1. PHYSIOLOGY 1 LECTURE 11 PROPAGATION of ACTION POTENTIALS

2. PROPAGATION of the ACTION POTENTIAL n The movement of the action potential along a biological membrane is dependent on the properties of the voltage gated ion channels and the electrical cable properties of the cell. n Basically, the action potential is regenerated at each new site along the membrane.

3. PROPAGATION of the ACTION POTENTIAL n Properties of the Voltage Gated Channels - –1. Threshold - Threshold is determined by the protein structure of the voltage gated channels –2. All or None Event - Once initiated the action potential goes to completion - protein cycle –3. Local Event

4. PROPERTIES OF VOLTAGE GATED CHANNELS n Threshold - n Changes in the electrical environment of the voltage gated ion channels causes the protein structure to change shape opening the channel.

5. PROPERTIES OF VOLTAGE GATED CHANNELS n All or none event - n Once the voltage gated ion channels have been stimulated they must cycle through the complete protein shape change in order to reset.

6. PROPERTIES OF VOLTAGE GATED CHANNELS n Local Event - n The cycling of a single voltage gated channel only moves 5 or 6 ions. This is insufficient to activate the entire membrane. However, if a second channel is close enough to be activated the action potential is propagated.

7. CABLE PROPERTIES OF CELLS ELECTRONIC CURRENT FLOW n 1. Ion movement across cell membranes n 2. Ions flow from positive to negative n 3. Resistance to current flow –A. Membrane resistance –B. Cytosolic resistance

8. CABLE PROPERTIES OF CELLS ELECTRONIC CURRENT FLOW n 4. Space and Time Constants - determines the velocity of propagation n 5. Decrement in signal strength n 6. Depolarization of adjacent membrane n 7. Threshold n 8. Generation of sequential “local” action potentials - Propagation

9. CABLE PROPERTIES OF CELLS ELECTRONIC CURRENT FLOW n Ion movement across cell membranes - n Movement of electrical charge implies current flow. Movement of a positive ion implies a negative charge is left behind. Like charges repel while unlike charges attract.

10. CABLE PROPERTIES OF CELLS ELECTRONIC CURRENT FLOW n Ions flow from positive to negative - n More properly stated positive ions flow form positive to negative while negative ions flow from negative to positive

11. CABLE PROPERTIES OF CELLS ELECTRONIC CURRENT FLOW n Resistance to Current Flow n Membrane Resistance (Rm) - The hydrophobic nature of the cellular membrane make ion diffusion difficult so most ions pass the cellular membrane by use of protein transporters. Since the transporters have maximum rates there exists resistance to ion movement.

12. CABLE PROPERTIES OF CELLS ELECTRONIC CURRENT FLOW n Resistance to Current Flow n Cytosolic Resistance - (Rc) n Ions must move through the cellular cytosol along with cytosolic proteins other ions and cellular products. Therefore, resistance occurs as interference to free movement begins to take effect.

13. CABLE PROPERTIES OF CELLS ELECTRONIC CURRENT FLOW n Space and Time Constants Both the space ( ) and time constants (  ) are mathematical means of normalizing data between different cell types (1/e). Both the space ( ) and time constants (  ) are mathematical means of normalizing data between different cell types (1/e). n They are the determents of the velocity of action potential propagation. Vel. = /  Vel. = / 

14. CABLE PROPERTIES OF CELLS ELECTRONIC CURRENT FLOW Space Constant - Space Constant - The space constant measures the distance it takes for the initial depolarization voltage of the action potential to decline by 1 / e or about 63%. The space constant measures the distance it takes for the initial depolarization voltage of the action potential to decline by 1 / e or about 63%.

15. CABLE PROPERTIES OF CELLS ELECTRONIC CURRENT FLOW Time Constant -  Time Constant -  n The time constant measures the time it takes for the initial action potential voltage to decline by 1 / e or about 63 %.

16. CABLE PROPERTIES OF CELLS ELECTRONIC CURRENT FLOW n VELOCITY OF PROPAGATION = (d x Rm /4 x Rc) 1/2 = (d x Rm /4 x Rc) 1/2  = Rm x Cm  = Rm x Cm n and Vel = /  Vel = /  n = 1 / Cm x (d / 4 x Rm x Rc) 1/2

17. CABLE PROPERTIES OF CELLS ELECTRONIC CURRENT FLOW n Decrement of Signal Strength - n The opening of the voltage gated ion channels during action potential generation only allow 5 to 6 ions to move, hence, as these disperse away from the area voltage falls.

18. CABLE PROPERTIES OF CELLS ELECTRONIC CURRENT FLOW n Depolarization of Adjacent Membrane - n Dispersal of the entering Na+ ions away from their entry site rises the membrane potential of adjacent membrane segments

19. CABLE PROPERTIES OF CELLS ELECTRONIC CURRENT FLOW n Threshold - n Threshold is an intrinsic property of the voltage gated transport proteins. n As the membrane potential changes the voltage gated channels twist to accommodate amino acid charges.

20. CABLE PROPERTIES OF CELLS ELECTRONIC CURRENT FLOW n Generation of Sequential “Local” Action Potentials - n If a second set of voltage gated ion channels exists close enough to the first set for the membrane potential to still be above threshold than a new action potential is generated - Propagation

21. Saltatory Conduction n 1. Effects of the myelin sheaths (Schwann cells) - Forces the action potential to jump from one node of Ranvier to the next. n 2. Influence of the space and time constants (Velocity of Conduction)

22. Saltatory Conduction n Myelin Sheaths - n Schwann cells wrap themselves around the axon several times forcing the voltage gated ion channels into the Nodes of Ranvier.

23. Saltatory Conduction n Effects of space and time constants - n Vel = 1 / Cm x (d / 4 x Rm x Rc) 1/2 n The myelin sheaths increase Rm but to a much greater degree they decrease the cell membrane capacitance (Cm), hence, significantly increasing velocity of conduction.