Action Potential PDF

The action potential Prof. Tarig Hakim Merghani  Intended learning outcomes Describe the development and the ionic basis of nerve action potential Describe the properties of action potentials (All or none law, Propagation, and Refractory period).  The Action Potential A...

The action potential Prof. Tarig Hakim Merghani  Intended learning outcomes Describe the development and the ionic basis of nerve action potential Describe the properties of action potentials (All or none law, Propagation, and Refractory period).  The Action Potential A sudden reversal of a membrane polarity, produced by a sufficient stimulus  The Action Potential Occurs only in the excitable tissues:  Nerves (to be conducted)  Muscles (to induce contraction) Action potential stimuli Stimuli:  Mechanical  Chemical  Electrical  Mechanism of AP The stimulus results in elevation of the RMP towards the threshold. This occurs due to insertion of some Na+ ions inside the cell during the stimulation  Mechanism of AP If the threshold is not reached (by a subthreshold stimulus), AP never occurs. Here the recorded change in the potential is called a local potential.  Mechanism of AP When the threshold is reached by a threshold stimulus or a supra- threshold stimulus, AP appears.  Mechanism of AP At the threshold, fast voltage gated Na+ channels open resulting in Na+ influx (both gates of Na+ channels are open)  Mechanism of AP Influx of Na+ results in depolarization  Mechanism of AP At the peak of AP, Na+ channels become inactivated, whereas voltage gated K+ channels are activated, causing K+ efflux= repolarization  Mechanism of AP Repolarization is caused by K+ efflux  Mechanism of AP The slower part of the repolarization is known as after depolarization (misleading name). The part below the RMP is known as hyperpolarization  Mechanism of AP The part of repolarization below the RMP is known as hyperpolarization. It is caused by extended K+ efflux due to slow closure of K+ channels  Mechanism of AP Then the membrane potential returns to the resting level by unexplained mechanism. This phase is known after hyperpolarization. After hyperpolarization is associated with increased Activity of Na+/K+ pumps  Mechanism of AP Passive process (No energy is utilized) During the action potential, the concentrations of sodium & potassium ions in ICF & ECF do not change (because the amounts that cause depolarization & repolarization are very little). Phases of a recorded AP AP potential can be recorded by a cathode ray oscilloscope A typical AP appears as follows: Name the events of AP indicated by the arrows & mention the cause of each What do you call the electrical recordings number 1-2, and explain Name the events of AP indicated by the numbers & mention the cause of each  Characteristics of the AP All or none law  Sub threshold stimuli can not elicit an AP, whereas threshold & supra threshold stimuli can elicit AP of the same magnitude Characteristics of the AP The refractory period  Periodof time during which an ordinary stimulation fails to generate an AP Characteristics of the AP Refractory period is divided into:  Absolute (AP is never generated).  Relative (AP may be generated by stronger stimulus)  Characteristics of the AP Propagation (Conduction of AP) Two types:  Continuous conduction (in unmyelinated neurons)  Saltatory conduction (in myelinated neurons) The velocity of conduction depends on: Degree of myelination:  Conduction is faster in thick myelinated (type A neurons) > thin myelinated (type B neurons) > un-myelinated (type C neurons). The velocity of conduction depends on: Distance between nodes of Ranvier:  Conduction is faster when the distance is increased. Diameter of the axon:  Conduction is faster when the diameter is larger.  Latent period The latent period ca be used for calculation of nerve conduction velocity.  Divide distance between stimulating and recording electrodes by latent period  Characteristics of the AP Stereotyped  APsin a cell show the same size and shape  Local potentials = Graded potentials What are the differences between APs & local potentials Stimulus  SubthresholdVs threshold and suprathreshold stimuli Conduction  LocalizedVs propagated along axons of neurons What are the differences between APs & local potentials Grading  Vary in amplitude Vs all or none Summation  Can be summated Vs refractory periods prevent summation  Types of action potentials AP in a nerve or a skeletal muscle AP in cardiac muscle Note the plateau phase (phase 2) that prolongs the refractory period AP in a pacemaker tissue Note the prepotential phase