2.117 Cajal间质细胞介导胃动素兴奋胃起搏区运动的作用

目的胃肠壁内的Cajal间质细胞(interstitial cells of Cajal,ICC)是胃肠慢波电位的启动者.本研究旨在观察大鼠ICC在胃起搏区收缩活动的特殊作用,以及兴奋性脑肠肽胃动素对ICC引起胃平滑肌细胞收缩活动的调节.方法采用大鼠胃体上1/3起搏区及胃窦环行肌3×10mm2肌条在95%O2,5 %CO2的恒温37℃灌流Kreb液中,通过张力换能器输入生理记录仪记录胃肌条的机械运动. 结果 1.带有ICC的胃肌正常自发收缩活动胃肌条孵育60min后出现稳定的收缩活动.胃体起搏区和胃窦收缩频率分别为3.9 2±0. 63次/min和4.14±0.50次/min.收缩振幅分别为0.25±0.05g和0.32±0.09g(均n=18) .胃窦收缩频率和振幅较胃体起搏区分别增高(5.61±0.84)%和(28.00±7.92)%.2.胃动素对带有ICC胃环行肌的作用胃动素0.0 3、0.06和0.12μg/mL明显增加胃体起搏区和胃窦的收缩频率和振幅,呈剂量依赖性增加 .在胃动素0.12μg/mL时,胃体起搏区比生理盐水对照组(NS)频率增加(30.84±6.52)% ,振幅增加(387.41±53.25)%.胃窦比NS组频率增加(14.62±3.71)%,振幅增加(135. 56±25.14)%(均P＜0.01,均n=18).结果显示,同剂量的胃动素对胃体起搏区的收缩作用比胃窦区强,两者差异显著(P＜0.01),表明胃体起搏区对胃动素较胃窦有较高的敏感性.3.破坏ICC对胃肌收缩活动的影响用化学药物美蓝(50μmol/L)+光照(50mW/cm2)破坏胃环肌层ICC细胞后,胃体起搏区平滑肌自发收缩频率和振幅急剧下降,分别比破坏ICC前下降(82.35±12.54)%和(85.41±1 4.37)%.而胃窦平滑肌的频率和振幅则分别下降(73.24±16.31)%和(75.82±11.33)%( 均P＜0.001,均n=18).结果显示胃体起搏区比胃窦区动力下降更明显,表明胃体起搏区平滑肌细胞更受ICC的控制.4.破坏ICC对胃动素增强胃动力的影响破坏环行肌ICC后几乎完全阻断胃动素对胃体起搏区和胃窦平滑肌的收缩作用.结论 ICC对胃平滑肌细胞的机械运动有直接的起搏作用,并通过脑肠肽胃动素进行化学调控,在胃体的起搏区其作用更为明显,证明胃体上1/3区域是胃运动的原动力部位.     

Cajal间质细胞介导促胰液素抑制胃平滑肌运动

目的　观察大鼠ICC介导促胰液素抑制胃平滑肌细胞收缩作用。方法　采用大鼠胃体上 1 3起博区及胃窦环行肌 8× 2mm肌条在 95 %O2 ,5 %CO2 的恒温 37℃灌流Kreb s液 ,通过张力换能器输入生理记录仪记录胃肌条的机械运动。用美蓝 (methyleneblue)加光照破坏ICC ,观察促胰液素作用 ,并用鼠抗c kit抗体(ACK2 )免疫组化方法对破坏平滑肌条ICC进行鉴定。结果　 1 带有ICC的胃肌正常自发收缩活动　带有ICC的大鼠胃体起博区肌条和胃窦肌条记录到稳定的收缩活动。胃体起博区和胃窦收缩频率分别为 4 2 4± 0 82次 min和 4 5 3± 0 94次 …     

2.115Cajal间质细胞介导促胰液素抑制胃平滑肌运动

目的观察大鼠ICC介导促胰液素抑制胃平滑肌细胞收缩作用. 方法采用大鼠胃体上1/3起博区及胃窦环行肌8×2mm肌条在95%O2,5%CO2的恒温37℃灌流Kreb's液,通过张力换能器输入生理记录仪记录胃肌条的机械运动.用美蓝(methylene blue)加光照破坏ICC,观察促胰液素作用,并用鼠抗c-kit抗体(ACk2)免疫组化方法对破坏平滑肌条ICC进行鉴定.     

