PSIO 5th Edition Lecture 34Outline of Last Lecture Neurophysiology : The Resting PotentialOutline of Current Lecture I. Neurophysiology: The Action PotentialII. Graded PotentialsIII. DepolarizationIV. Refractory PeriodsCurrent Lecture1. List in order the principal events associated with an action potential.2. Define the phrase all or none in the context of the neuronal action potential.3. Compare and contrast the characteristics of graded vs. action potentials.4. Discuss the basis of absolute and relative refractory periods.Neuronal Action PotentialA. Neurons are “excitable cells” - Can change membrane potential to generate and electrical signalB. Principal Mechanism – Voltage gated Na+ channelChain of Events in the Generation of the Action Potential1. Local change in membrane potential – such local changes can be hyperpolarizing (more negative) or depolarizing (less negative) – an Action Potential begins with a local depolarization.Graded Potentials- - occur in dendrites and cell body of neuron- size varies with strength of stimulus- usually generated by chemically and mechanically gated channelsTwo Types: Inhibitory Post Synaptic Potentials (IPSP) and Excitatory Post Synaptic Potentials (EPSP)2. Depolarization to a “threshold” value indices a population of voltage- gated Na channels within the local region of membrane to open3. Critical Point – shortly after Na Channels open – they spontaneously close “inactivation”4. Depolarization also opens more slowly a second population of channels : Voltage gated KchannelsRefractory Periods : periods during which it is difficult or impossible to generate a second AP1. Absolute Refractory Period – immediately following inactivation of Na- channels, membrane cannot be restimulated to produce AP; involves resetting of Na channels2. Relative Refractory Period – a period during which a new AP can be produces, but it takes a larger than normal stimulation; involved resetting of K channelsImportance of Refractory Periods1. Estabilishing maximum rate of Action Potentials2. Influence the characteristics of AP propagation – forward propagation from axon hillock to axon
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