Action Potential Generation and Propagation

24 Oct. 2012 Ashok Solanki 1
Action Potentials-
the language of excitable tissue
How neurons conduct impulses
How the muscle contract?
How the heart pump?

Resting membrane potential created
by semi-permeable membrane and
ions
• Intracellular
– Na 50
– K 400
– Cl 52
• Resting membrane
potential created by semi-
permeable membrane
and ions
• Intracellular
– Na 50
– K 400
– Cl 52
2Ashok Solanki24 Oct. 2012

Cell – the functional unit.
• 100 trillion cells organize systems of the body.
Ashok Solanki 324 Oct. 2012

AP OBEYS ALL OR NONE LAW
ALL-OR-NONE RESPONSE
A stimulus below the
threshold also will not
stimulate the neuron
once a threshold limit is
reached any stronger
stimulus will not increase
the cell's response
24 Oct. 2012 6Ashok Solanki

What starts an Action Potential??
• STIMULATION (chemical, electrical, mechanical)
opens Na+ channels
– low intensity stimulation opens few channels,
– local, graded potential
– resting potential restored without action potential

THE NEURON MEMBRANE AT REST
• Neuron maintains a resting membrane
potential of about -70 millivolts across the
cell membrane
• Sodium(Na+) and potassium(K+) are the main
ions involved
• Na+ and K+ cannot pass through the lipid
bilayer membrane
• move through the membrane by using

ACTION POTENTIAL
• What is it?
• Excitable tissue.
• All or none law of A.P.
• Change in RMP.
• Role of ions?
• Propagation of.
• Propreties of A.P.

Distribution of important ions in ECF & ICF
Na+ K+ Cl-
INSIDE 14 mEq / L 120 mEq / L 8 mEq / L
OUTSIDE 142 mEq / L 4.5 mEq / L 107 mEq / L

Action Potential
• ALL-OR-NONE phenomenon
• All Action Potentials are the same intensity.
– stronger sensations result from more
impulses, not stronger impulses.
– more impulses from same neuron
– more impulses from many neurons

Na+ / K+ PUMP
Membrane proteins actively transport sodium
out of the cell
potassium in Three Na+ are pumped out for
every two K+ pumped in
result is the cell has more Na+ on the outside
and more K+ on the inside

How neurons conduct impulses:
• Membrane potential (as
seen in muscle cells)
• K+ diffuses out of neurons
faster than Na+ diffuses
in,
• Na-K pump moves 3Na+
back out for 2K+ back in
• Cl-, phosphate, protein
anions balance cations
• “Resting potential”
= - 70 mV

• Action potential
– describes events at one
point of nerve fiber
– 1: stimulus to
threshold potential
– 2: Na+ channels
open, Na+ diffuses in
• Polarity briefly
reversed, to +30 mV
– 3: Na+ channels close

The Lipid Barrier of the Cell Membrane, and Cell Membrane Transport
Proteins
• Active Transport" of Substances Through Membranes
• Primary Active Transport and Secondary Active Transport
• Co-Transport of Glucose and Amino Acids Along with Sodium Ions
• Sodium Counter-Transport of Calcium and Hydrogen Ions
• Na+-K+ pump performs a continual surveillance role in maintaining
normal cell volume.
• A Positive-Feedback Cycle Opens the Sodium Channels
• Threshold for Initiation of the Action Potential
• A major function of the voltage-gated calcium ion channels is to
contribute to the depolarizing phase on the action potential in
some cells

What starts an Action Potential??
• STIMULATION opens
Na+ channels
– higher magnitude
stimulation opens more
channels, local potential
exceeds threshold at
trigger zone,
– Na+ floods in
depolarization

STIMULATED NEURON
Nerve cells are unique in their ability to carry a signal
using membrane potential changes
Stimulation of a neuron opens some of the membrane
proteins (a.k.a. Na+gates)
allows Na+ to pass freely into the cells
free flow of Na+ into the cell causes a reversal of
membrane polarity
polarity reversal is called the action potential

Resting Potential
• At rest, the inside of the cell is at -70 microvolts
• With inputs from dendrites inside becomes more positive
• If resting potential rises above threshold, an action potential
starts to travel from cell body down the axon
• Figure shows resting axon being approached by an AP

How Neurons Communicate
• Action Potential is the electrical process
that neurons use to communicate with
each other
• Action Potentials are based on movements
of ions (charged particles) between the
outside and inside of the axon
• Action Potential is an All or Nothing Process
(like a gun firing)

Conduction of an Action Potential
• Propagation of A.P. along neuron membrane
• Na+ diffuses, attracted to negative charges in
front of impulse
• A.P. at "A"

What Keeps Impulse Going
the Same Way ?
• Limits to stimulation of neuron/membrane
• Absolute Refractory Period
• Relative Refractory Period

Ashok Solanki 31
35. Regarding the ionic basis of action
potential
in cardiac muscle cells, which one of the
following is incorrect?
A. Phase 0: Na influx
B. Phase 1: K influx
C. Phase 2: Ca influx
D. Phase 3: K efflux
24 Oct. 2012

GOLDMAN–HODGKIN–KATZ
EQUATION
• V = 60mV log10 PNaNao
• + + PKKo
• + + PClCli
• −
• _________ ________
• PNaNai
• + + PKKi
• + + PClClo

– 3: K+ channels open,
K+ diffuses out,
Potential returns to
zero
– 4: All channels
closed, Na-K pump
moves Na+ back out
& K+ back in
– Hyperpolarization
– Resting potential
restored

• Action potential
– describes events at one
point of nerve fiber
– 1: stimulus to
threshold potential
– 2: Na+ channels open,
Na+ diffuses in
• Polarity briefly
reversed, to +30 mV
– 3: Na+ channels close

Two Ionic Equilibria and Resting Membrane
Potentials
• The resting membrane potential plays a
central role in the excitability of nerve and
muscle
• An action potential is a rapid change in the
membrane potential followed by a return to
the resting membrane potential

Generation and Conduction of Action Potentials
• An action potential is propagated with the same
shape and size along the whole length of a nerve
or muscle cell
• The action potential is the basis of the signal-
carrying ability of nerve cells
• In muscle cells, an action potential allows the
entire length of these long cells to contract
almost simultaneously.
• Voltage-dependent ion channel proteins in the
plasma membrane are responsible for action
potentials.

Voltage dependent ion channels
• Extracellular Na activation gate with
intracellular inactivation gate and slow K
activation gait
• Conformational changes due to membrane
potential changes influence ion permeability

Nomenclature
• Polarized membrane: Intracellular potential is
negative relative to extracellular space
• Depolarization = less polarization of the
membrane -80mV -> +20mV
• Hyperpolarization = more polarization of
membrane -80mV -> -100mV

Cell membrane
• Necessary for life as we know it
• Border role for cell
– Separates intracellular from extracellular milleau
• Allows ion and protein concentration
gradients to exist
– Creates electric charge gradients

LECTURE SUMMARY

Action Potential Generation and Propagation

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Action Potential Generation and Propagation