胞内cAMP在促胰液素和生长抑素引起胃平滑肌细胞舒张中的作用

用大鼠游离平滑肌细胞，观察促胰液素和生长抑素对胃平滑肌细胞的舒张效应及胞内ｃＡＭＰ的作用。结果表明：（１）促胰液素对胃体和胃窦平滑肌细胞的作用均为舒张反应。这一作用只引起环肌舒张，而对纵肌细胞无直接作用。促胰液素抗血清可阻断促胰液素对胃环肌细胞的舒张反应。Ｆｏｒｓｋｏｌｉｎ可加强促胰液素对胃环肌细胞的舒张作用，而ｃＡＭＰ抑制剂则减弱其舒张作用。促胰液素作用于胃环肌细胞后胞内ｃＡＭＰ含量显著升高。（２）生长抑素对胃体和胃窦平滑肌细胞无直接的舒张作用，但可以抑制由五肽胃泌素引起的胃平滑肌细胞的收缩作用，这一作用可被生长抑素抗血清所阻断。生长抑素作用下的胃平滑肌细胞内ｃＡＭＰ无显著变化。上述结果提示：促胰液素通过特异受体引起胃平滑肌细胞舒张，其收缩作用通过胞内ｃＡＭＰ介导。生长抑素对五肽胃泌素所致胃平滑肌细胞收缩的抑制作用不通过ｃＡＭＰ途径。     

G蛋白介导促胰液素对大鼠胃平滑肌细胞的舒张作用

本研究应用人鼠游离胃平滑肌细胞和采用[35S]GTPγS结合法测定肌细胞的G蛋白,观察促胰液素舒张胃个沿肌细胞中的G蛋白的变化及其信号转导通路。结果显示：（1）促膝液素对胃体、胃窦平滑肌细胞均有舒张作用,促膝液素抗血清可阻断促腔激素对胃窦平滑肌细胞的舒张反应,Forskolin与cAMP抑制剂分别加强和抑制促胰液素的这一作用。（2）Gαs抗体可抑制促胰液素的舒张作用,促胰液素明显引起Gαs抗体与[35S]GTPγS结合的增加,表明胃平滑肌细胞中促胰液素受体与Gs蛋白相偶联,激活cAMP系统介导促胰液素舒张胃平滑肌细胞。     

比较雷尼替丁和西米替丁对大鼠胃肌条运动的影响

雷尼替丁增高大鼠离体胃底和胃体平滑肌条的张力，增大胃窦环行肌的收缩幅度，阿托品明显减弱雷尼替丁对胃窦部的作用，西米替丁降低胃底和胃体肌条的张力，减小胃窦纵行肌和环行肌的收缩幅度，消炎痛未能阻断西米替丁的这些作用。     

牵张应变对小肠Cajal间质细胞起搏电流的作用

目的研究牵张应变刺激对体外培养的小肠Cajal间质细胞(ICC)起搏电流的影响。方法利用II型胶原酶消化并在含有干细胞因子的SmGM培养基中培养ICC,72h后采用免疫荧光细胞化学方法鉴定培养的ICC。利用传统全细胞记录模式膜片钳技术记录正常小肠ICC起搏电流,然后采用对细胞直接施加正压和灌流低渗溶液的两种方法给细胞膜施加张应变刺激,以观察牵张应变对小肠ICC起搏电流的影响。结果培养72h后的ICC在光镜下,胞体呈三角形或梭形,且自胞体发出2￣4条长突起,并与邻近ICC突起相互连接成网络状;荧光显微镜下观察ICC胞体和突起酪氨酸激酶受体c-kit表达呈阳性;在膜电位钳制在-60mV的条件下,可以记录到自发而节律性内向电流,即起搏电流;在传统全细胞记录模式下,两种张应变刺激均可以激活一种内向电流同时明显增加起搏电流的振幅及频率。结论牵张应变对胃肠壁的刺激可能作用于胃肠平滑肌节律性运动的起搏细胞ICC并改变其电生理特性,从而调节胃肠平滑肌运动的基础张力和频率。     

