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Biology for Advanced Level Secondary Schools

(c) In response to the increase of calcium concentration inside the axon, synaptic vesicles
fuse with the presynaptic membrane and release neurotransmitters into the gap between
the cells which is called the synaptic cleft. The delivery of neurotransmitters into the cleft
is an example of exocytosis (Figure 4.18).

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Synaptic vesicles
fuse with presynaptic
membrane and rapture

Neurotransmitters

Figure 4.18: Presynaptic membrane fused with synaptic vesicles

(d) The vesicles then return to the cytoplasm and are refilled with transmitter substance. The
neurotransmitter diffuses across the synaptic cleft, a process which takes 0.5 ms per
synapse. Upon reaching the postsynaptic membrane, it binds with receptor molecules,
which recognise the molecular structure of the acetylcholine molecule (Figure 4.19).

Diffusion of transmitter molecules
across cleft

Transmitter molecules attached to
receptor sites

Channels open up and allow
ions to enter from synaptic cleft
(permeability of postsynaptic
membrane increases)

Figure 4.19: Transmitter molecules attached to receptor sites on postsynaptic knob

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