RESTING MEMBRANE POTENTIAL AND ACTION POTENTIAL Dr.Sidra Qaiser
Learning Objectives • Students should be able to: • Define resting membrane potential and how it is generated. • Relate Nernst Equilibrium potential for sodium, potassium and chloride ion with resting membrane potential. • Describe the role of leak channels and sodium potassium pump in the generation of resting membrane potential.
Students should be able to: • Define action potential. • Describe the phases of action potential. • Explain the ionic basis of electrical events in an action potential and types of channels involved in it.
Describe the properties of action potential (all or none law, monophasic biphasic and compound potentials, electro tonic potential) and variation in action potential in different tissues like smooth, skeletal and cardiac muscles. • Illustrate difference between graded potential and action potential with the few examples (motor end plate potential, excitatory post synaptic potential, inhibitory post synaptic potential). • How action potential is propagated through mylinated and unmylinated nerve fibers. • What are the factors affecting the spread of conduction of action potential.
What is the difference in ionic composition of ICF and ECF? • What is potential difference? • What is the charge on cell membrane?
The cell membranes of all body cells in the resting condition are, polarized which means that they show an electrical potential difference, commonly used term for potential difference is only potential. • Membrane potential refers to a separation of charges across the membrane or a difference in the relative number of cations and anions in the ICF and ECF.
Basic Physics of Membrane Potentials • Diffusion potential • Equilibrium potwential
Nernst Equation • Relation of diffusion potential to the concentration difference…… resulting in Nernst (equilibrium) potential • For any univalent ion at body temperature of 37° C • EMF (mV)= +/-61log (Conc.inside/Conc.outside) • Calculate for K+ and Na+ • K= -61log(140/4) • Na= -61log(14/142) • Sign is –ve shows the polarity inside the cell.
For potassium • If Ko = 5 mM and Ki = 140 mM EK = -61 log(140/4) EK = -61 log(35) EK = -94 mV
For Sodium • If Nao = 142 mM and Nai = 14 mM EK = -61 log(14/142) EK = -61 log(0.1) EK = +61 mV
Factors Affecting RMP • 3 factors • Polarity of each ion • Membrane permeability of the ions • Concentrations of respective ions on both sides: (i= inside), (o= outside)
What is the role Na-K pump? • Electrogenic pump • Concentration gradient • Contributes -4mV.
Action potential These are rapid transient changes in the membrane potential that spread rapidly along the nerve fiber membrane .
Afterdepolarisation: The descending limb of action potential dosenot reach to the baseline abbruptly, but it shows a delay of few seconds. • Decrease rate of K efflux. • Afterhyperpolarisation: The descending limb of action potential dips a little below the baseline of RMP. • Continued K efflux.
Latent period • After a stimulus is applied to a nerve, there is a latent period before the start of the action potential. This interval corresponds to the time it takes the impulse to travel along the axon from the site of stimulation to the recording electrodes. Its duration is proportionate to the distance between the stimulating and recording electrodes and inversely proportionate to the speed of conduction.
Effects of myelination • Velocity • Energy • Sites