Download Resting membrane potential is

Electrical Properties of Nerve
Cells
10.5.12
The resting membrane potential
Action potential mechanism
The rapid opening of
voltage-gated Na+ channels
allows rapid entry of Na+,
moving membrane potential
closer to the sodium
equilibrium potential (+40 mv)
A cell is
“polarized”
because
its interior
is more
negative
than its
exterior.
Repolarization is movement back
toward the resting potential.
The slower opening of
voltage-gated K+ channels
allows K+ exit,
moving membrane potential
closer to the potassium
equilibrium potential (-90 mv)
The rapid opening of voltage-gated Na+ channels
explains the rapid-depolarization phase at the
beginning of the action potential.
The slower opening of voltage-gated K+ channels
explains the repolarization and after hyperpolarization
phases that complete the action potential.
Important
Potentials
•
•
•
•
Resting membrane potential is -70mV
Depolarization peak is at +40mV
Hyperpolarization peak is at -90mV
Threshold potential is about -55mV
• +40mV is the Na+ equilibrium potential
• -90mV is the K+ equilibrium potential
Na+ equilibrium potential:
K+ in Compartment 2,
Na+ in Compartment 1;
BUT only Na+ can move.
Ion movement:
Na+ crosses into
Compartment 2;
but K+ stays in
Compartment 2.
At the sodium
equilibrium potential:
buildup of positive charge in Compartment 2
produces an electrical potential that exactly
offsets the Na+ chemical concentration gradient.
K+ equilibrium potential:
K+ in Compartment 2,
Na+ in Compartment 1;
BUT only K+ can move.
Ion movement:
K+ crosses into
Compartment 1;
Na+ stays in
Compartment 1.
At the potassium
equilibrium potential:
buildup of positive charge
in Compartment 1 produces an electrical potential that
exactly offsets the K+ chemical concentration gradient.
Graded Potential
• A weak stimulus can “depolarize” or “hyperpolarize” the
membrane generating a membrane potential which is not
enough to generate an action potential. This is known as
graded potential
• Graded potential causes potential change in limited areas
• The graded potential spreads along the membrane by
changing the charge on the membrane capacitance and by
flowing through opened channels
Graded Potential
• As the current flows along the membrane, some of the
current leaks through open channels in the neighboring areas.
As a result the membrane potential progressively decreases
with increasing distance from the source point
• This spatial pattern is exponential and the distance where the
voltage changes to 37% of its original value is the “ length
constant”
The size of a
graded potential
(here, graded
depolarizations)
is proportionate
to the intensity
of the stimulus.
Graded potentials can be:
EXCITATORY
or
INHIBITORY
(action potential (action potential
is more likely)
is less likely)
The size of a graded potential is proportional to the size of the stimulus.
Graded potentials decay as they move over distance.
Remember:
1. Membrane potential changes due to change in
stored charge on membrane capacitor
2. Membrane conductance changes due to flow of
ions through gated channels during graded and
action potentials
Excitable cells
As most neurons and muscle cells are much longer
than their length constants, the graded impulses
disappear when flowing along the cell, thus the
responses cannot deliver signals from one end to the
other in the cell
Excitable cells are distinguished by their ability to
generate active potentials that can propagate
without losing their amplitude
Excitable nerve cells
• A typical neuron has a dendritic region and an axonal
region.
• The dendritic region is specialized to receive
information whereas the axonal region is specialized
to deliver information.
• Nerve cells have a low threshold for excitation. The
stimulus may be electrical, chemical or mechanical
Dendrites: receive information and undergo graded
potentials.
Neuron
Axons: undergo action potentials to deliver information,
typically neurotransmitters, from the axon terminals.
Two types of physicochemical disturbances
1. Local, non-propagated potential (Graded potentials)
2. Propagated potentials, Action potentials or nerve
impulse
All-or-None Principle
• The all or none feature of action potential implies
that stimulus less than certain threshold level of
depolarization results in a graded response which
would not be transferred. However a stimulus big
enough to move the membrane potential beyond the
threshold will generate action potential that can
propagate to distant regions of the cells
• Threshold potential of-55mV corresponds to the potential to
which an exccitable membrane must be depolarized in order to
initiate an action potential
•
•
•
Throughout depolarisation, the Na+ continues to rush inside
until the action potential reaches its peak and the sodium
gates close.
If the depolarisation is not great enough to reach threshold,
then an action potential and hence an impulse are not
produced.
This is called the All-or-None Principle.
Spatial or temporal summation
• Graded responses can interact with each other and can be
spatially or temporally summed
• If two graded potentials occur at the same time in close
enough /same places, their effects add up. This is called
“spatial summation”
• If two graded potentials occur at the same place in
succession, their effects add up. This is called temporal
summation
• As an analogy, spatial summation is like using many shovels
to fill up a hole all at once. Temporal summation is like using
a single shovel to fill up a hole over time. Both methods work
to fill up the hole
Graded and action potential in
neurons
• In neurons, the axon hillock (initial point of axon) has
the lower threshold with relatively high densities of
Na+ channels and is thought to be the principal
trigger zone
• The graded responses produced throughout the
dendrites or cell body is summed spatially and
temporally, and if the summed response is large
enough to pass the threshold, an action potential will
be generated at axon hillock.
The propagation of the action potential from the dendritic
to the axon-terminal end is typically one-way because the
absolute refractory period follows along in the “wake”
of the moving action potential.
One–way propagation of the AP
One–way propagation of the AP
One–way propagation of the AP