THE SYNAPSE
Where nerve
impulses convert to
neurotransmitters
The Sanger Institute
The synapse is where the nerve impulse
passes from one cell to the next
 The electrical signal (the action potential)
stops and a chemical signal takes over to
cross the gap between the cells
 The chemical messenger is called a
neurotransmitter
 The neurotransmitter crosses the gap by
diffusion, which creates a small delay

© 2008 Paul Billiet ODWS
Designer signals
The advantage of using neurotransmitter is
that the nerve impulse can be given some
more specificity
 Neurotransmitters can also control the
operation of the nervous system by
inhibition or excitation
 Many drugs that try to cure problems in the
nervous system operate at synapses

© 2008 Paul Billiet ODWS
Neurotransmitters and
hormones
In many ways neurotransmitters are
hormones working over a very short
distance (about 20nm)
 Some of them work both at synapses and
in the circulatory system

© 2008 Paul Billiet ODWS
1. Action potential arrives at
terminal button
Vesicle storing
neurotransmitter
Ca2+
channel
Membrane receptor
for neurotransmitter
© 2008 Paul Billiet ODWS
Dept of Biology, Saint Louis University
3. Ca2+ stimulates
vesicles to fuse
with membrane
2. Depolarisation
opens Ca2+ channels
Ca2+ enters terminal
button
Ca2+
© 2008 Paul Billiet ODWS
Ca2+
Ca2+
Ca2+
4. Exocytosis of
neurotransmitter
It diffuses 20nm across
the synaptic cleft
The passage across the
synapse
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An action potential travels down an axon to the
terminal buttons or synaptic knobs at the end
The action potential depolarises the membrane of
a terminal button causing voltage-gated Ca2+
channels to open
Ca2+ ions flood into the terminal button
This stimulates hundreds of synaptic vesicles,
packed with neurotransmitter, to fuse with the
membrane of the terminal button
By exocytosis
The Ca2+ ions are then pumped out again
© 2008 Paul Billiet ODWS
5. Neurotransmitter
receptor sites on the
postsynaptic membrane
are ion channels.
They open when the
neurotransmitter binds
6. Localised
depolarisation as
ions leak in or out
of membrane.
© 2008 Paul Billiet ODWS
The passage across the
synapse
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The neurotransmitter diffuses across cleft to
postsynaptic membrane
The neurotransmitter molecules bind with
specific receptor sites on postsynaptic
membrane
The receptor sites are part of a ligand-gated ion
channel
These channels let Na+ ions in or K+ ions out
causing localised depolarisation of the
membrane
© 2008 Paul Billiet ODWS
7. Action potential
generated which
travels down the
postsynaptic cell.
© 2008 Paul Billiet ODWS
8. Neurotransmitter
destroyed by
enzymes in the cleft.
Stops signal being
perpetuated.
A new action potential
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If the localised depolarisations build up to the
nerve cell threshold, a full action potential will be
produced
This will travel away, down the postsynaptic
neurone
The action of the neurotransmitters stops:
(i) as they dilute by diffusion in the synaptic cleft
(ii) by hydrolysis through the action of enzymes
there
Important: The signal must not be perpetuated
indefinitely
© 2008 Paul Billiet ODWS
The neuromuscular junction
is a synapse
The motor end plate is the terminal button
of a motor neurone that makes contact
with a muscle cell
 The motor end plate releases the
neurotransmitter acetylcholine that
ultimately causes the muscle cell to
contract

© 2008 Paul Billiet ODWS
Motor end plates
© David B. Fankhauser, Ph.D., Professor of Biology and Chemistry, University of Cincinnati Clermont College