1、2.2.3.3 动作电位的传播动作电位的传播 propagation of AP 局部电流local circuit current是导致动作电位传导和阈下反应传播的原因。动作电位传导动作电位传导去极化达到了阈电位阈下反应传播阈下反应传播去极没有达到阈电位，不能产生动作电位的情况下，阈下反应可通过局部电流在短距离内传播。（前面讲过阈下反应是衰减传导的）。影响动作电位传导速度的因素影响动作电位传导速度的因素神经和肌细胞具有电缆的某些特性rm/rin的比值越高，跨质膜的电流损失就越小，信号能够无明显衰减地通过电紧张性传送的距离就越远。传导速度由细胞阻抗和电容决定传导速度由细胞阻抗和电容决定直径越大
2、的细胞（轴突axon）传导速度越快。这主要是因为细胞（轴突）直径增大时，胞质的纵向电阻越小，因此局部电流越大。只有少数动物采取增大轴突直径的策略来增大传导速度，如枪乌贼。在脊椎动物中，有些神经纤维外包裹着髓鞘myelin 5maiEli(:)n sheath，这种神经纤维称作有髓神经纤维。髓鞘化导致传导速度显著增加髓鞘化导致传导速度显著增加在有髓神经纤维，由于大量的膜缠绕着轴突，增加了有效膜阻抗，以致rm/rin增大。另外，具有髓鞘缠绕的膜较裸露轴突膜的电容要小得多，局部电流可以更快地使膜去极化。以上两因素均可增大传导速度。在有髓纤维，动作电位只在郎飞氏节(node of Ranvier)处再
3、生，双由于以上电学特性，动作电位可以以小的衰减和高的速度由一个郎飞氏节传导至下一个郎飞氏节。这种传导称作跳跃式传导saltatory conduction2.2.3.4 缝隙连接缝隙连接 gap junction缝隙连接也称作电突触，在中枢和外周神经系统中广泛存在，许多非神经细胞间也存在，如心肌等。在电突触中一个细胞的膜电位变化以电流的形式直接传递给另一个细胞。通常允许双向传递。电紧张电位电紧张电位 被动 局部电位局部电位 少量离子通道开放，没有产生再生性循环 动作电位动作电位 产生再生性循环 去极化的局部电位多是由于去极化电紧张电位和少量离子通道开放产生的主动反应叠加而形成的。仍具有电紧张电
4、位电学特征不表现“全或无”特征；在局部形成电紧张传播Electrotonic propagation，范围不超过几个毫米 可以叠加 summation（总和），包括spatial summation;temporal summation 2.2.4 局部电位局部电位 local potential空间总和时间总和2.2.5 可兴奋细胞及其兴奋性可兴奋细胞及其兴奋性2.2.5.1 excitation and excitable cells excitation 动作电位本身或动作电位产生的过程 excitable cells 受刺激后能产生动作电位的细胞 including:neurons,m
5、uscle fibers,gland cells 2.2.5.2 Excitability and threshold stimulus stimulus 的参数有3 个，强度、时间、强度对于时间的变化率。实验时一般用electrical stimulus，原因是这三个参数都好控制。应用是一般采用方波。threshold stimulus,threshold intensity注意与threshold potnetial的区别刺刺 激激双双刺激：刺激：强度强度1 1，波宽，波宽1 1 强度强度2 2，波宽，波宽2 2，波间隔，波间隔串刺激：强度，波宽，频率，串长串刺激：强度，波宽，频率，串长连
6、续连续(复复)刺激：强度，波宽，频率刺激：强度，波宽，频率双刺激串刺激连续刺激 强度时间曲线 strength-duration curve:引起组织excitation所需的电小刺激强度与该刺激的作用时间基本上是一个反比例关系。It强度时间曲线 固定刺激时间，能够引起兴奋的最小刺激强度称阈强度threshold intensity,对应的刺激称阈刺激。强度大于阈强度的刺激称阈上刺激，subthreshold stimulus 强度小于阈强度的刺激称阈下刺激 suprathreshold stimulus。2.2.5.3 The change of excitability after exc
7、itation当细胞在接受一次有效刺激产生action potential(excitation)的当时和以后一小段时间内兴奋性经历一系列有次序的变化，然后恢复正常。依次是：绝对不应期、相对不应期、超常期和低常期。对于nerve and muscle cells,absolute refractory period:0.52 ms,relative refractory period:a few ms supranormal period and subnormal period 3050 ms 蛙的有髓神经纤维最多 500次s Na+通道的电压依赖性失活是造成不应期和细胞对刺激通道的电压依赖
8、性失活是造成不应期和细胞对刺激适应的原因。适应的原因。失活门在激活门开放后迅疾关闭。一旦Na+通道失活，它只能在膜电位恢复到（或接近恢复到）静息电位水平后才能够再次被激活。在绝对不应期，大量Na+离子通道被电压失活；在相对不应期的早期，一些Na+离子通道仍被电压失活被太缓慢去极化的细胞可能不会产生动作电位被太缓慢去极化的细胞可能不会产生动作电位factors that affect the excitability of cells RP threshold potential Ca2+concentration2.4 Contrction of muscle cells 肌肉组织Muscle
9、横纹肌 striated muscle 平滑肌 smooth muscle 骨骼肌 skeletal muscle心肌 cardiac muscle随意肌voluntary muscle2.4.1 striated muscle 2.4.1.1 Transmission at neuromuscular junction Traditionally,has been reserved for neuron-to-neuron contacts,and junction is used more broadly for contacts between any excitable cells.H
