Single Endplate Research Articles

Enhancing braced frames’ lateral ductility by new beam-end splices of supported blade or sliding teeth: FE analysis

For overcoming premature damages happened to steel beam-column-brace joints under significant story-drifts, researchers proposed different hinge-splices. In this paper, at first, the bolted beam-hinges of web double-angle splice (WLS), top-flange double-plate splice (TFS), and TFS with web shear-plate (TFS-W) are evaluated through validated finite-element models, in diagonal buckling-restrained braced frames. As a result, WLS inevitably possessed appreciable bending-rigidity, and its bottom bolts failed under one cycle of 2%-drift. Moreover, TFS suffered gradual beam-subsidences leading to drastic splice-bucklings, and TFS-W sustained additional fixity and web-slot widenings, under low-to-high gravity-loads and double cycles at 1, 2, 3, and 4% drifts. Hence, TFS’s shear performance was improved by adding web end-plates standing on cantilever-plates welded across beam gap, which both precluded the beam subsidence and facilitated cyclic splice-rotations. Subsequently, innovative supported-blade splices (SBSs) and sliding-teeth splices (STSs) are developed with four versatile configurations, and analyzed under the aforementioned incremental lateral-loading protocol. In SBSs, single axial blade-plates, and clamping support-plates or bolted connector-plates were economically incorporated. Alternatively, STSs utilized double shear-plates with multiple cantilever teeth inserted into fit socket-holes created in single end-plates. In consequence, SBSs effortlessly transmitted large gravity-loads and considerable beam-axial forces, while allowed maximal beam-column rotations with blade-plate bending smoothly along its slit/slotted mid-length. Furthermore, during the transfer of substantial shear-forces from brace to column or beam, STSs rotated steadily around their welded/bolted collector-plates by sliding their long teeth inwards/outwards the corresponding sockets, which merely caused superficial yieldings at tooth-socket interfaces.

Influence of coronal-morphology of endplate and intervertebral space to cage subsidence and fusion following oblique lumbar interbody fusion

BackgroundEndplate morphology is considered to be one of the influencing factors of cage subsidence after lumbar interbody fusion (LIF). Previous radiographic evaluations on the endplate mostly used sagittal X-ray or MRI. However, there are few studies on the CT evaluation of the endplate and intervertebral space (IVS), especially the evaluation of coronal morphology and its influence on subsidence and fusion after LIF. We aimed to measure and classify the shapes of the endplate and IVS using coronal CT imaging and evaluate the radiographic and clinical outcomes of different shapes of the endplate/IVS following oblique lateral lumbar interbody fusion (OLIF).MethodsA total of 137 patients (average age 59.1 years, including 75 males and 62 females) who underwent L4-5 OLIF combined with anterolateral fixation from June 2018 to June 2020 were included. The endplate concavity depth (ECD) was measured on the preoperative coronal CT image. According to ECD, the endplate was classified as flat (< 2 mm), shallow (2–4 mm), or deep (> 4 mm). The L4-5 IVS was further classified according to endplate type. The disc height (DH), DH changes, subsidence rate, fusion rate, and Oswestry Disability Index (ODI) in different endplate/IVS shapes were evaluated during 1-year follow up.ResultsThe ECD of L4 inferior endplate (IEP) was significantly deeper than that of L5 superior endplate (SEP) (4.2 ± 1.1 vs 1.6 ± 0.8, P < 0.01). Four types of L4-5 IVS were identified: shallow-shallow (16, 11.7%), shallow-flat (45, 32.9%), deep-shallow (32, 23.4%), and deep-flat (44, 32.1%). A total of 45 (32.9%) cases of cage subsidence were observed. Only one (6.3%) subsidence event occurred in the shallow-shallow group, which was significantly lower than in the other three groups (19 shallow-flat, 6 deep-shallow, and 19 deep-flat) (P < 0.05). Meanwhile, the shallow-shallow group had the highest fusion rate (15, 93.8%) and the highest rate of reach minimal clinically important difference (MCID) ODI among the four types. For a single endplate, the shape of L4 IEP is the main influencing factor of the final interbody fusion rate, and the shallow shape L4 IEP facilitates fusion ( OR = 2.85, p = 0.03). On the other hand, the flat shape L5 SEP was the main risk factor to cage subsidence (OR = 4.36, p < 0.01).ConclusionThe L4-5 IVS is asymmetrical on coronal CT view and tends to be fornix-above and flat-down. The shallow-shallow IVS has the lowest subsidence rate and best fusion result, which is possibly because it has a relatively good degree in matching either the upper or lower interface of the cage and endplates. These findings provide a basis for the further improvements in the design of OLIF cages.

