Membrane potentials XIA Qiang, MD & PhD Department of Physiology Room 518, Block C, Research Building School of Medicine, Zijingang Campus Tel: (Undergraduate school), (Medical school)

OUTLINE Resting potential Graded potential Action potential Refractory period

Electrocardiogram ECG

Electroencephalogram EEG

Electromyogram EMG

Extracellular Recording

Intracellular Recording

Opposite charges attract each other and will move toward each other if not separated by some barrier.

Only a very thin shell of charge difference is needed to establish a membrane potential.

Resting membrane potential A potential difference across the membranes of inactive cells, with the inside of the cell negative relative to the outside of the cell Ranging from –10 to –100 mV

Depolarization occurs when ion movement reduces the charge imbalance. A cell is “polarized” because its interior is more negative than its exterior. Overshoot refers to the development of a charge reversal. Repolarization is movement back toward the resting potential. Hyperpolarization is the development of even more negative charge inside the cell.

unequal ion distribution (chemical gradient) across the membrane selective membrane permeability (cell membrane is more permeable to K + ) Na + -K + pump

chemical driving force electrical driving force electrochemical balance

The Nernst Equation: K + equilibrium potential (E K ) (37 o C) R=Gas constant T=Temperature Z=Valence F=Faraday’s constant

Begin: K + in Compartment 2, Na + in Compartment 1; BUT only K + can move. Ion movement: K + crosses into Compartment 1; Na + stays in Compartment 1. buildup of positive charge in Compartment 1 produces an electrical potential that exactly offsets the K + chemical concentration gradient. At the potassium equilibrium potential:

Begin: K + in Compartment 2, Na + in Compartment 1; BUT only Na + can move. Ion movement: Na + crosses into Compartment 2; but K + stays in Compartment 2. buildup of positive charge in Compartment 2 produces an electrical potential that exactly offsets the Na + chemical concentration gradient. At the sodium equilibrium potential:

Mammalian skeletal muscle cell -95 mV-90 mV Frog skeletal muscle cell -105 mV-90 mV Squid giant axon -96 mV-70 mV E k O bserved RP Difference between E K and directly measured resting potential

Goldman-Hodgkin-Katz equation

Electrogenic Hyperpolarizing Role of Na + -K + pump: Establishment of resting membrane potential: Na+/K+ pump establishes concentration gradient generating a small negative potential; pump uses up to 40% of the ATP produced by that cell!

Origin of the normal resting membrane potential K + diffusion potential Na + diffusion Na + -K + pump

Electrotonic Potential

Graded potential Graded potentials are changes in membrane potential that are confined to a relative small region of the plasma membrane

The size of a graded potential (here, graded depolarizations) is proportionate to the intensity of the stimulus.

Graded potentials can be:EXCITATORYorINHIBITORY (action potential(action potential is more likely)is less likely) The size of a graded potential is proportional to the size of the stimulus. Graded potentials decay as they move over distance.

Graded potentials (Local response, local excitation, local potential) Not “all-or-none” Electrotonic propagation: spreading with decrement Summation: spatial & temporal

Threshold Potential: level of depolarization needed to trigger an action potential (most neurons have a threshold at -50 mV)

Action potential Some of the cells (excitable cells) are capable to rapidly reverse their resting membrane potential from negative resting values to slightly positive values. This transient and rapid change in membrane potential is called an action potential Excitable cells: a cell in which the membrane response to depolarisations is nonlinear, causing amplification and propagation of the depolarisation (an action potential).

Negative after- potential Positive after-potential Spike potential After-potential A typical neuron action potential

Ionic basis of action potential

(1) Depolarization: Activation of voltage-gated Na + channel Blocker: Tetrodotoxin (TTX)

(2) Repolarization: Inactivation of Na + channel Activation of K + channel Blocker: Tetraethylammonium (TEA)

The rapid opening of voltage-gated Na + channels explains the rapid-depolarization phase at the beginning of the action potential. The slower opening of voltage-gated K + channels explains the repolarization and after hyperpolarization phases that complete the action potential.

An action potential is an “all-or-none” sequence of changes in membrane potential. Action potentials result from an all-or-none sequence of changes in ion permeability due to the operation of voltage-gated Na + and K + channels. The rapid opening of voltage-gated Na + channels allows rapid entry of Na +, moving membrane potential closer to the sodium equilibrium potential (+60 mv) The slower opening of voltage-gated K + channels allows K + exit, moving membrane potential closer to the potassium equilibrium potential (-90 mv)

Click here to play the Voltage Gated Channels and Action Potential Flash Animation

Mechanism of the initiation and termination of AP

Re-establishing Na + and K + gradients after AP Na + -K + pump “Recharging” process

Properties of action potential (AP) Depolarization must exceed threshold value to trigger AP AP is all-or-none AP propagates without decrement

Voltage Clamp How to study ? Nobel Prize in Physiology or Medicine 1963 "for their discoveries concerning the ionic mechanisms involved in excitation and inhibition in the peripheral and central portions of the nerve cell membrane" Eccles Hodgkin Huxley

Currents recorded under voltage clamp condition

Patch Clamp Nobel Prize in Physiology or Medicine 1991 "for their discoveries concerning the function of single ion channels in cells" Erwin Neher Bert Sakmann

Conduction of action potential Continuous propagation in the unmyelinated axon

Click here to play the Action Potential Propagation in an Unmyelinated Neuron Flash Animation

Saltatory propagation in the myelinated axon

Saltatorial Conduction: Action potentials jump from one node to the next as they propagate along a myelinated axon.

Click here to play the Action Potential Propagation in Myelinated Neurons Flash Animation

Excitation and Excitability To initiate excitation (AP) Excitable cells Stimulation  Intensity  Duration  dV/dt Excitability: the ability to generate action potentials is known as EXCITABILITY

Strength-duration Curve

Four action potentials, each the result of a stimulus strong enough to cause depolarization, are shown in the right half of the figure. Threshold intensity & Threshold stimulus

Refractory period following an AP: 1. Absolute Refractory Period: inactivation of Na + channel 2. Relative Refractory Period: some Na + channels open A refractory period is a period of time during which an organ or cell is incapable of repeating a particular action potential

Factors affecting excitability Resting potential Threshold Channel status

The propagation of the action potential from the dendritic to the axon-terminal end is typically one-way because the absolute refractory period follows along in the “wake” of the moving action potential.

SUMMARY Resting potential: K + diffusion potential Na + diffusion Na + -K + pump Graded potential Not “all-or-none” Electrotonic propagation Spatial and temporal summation

Action potential Depolarization: Activation of voltage-gated Na+ channel Repolarization: Inactivation of Na+ channel, and activation of K+ channel Refractory period Absolute refractory period Relative refractory period

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