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Coordination and Irritability

(e) The arrival of acetylcholine at the excitatory postsynaptic potential in
postsynaptic membrane changes the post synaptic neurone. Once the
the shape of the receptor site. This neurotransmitter has depolarised the
initiates ion channels to open up. post synaptic neurone, it is hydrolysed
The excitatory synapse which opens to form acetyl and choline by the
ion channels on the postsynaptic enzyme “acetylcholinesterase” which
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membrane allows sodium ions to enter is found in the postsynaptic membrane.
and potassium ions to leave. This This prevents the successive impulse
creates a new potential known as the merging at the synapse (Figure 4.20).

Hydrolytic enzymes such as
acetylcholinesterase break down
transmitter molecules

Local depolarisation
and propagation of an action potential

Figure 4.20: Local depolarisation and propagation of an action potential in postsynaptic membrane

The resulting acetyl and choline diffuses The excitatory postsynaptic potential
across the synaptic cleft into the synaptic builds up as more neurotransmitter

knob of the presynaptic neurone where substance arrives until sufficient
they get stored in their vesicles for further depolarisation occurs to exceed the
use. The process is facilitated by energy threshold value and so generate an action
produced by numerous mitochondria potential in the postsynaptic neurone. This
in the knob. The overall depolarising is called temporal summation. All events
effect of several Excitatory Postsynaptic described are for excitatory synapse,
Potential (EPSPs) is called summation. some known inhibitory synapses respond
to neurotransmitter by opening potassium
When two or more EPSPs simultaneously ion channels and leaving the sodium ion

arise at different regions, either on the channels closed. Therefore, potassium
same or different neurones and produce moves out causing the membrane to be
sufficient depolarisation which can start polarised. Consequently, it prevents the
an action potential in the postsynaptic threshold value to be exceeded. Thus, no
neurone, the phenomenon is known as action potential will be created.
spatial summation.

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