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Coordination and Irritability

Mechanism of synaptic transmission
(a) An action potential arrives at the end of the axon and induces changes in the cell
membrane (Figure 4.16).

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Axon of presynaptic
neurone

Synaptic knob
Mitochondrion
Synaptic vesicle
Membrane of
postsynaptic neurone Synaptic cleft
Presynaptic
membrane Postsynaptic
Area of membrane
receptor sites
Figure 4.16: Conduction of nerve impulses at the presynaptic knob and action potential

(b) Depolarisation at the synaptic knob after arrival of nerve impulses creates the action
potential, which opens voltage-gated calcium channels located near the synapse in
the presynaptic membrane, thus increasing the permeability of the membrane to
calcium (Ca ) ions. The electrochemical gradient for Ca results in the inflow of
2+
2+
calcium ions through the open channels (Figure 4.17).

Permeability of presynaptic
Ca 2+ Ca 2+ Ca 2+ membrane to Ca increases
2+

Figure 4.17: Permeability of presynaptic membrane to the calcium ions

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