Page 227 - Biology_F5

Biology for Advanced Level Secondary Schools

Outside
+
(K )=20 nM Na / K pump K channel
+
+
+
(Na )=450 nM
+
+ + + + + + + + +
FOR ONLINE READING ONLY
Membrane
potential
-60 mV

- - - - - - - - -

ATP
(K )=400 nM Inside
+
ADP
(Na )=50 nM
+
Figure 4.7: A membrane at resting potential (Polarised membrane)

Action potential inside the axon, which consequently
An action potential is a rapid, temporary becomes more depolarised. Since sodium
change in membrane potential. It may gates are sensitive to depolarisation,
qualify as the most important type of the greater the depolarisation, the more
electrical signal in cells. In a neurone, sodium gates are opened, allowing more
action potential is generated by special sodium ions to enter into the cell, hence
types of voltage-gated ion channels greater depolarisation. This is called a
embedded in cell’s plasma membrane. A positive feedback loop. Positive feedback
neurone displaying the nerve impulse is loop causes acceleration in the entry of
said to have an action potential and the sodium to the potential difference peaks
cell is depolarised. Stimulation of axon by at about +40 mV. This peak corresponds
electrical impulse may result into a change to the maximum concentration of sodium
in potential difference of about +40 mV inside the axon. The total depolarisation
across the axon membrane, from more associated with the action potential has
negative inside to more positive inside. therefore been from -70 mV to +40 mV.
The action potential has the following
Action potential is generated by a sudden three distinct phases:
opening of sodium channels. This occurs
in response to stimulus which brings (a) Depolarisation
about a slight depolarisation or loss of This is the decrease in voltage across
charge of the axon membrane. Opening the membrane. It occurs when there is a
of Na gates increases permeability stimulus that leads to the opening of the
+
of the axon membrane to sodium ions sodium channel to increase inflow of
which enter the axon by diffusion. This sodium ions. The process makes the inside
increases the number of positive ions of the cell less negative.

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