Learning ObjectivesStudents should be able to:

Define resting membrane potential and how it is generated.

Relate Nernst Equilibrium potential for sodium, potassium and chloride ion with resting membrane potential.

Describe the role of leak channels and sodium potassium pump in the generation of resting membrane potential. 

Students should be able to:Define action potential.Describe the phases of action

potential.Explain the ionic basis of electrical

events in an action potential and types of channels involved in it.

 

Describe the properties of action potential (all or none law) and variation in action potential in different tissues like smooth, skeletal and cardiac muscles.

Illustrate difference between graded potential and action potential with the few examples (motor end plate potential, excitatory post synaptic potential, inhibitory post synaptic potential).

How action potential is propagated through mylinated and unmylinated nerve fibers.

What are the factors affecting the spread of conduction of action potential.

What is membrane potential?

The cell membranes of all body cells in the resting condition are, polarized which means that they show an

electrical potential differencecommonly used term for potential

difference is only potential.Membrane potential refers to a

separation of charges across the membrane or a difference in the relative number of cations and anions in the ICF and ECF.

RESTING MEMBRANE POTENTIAL

Basic Physics of Membrane PotentialsDiffusion potential

Is the potential difference generated across a membrane because of conc. difference of an ion

It can be generated only if the membrane is permiable to the ion

Diffusion potentials are created by very few ions which do not result in changes in concentration of diffusing ions

Equilibrium potentialIs the diffusion potential that exactly balances

(opposes) the tendency for diffusion caused by a concentration difference

An electrochemical equilibrium , i.e the chemical and electrical driving forces that act on an ion are equal and opposite, therefore, no net diffusion of the ion occur

Nernst EquationRelation of diffusion potential to the

concentration difference…… resulting in Nernst (equilibrium) potential

For any univalent ion at body temperature of 37° C

EMF (mV)= +/-61log (Conc.inside/Conc.outside)Calculate for K+ and Na+

K= -61log(140/4)Na= -61log(14/142)For a positive ion you use negative and vice versaSign shows the polarity inside the cell.

For potassiumIf Ko = 4 mM and Ki = 140 mM

EK = -61 log(140/4)

EK = -61 log(35)

EK = -94 mV

For Sodium If Nao = 142 mM and Nai = 14 mM

EK = -61 log(14/142)

EK = -61 log(0.1)

EK = +61 mV

Role of multiple ions

Factors Affecting RMP3 factors

Polarity of the electrical charge of each ion

Membrane permeability of the ions (p)Concentrations [c] of respective ions on

both sides: (i= inside), (o= outside)

What is the role Na-K pump?

Electrogenic pumpConcentration gradient Contributes -4mV.

Action potentialThese are rapid transient changes in the membrane potential that spread rapidly along the nerve fiber membrane .

Graded potentials

Graded potentials

Stages of Action potential

Afterdepolarisation: The descending limb of action potential does not reach to the baseline abbruptly, but it shows a delay of few seconds.Decrease rate of K efflux.

Afterhyperpolarisation: The descending limb of action potential dips a little below the baseline of RMP.Continued K efflux.

Propagation of Action potential

Unmyelinated nerve fiber

Myelination

Myelinated nerve fiber

Effects of myelinationHigh Velocity

action potentials occur at the unmyelinated nodes of Ranvier that occur between the myelinated internodes

Thus an action potential is propagates along the axon of a neuron at rates significantly higher than would be possible without the myelination of the axon (200 m/s compared to 2 m/s).

Energy efficiencyhelps in reducing energy expenditure, because the

amount of sodium and potassium ions that need to be pumped to bring the concentrations back to the resting state following each action potential is decreased

Sites Occurs only at the noded of ranvier

Effect of electrolytesSodium:Decreasing the external Na+

concentration reduces the size of the action potential but has little effect on the resting membrane potential. The lack of much effect on the resting membrane potential would be predicted, since the permeability of the membrane to Na+ at rest is relatively low.

Potassium:Conversely, increasing the external K+ concentration decreases the resting membrane potential.

Magnitude of stimulusSub threshold stimulusThreshold stimulusSuprathreshold stimulus

Magnitude of stimulusIt is possible to determine the minimal

intensity of stimulating current (threshold intensity) that, acting for a given duration, will just produce an action potential.

Action potential fails to occur if the stimulus is subthreshold in magnitude,produces graded potentials.

Suprathreshold stimuli produce action potential during relative refractory period.

Refractory period

"All-or-None" Law

The action potential fails to occur if the stimulus is subthreshold in magnitude, and it occurs with constant amplitude and form regardless of the strength of the stimulus if the stimulus is at or above threshold intensity. The action potential is therefore "all or none" in character and is said to obey the all-or-none law.

Cardiac muscles

Plateau greatly prolongs the period of

depolarization.

This type of action potential with plateau

is seen in heart muscle fibers.

Opening of fast channels causes the spike

portion of the action potential.

The slow, prolonged opening of the slow

calcium-sodium channels mainly allows

calcium ions to enter the fiber.

This is largely responsible for the plateau

portion of the action potential.

Smooth muscles

Sensitive to stretchSlow wave potentialSpike potential

Inhibition of excitabilityStabilizers - Membrane stabilizing factors

decrease excitabilityHigh extracellular fluid calcium ion conc.Decrease membrane permiability to sodium

ions & simulteneously reduces excitabilityLocal anaesthetics

Procaine & tetracaineActs directly on sodium channel activation

gatesMaking it much more difficult for the gates

to open

Membrane potential & action potentialsAre recorded by cathode Ray Oscilloscope

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