Classifications in Brief: AO Thoracolumbar Classification System.

Classifications in Brief: AO Thoracolumbar Classification System.

Friday, September 28, 2018 3:00 PM–4:00 PM abstracts: the gravity of obesity: 232. The impact of body mass index on severity of cervical spine fracture

BACKGROUND CONTEXT Several studies have analyzed the impact of obesity on severity and specific injury predispositions due to MVA using the Abbreviated Injury Scale (AIS). Although AIS grades individual regions according to severity from minor to maximum, it does not reflect specific description of injury in each region. In the case of cervical spine injury, the type of fracture pattern and spinal cord injury (SCI) cannot be determined from the AIS score. Therefore, the knowledge on impact of obesity on severity of cervical spine injury is limited from prior trauma studies.This information will be useful for providers in the ED to ascertain risk of severe cervical spine injury due to patient's BMI. In addition to risk estimation, fracture pattern, SCI, and patient-specific comorbidities are among important considerations in deciding treatment strategy. PURPOSE The objective of our study was to study the impact of BMI on severity of cervical spine injury after adjusting for injury and patient related variables. STUDY DESIGN/SETTING Retrospective cohort study. PATIENT SAMPLE Patients with traumatic cervical spine fracture presenting to a level I trauma center between January 1, 2010 and December 31, 2014. OUTCOME MEASURES Severity of cervical spine fracture according to AO Spine sub-axial cervical spine fracture classification. METHODS Approval from Institutional Review Board (IRB) was taken before identifying patients for data collection. Review of electronic medical record (EMR) was done to collect patient information. Patients were selected per the following inclusion criteria: (i) traumatic injury that resulted in sub-axial cervical spinal fracture, (ii) patient age 18 to 80 years, (iii) availability of cervical computed tomography (CT) scan, (iv) complete health record of patient clinical and injury characteristics. Patients were excluded if they had isolated atlanto-axial injury or did not have a cervical CT scan. Patients who had suspected pathological fracture were also excluded. CT scans of the cervical spine were assessed by an observer blinded to clinical information.Cervical spine injuries were categorized as severe (S) if they had burst fracture involving both end plates, tension band injuries, and dislocation injuries (Types A4, B0, B1, B2, B3) and/or associated SCI. Injuries with insignificant fractures or single endplate burst morphology (Types A0, A1, A2, A3) and no SCI were categorized as less severe (LS) for our analysis. The association between BMI and severity of cervical spine fracture was studied by multiple-variable logistic regression analysis using a conditional stepwise backward elimination model. The dependent variable was severity of injury with less severe (LS) fracture serving as reference. Independent variable of interest included BMI which was entered as a continuous variable and forced to be in the model at all steps. Other covariates included in the model included age, gender, ethnicity, smoking status, medical comorbidities, mechanism of injury, and other significant organ system injury. All variables were initially entered in the model and backward elimination removal criteria was set at p>.1. RESULTS A total of 751 patients with cervical spine fracture were identified, out of which 291 patients met our inclusion criteria and had adequate records available for analysis. The mean age of included patients was 46.1±19.3 years, and 212 patients (75.8%) were males.Based on morphology of fracture and SCI, 106 (36.4%) were categorized as severe (S) injury. The remaining 185 (63.6%) patients were classified as less severe (LS) injury.%). MVA was the most common injury mechanism (n=177, 60.8%) followed by fall from height (n=85, 29.2%) and random traumatic events (n=29, 10.0%).Our multiple-variable logistic regression model found that increasing BMI was not associated with a likelihood of severe cervical spine injury after adjusting for other clinical and injury parameters (OR 1.03, 95% CI: 0.97–1.08, p=.34). A sub-analysis of MVA cases was done to include side of impact, rollover, protection, and ejection of occupant as covariates in addition to other patient variables. Although trending towards significance, the odds ratio of severe cervical spine injury due to increasing patient BMI was 1.08 (95% CI: 0.99–1.18, p=.06). In this analysis, the presence of rollover was also represented in the final model with OR 2.55 (95% CI: 0.98–6.66, p=.06). CONCLUSIONS In conclusion, our findings indicate that patients with higher BMI may be predisposed to more severe cervical spine fracture and/or SCI after MVA, especially when rollover is present. The same risk may not be present in other common modes of injury such as nonrollover MVA and falls. Further prospective studies with larger sample size, use of clinically relevant outcome parameters (fracture classification, neural deficit), and comprehensive measurement of injury related variables may be helpful to corroborate our findings.

