The Synapse
The junction between two
neurons is termed a synapse
(synapsis = point of contact)
The narrow gap between the
two neurons at the synapse
is the synaptic cleft; the cleft
is filled with extracellular
fluid and spans an area of
= synapse
approximately 20 nm
A neuron that conducts impulses toward a synapse is
called a pre-synaptic neuron
A neuron that conducts impulses away from a
synapse is called a post-synaptic neuron
The Synapse
axon terminal
synaptic
knob
A single neuron may
have many thousands of
synaptic junctions on its
dendrites and cell body
mitochondrion
synaptic vesicle
(contains neurotransmitter)
pre-synaptic synaptic post-synaptic membrane
(with receptors for
membrane
cleft
neurotransmitter)
Vesicles packed with
neurotransmitter
Mitochondrion
Pre-synaptic
membrane
T.S. Synapse from x 26,500
Events at the synapse
Voltage-gated calcium ion
channels open in the
pre-synaptic membrane
An action potential travels down
the axon of the neuron to the
synaptic knob and depolarises
the pre-synaptic membrane
Calcium ions diffuse
into the synaptic knob
post-synaptic
membrane
calcium ions
in the
extra-cellular
fluid
Events at the synapse
Voltage-gated calcium
channels open in the
pre-synaptic membrane
Calcium ions diffuse
into the synaptic knob
post-synaptic
membrane
An action potential travels down
the axon of the neuron to the
synaptic knob and depolarises
the pre-synaptic membrane
The uptake of calcium ions
triggers the fusion of
the synaptic vesicles with the
pre-synaptic membrane
Events at the synapse
Voltage-gated calcium
channels open in the
pre-synaptic membrane
Calcium ions diffuse
into the synaptic knob
The uptake of calcium ions triggers
the fusion of the synaptic vesicles
with the pre-synaptic membrane
Neurotransmitter is released
into the synaptic cleft
by EXOCYTOSIS
Neurotransmitter
diffuses across the cleft
and binds to specific
protein receptors
embedded in the
post-synaptic
membrane
post-synaptic
membrane
receptors in the
post-synaptic
membrane
Events at the synapse
Binding of neurotransmitter opens
Na+ gates in the membrane and
there is an influx of Na+ into the
post-synaptic neuron
post-synaptic
membrane
Neurotransmitter diffuses across
the cleft and binds to specific
protein receptors
embedded in the
post-synaptic membrane
An excitatory post-synaptic
potential (EPSP) builds
up across the membrane
and if this reaches
threshold, an action
potential is triggered in
the post-synaptic neuron
sodium
ions
Depolarisation of the post-synaptic membrane
Events at the synapse
Following activation of the
post-synaptic membrane,
neurotransmitter is removed from
the synaptic cleft to enable
further stimulation to occur
post-synaptic
membrane
The neurotransmitter, acetylcholine,
is hydrolysed by the enzyme
acetylcholinesterase, which is
located at the surface of the
post-synaptic membrane
The neurotransmitter,
noradrenaline, is actively
transported back into
the axon terminals
sodium
ions
Depolarisation of the post-synaptic membrane
Features of Synaptic Transmission
Three important features of
synaptic transmission are:
• Unidirectionality
• Summation
• Inhibition
Unidirectionality
Unidirectionality describes the one-way
transmission of nerve impulses between neurons
Neurotransmitter is stored and
released only on the pre-synaptic
side of the synaptic cleft
Receptors for neurotransmitter
are only located on the
post-synaptic membrane
synaptic vesicle
(contains neurotransmitter)
This arrangement allows
for the transmission of
impulses between neurons
in one direction only
post-synaptic membrane
(with receptors for neurotransmitter)
A motor neuron
cell body may have
thousands of
synaptic junctions
Depolarisation of the
post-synaptic membrane at an
excitatory synapse only occurs
when sufficient neurotransmitter
has accumulated at the
post-synaptic membrane
A post-synaptic neuron will only
fire when many excitatory and
inhibitory inputs are added
together; this phenomenon is
called SUMMATION
Spatial summation involves
the simultaneous release of
transmitter from several
synaptic knobs innervating
the same neuron – in this way
sufficient neurotransmitter is
made available to depolarise
the post-synaptic membrane
Summation may occur
in two ways; spatial
summation and
temporal summation
Temporal summation involves a
series of impulses reaching a
synapse in rapid succession,
such that the effects of each
impulse are added together; the
impulses arrive at the same
location at different times
Summation may occur
in two ways:
spatial summation and
temporal summation
The nature of the
neurotransmitter determines
the response of the
post-synaptic membrane
During hyperpolarisation, the
post-synaptic membrane
potential becomes more negative
than its resting potential and
results from either the efflux of
positive charge or the influx
of negative charge
Inhibition occurs at
synapses where transmitter
release results in the
hyperpolarisation of the
post-synaptic membrane
Chemicals and Neurotransmission
A variety of different chemicals
affect synaptic transmission – these
may be broadly classified as:
• Agonists – chemicals that mimic
neurotransmitters by binding to receptor
sites and producing a response similar to
that of the neurotransmitter
• Antagonists – chemicals that block
neurotransmitters by binding to receptor
sites and preventing synaptic transmission
Chemical Agonists and Neurotransmission
Amphetamine is a dopamine agonist; this chemical
increases the amount of dopamine released at synapses
in the brain, and can produce the symptoms of
schizophrenia and lead to addiction
Chemical Antagonists and Neurotransmission
Curare is a poison from a South American Vine,
that functions as an antagonist
It binds to acetylcholine receptor sites at neuromuscular
junctions, and blocks neurotransmitter action