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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)



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