豚鼠输尿管Cajal间质细胞毒蕈碱受体的表达及功能研究

目的研究毒蕈碱受体在豚鼠输尿管Cajal间质细胞（ICC）上的表达,初步探讨ICC在胆碱能神经递质传递中的作用。方法将20只豚鼠随机分成对照组和实验组,进行输尿管肌条实验,观察在卡巴胆碱作用下输尿管肌条的收缩幅度和频率的变化;10只豚鼠,将输尿管进行固定、撕片和荧光染色,检测豚鼠输尿管毒蕈碱受体亚型（M1-M5）在ICC上的表达情况。结果卡巴胆碱作用后,实验组肌条的收缩幅度为0 g,频率为0次/分,对照组的幅度为（0.106±0.021）g,频率为（4.900±0.875）次/分,两组差异显著（P〈0.01）。免疫荧光双标显示,输尿管ICC仅表达M2、M3受体亚型。结论胆碱能神经可能通过ICC来调控输尿管平滑肌。     

八肽胆囊收缩素对大鼠胃平滑肌细胞收缩活动的作用

通过游离平滑肌细胞的制备，观察ＣＣＫ－８对大鼠胃体，胃窦和幽门平滑肌细胞的直接作用并初步探讨其作用机制，结果表明：１．ＣＣＫ－８通过结合特异的受体在而发挥对胃窦，幽门平滑肌细胞的收缩作用和对胃体平滑肌细胞的舒张作用。２．ＣＣＫ－８引起胃窦，幽门有细胞收缩性过程中，胞内Ｃａ＾２＋释放可能起重作用，而ＣＣＫ－８引起胃体肌细胞舒张则可能与胞浆，ｃＡＭＰ浓度的改变有关系。     

