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Similaire à Membrane potentials of a nerve impulse (20)
Membrane potentials of a nerve impulse
- 2. Ion Concentration in cell – Concentration outside
mmoldm-3 cell -mmoldm-3
K+ 150.0 2.5
Na+ 15.0 145.0
Cl- 9.0 101.0
In this condition the condition of the axon is said to be polarised.
Sodium ions are transported out of the axon and Potassium ions are
actively transported into the axon both by the sodium-potassium
pump. An equilibrium is established in which the chemical and electrical
gradients are balanced and henceforth there is no net movement of
ions. Resting potential is usually at -70mV inside the axon.
©HannahRTweddle 26/02/2013
- 3. An abrupt but short-lived
reversal of the resting
potential of a neurone.
It occurs at a specific point
and passes along a neurone
as a wave.
It is when the neurone sends
an explosion of electrical
activity.
The membrane is said to
have become depolarised at
this point.
©HannahRTweddle 26/02/2013
- 4. When stimulisation happens depolarisation happens.
Stimulus causes the membrane at one part of the neurone
to increase in permeability to Na+
Na+ voltage gated channels open and Na+ enter the axon
down the electrochemical gradient by diffusion. This
causes resting potential to move to 0mV.
When depolarisation reaches -30mV (or the threshold
limit) more Na+ channels open which causes Na+ to rush in
by diffusion the cell becomes more positive.
©HannahRTweddle 26/02/2013
- 5. Once the action potential of
around 40mV has been
established the voltage gates on
the Na+ close this causes K+ to
begin to open.
©HannahRTweddle 26/02/2013
- 6. With the K+ voltage channels now
open the electrochemical gradient
that was preventing the further
movement of potassium is reversed.
K+ rushes out making the inside of
the cell more negative
Since this restores the original
polarity within the axon it is called
repolarisation
©HannahRTweddle 26/02/2013
- 7. The outward diffusion of the K+ ions
causes a temporary overshoot of the
electrical gradient.
Inside the axon becomes more negative
(relative to the outside) . This is
hyperpolarisation.
The gates on the K+ ions now close and
the activities of the sodium potassium
pumps once again cause sodium ions to
be pumped out and potassium ions to be
pumped in.
©HannahRTweddle 26/02/2013
- 8. The term “action and resting potential” can be
quite misleading
The diffusion of the ions in “active potential” is
primarily down to diffusion – which is a passive
process.
The diffusion of the ions in “resting potential” is
down to active transport – which is a passive
process.
In this case the term “action potential” simply
means the nerve is transmitting a nerve impulse,
where as in “resting potential” it no longer is
transmitting a nerve impulse.
©HannahRTweddle 26/02/2013