10、owever,with the realization that synapses and other junctions operate by similar mechanisms,the distinction in terminology has seemed less important,and synapse is often used losely to connote neoron-effector organ junction as well as those between neorons.The axon terminal and muscle cell are seper
11、ated by 60 nm synaptic cleft.The presynaptic terminals are filled with thousands of 50-nm-diameter synaptic vesicles and many mitochondria.A muscle fiber many centimeters long may have an end-plate reegion only 100 to 500 m long.A few vertebrate muscles have more than one end-plate region innervated
12、 by different motoneurons,but most muscle fibers have only a single endplate innervated by one axon.Structure of neuromuscular junctionNeuromuscular TransmissionNeuromuscular transmission begins when:1)An AP propagating down the axon invades and depolarizes the presynaptic terminal region.This cause
13、s:2)A flow of Ca2+into the boutons through voltage-activated Ca2+channels.The elevated Ca2+triggers:3)The fusion of synaptic vesicles with pre-synaptic membrane specializations and the release of packets or quanta of transmitter(Ach)into the cleft.These are the pre-synaptic events.Post-synaptic Even
14、ts of Neuromuscular Transmission:4)ACh diffuses across the cleft.This takes time(up to several hundred s).5)ACh binds to Nicotinic Acetylcholine receptors in the post-synaptic membrane,opening monovalent cation channels.6)Channel opening depolarizes the post-synaptic membrane,creating an endplate po
15、tential(EPP).7)EPPs spread passively to the excitable muscle membrane.8)Voltage-activated Na+channels open giving rise to a muscle AP.9)Termination of the transmission process occurs via hydrolysis of ACh by acetylcholinesterase present in the junction.Please note:End-plate potential is not a“all or
16、 none type It can cause action potential of the excitable muscle membrane 筒箭毒，银环蛇毒可以阻断神经肌肉接头处的传递 2.4.1.2 The structure of striated musclemyofibril and sarcomeres:kmi Isotropic(等方性的）band(I band)or light band anisotropic band(A band)or dark band myosin myofilament （粗肌丝）actin myofilament（细肌丝）sarcotubul
17、ar system transvers tubules(T tubules)Sarcoplasmic reticulum,SR(a special type of smooth ER found in smooth and striated muscle)Juctional SR or terminal cisterna Triad(the T-tubule and its two terminal cisternae form a triad)The structure of striated muscle2.4.1.3 Mechanism of muscle contraction2.4.
