Action Potential
Excitable tissue – Nerve ,muscle contd…..
Learning objectives
• Recognize the Terminologies used in describing
the Action Potential.
• Describe the roles of Voltage gated Na+ channels
and K+ channels in the generation of the action
potential.
• Describe and plot action potential
• Discuss the relative kinetics of Na+, Ca2+, and K+
channel gating.
• List properties of action potential.
• Describe how changes in serum calcium affect
the nerve action potential.
Terminology
• The usual resting membrane potential is oriented with the
cell interior negative.
• Depolarization: it’s the process of making the membrane
potential LESS NEGATIVE
• Depolarization makes the interior of the cell less negative,
or it may even cause the cell interior to be positive.
• Hyperpolarization: is the process of making the
membrane potential more negative
• Threshold potential : it’s the membrane potential at
which occurrence of the AP is inevitable.
• The threshold potential is less negative than the
resting membrane potential (an inward current is
needed for this to happen)
• At threshold potential, the net inward current
becomes larger than the net outward current to
sustain the threshold.
Action potential
 The sequence of rapid changes in the membrane

potential that spread rapidly along the nerve fiber
when a threshold stimulus is applied following its
restoration to the resting level is called action
potential
Action potential is a property of excitable cells (i.e.,
nerve, muscle) that consists of a rapid
depolarization, or upstroke, followed by
repolarization of the membrane potential.
• Overshoot: that portion of the AP where the
membrane potential is positive (cell interior
positive)
• Undershoot: is that portion of the AP, following repolarization where the membrane potential is more
negative than at rest.
1.
VOLTAGE GATED Na⁺ AND K⁺ CHANNELS
Voltage gated Na⁺ channels
↙
↘
Activation gate (outside)
Inactivation gate( inside )
Delayed 10,000 of a sec. close at R M P.
Conformationalchange increases permeability by 500–5000 times
2. Voltage gated K⁺ channels
Opening corresponds with the closure of Na⁺ gates
3. Other ions : Ca⁺
Ca⁺pump acts along with Na⁺ pump in heart and smooth muscle
Voltage gated Ca⁺ channel – slow
VOLTAGE GATED NA AND K
CHANNELS
Generation of Action Potential
• Tetrodotoxin (TTX) and lidocaine block these
voltage-sensitive Na+ channels and abolish action
potentials.
overshoot is the brief
portion at the peak of
the action potential
when the membrane
potential is positive
undershoot
Action Potentials
• PHASES:
• threshold excitation (depolarisation)
• rising phase
• falling phase (Repolarisation)
• undershoot (hyperpolarisation)
Membrane Conductance
• Definition: Membrane conductance refers to the number of
channels that are open in a membrane.
• For example, Na+ conductance is proportional to the
number of open channels that will allow the Na+ to pass
through the membrane.
• General properties If conductance is increasing, channels
are opening, and if conductance is decreasing, channels are
closing.
• The rate at which ions move across a membrane depends on
the number of open channels and the net force. When ions
flow through channels, the cell’s membrane potential
changes. However, under physiologic conditions, too few
ions flow to produce a significant effect on the ion’s
extracellular concentration or the concentration gradient
across the membrane.
• Inward current is the flow of positive charge into
the cell. Inward current depolarizes the membrane
potential
• Outward current is the flow of positive charge out
of the cell. Outward current hyper polarizes the
membrane potential
Cadiac muscle
Action Potential
Smooth Muscle Action potential
Membrane potential (mV)
Action potential
0
Ca2+ influx
K+ efflux
-45
Threshold of
VOC’s
-50
200 msec
Characteristics of an AP
1. Stereotypical size and shape
2. Refractory period
3. Propagation
4. All or none response
1.Stereotypical size and shape
• Stereotypical size and shape: Each normal action
potential for a given cell type looks identical,
depolarizes to the same potential, and repolarizes
back to the same resting potential.
Types & duration of AP
1.Spike potential (Nerve fibre
,skeletal muscle ) – 10 to
50m sec
2.Plateau type (Myocardial
cell & smooth muscle cell )
– 250 to 350m sec
3.Pace maker type
(Conducting system of heart
& smooth muscle fibres ) –
100 to 150m sec
2. Refractory period
• Refractory period: the period during which another
normal action potential cannot be elicited in an
excitable cell.
• Refractory periods can be absolute or relative.
Refractory period
Absolute refractory period
(functional refractory
period)--
Relative refractory period-
• The absolute refractory
period is that period
during which no matter
how strong the
stimulus, it cannot
induce a second action
potential
• The relative refractory
period is that period
during which a greater
than normal stimulus is
required to induce a
second action potential.