A novel in vivo porcine model of intervertebral disc degeneration induced by cryoinjury.

Degenerative disc disease involves sequential events that lead to the loss of cells, a decrease in disc matrix production, disc dehydration, and alteration of its biomechanical properties. The aim of this study was to determine whether cryoinjury of the nucleus pulposus performed through endplate perforation contributes to disc degeneration and to compare this technique with standard methods. Under general anesthesia, the lumbar discs of six pigs were exposed and randomly submitted to needle puncture of the annulus fibrosus (NeP), isolated endplate injury (EP), or cryoinjury using a 2.5-J Thompson cryoprobe applied through a single endplate perforation (EP+cryo). The remaining discs served as controls. Animals were sacrificed at two months and the harvested lumbar spines were submitted to CT scan and MRI investigations. Histologic analysis was performed to assess the degree of disc degeneration. CT scan showed that decrease in average disc height was more important after cryoinjury (49.3%) than after endplate perforation (16.9%) (P < 0.0001) or needle puncture (19.4%) (P < 0.0001). On MRI, the dehydration ratio was significantly more important after EP+cryo (60%) than after NP (40%) or EP (30%) (P < 0.0001). After cryoinjury, the histologic score developed for this study was significantly higher than after needle puncture or endplate perforation (P < 0.0001). Imaging and histological analysis showed that disc cryoinjury applied through endplate perforation was superior to the classical NeP and EP models to induce experimental disc degeneration. This model appears suitable for testing safety and efficacy of novel treatments of intervertebral disc degeneration.

Clinical importance of posterior vertebral height loss on plain radiography when conservatively treating osteoporotic vertebral fractures

PurposeTo predict spinal canal compromise, the assessment of plain radiography with magnetic resonance imaging (MRI) can aid the detection of vertebral body collapse and prevent the development of neurological deficits. MethodsPatients who suffered osteoporotic vertebral fractures (OVFs) between January 2012 and December 2014 underwent consecutive radiological assessments, including measurements of anterior height loss (AHL), posterior height loss (PHL), and the kyphotic angle (KA). The fracture morphology was classified by AOSpine thoracolumbar spine injury classification system. MRI was performed at the initial assessment and the extent of canal encroachment (CE) was calculated in all patients. Follow-up computed tomography (CT) or MRI was performed in patients exhibiting significant height loss in follow-up radiography. The fracture patterns in T1- and T2-weighted MRI were also assessed. ResultsA total of 485 patients visited our institute for treatment of OVFs and 97 were enrolled; 15 were male and 82 were female. The mean age at initial visit was 70.3±14.6years. The initial spinal CE was correlated with the initial PHL and the initial AHL. The follow-up CE was correlated with age, the initial PHL, and the difference between the initial and last PHL (ΔPHL(initial-last)). OVFs with both endplate fractures have a greater tendency of posterior wall collapse than those with single endplate fracture. On initial T1-weighted sagittal MRI, a diffuse low signal change pattern of the fractured vertebra was correlated with PHL. Delayed neurological deficits developed in four patients. These patients underwent surgical intervention. ConclusionsIn patients with simple compression fractures, attention should be paid to the posterior vertebral body and both endplates as well as the T1-weighted MRI findings to allow early detection of spinal canal compromise, which can have devastating consequences.