2.119 比较西沙必利与莫沙必利对胃和结肠运动及平滑肌细胞的收缩作用

目的已知西沙必利(cisapride)和莫沙必利(mosapride citrate)均为新一代胃肠促动力剂,两者都是苯甲酰胺衍生物,均作用于胃肠5-HT4受体促进乙酰胆碱释放 ,从而增强胃肠运动.本研究拟采用大鼠在体、离体及胃肠单个平滑肌细胞技术,比较西沙必利和莫沙必利促进胃和结肠动力作用的异同及其作用机制.方法 1.在体实验,采用清醒大鼠进行慢性实验(n=8),埋植高灵敏度应力传感器记录胃结肠消化间期移行性复合运动(MMC).应力传感器分别位于胃窦和远端结肠. 用多导生理记录仪记录、处理和分析.2.离体实验,采用大鼠离体胃窦和远端结肠的肌条( n=12)动力测定技术.用特制灌流肌槽,加进西沙必利或莫沙必利分别作用于胃窦和远端结肠的肌条,其收缩活动通过张力换能器记录在X-Y记录仪上.3.细胞实验,采用胶原酶技术分离大鼠胃窦和远端结肠的单个平滑肌细胞(n=12),观察西沙必利或莫沙必利对细胞的直接收缩作用,并比较二者作用的异同.结果 1.西沙必利与莫沙必利对胃窦和远端结肠MMC作用的比较空腹时,大鼠胃窦和远端结肠可以记录到典型的MMC活动.正常MMC Ⅲ相平均振幅为25.12±4.25g.当分别静脉给予不同剂量西沙必利或莫沙必利0.125、0 .25、0.5、1.0、2.0mg/kg时均增强胃窦和远端结肠MMC Ⅲ相收缩,两者均随剂量增加而使运动振幅逐渐增加:在胃窦,西沙必利组按上述剂量比对照组分别增加:(11.66±1. 64)%、(29.14±3.17)%、(65.68±8.95)%、(110.10±15.21)%、(142.29±18.55)% (均P＜0.01).莫沙必利组按上述相同剂量比对照组分别增加:(4.0±0.54)%、(9. 5±2.35)%、(29.89±4.67)%、(72.90±11.24)%、(81.64±14.97)%(均P＜0.0 5).在远端结肠,西沙必利组(同上述剂量)比对照组运动振幅分别增加:(38.11±3.34)% 、(62.79±7.41)%、(86.51±8.24)%、(114.41±10.34)%、(151.62±12.64)%(均 P ＜0.01).莫沙必利组(同上述剂量)比对照组分别增加:(2.31±0.81)%、(3.14±0. 60)%、(6.27±0.47)%(P＜0.05)、(8.83±1.64)%(P＜0.05)、(26.62±0. 98)%(P＜0.01).结果表明,西沙必利与莫沙必利对增加胃和结肠MMC收缩均有剂量效应关系,但西沙必利对胃窦和远端结肠的促动力作用强于莫沙必利.2.西沙必利与莫沙必利对胃窦和远端结肠平滑肌收缩活动的比较胃窦和远端结肠平滑肌正常自发收缩波振幅分别为0.32±0.07g和0.21±0.04g.西沙必利引起胃窦和远端结肠平滑肌收缩的阈浓度为0.05mg/mL,最大反应浓度为0.6mg/mL.莫沙必利的阈反应浓度为0.1mg/mL,最大反应浓度为0.6mg/mL.阈浓度时,西沙必利增加胃窦和远端结肠振幅分别为(45.16±12.25)%和(85.71±20.15)%(均P＜0.01).莫沙必利增加胃窦和远端结肠振幅分别为(25.80±6.15)%和(23.65±5.45)%(均P＜0.05).给予最大反应浓度时,西沙必利增加胃窦和远端结肠分别为(187.09±16.24)%和(328.57±46.19)% (均P＜0.01).莫沙必利增加胃窦和远端结肠分别为(125.80±21.42)%、(95.23±2 7 .36)%(均P＜0.05).结果表明,西沙必利和莫沙必利二者均可增加胃窦和远端结肠平滑肌收缩活动,但莫沙必利阈浓度比西沙比利高,莫沙必利对胃特别是远端结肠的促动力作用明显低于西沙必利.3.西沙必利与莫沙必利对胃窦和远端结肠单个平滑肌细胞作用的比较在倒置显微镜下,游离的大鼠胃窦和结肠平滑肌细胞平均长度为98μm,直径为15μm.给予促动力剂西沙必利或莫沙必利均可使平滑肌细胞长度大大缩小.西沙比利和莫沙必利引起平滑细胞收缩的阈反应浓度分别为0.125mg/mL和0.25mg/m L,最大反应浓度均为2mg/mL.二者随剂量增加而肌细胞收缩反应逐渐增加,有剂量依赖性 .在阈浓度时,西沙必利(0.125mg)引起胃肌细胞收缩(13.42±3.62)%(P＜0.01), 结肠肌细胞收缩(19.42±3.51)%(P＜0.01).莫沙必利(0.25mg)引起胃肌细胞收缩( 8.46±1.63)%(P＜0.05),结肠细胞收缩(4.86±0.96)%(P＜0.05).最大浓度时,西沙必利(2mg)增加胃肌细胞收缩(45.24±7.71)%(P＜0.01),增加结肠肌细胞收缩(51.25±6.54)%(均P＜0.01).莫沙必利(2mg)增加胃肌细胞收缩(32.14±8 .47)%(P＜0.01),增加结肠肌细胞收缩(17.43±3.52)%(P＜0.05).上述结果表明,西沙必利和莫沙必利均对胃窦和远端结肠平滑肌细胞有直接收缩作用,但前者作用比后者强.4.受体拮抗剂对西沙必利或莫沙必利增强胃窦和远端结肠收缩效应的影响给予5-HT4受体拮抗剂(GR113808(100μg/kg/mL)或胆碱能M受体阻断剂阿托品(10mg/kg/mL)均可阻断西沙必利或莫沙必利对在体、离体及细胞收缩的兴奋作用.结论西沙必利和莫沙必利均可兴奋胃窦和远端结肠运动,二者在相同剂量下,西沙必利促动力作用优于莫沙必利.西沙必利和莫沙必利均通过5-HT4受体起作用,二者对胃肠促动力强度的差异可能与其作用于胃肠的受体亚型不同有关.     

