糖尿病大鼠膈肌功能和形态学变化

目的:观察四氧嘧啶诱导的糖尿病4周大鼠膈肌收缩功能和形态结构的改变。方法:应用体外大鼠膈肌肌条,对其单收缩动力学、最大强直张力(P₀)、张力-频率曲线、疲劳指数(FI)的变化以及应用H.E、Heidenhain铁矾苏木素法和标准脱氢酶染色法对膈肌组织的形态学变化进行观察。结果:糖尿病大鼠膈肌最大颤搐张力(Pt)、收缩时间(CT)、半舒张时间(RT_1/2)及FI均明显低于对照组,而两组P₀无明显变化。予25、50、75、100、125Hz频率刺激膈肌时,糖尿病大鼠膈肌张力明显低于对照组。在膈肌疲劳后予氨茶碱浸浴膈肌,再予25、50、75、100和125Hz频率分别刺激膈肌,糖尿病大鼠膈肌张力均明显低于对照组。H.E及Heidenhain法染色可见糖尿病大鼠膈肌萎缩,肌横纹模糊。标准脱氢酶染色法显示糖尿病大鼠膈肌α-甘油磷酸脱氢酶呈弱阳性,光密度明显低于对照组(P＜0.01)。结论:糖尿病4周大鼠膈肌出现收缩功能减弱、易疲劳和结构破坏。     

家兔陈旧性心肌梗死心室肌细胞钾离子电流的变化

目的:探讨陈旧性心肌梗死(HMI)心律失常的发生机制,观察HMI非梗死区心肌细胞动作电位时程(APD)、瞬时外向钾电流(I_to)、延迟整流钾电流(I_K)和内向整流钾电流(I_K1)的变化。方法: 12只家兔随机分为2组,陈旧性心肌梗死组(HMI)开胸结扎冠状动脉左回旋支,假手术组开胸但不结扎冠状动脉。3个月后应用全细胞膜片钳技术记录非梗死区心肌细胞的APD、I_to、 I_K和I_K1。 结果: (1)HMI组心肌细胞的膜电容明显高于假手术组;(2)HMI组心肌细胞的APD显著延长,并有早期后除极(EAD)出现;(3)HMI组心肌细胞I_to 、I_K,tail和I_K1的电流密度分别为(4.03±0.33)pA/pF、(1.14±0.11)pA/pF和(17.6±2.3)pA/pF,显著低于假手术组的(6.72±0.42)pA/pF、(1.54±0.13)pA/pF和(25.6±2.6)pA/pF(P<0.01)。结论: HMI非梗死区心室肌细胞I_to 、I_K,tail、和I_K1的电流密度的降低是其APD延长和EAD出现的离子流基础,而APD延长和EAD的出现,可能在HMI恶性心律失常的发生中起着重要作用。     

颤搐性气道内压对膈肌功能的评价作用

目的:探讨颈部膈神经根磁波刺激诱发的颤搐性气管内压［Ptra(t)］ 对膈肌功能的评价作用。 方法： 对10例因腹部手术需要全麻患者，在全麻前、全麻中观察磁波刺激膈神经诱发的Ptra(t)、颤搐性口腔压［Pm(t)］和颤搐性跨膈压［Pdi(t)］的动态变化规律。结果:(1)Pm(t)测定值不稳定，与Pdi(t)的变化不一致；Ptra(t)的变化与Pdi(t)的变化一致。(2)全麻前、中和复苏后的Pdi(t)分别为:（21.4±5.0）cmH2O、(8.7±3.1) cmH2O和（22.9±7.6）cmH2O,全麻中降低（56.8±20.5）%,明显低于全麻前（P<0.01），而复苏后与全麻前没有统计学差异。(3)Ptra(t)与Pdi(t)呈高度线性关系(r=0.802，P<0.01)。(4)Ptra(t)/Pdi(t)的比值为（1.07±0.18），其变异系数为（CV）（10.67±4.03）%。 结论:Ptra(t)可用于动态监测气管插管或气管切开患者的膈肌功能。     

