Properties of action potential

1. Effect of ions on Action Potential
&
Properties of Action Potential

2. Roles of Other Ions During the Action Potential
• Sodium and potassium ions do play vital role in generating
the action potential
• Other types of ions, negative anions, calcium & potassium
ions must be considered as well

5. Impermeant Negatively Charged Ions (Anions)
Inside the Nerve Axon
• Inside the axon are many
negatively charged ions that
cannot go through the membrane
channels (protein molecules,
organic phosphate compounds &
sulfate compounds).
• These negatively charged
maintain negativity iniside the
cell

6. Calcium ions
• Calcium ion concentration is more (10,000 times) in
the extracellular fluid than in the intracellular fluid.
• Tremendous diffusion gradient and electrochemical
driving force for the passive flow of calcium ions
into the cells
• Cells do have Calcium pump which pumps calcium
ions out of cell
• Some cells (cardiac muscle and smooth muscle)
have Voltage-gated calcium channels (slow
channels)
• When the channels open in response to a stimulus,
calcium ions flow to the interior of the cell cause
sustained depolarization.

7. Role of calcium in membrane stability
• Calcium (normal levels) stabilizes the membrane:
• Inner side of sodium channels is highly negatively
charged.
• calcium ions appear to bind to the exterior surfaces of
the voltage gated sodium channel protein.
• Positive charges of these calcium ions, alter the
electrical state of the sodium channel protein →
altering the voltage level required to open the sodium
gate.
• The concentration of calcium ions in the extracellular
fluid also has a profound effect on the voltage level at
which the sodium channels become activated

8. Normal levels of calcium in plasma
• Total calcium -2.4 mmol/L (9.4mg/dl)

9. Hypocalcemia
Increased Permeability of the Sodium Channels When There Is a Deficit of
Calcium Ions
• When there is a deficit of calcium ions in ECF, the
sodium channels become activated by very little
increase of the membrane potential from its
normal level.
• Only 50% below normal value of calcium → nerve
fiber becomes highly excitable, sometimes
discharging repetitively without provocation →
muscle “tetany”

10. Tetany (pathophysiology)
Decreased calcium in ECF
↓
No complete closure of activation gates of
sodium channels at rest
↓
Sodium ions leak into membrane from ECF
↓
Membrane potential becomes less negative &
near to threshold
↓
On slight stimulation action potential (tetany).

11. Effect of hyperventilation
Hyperventilation
↓
Alkalosis
↓
Calcium Binds with proteins
↓
Decreased ionized Calcium
↓
Hypocalcemia
↓
Tetany

12. Inhibition of Excitability—Stabilizers and Local
Anesthetics
• Local Anesthetics: procaine and tetracaine
• Mechanism of action: act directly on the activation gates of
the sodium channels, making it much more difficult for
these gates to open and thereby reducing membrane
excitability.
• Nerve impulses fail to pass along the anesthetized nerve

13. Hypercalcemia
• can decrease excitability (high extracellular fluid calcium ion
concentration decreases membrane permeability to sodium
ions and simultaneously reduces excitability.
Hypercalcemia
↓
Nervous
system depression, cardiac arrhythmias, lethargy.

14. Effect of potassium on membrane excitability

15. Effect of hypokalemia
• Hypokalemia: decrease in plasma potassium level below 3.5
mmol/L.
Hypokalemia
↓
RMP more –ve
↓
Membrane hyperpolarized
↓
Less excitable.

16. Effect of hyperkalemia
• Hyperkalemia: serum potassium concentration greater than
approximately 5.0-5.5 mEq/L
• Hyperkalemia → RMP less –ve → initially membrane
hypopolarized and is excitable, if more and more
hyperkalemia → loss of RMP or intracellular negativity →
membrane not excitable → death.

17. Properties of Action potential
1. Sudden abrupt onset
2. Limited amplitude (AP from -90 to+ 35-40 mv)
3. Short duration 0.2- 3.0 msec in nerve fiber 2-5 msec in skeletal
muscle, longer in cardiac (200 to 300 msec)and even longer in
smooth muscle fibers
4. Self propagative
5. Obeys all or none law: action potential is produced with its
maximum amplitude, if subthreshold stimulus not produced at all
6. Has refractory period. (when there wont be response to 2nd
stimulus).