Is there a regional difference in morphology interpretation of A3 and A4 fractures among different cultures?

OBJECT The aim of this study was to determine if the ability of a surgeon to correctly classify A3 (burst fractures with a single endplate involved) and A4 (burst fractures with both endplates involved) fractures is affected by either the region or the experience of the surgeon. METHODS A survey was sent to 100 AOSpine members from all 6 AO regions of the world (North America, South America, Europe, Africa, Asia, and the Middle East) who had no prior knowledge of the new AOSpine Thoracolumbar Spine Injury Classification System. Respondents were asked to classify 25 cases, including 6 thoracolumbar burst fractures (A3 or A4). This study focuses on the effect of region and experience on surgeons' ability to properly classify these 2 controversial fracture variants. RESULTS All 100 surveyed surgeons completed the survey, and no significant regional (p > 0.50) or experiential (p > 0.21) variability in the ability to correctly classify burst fractures was identified; however, surgeons from all regions and with all levels of experience were more likely to correctly classify A3 fractures than A4 fractures (p < 0.01). Further analysis demonstrated that no region predisposed surgeons to increasing their assessment of severity of burst fractures. CONCLUSIONS A3 and A4 fractures are the most difficult 2 fractures to correctly classify, but this is not affected by the region or experience of the surgeon; therefore, regional variations in the treatment of thoracolumbar burst fractures (A3 and A4) is not due to differing radiographic interpretation of the fractures.

Is there a Regional Difference in Morphology Interpretation of A3/A4 Fractures among Different Cultures?

Introduction The AOSpine Thoracolumbar Spine Injury Classification System was recently published, but before establishing a treatment algorithm to accompany the classification, further investigation on the cause of current regional treatment variations is required. The objective of this study is to determine if the ability of a surgeon to correctly classify A3 (burst fractures with a single endplate involved) and A4 (burst fractures with both end plates involved) fractures were affected by either the region or the experience of the surgeon. Material and Methods A survey was sent to 100 AOSpine members from all six AO regions of the world (North America, South America, Europe, Africa, Asia, and the Middle East) that had no prior knowledge of the new AOSpine Thoracolumbar Spine Injury Classification System. Respondents were asked to classify 25 cases, including 6 thoracolumbar burst fractures (A3 or A4). The current analysis focuses on the effect of region and experience on surgeons' ability to properly classify these controversial fractures. Results All 100 surveyed surgeons completed the survey, and no significant regional variability in the ability to correctly classify burst fractures was identified ( p &gt; 0.50). Further analysis demonstrated that no region predisposed surgeons increasing the severity of burst fractures. Similarly, experience did not affect surgeons' ability to correctly classify burst fracture ( p &gt; 0.21). Conclusion Regional variation in the treatment of thoracolumbar burst fractures (A3 and A4) is not because of the differing radiographic interpretation of the fractures.

Effects of rocuronium and vecuronium on initial rundown of endplate potentials in the isolated phrenic nerve diaphragm preparation of rats.

