Page 229 - Biology_F5
P. 229
Biology for Advanced Level Secondary Schools
In summary, an action potential occurs due to opening or closing of specific channels
in the plasma membrane in response to changes in voltage. An action potential always
has the same three-phase form; even though the size of the resting potential, threshold
potential, and peak depolarisation may vary among the species or even among different
types of neurones in one species.
FOR ONLINE READING ONLY
Conduction of nerve impulse along the axon
Task 4.4
Search from library and internet sources the animations/simulations on conduction
of nerve impulses along the axon. Observe the process, then note down how the:
(a) Resting potential gives the neuron a break
(b) Polarisation of the neurons’ membrane takes places
(c) Action potential occur
(d) Repolarisation occurs.
The mechanism for impulse transmission along the axon involves the following steps:
(a) Polarisation of the neurone’s membrane
Sodium is in high concentration on the outside, whereas potassium is in high concentration
inside the membrane. Cell membranes surround neurones, like any other cell in the body
which has a membrane. When a neurone is not stimulated, its membrane is said to be
polarised. Being polarised means that the electrical charge outside the membrane is
positive while the electrical charge inside the membrane is negative (Figure 4.9).
Outside of axon Plasma membrane of axon
Inside of axon
Figure 4.9: Polarised axon membrane
(b) The resting potential gives the neurone a break
When the neurone is polarised, it is said to be at its resting potential. It remains in this
state until when the stimulus comes along (that is, when it is stimulated). When action
potential is initiated, a region of the membrane depolarises. As the result the adjacent
region becomes depolarised as well (Figure 4.10).
Passive depolarisation spreads
Na +
Figure 4.10: Propagation of nerve impulse across a membrane (depolarisation)
Form Five Student’s Book
222