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WEEK 2:
ORIGIN OF BIOPOTENTIALS
Dr. Maria Tahamont


Fluid Mosaic Model -Phospholipid bilayer with proteins, cholesterol, glycoproteins

Selectively permeable-allows the passage of some not all ions

Forms barrier that separates cell from its environment

Controls what enters and exits the cell

Site of signal conduction


The potential is a difference in charge across the surface of the membrane

Due to a difference in concentration of ions and the selective permeability of the membrane

Permeability is controlled by gated channels in the membrane (these channels are proteins or protein complexes)

Movement of ions across the membrane causes an electrical current to travel along the membrane


At rest the membrane is polarized, slightly negative in and positive out

Due to the unequal distribution of Na+ and K+ across the membrane

There is more Na+ outside the membrane

There is more K+ inside the membrane

At rest the membrane is 50 to 100 times more permeable to K+ than to Na+

For the most part, the membrane is impermeable to negative ions

E
K
=
RT
___
F ln
C
Na+i
P
Na+
+ C
Ki
P
K+
+ C
Cl-o
P
Cl-
___________________________
C
Na+o
P
Na+
+ C
K+o
P
K+
+ C
Cl-i
P
Cl-
When the membrane is permeable to several different ions, the diffusion potential depends on three factors:

1.
the polarity of the electrical charge of each ion

2.
the permeability of the membrane (P) to each ion

3.
the concentrations ( C) of the respective ions on the outside ( o ) inside ( i ) and

Selective Permeability to
Na + / K +
K+
Na+
+ + + + + + + + + + + + +
_____________________________
- - - - - - - - - - - - - - - - - - - - - - - - -
K+
At rest the membrane is permeable to K+
Impermeable to anions (negative ions)
Selectively permeable to Na+
Na+

Selective Permeability to
Na+ / K+

K+ tends to leak out of the membrane due to the steep concentration gradient and the fact that the membrane is permeable to K+

K+ does leak out caring the positive charges with it but K+ movement is constrained by the pull of the anions and other negative charges inside the cell

The anions accumulate at the inner surface of the cell membrane hence the negative charge on the inside of the membrane

Remember there is a gradient for Na+ across the membrane as well.

Selective Permeability to
Na + / K +

Na+, K+ and Cl- are the most important ions involved in membrane potentials in neurons and muscles

The permeability of the membrane is the key to which ion influences the potential at any given time

The permeability of the membrane changes rapidly during the conduction of impulses along the membrane


Na+ outside 142 mEq/l

Na+ inside 14 mEq/l

Na+ i / Na+ o = 0.1

K+ outside 4 mEq/l
 K+ inside 140 mEq/l

K+ i / K+ o = 35

Selective Permeability to
Na+ / K+

At rest the membrane is freely permeable to K+
 There is a significant concentration gradient for K+ to move from the inside to the outside
 Some K+ does move out following the gradient
 But the membrane is impermeable to anions

The negative charges provide a brake, that slows the movement of the positive charges out of the membrane
 In addition there are Na+ ions, positively charges outside the membrane which move toward the membrane since the Na+ gradient is from the outside to the inside


The action potential is the rapid change in the membrane potential

Two phases depolarization and repolarization

There is a rapid change in the permeability to Na+

Na+ rushes in following its concentration gradient
 This brings a significant number of positive charges into the cell

Changes the charge at the inside of the membrane from negative to positive


Extra cellular anions follow but are prevented from entering by the membrane

There is a rapid change in the permeability of the membrane to Na+

Na+ rushes across the membrane, moving positive charges into the cell

Again the anions outside the cell can not move across the membrane

The in rush of positive charges removes the brake on the K+ and K+ follows the gradient and rushes outside (repolarization)

Editor’s note:
Too darn big picture…won’t fit into the slide…
Let me put it elsewhere…Click here to get it…
If it doesn’t work, click Plan B at the bottom of the screen..
RP


Hyperpolarization (positive after potential) causes the inside of the membrane to become slightly more negative than the resting potential

Mainly due to K+ channels remaining open for several milli seconds after repolarization

Extends refractory period


Period of time after an action potential when an excitable cell cannot generate another action potential
 Absolute refractory period-can’t stimulate the cell to generate an action potential
 Relative refractory period-can be stimulated again but only with a stronger than normal stimulus


Insert bme4

Editor’s note: Ditto….Same thing…
Too darn big picture…won’t fit into the slide…
Let me put it elsewhere…Click here to get it…
If it doesn’t work, click Plan B at the bottom of the screen..
RP


Additional reading assignment: Search on the web or elsewhere and read about
 Donnan Equilibrium,
 Goldman Equation,
 Hudgkin-Huxley model of the action potential.
 There may be an announced quiz to find out if you did…!