Rocuronium and vecuronium, two non-depolarizing neuromuscular blockers, have been widely used in surgery procedures. However, their electrophysiological properties need to be more widely explored. We examined the effects of rocuronium and vecuronium on initial rundown of endplate potential amplitudes in the non-uniform stretched muscle preparation of the rat isolated phrenic nerve diaphragm. More specifically, the endplate potentials were recorded with one microelectrode from a single endplate. The effects of rocuronium or vecuronium each at 4 concentrations (0.5 ×, l ×, 2 ×, 4 × EC95; EC95 = concentration of the drug required to produce the inhibitory effect by 95%) on the amplitude of endplate potentials and its rundown were observed. Treatment of the isolated rat phrenic nerve-diaphragm preparation with rocuronium (2.5–20 μg/ml) or vecuronium (0.5–4 μg/ml) decreased the amplitude of endplate potentials and inhibited its rundown in a concentration-dependent manner. At the concentration (2.5 μg/ml for rocuronium and 0.5 μg/ml for vecuronium) that did not alter the endplate potential amplitude, the onset of reduced endplate potential rundown was 3 and 5 min after administration of rocuronium or vecuronium, respectively. The results suggest that rocuronium and vecuronium block the neuromuscular junction presynaptically and that rocuronium does it faster than vecuronium.

&lt;strong&gt;Three new serpulids (Polychaeta: Serpulidae) from the Brazilian Exclusive Economic Zone&lt;/strong&gt;

Three new species of serpulids from Brazil are described herein, two of which were described, but not named, by Zibrowius are here considered as new species and given specific names. Vermiliopsis zibrowii sp. nov. is characterized by an operculum with a single endplate or several septa progressively nearer to each other towards the tip and which are separated by conspicuous ribs. Pseudovermilia harryi sp. nov. has an operculum with several septa separated by short ribs and its tube is different from similar congeners. Filogranula revizee sp. nov. has an operculum with a sunken funnel and, frequently, a slender, distally sharp spine. Comparisons between these three species and related species are provided

On the mechanism of action of muscle fibre activity in synapse competition and elimination at the mammalian neuromuscular junction

Activity-dependent competition plays a crucial role in the refinement of synaptic connections in the peripheral and central nervous system. The reduction in number of axons innervating each neuromuscular junction during development, i.e. synapse elimination, appears to be one such competitive activity-driven event. Recently, we showed that asynchronous firing of competing presynaptic terminals is a key player in synapse elimination. Although some previous studies suggested that activity of the postsynaptic cell may be an intermediary in the disposal of redundant presynaptic inputs, the mechanism involved remains unknown. In the present study, in order to assess the role of evoked muscle activity in this process, we inhibited the generation of postsynaptic action potentials in muscle fibers in vivo, through the overexpression of inwardly rectifying Kir2.1 and Kir2.2 channels, via electroporation of the soleus muscle in the mouse hindlimb. Electrophysiological and morphological data show that overexpression of potassium channels in the endplate region of neonatal muscle fibres induces membrane hyperpolarization and an increase in conductance, inhibition of the action potential mechanism and prolonged persistence of polyneuronal innervation. These changes are not seen in muscle fibres with overexpression of a non-conducting Kir2.1 mutant. Our results are compatible with the interpretation that the block of action potential generation, even in single endplates, can inhibit synapse elimination through local signalling.

The BMP Ligand Gbb Gates the Expression of Synaptic Homeostasis Independent of Synaptic Growth Control

Inhibition of postsynaptic glutamate receptors at the Drosophila NMJ initiates a compensatory increase in presynaptic release termed synaptic homeostasis. BMP signaling is necessary for normal synaptic growth and stability. It remains unknown whether BMPs have a specific role during synaptic homeostasis and, if so, whether BMP signaling functions as an instructive retrograde signal that directly modulates presynaptic transmitter release. Here, we demonstrate that the BMP receptor (Wit) and ligand (Gbb) are necessary for the rapid induction of synaptic homeostasis. We also provide evidence that both Wit and Gbb have functions during synaptic homeostasis that are separable from NMJ growth. However, further genetic experiments demonstrate that Gbb does not function as an instructive retrograde signal during synaptic homeostasis. Rather, our data indicate that Wit and Gbb function via the downstream transcription factor Mad and that Mad-mediated signaling is continuously required during development to confer competence of motoneurons to express synaptic homeostasis.