妊娠及分娩对盆底肌力的影响

目的比较妊娠及分娩后妇女和正常妇女的盆底肌力,了解妊娠及分娩对盆底肌力的短期影响。方法将183例在本院就诊的孕产妇分为顺产组和剖宫产组,使用盆底肌电生理仪分别检测其产前及产后盆底肌肌力,并与166例正常妇女进行评估及比较。结果妊娠组盆底肌力与正常组比较明显降低,差异有统计学意义（P〈0.05）;妊娠组和正常组中的Ⅰ类与Ⅱ类肌纤维肌力均降低,但差异无统计学意义（P〉0.05）;产后6～8周顺产组盆底肌力较剖宫产组肌力明显降低,差异有统计学意义（P〈0.05）;产后12～14周顺产组盆底肌力与剖宫产组盆底肌力均降低,差异无统计学意义（P〉0.05）。结论顺产及选择性剖宫产对盆底肌肌力的影响在产后3个月无明显差异。     

胸锁乳突肌亚部肌构筑、肌梭和神经入肌点定位

目的:为胸锁乳突肌亚部的临床应用提供形态学依据。方法:肌构筑法,组织学HE染色和体视学法。结果:(1)胸骨头亚部的肌重与生理横切面积分别是锁骨头两亚部之和的1.39倍与1.33倍;锁骨头深亚部的肌纤维长仅为胸骨头亚部肌纤维长的82%。(2)胸骨头亚部和锁骨头浅亚部的肌梭密度显著小于锁骨头深亚部。(3)副神经胸锁乳突肌支绕锁骨头深亚部后缘(占65%)入肌时距乳突尖(4.39±0.42)cm,而穿锁骨头深亚部(占35%)入肌时距乳突尖(3.96±0.34)cm。结论:胸骨头亚部是胸锁乳突肌肌力的主要提供者。锁骨头深亚部更多参与维持头部姿势。     

Electrogenesis of muscle fibers of the rat masseter muscle

It was shown by intracellular recording of resting membrane potentials (RMP) and action potentials that the superficial layers of the rat masseter muscle contain chiefly fibers with a high MPP and small overshoot, whereas the deep layers contain mainly fibers with a low MPP but a high overshoot. The excitability of the cytoplasmic membrane of muscle fibers with different MPP levels was found to be similar with respect to its electrical parameters. It is suggested that the rat masseter muscle contains a high proportion of fast phasic fibers in its superficial layers and slow phasic fibers in its deep layers.Department of Pathological Physiology, N. A. Semashko Moscow Medical Stomatologic Institute. (Presented by A. D. Ado.) Translated from Byulleten' Éksperimental'noi Biologii i Meditsiny, Vol. 88, No. 9, pp. 265–267, September, 1979.     

Effect of single and periodic contusion on the rat soleus muscle at different stages of regeneration

This work analyzed the rat soleus muscle after single and recurrent contusions at different stages of regeneration. A noninvasive contusion was produced by a type of drop‐mass equipment. The posterior region of the right hind limb received a trauma and both right and left soleus muscles were analyzed 1, 4, and 6 days after a single contusion (1×), and 6 and 30 days after periodic contusions (10×, one trauma per week for 10 weeks). Single contusion: there was no significant difference between right and left soleus muscle weight. All animals showed abundant signs of acute damage in the right soleus. AChE activity was identified in regeneration segments of the right soleus. Periodic contusions: there was an increase in the right soleus muscle weight (α = 5%) only in the animals evaluated 6 days after periodic contusions. The right soleus muscle showed a high incidence of chronic signs of damage, such as split fibers and a centralized nucleus, which predominated when compared with the acute signs. Right soleus muscles showed split fibers with AChE activity in both the proximal and middle regions. There was no difference in the incidence of muscle fiber types (I, II, and IIC) between right and left soleus muscles after periodic contusions. Skeletal muscle contusion is common in humans, especially in sport activities, where repetitive traumas are also frequent. The results of this work indicate that despite the regeneration process there is an important change in the morphological aspect of regenerated muscle fibers, which possibly affect muscle performance. Anat Rec 254:281–287, 1999. © 1999 Wiley‐Liss, Inc.     

