PHYSIOLOGY 1 LECTURE 13 PROPAGATION of ACTION POTENTIALS

n Objectives: The student should know –1. General properties of action potentials –2. Local or electrotonic –3. General action potential propagation –4. The cable properties of the cell –5. Factors that determine the velocity of conduction or propagation

PROPAGATION of ACTION POTENTIALS n I. Introduction n Basic, the action potential is continually regenerated at each new ion channel site along the membrane. –A. No degradation of wave form –B. No change in shape of wave form –C. All or none event –D. Self reinforcing signal

PROPAGATION of the ACTION POTENTIAL n Action Potential Propagation - n is dependent on - –1. Properties of ion conducting voltage gated channels –2. Cable properties of the cell n a. Resistance –1) Cell membrane –2) Ion concentrations –3) Cell diameter n b. Capacitance

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 – only 5 to 6 ions move per cycle – effects local area

PROPAGATION of the ACTION POTENTIAL

PROPERTIES OF VOLTAGE GATED CHANNELS n Local Event- –a. Ion channels open and polarity changes in only one small section of membrane –b. If there are ion channels close to the depolarized area and threshold is reached the adjacent area can be activated –c. Movement along the membrane is a continually occurring sequential “local response” and the mechanism for conduction is called “electronic conduction”.

PROPERTIES OF VOLTAGE GATED CHANNELS n 4. AP at one location in the membrane acts as a stimulus for production of an AP at an adjacent region of the membrane n 5. Generation of “new” AP at each site (self reinforcing signal) n 6. Propagation - –a. Propagation involves protein cycle

PROPERTIES OF VOLTAGE GATED CHANNELS n Local Current Flow - Threshold - “new” AP - Local Current Flow - Threshold - “new” AP - Local Current Flow - Threshold - “new” AP - etc. –b. Characteristics of AP are the same along the cell membrane –c. Since the size & shape of all AP are the same on a cell membrane the frequency of AP can be used to code for information

CABLE PROPERTIES OF CELLS ELECTRONIC CURRENT FLOW n Nerve and muscle cells have the properties of an electrical cable –Perfect cable: Insulation surrounds the core conductor and prevents loss of current to the surrounding medium (environment) –Nerve and muscle cells; n Plasma membrane - insulation n Cytoplasm - core conductor

CABLE PROPERTIES OF CELLS ELECTRONIC CURRENT FLOW Not perfect cables 1. Leaky membrane – insulation not perfect 2. Resistance to current flow –A. Membrane resistance (Rm) –B. Cytosolic resistance (Ri or Rc)

CABLE PROPERTIES OF CELLS ELECTRONIC CURRENT FLOW n Membrane Resistance (Rm) –a. Rm is greater than Rc –b. Rm is inversely related to membrane permeability –c. Rm is resistant to ionic current flow –d. Nonpolar nature of membrane - ions have difficulty penetrating –e. Ion transporters have a maximum cycle rate (part of resistance)

CABLE PROPERTIES OF CELLS ELECTRONIC CURRENT FLOW

n Cytoplasmic Resistance (Ri or Rc) n Resistance to ion flow in the cytoplasm n Proteins, ions and other molecules provide resistance to free movement

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

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. = / 

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%.

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 %.

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

CABLE PROPERTIES OF CELLS ELECTRONIC CURRENT FLOW n Space and Time Constants Predict Conduction Velocities –a. If space constant is large - potential will spread a greater distance along the axon and bring distant regions to threshold sooner –b. If time constant is small the AP will be initiated sooner and velocity of propagation is faster

CABLE PROPERTIES OF CELLS ELECTRONIC CURRENT FLOW –c. It is desirable to have axons that have high propagation velocities - rapid information transmission has great survival value –d. Compression of nerves - alters the velocity of conduction by decreasing cell diameter and hence decreasing the space constant and thus decreasing propagation velocities. This is the scientific basis of chiropractic adjustment

CABLE PROPERTIES OF CELLS ELECTRONIC CURRENT FLOW

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)

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.

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.

Saltatory Conduction 1) AP is conducted with little decrement and at great speed from node - node 1) AP is conducted with little decrement and at great speed from node - node 2) AP can only be regenerated at node rather than point to point along the fiber 2) AP can only be regenerated at node rather than point to point along the fiber 3) Because AP appears to "jump" from one node to the next -- called saltatory conduction 3) Because AP appears to "jump" from one node to the next -- called saltatory conduction