Preliminary observations on the microarchitecture of the human abdominal muscles

Precise knowledge of muscle architecture and innervation patterns is essential for the development of accurate clinical and biomechanical models. Although the gross anatomy of the human abdominal muscles has been investigated, the finer details of their microanatomy are not well described. Fascicles were systematically sampled from each of the human abdominal muscles, and small fiber bundles from selected fascicles stained with acetylcholinesterase to determine the location of motor endplate bands, myomyonal junctions, and myotendinous junctions. Statistical analysis was used to ascertain the association between fascicular length and number of endplate bands. The number of endplate bands along a fascicle was variable between different portions of each muscle, but was strongly correlated with fascicular length (r = 0.814). In fascicles less than 50 millimeters (mm) in length, only a single endplate band was generally present, while multiple endplate bands (usually two or three) were found in fascicles longer than 50 mm. The presence of myomyonal junctions throughout the longer (>50 mm) fascicles verified that they were composed of short, intrafascicularly terminating fibers, while shorter fascicles comprised fibers spanning the entire fascicular length. This preliminary study provides evidence that multiple endplate bands are contained in some regions of the abdominal muscles, an arrangement that differs from most human appendicular muscles. It is not clear whether the variations in the described fine architectural features reflect regional differences in muscle function.

Schwann cell dynamics with respect to newly formed motor-nerve terminal branches on mature (Bufo marinus) muscle fibers.

A study has been made of the formation of synaptic terminals from long processes formed at the end of motor nerve branches of endplates in mature amphibian (Bufo marinus) muscle. Injection of fluorescent dyes into individual motor axons showed the full extent of their branches at single endplates. Synaptic vesicle clusters at these branches were identified with styryl dyes. Some terminal branches consisted of well separated varicosities, each possessing a cluster of functioning synaptic vesicles whilst others formed by the same axon consisted of closely spaced clusters of vesicles in a branch of approximately uniform diameter. All the varicosities gave rise to calcium transients on stimulation of their parent axon. Both types of branches sometimes possessed short processes (<5 microm long) or very long thin processes (>10 microm long) which ended in a bulb that possessed a functional synaptic vesicle cluster. These thin processes could move and form a varicosity along their length in less than 30 min. Injection of a fluorescent dye into terminal Schwann cells (TSCs) at an endplate showed that they also possessed very long thin processes (>10 microm long) which could move over relatively short times (<30 min). Injecting fluorescent dyes into both axons and their associated TSCs showed that on some occasions long TSC processes were accompanied by a long nerve terminal process and at other times they were not. It is suggested that the mature motor-nerve terminal is a dynamic structure in which the formation of processes by TSCs guides nerve terminal sprouting.

Decreased calcium influx into the neonatal rat motor nerve terminals can recruit additional neuromuscular junctions during the synapse elimination period