心通胶囊对心肌缺血大鼠心室肌亚硝酸盐和cGMP含量的影响

目的:探讨心通胶囊对实验性大鼠心肌缺血的预防效果及其与一氧化氮形成的相关机制。方法:应用垂体后叶素致大鼠急性心肌缺血模型,以心电图上ST段的抬高作为心肌缺血的指标。测定大鼠心肌缺血心室肌组织一氧化氮(NO)代谢产物(NO₂^-/NO₃^-)和cGMP含量。结果:急性心肌缺血组大鼠的心室肌组织NO₂^-/NO₃^-和cGMP含量分别为(486±59)nmol/gprotein和(0.38±0.08)nmol/gprotein,明显低于正常对照组大鼠的NO₂^-/NO₃^-和cGMP含量,有显著差异(P＜0.01);急性心肌缺血前使用心通胶囊组的心室肌组织NO₂^-/NO₃^-和cGMP含量为(845±105)nmol/gprotein和(0.51±0.10)nmol/gprotein明显高于缺血组(P＜0.01)。与正常组比较,缺血组的心电图ST段抬高明显抬高(P＜0.05);心肌缺血前使用心通胶囊,可使心肌缺血得以改善。结论:心通胶囊提高急性心肌缺血大鼠心室肌组织的NO含量,进而使cGMP含量升高,达到改善心肌缺血的作用。     

犬右心室M细胞复极1期瞬间外向钾电流特性的研究

目的:探讨犬右心室M细胞复极1期瞬间外向钾电流(Ito₁)的电生理特性。方法:应用膜片钳技术,对犬右心室M细胞复极1期Ito₁离子流的电流强度、动力学过程和动作电位切迹进行定量观察。结果:(1)犬右心室M细胞Ito₁离子流的激活过程呈明显的电压依赖性,在37℃、刺激周长为5000ms、去极化电压为0mV和+70mV时,其峰值Ito₁离子流的强度和密度分别为(690±380)pA和(3130±1910)pA,差异显著(P＜0.01);(2)犬右心室M细胞Ito₁离子流具有明显的频率依赖性,在37℃、去极化试验电压为+70mV和基础刺激周长为500ms及5000ms时,Ito₁离子流的强度分别为(1690±830)pA和(3130±1910)pA,差异显著(P＜0.01),并与动作电位的"尖峰-和穹隆"幅度的增加相对应。结论:强大的Ito₁离子流及其介导的"尖峰-和穹隆"状动作电位图形是犬右心室M细胞一个突出的电生理特点之一。     

改变左心室后负荷和离体兔心脏对阈下条件串刺激引起的心室不应期变化的影响

目的:探讨改变左心室后负荷和兔离体灌注心脏对阈下条件串刺激引起的心室不应期变化的影响。方法:采用阈下条件刺激(S_t)法心室不应期测定技术,测定改变左心室后负荷和去除神经,体液因素作用时,S_t法心室不应期强度间期关系特征。结果:减小左室后负荷,可使S_t法测定的心室不应期延长(P＜ 0.05, P＜ 0.01);且在各刺激强度的S_t,S_t(200 Hz,100 ms)和S_t(50 ms, 100 Hz)测定的心室不应期最长(P＜ 0.05,P＜0.01)。结论:改变左心室后负荷和去除神经体液因素作用可使S_t法心室不应期强度间期关系特征发生改变。     

解热醒神注射液对内毒素性发热家兔下丘脑cAMP、IL-1β含量的影响

目的:观察解热醒神(JRXS)注射液对内毒素(ET)性发热1h家兔体温、下丘脑cAMP、IL-1_β含量的影响。方法:复制家兔ET性发热模型,用放免法检测下丘脑及脑脊液cAMP、下丘脑IL-1_β的含量。结果:ET组的ΔT[(0.40±0.11)℃]、TRI₁(1.78±0.79)、下丘脑cAMP含量[(2.90±0.40)nmol/g]、脑脊液cAMP含量[(32.10±4.51)nmol/L]、下丘脑IL-1_β含量[(6.08±0.79)ng/g]显著高于生理盐水(NS)组及JRXS+ET组(P＜0.01)。JRXS+ET组ΔT[(0.10±0.10)℃]、TRI₁(0.36±0.64)、下丘脑cAMP含量[(1.37±0.27)nmol/g]、脑脊液cAMP含量[(14.40±3.69)nmol/L)]、下丘脑IL-1_β含量[(2.90±0.37)ng/g]接近NS组,但与ET组比较有非常显著差异(P＜0.01);4组下丘脑及脑脊液cAMP含量、下丘脑IL-1_β含量变化与体温变化呈正相关。结论:JRXS能明显抑制ET引起的发热反应,其抑制效应的机制与降低ET引起的下丘脑cAMP、IL-1_β含量增多有关。     