31
3. Accommodation
• When a nerve or muscle cell is depolarized slowly or is
held at a depolarized level, the usual threshold
potential may pass without an action potential having
been fired called accommodation.
• occurs because depolarization closes inactivation gates
on the Na+ channels and if depolarization occurs slowly
enough, the Na+ channels close and remain closed.
• Example: is demonstrated in hyperkalemia, in which
skeletal muscle membranes are depolarized by the high
serum K+ concentration. Although the membrane
potential is closer to threshold, action potentials do not
occur because inactivation gates on Na+ channels are
closed by depolarization, causing muscle weakness.
The voltage-gated sodium
channels have two gates:
1. A voltage-sensitive gate opens as
the cell is depolarized.
2. A second, time-sensitive
inactivation gate stops the
movement of sodium through the
channel after the channel has been
open for a certain time
Depolarization closes
inactivation gates on the
Na+ channels
4. Propagation of action potentials
• Occurs by the spread of local currents to adjacent
areas of membrane, which are then depolarized to
threshold and generate action potentials
Propagation:
• Propagation: An action potential at one site causes
depolarization at adjacent sites, bringing those
adjacent sites to threshold. Propagation of action
potentials from one site to the next is
nondecremental.
Initiation & propagation of AP
Factors affecting: Conduction Velocity of the Action Potential
. Size of the action potential:
• Cell diameter:
• Myelin: The greater the myelination, the greater the
conduction velocity.
• (Demyelination (e.g., multiple sclerosis, Guillain-Barre
syndrome): This would decrease the amplitude of the
action potential as it travels from node to node. If the
action potential arrives below a certain magnitude,
another action potential may not be generated and
transmission is blocked.)
• Thus, Large myelinated fibers =fast conduction
All or none response---Either the AP occurs
or does not occur.
• All-or-none response: An action potential either
occurs or does not occur. If an excitable cell is
depolarized to threshold in a normal manner, then
the occurrence of an action potential is inevitable.
If the membrane potential has not reached the
threshold, no action potential can occur.
Local Anesthetics.
Among the most important stabilizers
are the many substances used clinically as local
anesthetics, including procaine and tetracaine.
Most of these act directly on the activation gates of
the sodium channels, making it much more difficult
for these gates to open, thereby reducing membrane
excitability.
For a television game show, 16 contestants volunteer to be stranded
on a deserted island in the middle of the South China Sea. They
must rely on their own survival instincts and skills. During one of the
challenges, one team wins a fishing spear. They catch a puffer fish
and cook it over the open flames of their barbecue. None of them
are very skilled in cooking, but they enjoy the fish anyway. One of
the contestants, a worldwide traveler, comments that it tastes like
Fugu. After dinner, they all develop a strange tingling around their
lips and tongue. They all become weak, and their frailty progresses
to paralysis. They all die. What was the cause of death?
A
B
C
D
E
Tetrodotoxin
Botulism
Bacillus cereus food poisoning
Tetanus
Ciguatoxin
A well-meaning third year medical student accidentally pushes an unknown quantity
of KCl IV to a patient. If the concentration of potassium outside a neuron were to
increase from 4 mEq/L to 8 mEq/L, what would you expect to happen to the minimal
stimulus required for initiation of an action potential?
A The minimal stimulus required for initiation of an action potential would remain
the same
B The minimal stimulus required for initiation of an action potential would increase
C The minimal stimulus required for initiation of an action potential would decrease
D The minimal stimulus required for initiation of an action potential would stay the
same, but the amplitude of the peak of the action potential would increase
E The minimal stimulus required for initiation of an action potential would stay the
same, but the conduction velocity of the action potential down an axon would slow
If the extracellular K+ concentration is
increased from 4 meq/L to
10 meq/L
• a. The membrane potential will become more
negative
• b. The sodium conductance will increase
• c. The potassium conductance will increase
• d. The membrane will become more excitable
• e. The Na-K pump will become inactivated
Q : During the upstroke of the
nerve action potential
(A) there is net outward current and the cell interior
becomes more negative
(B) there is net outward current and the cell interior
becomes less negative
(C) there is net inward current and the cell interior
becomes more negative
(D) there is net inward current and the cell interior
becomes less negative
At which point on the action potential does the Na+current
exceed theK+ current?
a. Point A
b. Point B
c. Point C
d. Point D
e. Point E
At which point on the action potential is the membrane closest to
theNa+ equilibrium potential?
a. Point A
b. Point B
c. Point C
d. Point D
e. Point E