MEMBRANE POTENTIAL

بسم الله الرحمن الرحيم MEMBRANE POTENTIAL

Objectives • Define Membrane Potential? • Describe the electrical potential across the cell membrane. • Explain the Resting Membrane and its cause (Concentration of ions, permeability of ions) • Emphasize Role of Na+ - K+ pump • Define the term excitable tissue . • Outline the Types of Channels present in cell membrane – Stimuli which can open the Gated Channels – Electrical , chemical • Define terms Depolarization, Repolarization, Hyperpolarization.

Membrane Potential • Plasma membrane of all living cells has a membrane potential (polarized electrically) • Separation of opposite charges across plasma membrane • Due to differences in concentration and permeability of key ions

Membrane Potential • What is Membrane Potential ? • It is the electrical potential across the cell membrane. • It is due to the separation of opposite charges across the Membrane. • It is due to the number of Cations(+) charged particles and Anions(-) charged particles in intracellular fluid and extracellular fluid.

Membrane Potential

Molecular Gradients inside (in mM) 14 140 0.5 10-4 (pH 7.2) 10 5-15 2 75 40 outside (in mM) 142 4 1-2 1-2 (pH 7.4) 28 110 1 4 5 Na+ K+ Mg2+ Ca2+ H+ HCO3- Cl- SO42- PO3- protein

Active Transport outside inside K+ Na+ Na+ ATP K+ 3 Na+ 2 K+ ADP Remember: sodium is pumped out of the cell, potassium is pumped in...

K+ K+ Na+ Na+ Simple Diffusion outside inside

Membrane Potential (Vm) ( - charge difference across the membrane - outside inside …how can passive diffusion of potassium and sodium lead to development of negative membrane potential? K+ K+ Na+ Na+

Simplest Case Scenario: inside outside If a membrane were permeable to only K+ then… K+ K+ would diffuse down its concentration gradient until the electrical potential across the membrane countered diffusion. K+ The electrical potential that counters net diffusion of K+ is called the K+EQUILIBRIUM POTENTIAL (EK).

Simplest Case Scenario: inside outside If a membrane were permeable to only Na+ then… Na+ Na+ would diffuse down its concentration gradient until potential across the membrane countered diffusion. Na+ The electrical potential that counters net diffusion of Na+ is called the Na+ equilibrium potential (ENa).

In living cell effect of both Na+ and K+ must be taken into account • Greater the permeability of plasma membrane for the given ion , the greater I the tendency for that ion to drive the membrane potential towards the ion’s own equilibrium potential. • At rest membrane is 25 to 30 times more permeable to K+ than Na+, thus K+ influence the membrane potential at rest to much greater extent.

Vm -90 to -70 ENa +61 EK -94 Resting Membrane Potential 0 mV Why is Vm so close to EK? Ans. The membrane is far more permeable to K than Na.. The resting membrane potential is closest to the equilibrium potential for the ion with the highest permeability!

RESTING MEMBRANE POTENTIAL • Resting Membrane Potential (RMP) – is the potential across the cell membrane at rest. (unstipulated cell) Usually used to refer to the intracellular potential when compared to the extracellular potential. • Normal value of excitable cells -70 to -80mV • Presence of negatively charged proteins inside the cells ( impermeable )

RESTING MEMBRANE POTENTIAL • RMP established by – • concentration difference of different ions between ICF & ECF • Relative permeability of the cell membrane to the different ions. (more permeable to K) • The NaK ATPase pump which is electrogenic in nature

Membrane Potential • Effect of sodium-potassium pump on membrane potential • Makes only a small direct contribution to membrane potential through its unequal transport of positive ions

Membrane Potential • Nerve and muscle cells • Excitable cells • Have ability to produce rapid, transient changes in their membrane potential when excited • Resting membrane potential • Constant membrane potential present in cells of nonexcitable tissues and those of excitable tissues when they are at rest

Specialized Use Of Membrane Potential In Nerve & Muscle Cells • Nerve and Muscle can rapidly change their membrane permeabilities to the ions, when stimulated. • Therefore, bring changes in membrane potentials. • These rapid changes in membrane potential are responsible for producing nerve impulses in nerves and contraction in muscle cells.

SUMMARY • All living cells have membrane potential. • Cell is negative inside. • Nerve and Muscle are excitable tissues. • Nerves send electrical signal or nerve impulses. • Rapid changes in membrane potential in muscle cell cause muscle contraction. • Resting membrane potential in Neuron (nerve cell) is -70 mv.

Neural Communication • Membrane electrical states • Polarization • Any state when the membrane potential is other than 0mV • Depolarization • Membrane becomes less polarized(less negative) than at resting potential • Repolarization • Membrane returns to resting potential after having been depolarized • Hyperpolarization • Membrane becomes more polarized(membrane becomes more negative) than at resting potential

Types of Changes in Membrane Potential

overshoot 0 mV excitability repolarization + threshold depolarization resting potential -90 mV hyperpolarization - Resting and action potentials • There are some terms that need to be understood & remembered: • excitability • depolarization • hyperpolarization • overshoot • means positive to 0 mV • repolarization • towards resting potential • threshold (for action potential generation)

Neural Communication • Two kinds of potential change • Graded potentials • Serve as short-distance signals • Action potentials • Serve as long-distance signals

Channels & local potentials • The ionic basis of the action potential • membrane permeability • ion channels • types of channels • voltage-dependent channels • receptor operated (ligand-gated) channels.

References • Human physiology by Lauralee Sherwood, seventh,eighthedition • Text book physiology by Guyton &Hall,12thedition • Text book of physiology by Linda .s contanzo,third edition

MEMBRANE POTENTIAL