模拟缺血-再灌注对窦房结细胞自发性动作电位的影响及吡那地尔的干预作用

目的:探讨模拟缺血-再灌注对窦房结细胞动作电位的影响及K_ATP通道开放剂的干预效果。方法:取培养2d的乳鼠窦房结细胞进行实验,随机分为对照组、模拟缺血-再灌注组(I/R)、K_ATP通道开放剂pinacidil干预组(P+I/R)及K_ATP通道阻断剂5-HD干预组(5-HD+P+I/R及5-HD+I/R)。采用全细胞膜片钳技术记录窦房结细胞动作电位,测定最大舒张电位(MDP)、0期去极化速度(UV)、超射值(APO)、动作电位周期(IBI)及50%复极时程(APD₅₀)的变化。结果:①I/R组MDP(-55.2±4.5)mV明显低于对照组(P＜0.05),APD₅₀(64.3±3.8)ms明显短于对照组(P＜0.01),而UV(3.8±0.5)V/s及APO(14.2±2.0)mV则明显小于对照组(P＜0.01),但IBI无明显改变。②P+I/R组的MDP及APO明显大于I/R组,IBI明显长于、UV显著快于I/R组,但APD₅₀进一步缩短。③5-HD不能阻断pinacidil对模拟缺血-再灌注窦房结细胞MDP、IBI和APD₅₀的影响,但能使pinacidil引起的UV及APO改变发生逆转。结论:Pinacidil可能通过开放不同种类K_ATP通道对模拟缺血-再灌注窦房结细胞动作电位产生影响。     

急性心肌梗死后1周兔心室肌细胞钠离子通道活性的变化

目的:研究急性心肌梗死(AMI)后心室肌细胞钠离子通道活性的变化。方法:采用结扎兔冠状动脉左前降支方法建立AMI动物模型, 应用膜片钳全细胞记录方法, 观察AMI后1周心外膜梗死区心肌细胞钠通道电流(I_Na)的变化。结果:正常对照组I_Na电流密度峰值(-30mV)为(45.5±5.33)PA/PF(n=12), 心梗组为(22.48±4.62)PA/PF(n=14), 明显低于对照组, P＜0.01, I_Na电流-电压关系曲线在心梗组明显下移;心梗组I_Na失活曲线较对照组明显左移, 对照组V_0.5为(-76.2±5.3)mV(n=5), 心梗组为(-89.1±5.6)mV(n=6), P＜0.05;心梗组钠通道灭活后恢复时程明显慢于对照组, 恢复曲线下移。结论:AMI可导致梗死区心室肌细胞I_Na下降、钠通道动力学发生变化, 引起心肌传导速度下降和不应性延长, 可能是导致AMI后出现折返性室性心律失常的原因。     

增强型体外反搏对冠心病患者血浆内皮素和一氧化氮的影响

目的:探讨增强型体外反搏对冠心病患者血浆内皮素(ET)和一氧化氮(NO)的影响。方法:把62例确诊冠心病的患者随机分为体外反搏治疗组(29例)和药物治疗组(32例), 反搏组在常规药物治疗的基础上接受增强型体外反搏仪治疗36d(1h/d), 药物组接受常规药物治疗相同天数;分别于治疗前后应用放射免疫法测定患者的血浆ET含量, 应用硝酸盐还原酶法测定患者血浆NO^-₂/NO^-₃含量, 以间接反映NO的浓度;并测定30例健康人的ET和NO^-₂/NO^-₃值作为对照。结果:治疗前反搏组和药物组的ET水平(116.4±44.9)ng/L, (111.9±44.4)ng/L明显高于正常人(65.8±15.6)ng/L(P＜0.01)。治疗后反搏组ET水平(78.9±30.2)ng/L明显低于药物组(148.0±39.5)ng/L(P＜0.01)。NO^-₂/NO^-₃水平, 治疗前反搏组(64.4±14.8)μmol/L和药物组(67.0±24.0)μmol/L, 稍低于正常人(70.1±13.9)μmol/L, 但P＞0.05;治疗后反搏组(89.6±30.3)μmol/L高于正常人(P＜0.01), 药物组NO^-₂/NO^-₃(83.4±23.0)μmol/L与正常人比较无明显差异(P＞0.05)。体现血管收缩和舒张平衡关系的ET/(NO^-₂/NO^-₃)比值, 治疗前反搏组(1.9±0.8)和对照组(1.8±0.9)均高于正常人(1.0±0.3)(P＜0.01), 治疗后反搏组该值(0.9±0.4)下降(P＜0.01), 并接近正常人水平(P＞0.05), 而药物组(1.8±0.7)与治疗前比较无明显差异(P＞0.05)。结论:增强型体外反搏可改善冠心病患者的内皮功能。     

