Membrane potential

definition

• It is the difference in electrical potential between the two sides of the membrane surface under resting conditions.

• The inside is negative relative to the outside of the membrane (polarized state).

Measuring cell membrane potential

Resting membrane potential (RMP)

• myelinated nerve& skeletal muscle is -90 mV.

• The RMP in Medium-sized neuron is -70 mV.

• The RMP of the cardiac muscle is -60 Mv.

Causes of RMP

Selective permeability for:Na+K Na-K ATP pump

Diffusion potential

1 .Selective permeability

Inside of the cell:K+. is the main cation &proteins are the main anions.

The cell membrane is: It is more permeable to K+, less permeable to Na+ ,freely

permeable to Cl- & impermeable to proteins.

Cont” selective permeability

K+ ions:Although K+ is more concentrated inside the cell , it tends to

diffuse outside the cell according to

1-The concentration gradient 2- the high permeability of the cell membrane.

However the K+ outflux is limited by :1-Attraction between K+ & intracellular proteins.2-Repulsion between K+ & Na+ outside cell membrane.3- K+ -Na+ pump which derive K+ inside the cell

Na+ ions:Although Na+ is more concentrated outside the cell , it tends to diffuse inside the cell according to:

1-The concentration gradient.

2-Electrical gradient.

However the Na+ influx is limited by: 1-the low permeability of the cell membrane to Na. +

2.Na+ -K pump which pump Na outside the cell.

Cont” selective permeability

2.Na-K ATP pump

•Pumps 3 Na+ outside the cell for every 2 K+

•Electrogenic pump

Action potential

A short-term change in the electrical potential on the surface of a cell (e.g. a nerve cell or muscle cell) in response to stimulation, and spread rapidly along the nerve fiber membrane.

Th

e Nob

el Prize in

Ph

ysiology or Med

icine (1963)

“for their discoveries concerning the ionic mechanisms involved in excitation and inhibition in the peripheral and central portions of the nerve cell membrane”

Alan Lloyd Hodgkin Andrew Fielding Huxley

Basic Electrophysiological Terms

o Stimulus: a sudden change of the (internal or external) environmental condition of the cell. o includes physical and chemical stimulus.

o Threshold (intensity): the lowest or minimal intensity of stimulus to elicit an action potential

Basic Electrophysiological Terms:

Polarization: a state in which membrane is polarized at rest, negative inside and positive outside.

Depolarization: the membrane potential becomes less negative than the resting potential (close to zero).

Hyperpolarization: the membrane potential is more negative than the resting level.

Repolarization: restoration of normal polarization state of membrane.

Phases of action potential

1.resting stage

2 depolarization stage

3.repolarization stage

1.resting stage

• The membrane is polarized.

• all Na+ and K+ gated channels are closed.

2. depolarization stage

Depolarization phase

the voltage

gated fast Na+ channels open partially & when it reaches -55mV :Na+ channels are fully open and the Na+ moves inside the cell.

Activation gate

Inactivation gate

"for their discoveries concerning the function of single ion channels in cells"

Th

e Nob

el Prize in

Ph

ysiology or Med

icine (1991)

Erwin Neher Bert Sakmann

3.Repolarization stage

• Repolarization

• hyperpolarization

Properties of the Action Potential

“All or none” phenomenonA threshold or suprathreshold stimulus applied to a

single nerve fiber always initiate the same action potential with constant amplitude, time course and propagation velocity.

Propagation Transmitted in both direction in a nerve

fiber

Inhibition of action potential

Local anesthetics:• Proacaine and Tetracaine.

• Work on gate of Na channels: difficult to open.

Refractory periods

1. relative

2. Absolute.

      

                    

                    

                                                                                          

Slide 3 of 28

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