BIO 216 Membrane Potentials PDF

BIO 216 Membrane potential Composition of Intra cellular fluid (ICF) and Extra cellular fluid (ECF) What is the difference in ionic composition of ICF and ECF? What is the potential difference? What is the charge on cell membrane? Composition of ICF and ECF  What is the membrane poten...

BIO 216 Membrane potential Composition of Intra cellular fluid (ICF) and Extra cellular fluid (ECF) What is the difference in ionic composition of ICF and ECF? What is the potential difference? What is the charge on cell membrane? Composition of ICF and ECF  What is the membrane potential The cell membranes of all body cells in the resting condition are, polarized which means that they show an electrical potential difference. 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.  Electrical potentials Electrical potentials exist across the membranes of virtually all cells of the body. In addition, some cells, such as nerve and muscle cells, are capable of generating rapidly changing electrochemical impulses at their membranes, and these impulses are used to transmit signals along the nerve or muscle membranes. In still other types of cells, such as glandular cells, macrophages, and ciliated cells, local changes in membrane potentials also activate many of the cells’ functions. Membrane potential (MP) - a transmembrane potential difference that exists between the inner and outer surfaces of the plasma membrane. Resting potential (RP) - a membrane potential of excitable cells that are at rest. In other words, RP - a special case of membrane potential. Measuring the Membrane Potential RESTING MEMBRANE POTENTIAL ▪The membrane in this instance is permeable to the potassium ions but not to any other ions. ▪Because of the large potassium concentration gradient from inside toward outside, there is a strong tendency for extra numbers of potassium ions to diffuse outward through the membrane. ▪As they do so, they carry positive electrical charges to the outside, thus creating electropositivity outside the membrane and electronegativity inside because of negative anions that remain behind and do not diffuse outward with the potassium.  Factors Affecting Resting Membrane Potential 3 factors – Polarity of each ion – Membrane permeability of the ions – Concentrations of respective ions on both sides: (i= inside), (o= outside)  The diffusion potential level across a membrane that exactly opposes the net diffusion of a particular ion through the membrane is called the Nernst potential for that ion.  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) – Sign is –ve shows the polarity inside the cell.  For potassium If Ko = 5 mM and Ki = 150 mM EK+ = 61 log(5mM/150mM) EK+ = 61 log 1/30 Because log of 1/30 = -1.477 EK + = 61 (-1.477 ) EK + = -90 mV Eion = equilibrium potential for ion in mV  For Sodium If Nao = 150 mM and Nai = 15 mM ENa+ = 61 log(150/15) ENa+ = 61 log 10 Because log of 10 is 1, ENa+ = 61 (1) = 61mV Eion = equilibrium potential for ion in mV Role of multiple ions  When a membrane is permeable to several different ions, the diffusion potential that develops depends on three factors: (1) the polarity of the electrical charge of each ion, (2) the permeability of the membrane (P) to each ion, (3) the concentrations (C) of the respective ions on the inside (i) and outside (o) of the membrane. Thus, the following formula, called the Goldman-Hodgkin- Katz equation, gives the calculated membrane potential on the inside of the membrane when two univalent positive ions, sodium (Na+) and potassium (K+), and one univalent negative ion, chloride (Cl–), are involved.  Vm= - 86 mV  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. Graded potentials  Afterdepolarisation: The descending limb of action potential dosenot reach to the baseline abruptly, 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. "All-or-None" Law - The action potential fails to occur if the stimulus is subthreshold in magnitude, and it occurs with constant amplitude and form regardless of the strength of the stimulus if the stimulus is at or above threshold intensity. The action potential is therefore "all or none" in character and is said to obey the all-or-none law Absolute refractive period – unresponsive to further stimulation Relative refractive period – a second action can be produced only by a considerably stronger than usual trigger Compound Action potential END