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Action potential
1.
2. MEMBRANE POTENTIAL
The cell membranes of all body cells in the
resting condition are, polarized which means that
they show an electrical potential difference,
commonly used term for potential difference is
only potential.
Membrane potential refers to a separation of
charges across the membrane or a difference
in the relative number of cations and anions
in the ICF and ECF.
3. Resting Potential
In a resting neuron
(one that is not
conducting an
impulse), there is a
difference in
electrical charges on the outside and inside of
the plasma membrane. The outside has a
positive charge and the inside has a negative
charge.
4. a net positive charge outside &
a net negative charge inside. Such a
membrane is POLARISED
7. Factors Affecting RMP
3 factors
Polarity of each ion
Membrane permeability of the ions
Concentrations of respective ions on both sides:
(i= inside), (o= outside)
8. Action potential
These are rapid transient changes in the
membrane potential that spread rapidly
along the nerve fiber membrane .
9. Action Potential
When the cell membranes
are stimulated, there is a
change in the
permeability of the
membrane to sodium
ions (Na+
).
The membrane becomes
more permeable to Na+
and K+
, therefore
sodium ions diffuse into the cell down a concentration gradient. The entry
of Na+
disturbs the resting potential and causes the inside of the cell to
become more positive relative to the outside.
10. DEPOLARISATION
As the outside of the cell
has become more
positive than the inside
of the cell, the
membrane is now
DEPOLARISED.
When enough sodium ions
enter the cell to
depolarise the
membrane to a critical
level (threshold level)
an action potential arises
which generates an
impulse.
In order for the neuron to
generate an action
potential the membrane
potential must reach the
threshold of excitation.
15. All-or-None Principle
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.
16. Refractory Period
There are two types of
refractory period:
Absolute Refractory Period
– Na+ channels are
inactivated and no
matter what stimulus is
applied they will not re-
open to allow Na+ in &
depolarise the membrane to the threshold of an action potential.
Relative Refractory Period - Some of the Na+ channels have re-opened but the
threshold is higher than normal making it more difficult for the activated Na+
channels to raise the membrane potential to the threshold of excitation.
17. Speed of Nerve Impulses
Impulses travel very
rapidly along neurons.
The presence of a myelin
sheath greatly increases
the velocity at which
impulses are conducted
along the axon of a
neuron. In unmyelinated
fibres, the entire axon
membrane is exposed
and impulse conduction
is slower.
18. Nernst Equation
Relation of diffusion potential to the
concentration difference…… resulting in Nernst
(equilibrium) potential
For any univalent ion at body temperature of 37°
C
EMF (mV)= +/-61log (Conc.inside/Conc.outside)
Sign is –ve shows the polarity inside the cell.
20. What is the role Na-K pump?
Electrogenic pump
Concentration gradient
Contributes -4mV.
21. Contribution of Active
Transport – Factor 1
There are different numbers of potassium ions (K+
)
and sodium ions (Na+
) on either side of the
membrane. Even when a nerve cell is not
conducting an impulse, for each ATP molecule
that’s hydrolysed, it is actively transporting 3
molecules Na+
out of
the cell and 2 molecules
of K+
into the cell, at
the same time by
means of the
sodium-potassium pump.
22. Contribution of facilitated
diffusion
The sodium-potassium
pump creates a
concentration and
electrical gradient for Na+
and K+
, which means that
K+
tends to diffuse (‘leak’)
out of the cell and Na+
tends
to diffuse in. BUT, the membrane is much more permeable to K+
, so K+
diffuses out along its concentration gradient much more slowly.
23.
24.
25.
26. AFTERDEPOLARIZATION and
Hyperpolarization
Afterdepolarisation: The descending limb of action
potential dosenot reach to the baseline abbruptly, but
it shows a delay of few seconds.
Decrease rate of K efflux.
Afterhyperpolarisation: The descending limb of
action potential dips a little below the baseline of
RMP.
Continued K efflux.
27. Latent period
After a stimulus is applied to a nerve, there is a latent
period before the start of the action potential. This
interval corresponds to the time it takes the impulse
to travel along the axon from the site of stimulation to
the recording electrodes. Its duration is proportionate
to the distance between the stimulating and recording
electrodes and inversely proportionate to the speed of
conduction.
28. Effect of electrolytes
Sodium:Decreasing the external Na+
concentration
reduces the size of the action potential but has little
effect on the resting membrane potential. The lack of
much effect on the resting membrane potential would
be predicted, since the permeability of the membrane
to Na+
at rest is relatively low.
Potassium:Conversely, increasing the external K+
concentration decreases the resting membrane
potential.
Calcium
Negative ions:Intracellular proteins
30. Magnitude of stimulus
It is possible to determine the minimal intensity of
stimulating current (threshold intensity) that,
acting for a given duration, will just produce an action
potential.
Action potential fails to occur if the stimulus is
subthreshold in magnitude,produces graded
potentials.
Suprathreshold stimuli produce action potential
during relative refractory period.
35. Plateau formation
Plateau greatly prolongs the period of
depolarization.
This type of action potential with plateau is seen
in heart muscle fibers.
36. Plateau formation
Opening of fast channels causes the spike portion
of the action potential.
The slow, prolonged opening of the slow calcium-
sodium channels mainly allows calcium ions to
enter the fiber.
This is largely responsible for the plateau portion
of the action potential.