18. Stimulus
• A sudden change of the (internal or external) environmental
condition of the cell.
• Includes chemical, mechanical stimulus. inside the body.
• The electrical stimulus is often used for the physiological Lab
and research. outside the body
• Threshold (intensity): the lowest or minimal intensity of
stimulus to elicit an action potential.
• Threshold stimulus
• Subthreshold stimulus
• Suprathreshold stimulus:

19. What is local/graded potential?
• Occurs in small, specialized region of excitable
cell membranes
• Magnitude of graded potential varies directly
with the magnitude of the triggering event
• The Stronger a triggering event, the larger the
resultant graded potential.
• Graded Potential spread by passive Current
flow.
• Graded potentials die over short distances
• Decremental: gradually decreases
• If strong enough, graded potentials trigger
action potentials.

20. Examples of graded potential
• Motor end plate
• Receptor potential
• Excitatory post synaptic
potential
• Inhibitory post synaptic potential

21. Refractory period
• Absolute refractory period: Tissue does
not respond to 2nd stimulus what ever
the strength of stimulus
• During depolarization & first 2/3rd of
repolarization
• Due to inactivation of sodium channels.
(closure of inactivation gates of Na
channels, will not open till potential
reaches resting value.)
• Relative refractory period: Tissue may
respond to 2nd stimulus if it is of larger
intensity
• First 2/3rd of repolarization to the
beginning of after depolarization.

22. All-or-None Principle
• If a stimulus is threshold or
suprathreshold, action potential
is produced with its maximum
amplitude, if subthreshold
stimulus not produced at all

23. Properties of Nerve Fibers
1. Conductivity
2. Excitability
3. All or None law
4. Refractory period
5. Infatiguability
6. Summation

24. Conductivity
• Conductivity is the ability of nerve fibers to transmit
the impulse from the area of stimulation to the other
areas
• Action potential is transmitted through the nerve fiber
as nerve impulse

25. Excitability
• Property of Nerve Fiber
due to which it respond
to stimulus by generating
nerve signal.
• Stimulus – Mechanical,
Electrical, Chemical or
Thermal

26. Strength Duration Curve
• A graph between
electrical stimuli of
different intensities and
recording the time
needed by each stimulus
to start the response

27. Rheobase
• Minimum intensity of
stimulus applied for
adequate time produces
response

28. Utilization Time
•Minimum time that a current
equal to rheobase must act
to induce an AP is called
utilization time
•Stimulus having current
strength less than rheobase
will not reach threshold
value even if applied for
unlimited time

29. Chronaxie
• Minimum duration for
which intensity of double
the Rheobase should
applied to produce
response
•Nerve fibers have shorter
chronaxie than muscle
tissue means they are more
excitable

30. Significance of Strength Duration Curve
• Chronaxie is an important
parameter to determine the
condition of nerve fiber
Clinically, the damage of nerve
fiber is determined by measuring
the chronaxie
• Lesser is the chronaxie more is
the excitability
• Nerve fibers have a low
threshold for excitation than the
other cells

31. Infatigability
• Nerve fiber cannot be fatigued, even if it is stimulated
continuously for a long time.
• The reason is that nerve fiber can conduct only one
action potential at a time. At that time, it is
completely refractory and does not conduct another
action potential

32. Summation
• When one subliminal(weak)
stimulus is applied, it does not
produce any response in the
nerve fiber
• if two or more subliminal stimuli
are applied within a short
interval of about 0.5 millisecond,
the response is produced.
• The subliminal stimuli are
summed up together to become
strong enough to produce the
response

33. Compound Action Potential
• It is recording of AP from
mixed nerve.
• Mixed Nerve – which
contains different types of
nerve fibers with different
diameters
• So it’s the algebraic
summation of AP of many
axons