Download Lec. 13new_04 - Prop. Action Potentials

PHYSIOLOGY 1
LECTURE 13
PROPAGATION of ACTION
POTENTIALS
PROPAGATION of ACTION
POTENTIALS

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
I. Introduction
 Basic, the action potential is continually
regenerated at each new ion channel
site along the membrane.

– A.
– B.
– C.
– D.
No degradation of wave form
No change in shape of wave form
All or none event
Self reinforcing signal
PROPAGATION of the ACTION
POTENTIAL
Action Potential Propagation  is dependent on 
– 1. Properties of ion conducting voltage
gated channels
– 2. Cable properties of the cell


a. Resistance
– 1) Cell membrane
– 2) Ion concentrations
– 3) Cell diameter
b. Capacitance
PROPAGATION of the ACTION
POTENTIAL

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

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
PROPERTIES OF VOLTAGE
GATED CHANNELS
4. AP at one location in the membrane
acts as a stimulus for production of an
AP at an adjacent region of the
membrane
 5. Generation of “new” AP at each site
(self reinforcing signal)
 6. Propagation 
– a. Propagation involves protein cycle
PROPERTIES OF VOLTAGE
GATED CHANNELS

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

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;
 Plasma membrane - insulation
 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

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
CABLE PROPERTIES OF CELLS
ELECTRONIC CURRENT FLOW
Cytoplasmic Resistance (Ri or Rc)
 Resistance to ion flow in the cytoplasm
 Proteins, ions and other molecules
provide resistance to free movement

CABLE PROPERTIES OF CELLS
ELECTRONIC CURRENT FLOW
CABLE PROPERTIES OF CELLS
ELECTRONIC CURRENT FLOW
4. Space and Time Constants determines the velocity of propagation
 5. Decrement in signal strength
 6. Depolarization of adjacent
membrane
 7. Threshold
 8. Generation of sequential “local”
action potentials - Propagation

CABLE PROPERTIES OF CELLS
ELECTRONIC CURRENT FLOW
Space and Time Constants
 Both the space (l) and time constants
(t) are mathematical means of
normalizing data between different cell
types (1/e).
 They are the determents of the velocity
of action potential propagation.


Vel. = l / t
CABLE PROPERTIES OF CELLS
ELECTRONIC CURRENT FLOW


Space Constant - l
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
t

Time Constant -

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

VELOCITY OF PROPAGATION

l = (d x Rm /4 x Rc)1/2
 t = Rm x Cm

and
=l/t
 = 1 / Cm x (d / 4 x Rm x Rc)1/2
 Vel
CABLE PROPERTIES OF CELLS
ELECTRONIC CURRENT FLOW

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
1. Effects of the myelin sheaths
(Schwann cells) - Forces the action
potential to jump from one node of
Ranvier to the next.
 2. Influence of the space and time
constants (Velocity of Conduction)

Saltatory Conduction


Myelin Sheaths Schwann cells wrap
themselves around
the axon several
times forcing the
voltage gated ion
channels into the
Nodes of Ranvier.
Saltatory Conduction

Effects of space and time constants -
 Vel
= 1 / Cm x (d / 4 x Rm x
Rc)1/2

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
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