Diaphragm motor unit recruitment in rats

We hypothesized that considerable force reserve exists for the diaphragm muscle (DIAm) to generate transdiaphragmatic pressures (Pdi) necessary to sustain ventilation. In rats, we measured Pdi and DIAm EMG activity during different ventilatory (eupnea and hypoxia (10% O₂)–hypercapnia (5% CO₂)) and non-ventilatory (airway occlusion and sneezing induced by intranasal capsaicin) behaviors. Compared to maximum Pdi (Pdi_max generated by bilateral phrenic nerve stimulation), the Pdi generated during eupnea (21 ± 2%) and hypoxia–hypercapnia (28 ± 4%) were significantly less (p < 0.0001) than that generated during airway occlusion (63 ± 4%) and sneezing (94 ± 5%). The Pdi generated during spontaneous sighs was 62 ± 5% of Pdi_max. Relative DIAm EMG activity (root mean square [RMS] amplitude) paralleled the changes in Pdi during different ventilatory and non-ventilatory behaviors (r² = 0.78; p < 0.0001). These results support our hypothesis of a considerable force reserve for the DIAm to accomplish ventilatory behaviors. A model for DIAm motor unit recruitment predicted that ventilatory behaviors would require activation of only fatigue resistant units.     

Novel method for transdiaphragmatic pressure measurements in mice

The diaphragm muscle (DIAm) is responsible for breathing and determines the ability to generate both ventilatory and non-ventilatory behaviors. Size limitations of the mouse make transdiaphragmatic pressure (Pdi) measurement using a dual balloon system untenable. Adult C57BL/6J mice (n = 8) and C57BL/6 × 129 (n = 9), underwent Pdi measurements using solid-state pressure catheters spanning the thoracic and abdominal surfaces of the DIAm. Measurements were conducted during eupnea, hypoxia (10% O₂)–hypercapnia (5% CO₂), chemical airway stimulation (i.e., sneezing), spontaneously occurring deep breaths, sustained tracheal occlusion, and bilateral phrenic nerve stimulation. There was a difference in the Pdi generated across the range of ventilatory and non-ventilatory behaviors (p = 0.001). No difference in Pdi across behaviors was evident between mouse strains (p = 0.161). This study establishes a novel method to determine Pdi across a range of DIAm behaviors in mice that may be useful in evaluating conditions associated with reduced ability to perform expulsive, non-ventilatory behaviors.     

Diaphragm compound muscle action potential measured with magnetic stimulation and chest wall surface electrodes

To seek a method to reliably measure phrenic nerve conduction time (PNCT) with magnetic stimulation we investigated two stimulus sites, placing the magnetic coil at the cricoid cartilage (high position) or close to the clavicle (low position). We also compared compound muscle action potential (CMAP) recorded from three different sites: in the sixth to eighth intercostal spaces in the anterior axillary line (Ant-a); in the 8th intercostal space close to the midclavicular line; and with one electrode at the lower sternum and the other at the costal margin. Fourteen normal subjects were studied. The PNCT measured by magnetic stimulation in the high position recorded from (Ant-a) was 7.6+/-0.6 on the left side and 8.4+/-0.7 on the right. The PNCT recorded from all three sites become much shorter when the magnetic coil was moved from the high to the low position. Our results show that PNCT can be accurately measured with magnetic stimulation when care is taken to avoid coactivation of the brachial plexus.     

