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1、2.2.3.3 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。 动作电位传导去极化达到了阈电位 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,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,直径越大的细胞（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
2、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,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。
3、这种传导称作跳跃式传导saltatory conduction,2.2.3.4 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 被动 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 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
4、毫米 可以叠加 summation（总和），包括spatial summation; temporal summation,2.2.4 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,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, muscle fibers, gland cells,2.2.5.2 Excitability and threshold stim
5、ulus,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 激,双刺激： 强度1，波宽1 强度2，波宽2，波间隔 串刺激：强度，波宽，频率，串长 连续(复)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
6、个反比例关系。,I,t,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 excitation,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
7、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+
8、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,factors that affect the excitability of cells,RP threshold potential Ca2+ concentration,2.4 Contrction of muscle cells,肌肉组织 Muscle,横纹肌 striated muscle,平滑肌 smooth muscle,骨骼肌 skeletal muscle,心肌 cardiac muscle,随意肌 voluntary muscle,2.4.1 striated muscle,2.4.1.1 Transmissi
9、on at neuromuscular junction Traditionally, synapse has been reserved for neuron-to-neuron contacts, and junction is used more broadly for contacts between any excitable cells. However, with the realization that synapses and other junctions operate by similar mechanisms, the distinction in terminolo
10、gy 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 seperated by 60 nm synaptic cleft. The presynaptic terminals are filled with thousands of 50-nm-diameter synaptic vesicles
11、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 by different motoneurons, but most muscle fibers have only a single endplate innervated by one axon.,Structure of
12、 neuromuscular junction,Neuromuscular Transmission,Neuromuscular transmission begins when: 1) An AP propagating down the axon invades and depolarizes the presynaptic terminal region. This causes: 2) A flow of Ca2+ into the boutons through voltage-activated Ca2+ channels. The elevated Ca2+ triggers:
13、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 Events of Neuromuscular Transmission: 4) ACh diffuses across the cleft. This takes time (up to seve
14、ral 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 potential (EPP). 7) EPPs spread passively to the excitable muscle membrane. 8) Voltage-
15、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 none type It can cause action potential of the excitable muscle membrane
16、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 muscle,myofibril 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） sarcotubular 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 structur