Individual skeletal muscle fibers in newborn vertebrates are innervated at a single endplate by several motor axons. During the first postnatal weeks, the polyneuronal innervation decreases in an activity-dependent process of synaptic elimination by axonal competition. Because synaptic activity depends strongly on the influx of calcium from the external media via presynaptic voltage-dependent calcium channels, we investigate the relationship between calcium channels, synaptic activity and developmental axonal elimination. We studied how several calcium channel blockers affect (after 1 h of incubation) the total number of functional axons per muscle fiber (poly-innervation index) of the Levator auris longus muscle of 6-day-old rats. We determined the poly-innervation index by gradually raising the stimulus amplitude and recorded the recruitment of one or more axons that produced a stepwise increment of the endplate potential. The L-type channel blocker nitrendipine (1 μM) increased the mean poly-innervation index (35.79%±3.91; P<0.05). This effect was not washed out with normal Ringer, although the poly-innervation index returned to the control value when high-calcium Ringer (5 mM) was used. The P-type channel blocker ω-agatoxin-IVA (100 nM) also increased the number of recruitable endplate potentials (27.49%±1.78; P<0.05), whereas N-type channel blocker ω-conotoxin-GVIA (1 μM) was ineffective ( P>0.05). However, neither nitrendipine nor ω-agatoxin-IVA modified the poly-innervation index on high-calcium Ringer ( P>0.05 in both cases). A more intense inhibition of calcium influx (by the sequential use of two calcium channel blockers) did not recruit any additional silent synapses. Moderately increasing the magnesium ions (by 500 μM) in the physiological solution produces a synaptic recruitment (36.78%±2.1; P<0.05) similar to that with L- and P-type calcium channel blockers incubation. This magnesium effect was not washed with normal Ringer but a Ringer that is high in calcium can reverse it. The recruited endings were identified by selective activity-dependent loading with styryl dyes. Rhodaminated α-bungarotoxin-labeled acetylcholine receptors were present in the postsynaptic counterpart. Based on these findings we suggest that, before their complete retraction, functionally silent nerve terminals can be manifested or recovered if calcium influx is reduced by a calcium channel blocker or if external magnesium is increased. The normal activation of this calcium-dependent silencing mechanism during development may be related to the final loss of the supernumerary axons.

Pertussis toxin-sensitive G-protein and protein kinase C activity are involved in normal synapse elimination in the neonatal rat muscle.

Individual skeletal muscle fibers in most new-born rodents are innervated at a single endplate by several motor axons. During the first postnatal weeks, the polyneuronal innervation decreases in a process of synaptic elimination. Previous studies showed that the naturally occurring serine-protease thrombin mediates the activity-dependent synapse reduction at the neuromuscular junction (NMJ) in vitro and that thrombin-receptor activation may modulate nerve terminal consolidation through a protein kinase mechanism. To test whether these mechanisms may be operating in vivo, we applied external thrombin and its inhibitor hirudin, and several substances affecting the G protein-protein kinase C system (GP-PKC) directly over the external surface of the neonatal rat Levator auris longus muscle. Muscles were processed for immunocytochemistry to simultaneously detect acetylcholine receptors (AChRs) and axons for counting the percentage of polyinnervated NMJ. We found that exogenous thrombin accelerated synapse loss and hirudin blocked axonal removal. Phorbol-12-myristate-13-acetate, a potent PKC activator, had a similar effect as thrombin, whereas the PKC inhibitors, calphostin C and staurosporine, prevented axonal removal. Pertussis toxin, an effective blocker of GP function, blocked synapse elimination. These findings suggest that the normal synapse elimination in the neonatal rat muscle may be modulated, at least in part, by the pertussis-sensitive G-protein and PKC activity and that thrombin could play a role in the postnatal synaptic maturation in vivo.

From Plaque to Pretzel: Fold Formation and Acetylcholine Receptor Loss at the Developing Neuromuscular Junction

Although there has been progress in understanding the initial steps in the formation of synapses, less is known about their subsequent maturation (Sanes and Lichtman, 1999). Two alterations on the postsynaptic side of the mammalian neuromuscular junction occur during early postnatal life: acetylcholine receptors (AChRs) disappear from parts of the developing junction as all but one axonal inputs are removed, and the topography of the postsynaptic membrane becomes more complicated as gutters and folds are established. We have studied the maturation of the AChR distribution and postsynaptic topography simultaneously by imaging labeled AChRs at the mouse neuromuscular junction in a new way, using reflected light confocal microscopy. At birth postsynaptic receptors were localized in irregular patches within a spoon-shaped plaque. Beginning several days later, receptor regions within a single endplate were divided into differentiated and less organized compartments. Folds generally oriented orthogonal to the long axis of the muscle fiber were seen in developing gutters, although the orientation of the gutters seemed to be imposed by the branching pattern of the nerve. Eventually, superficial regions lacking AChR labeling were apparent in all junctions. In junctions denervated in the neonatal period both gutter formation and the disappearance of superficial receptors regions were prevented. We suggest that tension between growing muscle fibers and the relatively inelastic synaptic terminals that adhere to them causes the topographic features of the postsynaptic membrane. This view provides a mechanical explanation for gutters, folds, and the location of folds at sites of neurotransmitter release.

