Principles of Biomedical Systems & Devices / WEEK 2: ORIGIN OF BIOPOTENTIALS Dr. Maria Tahamont

The cell membrane  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

Overview of Membrane Potentials  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

The Resting Potential  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

Distribution of charges and ions

Goldman-Hodgkin-Katz Equation RT C Na+i P Na+ + C Ki P K+ + C Cl-o P Cl- E K =___ln ___________________________ F 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 inside (i) and outside (o)

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

Concentration gradients  Na+ outside 142 mEq/l  Na+ inside 14 mEq/l  Na+ i / Na+ o= 0.1  K+ outside 4 mEq/l  K+ inside140 mEq/l  K+ i / K+ o= 35

 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 Selective Permeability to Na+ / K+

Action Potential  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

Action Potentials  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)

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

Action Potential

Action Potentials  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

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

Propagation  Insert bme4

Cardiac Muscle

Micro anatomy of cardiac muscle cell

Cardiac Action Potential

Cardiac Action Potential 2

Conduction System of the Heart

EKG

The pressure volume relationships in the heart 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…here If it doesn’t work, click Plan B at the bottom of the screen.. RP

Homework  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…!