Download Principles of Physiology: Understanding Resting and Action Potentials - Prof. Bing Zhang and more Study notes Zoology in PDF only on Docsity! ZOO 3103 Principles of Physiology Spring 2011 Lecture 2: Resting & Action Potentials (part I) Today’s goals: Equilibrium and resting states: How they differ from each other Nernst and Goldman-Hodgkin-Katz (GHK) equations Neurons are specialized cells Neurons and muscles are electrical excitable cells in animal bodies because they produce Action Potentials (APs) APs are transient ‘expression’ of membrane potentials mediated by specific changes to ion permeabilities Basic properties of APs Computer simulation of resting potential and APs 1 Vm = -71 mV Predicted Vm = -81 mV Vm 5 mM 100 mM K+ +10 mV -150 mV Why??? A realistic resting membrane potential! Na+ 12 mM Na+ 120 mM K+ 125 mM K+ 5 mM [K]o + b [Na]o ( ) [K]i + b [Na]i = 58 log = 58 log ( ) 5 + 0.02 X 120 125 + 002 X 12 = 58 log ( ) 7.4 125.24 = -71 mV 5 Key Features of Resting Potentials 1. Select and different permeability to more than one ions; 2. Uneven distribution of [ion]s are still needed; 3. No equilibrium state is ever achieved for any of the permeable ions; 4. These ions negotiated a ‘peaceful co-existence’ deal: to live with only one membrane potential: resting potential; 5. Whoever that has the highest permeability has more ‘say’ on the resting potential (i.e. Vm close to its Eion); 6. To maintain chemical gradients, ATP-consuming Na/K pump is needed. Neuron Structure and Function Neurons are not all alike; they vary in structure and properties; Use the same basic mechanisms to send signals There are four functional neural zones 1. Signal reception: dendrites and cell body (soma) Incoming signal is received and converted to a change in membrane potential 2. Signal integration: axon hillock Strong signal action potential (AP) 3. Signal conduction: axon; some wrapped in myelin sheath AP travels down axon and to backward to dendrites too 4. Signal transmission: axon terminals Neurotransmitter is released
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Change is good: Membrane potentials can change dramatically in neurons & general terms used to describe these changes Anatomy of An Action Potential Resting pot. Resting pot. 0 mV overshoot Undershoot (hyperpolarization) Depolarization Repolarization “For their discoveries concerning the ionic mechanisms involved in excitation and inhibition in the peripheral and central portions of the nerve cell membrane” Basic Properties of the Action Potential 1. It is transient. The duration of an AP is usually less than 1 ms. 2. It is an ‘all-or-nothing’ event (hence it is voltage-dependent). 3. It propagates along axons (at rates ranging from 5 m/s to 100 m/s) and can backfire to cell body and dendrites. 4. It has refractory periods. Threshold Refractory periods Absolute refractory period – incapable of generating a new AP, upstream Na+ channels inactivated,membrane must repolarize so channel proteins can return to openable state Relative refractory period – more difficult to generate a new AP; some Na+ channels still inactive and increased # K+ channels are open The ionic basis of the action potential Unlike the resting membrane potential, AP is highly dependent on the extracellular [Na+]. Hodgkin & Katz, 1949. Neuroscience by Purves et al. http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=neurosci.TOC&depth=2 A Lethal War Between A Bullfrog and A Newt The Yin and Yang Cycles of the Action Potential AP PNa PK (yang) (yin) ENa 0 mV EK Vm +58 mV -70 mV -80 mV PK>> PNa PNa PNa PK PK>> PNa Anatomy of the AP: dynamic interplay between Na+ and K+ currents Electrical Equivalent Circuit ENa =58 mV EK = -81 mV + - Outside Inside + - + - Vm Ohm’s law: V=IR = I/G Hence, I = VG I ion = G V = G (Vm-Eion) ENa EK Driving force Challenge: How do you express Vm using Ohm’s law?