Neutrophil influx into guinea-pig airway lumen during cholinergic and non-cholinergic bronchoconstriction

Sensory nerve activation will produce adherence of neutrophils to tracheobronchial microvessels. The aim of the present study was to investigate whether this adherence would lead to an influx of neutrophils into the airway lumen. To do this, we studied the effects of 20 minutes of vagal stimulation (1 Hz, 5 ms, 5 V) in anaesthetized and tracheostomized guinea-pigs on both lung resistance, and the cell picture in bronchoalveolar lavage. Any changes were compared to those of intravenous methacholine influsion, producing similar changes in lung resistance. Since high pressure ventilation could produce lung damage, we also studied the effects of ventilation through an extracorporeal resistor, producing a similar change in transpulmonary pressure (45 ± 2 cmH₂O) as vagal stimulation (42 ± 4 cmH₂O). The total number of cells recovered in the lavage was not increased by vagal stimulation, methacholine influsion or the extracorporeal resistor. However, both vagal stimulation and methacholine influsion significantly increased the relative number of neutrophils in the lavage compared to sham stimulated animals (21 ± 11%, 13 ± 4% and 4±1% respectively), but the extracorporeal resistor had no effect (4 ± 2%). Our data suggests that prolonged bronchoconstriction per se may induce an influx of neutrophils into the airway lumen of the guinea-pig.     

The influence of small fibre muscle mechanoreceptors on the cardiac vagus in humans

We have previously shown that activation of muscle receptors by passive stretch (PS) increases heart rate (HR) with little change in blood pressure (BP). We proposed that PS selectively inhibits cardiac vagal activity. We attempted to test this by performing PS during experimental alterations in vagal tone. Large decreases in vagal tone were induced using either glycopyrrolate or mild rhythmic exercise. Milder alterations in vagal tone were achieved by altering carotid baroreceptor input: neck pressure (NP) or neck suction (NS). PS of the triceps surae was tested in 14 healthy human volunteers. BP, ECG and respiration were recorded. PS alone caused a significant decrease ( P < 0.05) in R–R interval (962 ± 76 ms at baseline compared to 846 ± 151 ms with PS), and showed a reduction in HR variability, which was not significant. The decrease in R–R interval with PS was significantly less ( P < 0.05, n = 3) following administration of glycopyrrolate (−8.1 ± 4.5 ms) compared to PS alone (−54 ± 11 ms), and also with PS during handgrip (+10 ± 10 ms) compared with PS alone (−74 ± 15 ms) ( P < 0.05, n = 5). Milder reductions in vagal activity (NP) resulted in a small but insignificant further decrease in R–R interval in response to PS (−107 ± 17 ms compared to PS alone −96 ± 13 ms, n = 5). Mild increases in vagal activity (NS) during PS resulted in smaller decreases in R–R interval (−39 ± 5.5 ms) compared to PS alone (−86 ± 17 ms) ( P < 0.05, n = 8). BP was not significantly changed by stretch in any tests. The results indicate that amongst muscle receptors there is a specific group activated by stretch that selectively inhibit cardiac vagal tone to produce tachycardia.     

Task failure from inspiratory resistive loaded breathing: a role for inspiratory muscle fatigue?

The use of non-invasive resistive breathing to task failure to assess inspiratory muscle performance remains a matter of debate. CO₂ retention rather than diaphragmatic fatigue was suggested to limit endurance during inspiratory resistive breathing. Cervical magnetic stimulation (CMS) allows discrimination between diaphragmatic and rib cage muscle fatigue. We tested a new protocol with respect to the extent and the partitioning of inspiratory muscle fatigue at task failure. Nine healthy subjects performed two runs of inspiratory resistive breathing at 67 (12)% of their maximal inspiratory mouth pressure, respiratory rate ( f_R), paced at 18 min^–1, with a 15-min pause between runs. Diaphragm and rib cage muscle contractility were assessed from CMS-induced esophageal (P_es,tw), gastric (P_ga,tw), and transdiaphragmatic (P_di,tw) twitch pressures. Average endurance times of the first and second runs were similar [9.1 (6.7) and 8.4 (3.5) min]. P_di,tw significantly decreased from 33.1 to 25.9 cmH₂O in the first run, partially recovered (27.6 cmH₂O), and decreased further in the second run (23.4 cmH₂O). P_es,tw also decreased significantly (–5.1 and –2.4 cmH₂O), while P_ga,tw did not change significantly (–2.0 and –1.9 cmH₂O), indicating more pronounced rib cage rather than diaphragmatic fatigue. End-tidal partial pressure of CO₂ (P_ETCO₂) rose from 37.2 to 44.0 and 45.3 mmHg, and arterial oxygen saturation (S_aO₂) decreased in both runs from 98% to 94%. Thus, task failure in mouth-pressure-targeted, inspiratory resistive breathing is associated with both diaphragmatic and rib cage muscle fatigue. Similar endurance times despite different degrees of muscle fatigue at the start of the runs indicate that other factors, e.g. increases in P_ETCO₂, and/or decreases in S_aO₂, probably contributed to task-failure.     