18、1.4 Excitation-contraction(EC)coupling T-tubules conduct signal APs into the cell Depolarization of T-tubule causes calcium release from SR Calcium activation of contraction Relaxation occurs as a result of the active transport of Ca2+into the SR by the Ca2+pump on the SR membrane 2.4.1.5 影响横纹肌收缩效能的
19、因素影响横纹肌收缩效能的因素 isometric contraction 等长收缩等长收缩 Tension in the muscles increasesThe muscle is unable to shorten isotonic contraction 等张收缩等张收缩Myofilaments are able to slide past each other during contractionsThe muscle shortensPreloadPreloadInitial lengthSingle twitch tension is determined by length of
20、 sarcomereAfterloadContractility肌肉收缩能力(Contractility)是指与负荷无关的决定肌肉收缩效能的肌肉本身的内在特性。与多种因素有关，如：Ca2+concentration of cytosol ATPase activity of myosin The level of other related proteinsThe variation of contractility is important for cardiac muscleThe force of skeletal muscle are affected by Number of mot
21、or units recruitedFrequency of stimulation SummationForce and the number of motor units recruitedFrequency of stimulation Simple twitch,summation,tetanusIndividual muscle fibers respond to a single stimulus in an all or none fashion-simple twitchSlide 6.26bCopyright 2003 Pearson Education,Inc.publis
22、hing as Benjamin Cummings Anaerobic glycolysis(continued)This reaction is not as efficient,but is fastHuge amounts of glucose are neededLactic acid produces muscle fatigueFigure 6.10bSlide 6.25Copyright 2003 Pearson Education,Inc.publishing as Benjamin Cummings Aerobic RespirationSeries of metabolic
23、 pathways that occur in the mitochondriaGlucose is broken down to carbon dioxide and water,releasing energyThis is a slower reaction that requires continuous oxygenFigure 6.10cSlide 6.27Copyright 2003 Pearson Education,Inc.publishing as Benjamin Cummings When a muscle is fatigued,it is unable to con
24、tract The common reason for muscle fatigue is oxygen debtOxygen must be“repaid”to tissue to remove oxygen debtOxygen is required to get rid of accumulated lactic acid Increasing acidity(from lactic acid)and lack of ATP causes the muscle to contract less骨骼肌可以根据收缩速度被分为快肌和慢肌骨骼肌可以根据收缩速度被分为快肌和慢肌 骨骼肌可以根据其
25、收缩速度被分为两种基本类型：快肌(fast twitch)和慢肌(slow twitch)。虽然有些肌肉几乎只含有快肌纤维或慢肌纤维，大多数肌肉是由这两种肌纤维混合而成的。但是另一方面，一个运动单位典型地只含有一种类型的肌纤维。因此，骨骼肌往往同时含有快肌运动单位和慢肌运动单位。在人体，快肌纤维直径较大，具有高的酵解能力和低的氧化能力。与此相反，慢肌纤维典型地具有较小的直径，具有较高的氧化能力和较低的酵解能力。这个代谢活动的基本差异是与慢肌纤维的抗疲劳特性相吻合的。与此相比，快肌纤维则易于疲劳。由于肌纤维疲劳特性的这种差异，在肌肉收缩的过程中，首先被募集的是慢肌纤维，然后才是快肌纤维。慢肌纤维
26、较小的直径也使得细胞的表面积体积比例得以提高，因此提高来自血液的氧和代谢物质的扩散及摄取能力，并有利于抗疲劳特性的提高。慢肌较高的氧化能力也表现在其由于丰富的血红蛋白、线粒体以及丰富的血管供应而形成的红色外表。与此相比，快肌由于更多地依赖无氧代谢，看上去显得苍白。决定肌纤维种类的一个重要因素是支配神经的类型。这个概念可以通过将支配神经进行互换的实验得到验证。在这个实验中，当原先支配慢肌运动单位的运动神经元被接到快肌纤维，会使快肌运动单位肌纤维的氧化能力增强，收缩速度降低，并表达慢肌肌球蛋白异构体。类似地，将慢肌纤维接上支配快肌运动单位的 运动神经元，慢肌运动单位肌纤维的特性也会转化成快肌纤维的
27、特性，包括快肌肌球蛋白的合成。2.4.2 Smooth Muscle Distribute widely throughout the body and is more variable in function than other muscle type.Homeostatic role Control fluid Sphincters Tonic contractions Support tubes Move products Slow contractions Little fatigue Low O2 useSmooth Muscles:Contrasted to Skeletal M
28、uscle2.4.2.1 Structure of Smooth Muscle Fiber 细肌丝明显多于粗肌丝，还有中间丝，没有Z盘，没有凹入的横管，肌丝主要被肌膜而不是肌质网包绕。2.4.2.2 Types of smooth muscle single-unit smooth muscle 单个单位平滑肌 多缝隙连接 multi-unit smooth muscle多单位平滑肌()很少缝隙连接2.4.2.3 Electric Activities of Smooth Muscle Resting potential -50-60 mV Slow waves(single unit SM)
29、Action potential depolarization caused by influx of both Na+and Ca2+2.4.2.4 Contracting mechanism of smooth muscle Stimulation Electrical stimulus Hormones ParacrinesSmooth Muscle Contraction:MechanismFigure 12-28:Smooth muscle contractionSmooth Muscles:CharacteristicsFigure 12-27:Anatomy of smooth muscleEnd of This Chapter
