Memb potential.sept 13. 1 st

24 Oct. 2012 Ashok Solanki 1

Action Potentials-

the language of excitable tissue

How neurons conduct impulsesHow the muscle contract?

How the heart pump?

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Resting membrane potential created by semi-permeable membrane and

ions• Intracellular

– Na 50– K 400– Cl 52

• Resting membrane potential created by semi-permeable membrane and ions

• Intracellular– Na 50– K 400– Cl 52

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Cell – the functional unit.

• 100 trillion cells organize systems of the body.

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AP OBEYS ALL OR NONE LAW

ALL-OR-NONE RESPONSEA stimulus below the threshold also will not stimulate the neurononce a threshold limit is reached any stronger stimulus will not increase the cell's response

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What starts an Action Potential??• STIMULATION (chemical, electrical, mechanical)

opens Na+ channels– low intensity stimulation opens few channels, – local, graded potential– resting potential restored without action potential

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THE NEURON MEMBRANE AT REST

• Neuron maintains a resting membrane potential of about -70 millivolts across the cell membrane

• Sodium(Na+) and potassium(K+) are the main ions involved

• Na+ and K+ cannot pass through the lipid bilayer membrane

• move through the membrane by using

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ACTION POTENTIAL

• What is it?• Excitable tissue.• All or none law of A.P.• Change in RMP.• Role of ions?• Propagation of.• Propreties of A.P.

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Distribution of important ions in ECF & ICF

Na+ K+ Cl-

INSIDE 14 mEq / L 120 mEq / L 8 mEq / L

OUTSIDE 142 mEq / L 4.5 mEq / L 107 mEq / L

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Action Potential

• ALL-OR-NONE phenomenon• All Action Potentials are the same intensity.

– stronger sensations result from more impulses, not stronger impulses.

– more impulses from same neuron– more impulses from many neurons

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Na+ / K+ PUMP

Membrane proteins actively transport sodium out of the cell

potassium in Three Na+ are pumped out for every two K+ pumped in result is the cell has more Na+ on the outside and more K+ on the inside

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How neurons conduct impulses:

• Membrane potential (as seen in muscle cells)

• K+ diffuses out of neurons faster than Na+ diffuses in,

• Na-K pump moves 3Na+ back out for 2K+ back in

• Cl-, phosphate, protein anions balance cations

• “Resting potential” = - 70 mV



• Action potential – describes events at one

point of nerve fiber– 1: stimulus to

threshold potential – 2: Na+ channels open,

Na+ diffuses in• Polarity briefly

reversed, to +30 mV– 3: Na+ channels close

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The Lipid Barrier of the Cell Membrane, and Cell Membrane Transport Proteins

• Active Transport" of Substances Through Membranes • Primary Active Transport and Secondary Active Transport • Co-Transport of Glucose and Amino Acids Along with Sodium Ions • Sodium Counter-Transport of Calcium and Hydrogen Ions • Na+-K+ pump performs a continual surveillance role in maintaining

normal cell volume. • A Positive-Feedback Cycle Opens the Sodium Channels • Threshold for Initiation of the Action Potential • A major function of the voltage-gated calcium ion channels is to

contribute to the depolarizing phase on the action potential in some cells

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What starts an Action Potential??

• STIMULATION opens Na+ channels– higher magnitude

stimulation opens more channels, local potential exceeds threshold at trigger zone,

– Na+ floods in depolarization

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STIMULATED NEURON

Nerve cells are unique in their ability to carry a signal using membrane potential changes

Stimulation of a neuron opens some of the membrane proteins (a.k.a. Na+gates)

allows Na+ to pass freely into the cells

free flow of Na+ into the cell causes a reversal of membrane polarity

polarity reversal is called the action potential24 Oct. 2012

Resting Potential

• At rest, the inside of the cell is at -70 microvolts• With inputs from dendrites inside becomes more positive • If resting potential rises above threshold, an action potential

starts to travel from cell body down the axon• Figure shows resting axon being approached by an AP24 Oct. 2012 27Ashok Solanki

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How Neurons Communicate

• Action Potential is the electrical process that neurons use to communicate with each other

• Action Potentials are based on movements of ions (charged particles) between the outside and inside of the axon

• Action Potential is an All or Nothing Process (like a gun firing)

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Conduction of an Action Potential

• Propagation of A.P. along neuron membrane• Na+ diffuses, attracted to negative charges in

front of impulse• A.P. at "A"


What Keeps Impulse Going the Same Way ?

• Limits to stimulation of neuron/membrane• Absolute Refractory Period• Relative Refractory Period

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35. Regarding the ionic basis of action potentialin cardiac muscle cells, which one of thefollowing is incorrect?A. Phase 0: Na influxB. Phase 1: K influxC. Phase 2: Ca influxD. Phase 3: K efflux

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GOLDMAN–HODGKIN–KATZEQUATION

• V = 60mV log10 PNaNao• + + PKKo• + + PClCli• −• _________ ________• PNaNai• + + PKKi• + + PClClo24 Oct. 2012



– 3: K+ channels open, K+ diffuses out, Potential returns to zero

– 4: All channels closed, Na-K pump moves Na+ back out & K+ back in

– Hyperpolarization– Resting potential

restored



• Action potential – describes events at one

point of nerve fiber– 1: stimulus to

threshold potential – 2: Na+ channels open,

Na+ diffuses in• Polarity briefly

reversed, to +30 mV– 3: Na+ channels close

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Two Ionic Equilibria and Resting Membrane Potentials

• The resting membrane potential plays a central role in the excitability of nerve and muscle

• An action potential is a rapid change in the membrane potential followed by a return to the resting membrane potential

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Generation and Conduction of Action Potentials

• An action potential is propagated with the same shape and size along the whole length of a nerve or muscle cell

• The action potential is the basis of the signal-carrying ability of nerve cells

• In muscle cells, an action potential allows the entire length of these long cells to contract almost simultaneously.

• Voltage-dependent ion channel proteins in the plasma membrane are responsible for action potentials.24 Oct. 2012

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Voltage dependent ion channels

• Extracellular Na activation gate with intracellular inactivation gate and slow K activation gait

• Conformational changes due to membrane potential changes influence ion permeability

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Nomenclature

• Polarized membrane: Intracellular potential is negative relative to extracellular space

• Depolarization = less polarization of the membrane -80mV -> +20mV

• Hyperpolarization = more polarization of membrane -80mV -> -100mV

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Cell membrane

• Necessary for life as we know it• Border role for cell

– Separates intracellular from extracellular milleau• Allows ion and protein concentration

gradients to exist– Creates electric charge gradients

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LECTURE SUMMARY

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Memb potential.sept 13. 1 st

Health & Medicine

Transcript of Memb potential.sept 13. 1 st