiber synapses on the developing Purkinje cells

258 the inferior olive (IO) that in turn produce climbing fiber synapses onto Purkinje cells.

259 ed of electrically-coupled neurons that make climbing fiber synapses onto Purkinje cells.

260 In cerebellar Purkinje cells, climbing fiber synapses provide an instructive signal fo

261 postnatal ages (P2 and P5) and was higher in climbing fiber synapses than in parallel fiber synapses

262 ence on the associative activation of PF and climbing fiber synapses.

263 ebellar Purkinje cells caused loss of distal climbing-fiber synapses and weakened climbing-fiber but

264 airments in their distribution and function (climbing-fiber synapses) to large decreases in synapse n

265 Climbing-fiber synapses, conversely, were unaffected.

266 antiexcitotoxic cellular response to strong climbing fiber synaptic activation, as occurs following

267 ficits along with an increased engulfment of climbing fiber synaptic elements by microglia were found

268 s this single isoform, we studied excitatory climbing fiber synaptic function onto PCs in Cacna2d2 KO

269 neuron dendrites.SIGNIFICANCE STATEMENT The climbing fiber synaptic input transiently depolarizes th

270 Moreover, significant climbing fiber synaptic refinement deficits along with a

271 ffect of NPC1 deficiency on microglia and on climbing fiber synaptic refinement during cerebellar pos

272 hysiology and function of the inferior olive/climbing fiber system and are interpreted to provide add

273 e primary role of the inferior olive and the climbing fiber system in timing is the encoding of tempo

274 ngs elucidate an unappreciated aspect of the climbing fiber teaching signal, and are consistent with

275 ffect of varying the number of pulses in the climbing fiber teaching signal.

276 set, suggesting that glutamate released from climbing fiber terminals escapes synaptic clefts and rea

277 Climbing fibers terminate only on smooth dendrites near

278 Within the rat AZ/PZ, climbing fibers terminated selectively within the dendri

279 ls in the sulcus were innervated by multiple climbing fibers than in the gyrus or bank subdivisions;

280 stronger influence of the background on the climbing fibers than on learning.

281 f the inferior olivary nucleus (IO) form the climbing fibers that excite Purkinje cells of the cerebe

282 ts of inferior olivary nuclei, the source of climbing fibers that innervate Purkinje cells.

283 s: complex spikes (CSs) are evoked by single climbing fibers that originate from the contralateral in

284 afferent mossy fibers and tertiary afferent climbing fibers that project to the uvula-nodulus (folia

285 Climbing fibers, the projections from the inferior olive

286 At climbing fiber to Purkinje cell (PC) synapses in cerebel

287 ses in rat brain slices at 34 degrees C: the climbing fiber to Purkinje cell synapse, the parallel fi

288 determine the mechanism of depression at the climbing fiber to Purkinje cell synapse.

289 rise to these characteristic features at the climbing fiber to Purkinje cell synapse.

290 are transmitted by mossy/parallel fibers and climbing fibers to cerebellar Purkinje cells that acquir

291 Our data suggest that the ability of climbing fibers to induce plasticity can be dynamically

292 s in inferior olivary (IO) neurons that send climbing fibers to innervate cerebellar Purkinje cells f

293 he next trial, and optogenetic activation of climbing fibers to mimic their encoding of performance e

294 At climbing fiber-to-Purkinje cell synapses, the number of

295 Climbing fibers typically fire approximately once a seco

296 ia cells encase synapses between presynaptic climbing fiber varicosities and postsynaptic Purkinje ce

297 llins as neurexin ligands for the excitatory climbing-fiber versus parallel-fiber synapses.

298 Individual climbing fibers were broadly responsive to different sen

299 nts are encoded, we recorded the activity of climbing fibers when monkeys were engaged in an eye move

300 in paired-pulse depression at the cerebellar climbing fiber, where glial ensheathment of synapses is