membrane potentials

Dr. Rashid Mahmood 1

Membrane potentials and

Resting Membrane Potential

By Prof. Dr. Rashid Mahmood

Objectives♦Goal /AimBy the end of this session students should be able to understand

the Physiology of Membrane potentials and Resting Membrane Potential

Student should be able to Define the electrochemical potential difference (Δμ) Use the

Nernst equation to determine whether an ion is in equilibrium across a membrane.

Explain the importance of the Gibbs-Donnan equilibrium. Compute the equilibrium transmembrane electrical potential

difference across a membrane that is permeable to only one ionic species.

Estimate a cell’s resting membrane potential by the Nernst equation & Goldman equation

Dr. Rashid Mahmood

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Lesson contents • Membrane potentials • DEFINITIONS

Excitation Stimulus Excitable Tissues Nerve impulse

• Types of Channels involved in Various Excitable Tissues• Basic Physics of Membrane Potentials• Nernst potential• Distribution of Ions across the membrane • Goldman Equation • Resting Membrane Potential• Origin of RMP



DEFINITIONS

• Excitation: • the process of eliciting the action potential• Stimulus: • Anything that excites

“Any Change in the environment” TYPES: a. Electrical

b. Mechanical & c. Chemical


DEFINITIONS• Excitable Tissues: • Any tissue that is capable of generating

rapidly changing electrochemical impulses at their membranes– Tissues which are capable of responding to

stimuli to highest degree than other tissues of the body in the form of electrical signals. These include

• Nerve &• Muscle

• Excitable tissues have LOW Threshold of Stimulation


DEFINITIONS

• Electrical potentials exist across the membranes of virtually all cells of the body

• Change in Electrical Potential• nerve or muscle membranes

– Nerve impulse• Propagated Action Potential

– Local • glandular cells, macrophages, and ciliated cells

• Transmission= – Conduction


Types of Channels involved in Various Excitable Tissues

Na+

• Voltage gated channels (fast) K+

• Slow Ca++- Na+ Channels

• Ligand Gated Channels• Na+ - K+ Pump• Mechanical Gated Channels (Hair Cells )• Na+ - K+ Leak Channels


Basic Physics of Membrane Potentials

Assessment Q.1• What is meant by Excitable Tissues?



Basic Physics of Membrane Potentials

• Membrane Potentials Caused by Diffusion• "Diffusion Potential" Caused by an Ion

Concentration Difference on the Two Sides of the Membrane

• Nernst Potential: • Relation of the Diffusion Potential to

the Concentration Difference

Gibbs-Donnan equilibrium



Nernst potential• Definition: 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

• Magnitude of Nernst potential: – determined by

• ratio of the concentrations of that specific ion on the two sides of the membrane.

• Directly proportional


Distribution of Ions across the membrane

– Na+ mainly extracellular--- 142 mEq/L (ICF: 14 mEq/L)– K+ mainly intracellular----- 140 mEq/L (ECF: 4 mEq/L)– Cl- mainly extra cellular– 103 mEq/L(ICF: 4 mEq/L)– Non-diffusible intracellular anions. -- HPO4 – -- SO4--– ---Intracellular proteins– (4 times as in the plasma)


Nernst Potential

• EMF=± 61log

• EMF is electromotive force• at normal body temperature of 98.6°F (37°C):

• For univalent ion • +ve for –ve ion• Conc. Inside= Ci• Conc. Outside= Co

Conc. insideConc. outside

Assessment Q.2• Calculate Nernst Potential for

Potassium ions.



Calculation of Nernst Potential for K+

• EMF=± 61log

• Conc. Inside= Ci= 140 mEq/L

• Conc. Outside= Co= 4 mEq/L• EMF= -61 log (140/4)• EMF= -61 log (35)• EMF= -61 (1.544)• EMF= -94 mv


Log of 35=

-1.544


Calculation of Nernst Potential for Na+

• EMF=± 61log

• Conc. Inside= Ci= 14 mEq/L

• Conc. Outside= Co= 142 mEq/L • EMF= -61 log (14/ 142)• EMF= -61 log (0.0986)• EMF= -61 (-1.00616031)• EMF= 61.366 mv


Log of 0.0986=

-1.006


Diffusion Potential of Na+ and K+

• POTASSIUM :94 millivolts, with negativity inside the fiber membrane.

• SODIUM: 61 millivolts positive inside the fiber.


Goldman Equation(Goldman-Hodgkin-Katz equation)

• EMF=-61log

• P= Permeability• More than one ion• Potential in ECF outside cell membrane=

zero

(CNa+i PNa+) + (CK+iPk+) + (CCl-o PCl-)

(CNa+o PNa+) + (CK+oPk+) + (CCl-i PCl-)

Assessment Q.3• Calculate Nernst Potential for Sodium

ions.


Resting Membrane Potential(RMP)

Definition: The resting membrane potential is the electrical potential difference across the plasma membrane of a normal living cell in its unstimulated state.

In most cells RMP is close to Nernst Potential for K+

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Channels involved in origin of RMP

Potassium leak channel



Video of Potassium channel: selectivity

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Contribution to RMP

• Contributed by1. K+ Diffusion Potential = -94 mv2. Na+ Diffusion Potential = +61mv3. Na+ K+ Pump = -4mv

Large nerve fibers= -90 mv

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Origin of RMP

Contribution by K+ Diffusion Potential

Contribution by Na+ K+ Pump

-4 mv

Contribution by Na+ and K+

Diffusion Potential (Goldman Equation)

Contribution by Na+ Diffusion Potential

Net RMP

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Types of Disturbances across the Cell Membrane

• TWO Types:• Non-Propagated Potentials:

– Synaptic– Generator

• Propagated• Action Potential

Action Potential

Summary• Electrical potentials exist across the membranes of virtually all cells of the

body• Nernst potential is 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

• Diffusion Potential of POTASSIUM is -94 millivolts, and of SODIUM is +61 millivolts

• Goldman Equation is used to calculate diffusion potential if more than one ions are taken into consideration

• Resting Membrane Potential (RMP) is defined as the electrical potential difference across the plasma membrane of a normal living cell in its unstimulated state.

• In case of large myelinated nerve fibers Contribution to RMP by K+ Diffusion Potential is -94 mv, by Na+ Diffusion Potential is +61mv; and Na+ K+ Pump contributes -4mv. The final calculation is done by putting the values in Goldman Equation


Assessment Q.4• In case of large myelinated nerve fibers

how much is the Contribution to RMP by 1. K+ Diffusion Potential 2.Na+ Diffusion Potential, and 3.Na+ K+ Pump


Next topic

Next topic: Action Potential & its propagation

Prior knowledge required for next lecture

Transport across Cell Membrane

Types of channel in Cell Membrane

© Prof. Dr. Rashid Mahmood 31

Learning Resources • Guyton and Hall (Text book of physiology), 13th Edition• Ganong (Text book of physiology), 24th Edition• Berne & Levy Principles of Physiology (Koeppen BM), 6th

Edition• Human Physiology Stuart Ira Fox, 13th Edition• Human Physiology : from cells to system Lauralee

Sherwood,9th Edition • Internet

© Prof. Dr. Rashid Mahmood 32

membrane potentials

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