final action potential

8/8/2019 Final Action Potential

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Bioelectric potentials


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Why bioelectric potentials?

To initiate, regulate, coordinate , performcomplex biological activities through the body,a uniform messaging system (Homeostasis )

Messaging units are bioelectric potentials

The language spoken and understood by allthe cells in our body


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Bioelectric potentials

Exists between interior and exterior of all cell membrane of theliving body

Generated bycharged ions lining the either side of the cellmembrane

The study of electrical phenomena in biological system is

called Bio-electricity.

The basic bioelectrical phenomena are the:

1.Membrane potential

Resting Membrane Potential

Action potential


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Recording membrane potential

Cathode ray

oscilloscope

Voltage clamptechnique


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It exists between the interior and the exterior

of all cell membranes of the living body.

Generated by the charged ions that line either

side of the cell membrane.

Not by the ions in the centre of the cytosol.

Generated due to the ionic diffusion across

the cell membrane.

Types 1. RMP , 2.LP , 3.AP, 4. SP


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We study them using the , Voltage clamp ,Patch clamp

techniques , channel blockers.

Potential = Difference in EMF across the cellmembrane.

Polarization = Normal polarity across membrane.

Depolarization = opposite to Polarization.

Repolarisation = restoration of membrane to polarized

state.

Partial depolarization = decreased negativity inside

cell. Stimulus = changes in the environment.


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Stimulus can be (1) Mechanical , (2) Osmotic(3) Chemical , (4) Thermal (5) Electrical

Stimulus Has Strength , Duration ,Frequency.

Duration Milliseconds , inversely related tostrength of stimulus.

Strength Volts Frequency Stimuli per second.


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RMP

The potential difference that exists across the

membrane when the cell is at rest. Normal value = -10 to 100 mV


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Genesis of RMP

It develops due to :

1.Membrane less permeable to Na

2.Membrane more permeable to K and Cl

Membrane not permeable to Non ionic ions

( Protein)

Na K Pump


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K moves easily out of the cell and makes theinside very very negative.(-94mV)

Na and Cl will try to enter and neutralize butin vain

Na K Pump moves out 3 Na and takes in 2

K inside. K diffusing out causes = - 94mV

Na diffusing in = + 8mV

Cl diffusing inside = 00 mV Na - K Pump = - 4 mV

Net = - 90mV


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Role of sodium potassium pump

Electrogenic pump

Pumps 3 Na+ outside

Pumps 2 K+ inside

Creates more negativity

inside


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Role of sodium potassium pump

Creates concentrationgradient

Na+ outside=142 mEq/L

Na+ inside =14 mEq/L

K+ outside =4 mEq/L

K+ inside = 140 mEq/L


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Resting membrane potential (RMP) When the cell is under resting condition, a potential

difference exists across the cell membrane

Because ofBrownian movement of ions

Helps in rapid communication

RMP is defined as being negative, varies from cellto cell

In the absence of RMP cells gain excess water & ions( swells up and gets destroyed)


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Resting membrane potential (RMP)

Evidences in favor of K+

Membrane permeability to

K+ is maximum

Changes in K+ concentration

alters RMP

Change in concentration of

Na+ or cl- does not influence

RMP

Equilibrium potential for K+

(EK) is almost equal to RMPi.e., -94 mv.


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Maintenance of the RMP

1.K+

can pass through its leak channels & the strongand persistent tendency ofK+ efflux makes inside of

the membrane negative

2.Na+ cannot pass thro leak channels (higher diameter

& water molecules attached to it )3.By operation ofNa+ K+ pump

Applied physiology

If this pump is paralysed, the RMP is lost.


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Measurement of RMP

RM

P ofsmooth muscle= -50mv RMP ofSAnodal cells = -60 mv

RMP ofnerve fibres = -90mv


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Theories of membrane potential

1.Nernst equation-

Membrane potential can be predicted

theoretically from ion concentrations on

either side of cell membrane after equilibrium

(Diffusion potential )

Bernstein Hypothesis

Year 1912

Role of potassium in RMPResting membrane is permeable only to K

not to any other ions


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Theories of membrane potential

2.Goldman Hodgkin Katz equation

RMP must be influenced by ions Na+ ,Cl other

than K+

But , the RMP does not change with change of

external Na+ concentration though it changes with

change of external K+ concentration

.


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Action potential

The sequence ofrapid changes in the

membrane potential that spread rapidly

along the nerve fiber when a threshold

stimulus is applied is called action potential.

Three stages

1. Resting stage

2. Depolarization stage

3. Repolarization stage


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Voltage gated sodium & potassium

channels


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Action potential


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Definitions ofAP

1. Stimulus artifact

Small fluctuation of the MP seen at the time

of the stimulus.2. Latent period

Time taken by the impulse to travel from

stimulating electrodes to recordingelectrodes.


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Definitions ofAP

Cell membrane at rest is said to be

polarized

3. Depolarization decrease in electro

negativity (15-20mv ) of the interior of the

resting cell

4. Repolarization return of RMP to more

negative value after depolarization

recovery of RMP after action potential5. Hyperpolarization increased

electronegativity within the cell


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Initiation & propagation of AP


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Types & duration of AP

1.Spike potential (Nerve fibre

,skeletal muscle ) 10 to50m sec

2.Plateau type (Myocardial

cell , smooth muscle cell )

250 to 350m sec

3.Pace maker type

(Conducting system of

heart , smooth muscle

fibres ) 100 to 150m sec


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Local potentials ( Graded potentials )

Potential changes to a sub threshold stimulus


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Examples of local potential1.Excitarory postsynaptic potential (EPSP)

2.Inhibitory postsynaptic potential (IPSP )

-- in synapses

3. End plate potential (EPP )

-- in Myoneural junction

4.Generator potential

--- in Receptor

5. Electrotonic potentials

Catelectrotonus depolarizationAnelectrotonus - hyperpolarization


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Local potentials


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Local potential Action potential

Can be summated Cannot be summated

Not propagated Propagated

Not follow all or none law Follows all or none law

Response to sub threshold stimulus Response to threshold stimulus &

above

Slow event Fast event

Longer duration Shorterduration

Simultaneous opening of Na+ & K+

channels

Opening ofvoltage gated Na+

channels

Passive Active


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Bio-electric potentials

The normal functioning of the nervous

system, beating of the heart ,contraction ofthe muscles, etc., are also associated withelectrical phenomena.

This is studied by :

1.ECG :Records of electrical activity ofheart

2.EEG :Records of electrical activity of brain

3.EMG :Records of electrical activity of muscles

4.Evoked response:A neural pathway isstimulated & the electrical response isrecorded at various levels.


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final action potential

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