The human temporalis muscle: Superficial,deep, and zygomatic parts comprise one structural unit

The structure of the temporalis muscle was examined in detail from cadaveric specimens (32 specimens from 16 subjects: 5 males, average age 80.6 years; 11 females, average age 88.6 years) and Computerized Tomography (CT) and T1‐weighted Magnetic Resonance (MR) images from normal clinical patients (10 females: average age 45.0 years). Three parts of the muscle were clearly delineated in all cadaveric specimens: (1) the classically recognized superficial part, (2) a zygomatic part, and (3) a complex deep part. In one female specimen, the superficial temporalis demonstrated extensive insertions into the zygomatic process and temporomandibular joint. The zygomatic temporalis originates from the zygomatic arch to insert into the superficial part of the temporalis as it inserts into the lateral surface of the coronoid process. In all specimens, the deep temporalis contained muscle bundles that originated from various crests along the anterior surface of the temporal fossa and inserted into the internal aspect of the coronoid process and retromolar triangle, interdigitating with the buccinator, mylohyoid, and superior constrictor muscles. The confluence of muscle fibers into the buccinator muscle was confirmed in all CT/MRI images. The deep and zygomatic parts described were regarded as accessory muscle bellies previously, but are demonstrably part of the temporalis muscle as a whole. Clin. Anat. 22:655–664, 2009. © 2009 Wiley‐Liss, Inc.     

Possible effects of fatigue on muscle efficiency

The efficiency of energy transduction is defined as the ratio of the work done by a muscle to the free energy change of the chemical processes driving contraction. Two examples of the experimental measurement of muscle efficiency are: (1) the classical method of Hill which measures the value during a steady state of shortening, (2) measuring the overall efficiency during a complete cycle of a sinusoidal process, which comes closer to the situation during natural locomotion. The reasons why fatigue might lower efficiency are the following. (1) The reduction in PCr concentration and increase in Pi and Cr concentration which are characteristic of fatigued muscle, reduce the free energy of PCr splitting. This will reduce the efficiency of the recovery process. It is not known whether the efficiency of the initial process is increased to compensate. (2) There is a general conflict between efficiency and power output when motor units are chosen for a task or when the timing of activation is decided. During fatigue more powerful units have to be used to achieve a task which is no longer within the scope of less powerful units. (3) The slowing of relaxation that is sometimes found with fatigue may make it impossible to achieve the short periods of activity required for optimum efficiency during rapid cyclical movements. A reason why fatigue might increase efficiency is that muscles are thought to be more efficient energy converters when not fully activated than when fully active. Full activation is often not achieved in muscle which is considerably fatigued. Available observations do not allow us to find where the balance between these factors lies. The conclusion is thus that experiments of both the types discussed here should be performed.     

