Action Potential Propagation
Action potentials are a means of sending
a rapid, ultimately non-decremental
signal from one part of a cell to another
• The last block considered the question of
how Aps are initiated at one spot of
excitable membrane
• This one will consider how they travel
throughout an excitable cell.
• Subsequent lectures will show how a
message that takes the form of an AP can
jump from one cell to another.
What is a decremental signal?
Passive or decremental signals lose intensity
exponentially as they spread within a cell
The length constant describes the rate
of voltage decrease with distance.
Vx=V0 (-x/rm/ri )
Where V0 is the membrane potential at the
point where the signal is initiated, Vx is the
membrane potential at some distance x away
along the axon, rm is the transmembrane
resistance and ri is the axoplasmic resistance.
The time constant tau is a measure of
how rapidly the membrane potential
can change
This equation is of the form used for any process of exponential
change – it just says that the voltage at time t is equal to the initial
voltage multiplied by e (the base of the natural logarithms) raised
to the power of t/tau.
Tau =RC
Where R is the membrane resistance and C is the membrane
capacitance
An AP spreads by sending a decremental
signal ahead of itself, that activates
voltage-sensitive Na+ channels in
adjacent membrane
The rate at which an action potential can spread
into adjacent membrane is entirely dependent on
how much adjacent membrane it can depolarize to
threshold.
This is determined almost entirely by geometry.
The speed of propagation of the AP is
dramatically increased by increasing the
diameter of the axon
Effect on the length constant: The larger the diameter of
the cell, the smaller the surface/volume ratio. Thus, as a
cylindrical cell is made larger, core resistance decreases
more rapidly than membrane resistance. As core
resistance gets lower relative to membrane resistance, less
of the initial current leaks out per unit distance.
Small-diameter
axon
Large diameter
axon
This fact has led to the evolution of giant axons
in some invertebrates and lower vertebrates
Giant axons of the order of 1 mm diameter can achieve
conduction velocities of the order of 20 m/sec. Such axons
are found in the pathway that drives escape responses in
squid.
Myelination
A second strategy to increase conduction
velocity of neurons involves decreasing the
membrane resistance through electrical
insulation. Such axons are said to be
myelinated. Myelination has reached its
highest evolution in vertebrates.
The insulation is formed by glial cells that
wrap processes around axons. Myelin-forming
cells in the CNS are called oligodendrocytes those in the peripheral nervous system are
called Schwann cells.
The speed advantage is gained because each length
of internode is crossed by a decremental current
The process is not decremental in the macroscopic
sense because an action potential regenerates the
signal at each node. This mode of conduction is
called saltatory (jumping)
Myelinated axons can achieve conduction velocities
as high as 120 m/sec. In vertebrates, myelination is
characteristic of pathways that carry information
about muscle length/joint position and rapidly
changing stimuli delivered to the body surface.
Multiple Sclerosis, a demyelinating disease
Altered Conduction Patterns with MS
What causes multiple sclerosis?
• It is an autoimmune disease in which the immune
system attacks oligodendrocytes or the myelin
itself within the central nervous system (the brain
and spinal cord). Conduction rate drops and
sometimes fails. Treatment usually involves drugs
that suppress the immune system.