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Biology for Advanced Level Secondary Schools
End of axon
Mitochondrion
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Synaptic knob
Synaptic vesicle containing
neurotransmitter substances
Presynaptic membrane
Synaptic cleft Protein receptor
Post synaptic membrane
Dendrite Ion channel
Figure 4.15: Structure of the chemical synapse
Synaptic transmission of nerve impulses from a muscle cell. This space is called
the synapse. The synapse located at the
Task 4.5 junction of a neurone and muscle fibre
From the library and internet sources, is called a neuromuscular junction. Such
search for simulations on the synapses can be classified based on their
mechanism of synaptic transmission. means of transmission of impulses across
When watching, note down how: their gaps. In this typology, two types are
(a) Action potential arrives at the end revealed, namely electrical and chemical
of the axon and induces changes synapses. An electrical synapse is the
in the cell membrane. one in which the physiological continuity
(b) Depolarisation at the synaptic between pre and post synaptic neurones
knob after the arrival of the nerve is provided by a special channel called a
impulse creates action potential. gap junction between the two neurones.
(c) Synaptic vesicles fuse with the These channels are capable of passing
synaptic membrane in response to an electrical current, causing voltage
the increase in Ca . changes in the presynaptic cell to induce
2+
(d) Vesicles returning to the cytoplasm voltage changes in the postsynaptic cell.
are refilled with transmitters. This synapse can be effective when the
neurones are very close together (2 nm).
The nerve impulse passes down the The main advantage of electrical synapse
dendrite, through the cell body, and is that it facilitates the rapid transfer
down the axon. At the end of the axon, of signals from one cell to the next. In
the impulse encounters a fluid-filled chemical synapses, impulse transmission
space separating the end of the axon is facilitated by chemical substances
from the dendrite of the next neurone or called neurotransmitters.
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