membrane and action potential [gen physio]
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MA. BEMERLY P. JARDIOLIN-SIRA, MDPATHOLOGIST
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Electric potentials exist across the
membranes of ESSENTIALLY ALL cells of
the bodyBut FEW CELLS are excitable they are
capable of self-generation of
electrochemical impulses at their
membranes impulses can be used totransmit signals along the membranes
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Macrophages, ciliated cells,
glandular cells : changes in their
membrane potentials playsignificant roles in controlling
many of the cells functions
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1. impermeable to intracellular1. impermeable to intracellular
proteins and other organic anions ( Aproteins and other organic anions ( A-- ))
2. moderately permeable to Na+2. moderately permeable to Na+3. freely permeable to3. freely permeable to ClCl-- and K+and K+
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NORMALLY: the K+ conc is GREAT INSIDE themembrane, while OUTSIDE the membrane,gradient is LOW
Because of the LARGE K+ concentration gradientfrom the inside toward the outside strongtendency for K+ to diffuse outward the outside
As K+ moves outside, it carries positive chargesto the outside => ELECTROPOSITIVITY OUTSIDE
& ELECTRONEGATIVITY INSIDE (due to theremaining negative ions that remain behind anddo not diffuse outward along with the K+)
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The new POTENTIAL DIFFERENCE (+
outside, - inside) repels the positively
charged K+ continually diffusing outwardback in the opposite direction, from the
outward toward the inside
Within Millisec, the potential change
becomes incredibly increased blockade of further net diffusion to the
exterior
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Potential Difference is 94 MV,
with negativity inside the fiber
membrane( if this a mamaliannerve fiber)
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IF Na+ is involved: Inc conc of Na+ OUTSIDE the membrane,
low conc of Na+ INSIDESodium is positively charged, and the
membrane is impermeable to all otherions, BUT highly permeable toNa+
With diffusion of Na+ towards the INSIDE=> change in polarity =>ELECTROPOSITIVITY INSIDE &ELECTRONEGATIVITY OUTSIDE
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Again, within Millisec, the membrane
potential rises high enough => Blockade
of further diffusion of Na+ to the inside Potential Difference is 61 MV,with
positivity inside the fiber membrane
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The concentration difference ofThe concentration difference of
IONS across a selectivelyIONS across a selectively
permeable membrane cause thepermeable membrane cause thecreation of MEMBRANEcreation of MEMBRANE
POTENTIALPOTENTIAL
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Particle sizeParticle size ions in the body are
hydrated so although K+ is
larger than Na+, hydrated Na+ islarger than hydrated K+
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Ion channels ions cross membranes via
ion channels rather than simple
pores and these channels arepassages through protein
molecules, therefore, there are
separate Na+, K+ & Cl- channels
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VOLTAGEVOLTAGE the ease with which ions
pass through some of these
channels is controlled by voltage- the passage of Na+ through
Na+ channels in excitable tissues
( muscles and nerves ) is greatly
increased by a decrease inmembrane potential
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NeurotransmittersNeurotransmitters these agents may
bind to receptors associated with the ions
and can bring ease with which ions passthrough some of the channels
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example: Cl ions are present in higher
concentration in the extracellular fluid
than in the cell interior=
>=>
they tend todiffuse into the cell
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example: the interior of the cell is
negative compared to the exterior
area, therefore, Cl- ions are pushedout of the cell
** so, after sometime an equilibrium is
reached and Cl influx will be equal
to Cl efflux
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K+ = concentration gradient is outward,
electrical gradient is inwardNa+ = chemical gradient is inward,
electrical gradient is also inward
Cl - = concentration gradient is inward,
electrical gradient is outward
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FOR K+ and Na+ ions => intracellular
concentration remains constantbecause there is an ACTIVE TRANS-
PORT of Na+ out of the cell against
its electrical and concentration
gradients and also an ACTIVETRANSPORT of K+ into the cell
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This ACTIVE TRANSPORT is called
SODIUM-POTASSIUM PUMP ==>==>
responsible for the active transport ofNa+ out of the cell and K+ into the
cell ( extrudes 3 Na+ from the cell for
each 2 K+ it takes into the cell )
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Ca++ = intracellular concentration is
very low => electrical and
concentration gradients areBOTH directed inward
** the distribution of ions across cell
membrane and the nature of thismembrane are responsible for the
presence of MEMBRANE POTENTIAL
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Na+ is actively transported out of the
cell and K+ is actively transported in
K+ diffuses back out of the cell becauseof its conc gradient and Na+ diffuses
back in
Since the permeability of the membrane
to K+ is much greater than it is to Na+ ATREST, the passive K+ efflux is much
greater than the passive Na+ influx
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Since the membrane is impermeable
to most of the anions in the cell, the
K+ efflux is not accompanied by anequal flux of anions and the
membrane is maintained in a
polarized state ( the interior of the
cell is negatively charged and theexterior area is positively charged )
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