Regeneration of specific innervation in Xenopus pectoralis muscle.

We investigated the motor unit organization and precision of reinnervation in the Xenopus pectoralis muscle following different manipulations, including crush or section of the posterior pectoralis nerve, foreign nerve innervation, and crush coupled with activity modulation or block. Most fibers have two neuromuscular junctions, and multielectrode recordings were used to identify the axonal origin of all inputs to both junctions on most or all fibers covering about 25% of the muscle surface. Following simple nerve crush, a highly organized innervation pattern was restored, indistinguishable from the normal pattern, including selective innervation of fibers of similar input resistance (R(in)), compact motor unit organization, and high incidence of exclusive innervation of both end plates on each fiber by the same axon (distributed mononeuronal innervation, or a/a pattern). Initial reinnervation was equally precise when nerve conduction in the regenerating nerve was blocked by tetrodotoxin. More distant or repeated nerve crush or nerve section delayed and reduced the precision of reinnervation, but the majority of fibers still received input to both end plates by the same axon, often in combination with others. A foreign nerve, the pectoralis sternalis, which in its own muscle forms only single end plates, showed less precise reinnervation, but still had an incidence of a/a innervation far above chance. These data imply the expression and recognition of remarkably precise chemospecific cues even in mature animals, superimposed on which is a further refinement by synapse elimination, probably based on an activity-dependent process.

Facilitation of nicotinic receptor desensitization at mouse motor endplate by a receptor-operated Ca 2+ channel blocker, SK&F 96365

When acetylcholinesterase was inhibited by neostigmine, SK&F 96365 (1-{ β-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenethyl}-1 H-imidazole hydrochloride) at 10 μM caused no effect on the amplitude of single endplate potentials (e.p.p.s) but shortened the decay time in mouse phrenic nerve-diaphragm preparations. However, SK&F 96365 inhibited high-frequency stimulation-evoked long-lasting depolarization of the endplate region and accelerated the run-down of trains of e.p.p.s which were eliminated within 1 s. After a train of stimulation, SK&F 96365 produced a post-tetanic depression of single e.p.p.s. The post-tetanic effect gradually dissipated with full restoration in 10–15 s. During a train of stimulation, SK&F 96365 also depressed miniature endplate potentials (m.e.p.p.s), which were restored after termination of stimuli in parallel with the recovery of e.p.p. The decay times of miniature endplate currents during recovery phases changed slightly. In control preparations not treated with neostigmine, however, SK&F 96365 did not alter the amplitude and decay time of m.e.p.p.s or e.p.p.s but accelerated the decay of succinylcholine-induced endplate depolarizations. The results suggest that SK&F 96365 facilitates nicotinic receptor desensitization in addition to blocking receptor-operated Ca 2+ channels.

Instability due to axial shear and surface impedance

The stability of plasma flow in the scrape-off layer of a tokamak, taking into account the surface sheath impedance and the axial shear in the E×B flow is analyzed. An interesting stability problem arises in the limit that end plates are sufficiently far apart, so that stability can be analyzed when the plasma is taken to interact with a single end plate. As parameters are varied, windows of instability are found, and it is shown that growth rates are maximized for an insulating end plate and are also quite sensitive to the ratio of the ion diamagnetic and E×B drift frequencies. Mixing-length estimates of the diffusivity are comparable to experimentally observed values.