Muscle fibre types in the suprahyoid muscles of the rat

Five muscle fibre types (I, IIc, IIa, IIx and IIb) were found in the suprahyoid muscles (mylohyoid, geniohyoid, and the anterior and posterior bellies of the digastric) of the rat using immuno and enzyme histochemical techniques. More than 90% of fibres in the muscles examined were fast contracting fibres (types IIa, IIx and IIb). The geniohyoid and the anterior belly of the digastric had the greatest number of IIb fibres, whilst the mylohyoid was almost exclusively formed by aerobic fibres. The posterior belly of the digastric contained a greater percentage of aerobic fibres (83.4%) than the anterior belly (67.8%). With the exception of the geniohyoid, the percentage of type I and IIc fibres, which have slow myosin heavy chain (MHCβ), was relatively high and greater than has been previously reported in the jaw‐closing muscles of the rat, such as the superficial masseter. The geniohyoid and mylohyoid exhibited a mosaic fibre type distribution, without any apparent regionalisation, although in the later MHCβ‐containing fibres (types I and IIc) were primarily located in the rostral 2/3 region. In contrast, the anterior and posterior bellies of the digastric revealed a clear regionalisation. In the anterior belly of the digastric 2 regions were observed: both a central region, which was almost exclusively formed by aerobic fibres and where all of the type I and IIc fibres were located, and a peripheral region, where type IIb fibres predominated. The posterior belly of the digastric showed a deep aerobic region which was greater in size and where type I and IIc fibres were confined, and a superficial region, where primarily type IIx and IIb fibres were observed.     

Muscle fibre types in the suprahyoid muscles of the rat

Five muscle fibre types (I, IIc, IIa, IIx and IIb) were found in the suprahyoid muscles (mylohyoid, geniohyoid, and the anterior and posterior bellies of the digastric) of the rat using immuno and enzyme histochemical techniques. More than 90% of fibres in the muscles examined were fast contracting fibres (types IIa, IIx and IIb). The geniohyoid and the anterior belly of the digastric had the greatest number of IIb fibres, whilst the mylohyoid was almost exclusively formed by aerobic fibres. The posterior belly of the digastric contained a greater percentage of aerobic fibres (83.4%) than the anterior belly (67.8%). With the exception of the geniohyoid, the percentage of type I and IIc fibres, which have slow myosin heavy chain (MHCβ), was relatively high and greater than has been previously reported in the jaw-closing muscles of the rat, such as the superficial masseter. The geniohyoid and mylohyoid exhibited a mosaic fibre type distribution, without any apparent regionalisation, although in the later MHCβ-containing fibres (types I and IIc) were primarily located in the rostral 2/3 region. In contrast, the anterior and posterior bellies of the digastric revealed a clear regionalisation. In the anterior belly of the digastric 2 regions were observed: both a central region, which was almost exclusively formed by aerobic fibres and where all of the type I and IIc fibres were located, and a peripheral region, where type IIb fibres predominated. The posterior belly of the digastric showed a deep aerobic region which was greater in size and where type I and IIc fibres were confined, and a superficial region, where primarily type IIx and IIb fibres were observed.     

The importance of frequency and amount of electrical stimulation for contractile properties of denervated rat muscles

Soleus (SOL) and extensor digitorum longus (EDL) muscles were denervated and directly stimulated for 23–69 days through implanted electrodes employing three different patterns. The stimulation was delivered in impulse trains where the pulse frequency differed (20, 75, and 150 Hz), while the train duration (0.3 s) and train repetition rate (1 min^-1) were identical. Consequently, the number of pulses varied such that higher frequency was combined with a higher amount of stimulation. In both SOL and EDL the high-frequency pattern resulted in shorter twitch time-to-peak, greater post-tetanic potentiation, and greater tetanic force than the low frequency. Isotonic shortening velocity was increased to the same extent by all the patterns in SOL whereas in EDL fast intrinsic shortening velocity was maintained by the low-frequency pattern while it was decreased by the high-frequency pattern. We attribute this unexpected effect on the EDL to the larger number of pulses in the high-frequency pattern. By combining the present findings with previous data on directly stimulated rat muscles we conclude: in SOL the twitch duration is influenced by both the frequency and the amount of impulse activity, higher frequencies and smaller amounts leading to faster twitches. The EDL twitch duration is similarly dependent on the amount of activity, but the role of frequency is more unclear. In both SOL and EDL the isotonic shortening velocity is reduced by increasing amounts of activity and there is no evidence that impulse frequency plays a role. In EDL force output is strongly influenced by the impulse frequency, low frequencies resulting in low force outputs irrespective of the amount of activity.