Graded Potentials
Passive electrotonic conduction
decays with distance
1.
Cytoplasm resistance
2.
Plasma membrane
resistance
3.
Charges leaks out
Length constant () is defined
as the distance over which a
steady-state potential shows a
63% drop in amplitude.
Stimulating electrode:
Introduces current that can
depolarize or
hyper-polarize
Recording electrode:
Records change in
Potential of the membrane
At a distance away
At Threshold Na influx equals K efflux
Voltage (mVolts) along Y axis
Time (msec)
Action Potential
Changes in Ion Permeability allows inward Na flux
and triggers an increased outward K flux through
voltage gated ion channels
Causes transient change in Membrane Potential
The change in ion permeability is triggered by
transient depolarization of the membrane
Action Potential shapes
Characteristics of an Action
Potential
• Triggered by depolarization
• a less negative membrane potential that
occurs transiently
• Understand depolarization, repolarization
and hyperpolarization
Threshold
• Threshold depolarization needed to trigger
the action potential
• 10-20 mV depolarization must occur to
trigger action potential
All or None
• Are all-or- none event
• Amplitude of AP is the same regardless of
whether the depolarizing event was weak
(+20mV) or strong (+40mV).
No Change in Size
• Propagates without
decrement along
axon
The shape (amplitude &
time) of the action
potential does not change
as it travels along the axon
Reverses Polarity
• At peak of action potential the membrane
potential reverses polarity
• Becomes positive inside as predicted by the
Ena Called OVERSHOOT
• Return to membrane potential to a more
negative potential than at rest
• Called UNDERSHOOT
Refractory Period
• Absolute refractory period follows an action
potential. Lasts 1 msec
• During this time another action potential
CANNOT be fired even if there is a transient
depolarization.
• Limits firing rate to 1000AP/sec
Ion Permeability
• Changes during action potential
• The plasma membrane becomes
permeable to sodium ions
– Permeability increases from 0.02 to 20=1000
fold increase
• Causes Em to approach Ena at positive
voltages = +20mV
Regenerative Process:
Once one Na channel
Opens, Na enters,
Depolarizes membrane,
More and more Na
Channels open leading to
More sodium influx &
causes upward &
depolarizing (more +)
phase of the AP
What does a sodium
Channel look like?
It is one large protein
With 4 domains that
Each loop through the
Plasma membrane 7
Times.
Property of Voltage Dependent
Sodium Channel
• Sodium channel opens for 1-2 millisecond
following threshold depolarization
• then inactivates and does not open even if
Vm is depolarized.
• This is called sodium channel inactivation
and contributes to the repolarization of Vm
Na Channel Gates
•M gate= activation gate
on Na channel; opens
quickly when membrane
is depolarized
•H gate- inactivation gate
on Na channel; Closes
slowly after membrane is
depolarized
•causes the absolute
refractory period for AP
propagation
K+ efflux starts to exceed Na+ influx and Vm reverses.
Inactivation gates are slowly closing.
No!
x
*
(~0.03% OP)
(~40% OP)
(~75% OP)
(~96% OP)
(~70% OP)
maximum Na+ influx
(~20%
(~7% OP)
OP)
*
Most fast gate closure is during 2nd half of repolarization.
Slow gates have reached minimum open probability.
Vm - ENa
(~1% OP)
(~45% OP)
(~70% OP)
(~7% OP)
maximum net influx
of positive charge
(f) During the hyperpolarization phase, inactivation gates
slowly reopen.
minimum
gNa & INa
(~15% OP)
(~55% OP)
(~15% OP)
Potassium Channel Property
• K channels open with a delay and stay open
for length of depolarization
• Repolarize the Vm to Ek= -75mV which is
why you have hyperpolarization.
• Also called a delayed rectifier channel
Gate on the Delayed Rectifier
Potassium Channel
•K channels have a
single gate (n) that stays
open as long as Vm is
depolarized.
• n gate on K channels
opens very slowly this
allows the Vm to
depolarize due to Na
influx; Na and K
currents do not offset
each other right away
Refractory Period
• Refractory period due to Na channel
inactivation and the high gk
• Subsequent Action potential cannot be
generated
Conductance = g
• How many charges (ions) enters or leaves
cell (inverse of resistance)
• due to:
– number of channels/membrane area
• Highest density at axon hillock
– number of open channels
– ion concentration on either side of membrane
– Measured in Siemens (S), in cells pS (pico; -12)
Action Potential: a transient and
rapid sequence of changes in
the membrane potential
Action Potentials
Can travel up to
100 meters/second
Usually 10-20 m/s
0.1sec delay between
muscle and sensory
neuron action potential
Two types of signal conduction within a single
neuron
1.
Passive (graded) electrotonic conduction:
depend on the movement of ions along the
two faces of the plasma membrane; decays
with distance.
2.
Active (regenerative) conduction (AP): depend
on the presence and activity of biological
molecules such as voltage-gated ion channels;
transmit without loss of signal strength.
Increasing conduction velocity of AP
1.
Increase axonal diameter
2.
Myelination
Conduction in Myelinated Axon
• Myelin prevents
movement of Na+
and K+ through
the membrane.
• Nodes of Ranvier
contain VG Na+
and K+ channels.
• Saltatory
conduction
(leaps).
• Fast rate of
conduction.
Nodes of Ranvier
One of the regularly spaced interruptions of
myelin sheath along an axon.
Saltatory conduction
Discontinuous conduction of action potentials that takes place at the
nodes of Ranvier in myelinated axons.