生理学第三章 细胞的基本功能3PPT.ppt

* ５、AP propagation-the local Circuit (局部电流) The propagation of AP along unmyelinated nerve fibers is usually explained with the local Circuit．Flow or Local Current theory． Fig. 2－6 depicts the propagation of action potentials in an unmyelinated nerve flber. 有髓纤维－－跳跃式传导Propagation AP along myelinated nerve fibers is usually exolained with the “Saltatory Conduction” theory（Fig.2－7）．At the point of excitation，the membrane is depolarized，the outside becoming negative and the inside becoming positive. This causes electrical currents flow from the adjacent areas towards the depolarized area in a manner essentially similar to that occurred in unmyelinated nerve fibers. However, since the myelin sheath is a good insulator, the electrical current must reach the next node of Ranvier in order to flow across the membrane to complete the circuit. The entry of electrical current at the node causes depolarization of the nodal region which again induces electrical current flow. Thus, the electrical current flows through the surrounding extracellular fluid and the axoplasm from node to node, exciting successive nodes one after another. The jumping of depolarization from node to node is called “Saltatory Conduction”. * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * 动作电位的特征． all or none phenomenon（“全或无”现象） 动作电位的大小和波形不随剌激强度和传导距离而改变 A stimulus of threshold or suprathresho1d intensity applied to a single nerve fiber always produces an action potential of constant amplitude and the amplitude remains constant a long the propagation path． The propagation velocity of a single nerve fiber also remains constant． Responses of a nerve trunk to stimulus of various intensities differ from that of a single nerve fiber． As the intensity of the stimulus is gradually increased from threshold value， the AP amplitude increases． At a particular value of stimulus strength the amplitude of the response becomes maximum；is no longer increases with greater intensitie