Effect of strength training on the relationship between magnetic resonance relaxation time and muscle fibre composition

Summary The effect of muscle hypertrophy on the relationship between magnetic resonance (MR) relaxation time and muscle fibre composition was investigated. Relaxation time and muscle fibre composition were measured in five subjects before and after a 20-week period of strength training. Muscle fibre composition in all subjects exhibited a significant shift to a predominance of fast-twitch (FT) fibres as a result of 20-week strength training (% area FT fibres: mean values from 49.8%, SD 17.9% to 57%, SD 5.6%; P<0.05). Longitudinal relaxation time (T₁) and transverse relaxation time (T₂) were prolonged significantly after strength training (T₁ mean values from 334.9 ms, SD 13.6 to 359.0 ms, SD 9.0, P<0.001; T₂ from 27.5 ms, SD 0.9 to 30.8 ms, SD 2.3, P<0.05). A constant relationship was observed in changes caused by strength training in muscle fibre composition (% area FT) and relaxation time, with a high correlation obtained between both parameters. These results indicate that MR relaxation time can be used for non-invasive estimation of muscle fibre composition.     

Reproducibility of twitch and sniff transdiaphragmatic pressures

Twitch transdiaphragmatic pressure (Tw Pdi) measured with magnetic stimulation of the phrenic nerve is used to follow up patients and to assess the effect of clinical treatments on diaphragm function. However the reproducibility of Tw Pdi on different occasions has been little studied. We investigated 32 normal subjects, measuring Tw Pdi elicited by bilateral magnetic stimulation of the phrenic nerves on two to 14 occasions. Sniff transdiaphragmatic pressure (sniff Pdi) was also measured. The mean value of Tw Pdi and sniff Pdi were 28+/-5 and 134+/-24 cm H(2)O, respectively. The within subjects coefficient of variation was 11% for both Tw Pdi and sniff Pdi. We conclude that there is a variability of Tw Pdi and the variability of Tw Pdi is the same as that of sniff Pdi.     

Episodic phrenic-inhibitory vagus nerve stimulation paradoxically induces phrenic long-term facilitation in rats

All respiratory long-term facilitation (LTF) is induced by inspiratory-excitatory stimulation, suggesting that LTF needs inspiratory augmentation and is the result of a Hebbian mechanism (coincident pre- and post-synaptic activity strengthens synapses). The present study examined the long-term effects of episodic inspiratory-inhibitory vagus nerve stimulation (VNS) on phrenic nerve activity. We hypothesized that episodic VNS would induce phrenic long-term depression. The results are compared with those obtained following serotonin receptor antagonism or episodic carotid sinus nerve stimulation (CSNS). Integrated phrenic neurograms were measured before, during and after three episodes of 5 min VNS (50 Hz, 0.1 ms), each separated by a 5 min interval, at a low (˜50 μA), medium (˜200 μA) or high (˜500 μA) stimulus intensity in anaesthetized, vagotomized, neuromuscularly blocked and artificially ventilated rats. Medium- and high-intensity VNS eliminated rhythmic phrenic activity during VNS, while low-intensity VNS only reduced phrenic burst frequency. At 60 min post-VNS, phrenic amplitude was higher than baseline (35 ± 5 % above baseline, mean ± S.E.M., P < 0.05) in the high-intensity group but not in the low- (-4 ± 4 %) or medium-intensity groups (-10 ± 15 %), or in the high-intensity with methysergide group (4 mg kg⁻¹, I.P.) (-11 ± 5 %). These data, which are inconsistent with our hypothesis, indicate that phrenic-inhibitory VNS induces a serotonin-dependent phrenic LTF similar to that induced by phrenic-excitatory CSNS (33 ± 7 %) and may require activation of high-threshold afferent fibres. These data also suggest that the synapses on phrenic motoneurons do not use the Hebbian mechanism in this LTF, as these motoneurons were suppressed during VNS.