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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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