Objectives Define Membrane Potential Describe the electrical 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 Ø 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 outside (in m. M) Na+ K+ Mg 2+ Ca 2+ H+ HCO 3 Cl. SO 42 PO 3 - 14 140 0. 5 10 -4 (p. H 7. 2) 10 5 -15 2 75 142 4 1 -2 (p. H 7. 4) 28 110 1 4 protein 40 5

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

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

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

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

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

• 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 is 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.

Resting Membrane Potential Vm -90 to -70 0 m. V ENa +61 EK -94 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 -80 m. V Ø Presence of negatively charged proteins inside the cells ( impermeable )

RESTING MEMBRANE POTENTIAL • RMP established by – Ø concentration difference of different ions between ICF & ECF Ø Relative permeability of the cell membrane to the different ions. (more permeable to K) Ø The Na. K 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

19 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 0 m. V – 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

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

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 qmembrane permeability qion channels Øtypes of channels Øvoltage-dependent channels Øreceptor operated (ligand-gated) channels.

References • Human physiology by Lauralee Sherwood, seventh, eighth edition • Text book physiology by Guyton &Hall, 12 th edition • Text book of physiology by Linda. s contanzo, third edition 26