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What is a synapse?
1. A junction either between two neurones, or a neurone and an
effector cell.
2. The gap between cells at a synapse is called a synaptic cleft.
3. The presynaptic neurone has a swelling called the presynaptic
knob. This contains vesicles filled with neurotransmitters
4. When an action potential reaches the end of a neurone it causes
neurotransmitters to be released into the synaptic cleft. They
diffuse across to the postsynaptic membrane and bind to
specific receptors.
5. When neurotransmitters bind to receptors they might trigger
am action potential, cause a muscle contraction, or, cause a
hormone to be excreted (from a gland cell)
6. Neurotransmitters are removed from the cleft so the response
doesn't keep happening. E.g., they're taken back into the
presynaptic membrane or they're broken down by enzymes and
the products are taken into the neurone
7. There are many different neurotransmitters. E.g., acetylcholine
and noradrenaline. Synapses that use acetylcholine are called
cholinergic synapses. They bind to cholinergic receptors and are
broken down by acetylcholinesterase.…read more

Slide 3

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How neurotransmitters transmit nerve
impulses between neurones
1. An action potential triggers
calcium influx
2. An action potential arrives at the synaptic
knob.
3. The action potential stimulates voltage-gated
calcium ion channels to open
4. Calcium ions diffuse into the synaptic knob,
they're pumped out afterwards by active
transport
2. Calcium influx causes
neurotransmitter release
1. The influx of calcium ions causes the
synaptic vesicles to move to the
presynaptic mb, then fuse with it.
2. The vesicles release the neurotransmitter
into the synaptic cleft by exocytosis…read more

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3. The neurotransmitter triggers an action potential in the
post synaptic neurone.
1. The neurotransmitter diffuses across the synaptic cleft and binds to
specific receptors on the postsynaptic mb.
2. This causes sodium ion channels in the postsynaptic neurone to open.
3. The influx of sodium ions into the postsynaptic mb causes
depolarisation. An action potential on the postsynaptic mb is generated
if the threshold is reached.
4. The neurotransmitter is removes from the synaptic cleft so the
response doesn't keep happening…read more

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Functions of Synapses
1. Synapses allow information to be dispersed when one
neurone connects to many neurones the information can be
dispersed to different parts of the body (synaptic divergence)
2. Synapses allow information to be amplified, when many
neurones connect to one information can be amplified
(synaptic convergence)
3. If a stimulus is weak, only a small amount of
neurotransmitter will be released and this may not be enough
to excite the postsynaptic mb to threshold level. Summation
is where the effect of the neurotransmitter released from
many neurones (or one that's been stimulated a lot in a short
period of time) is added together.
4. Synapses make sure impulses only travel in one direction
because the receptors for neurotransmitters are only on the
postsynaptic mb.…read more

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Effects of chemicals on synapses
· Nicotine- has a similar shape to acetylcholine so fits
into acetylcholine receptors on postsynaptic mbs and
causes action potentials. However, it isnt rapidly broken
down by enzymes so remain on receptors for longer,
large doses can be fatal
· Botulinum toxin (botox) acts on the presynaptib mb
and prevents the release of acetylcholine. Can be fatal if
ingested but if injected can help smooth wrinkles or
relax muscles that cause eyelids to permananty
contract.
· Organophosphorous insecticides- inhibit the action
of acetylcholinesterase, allowing continuous production
of action potentials. Several nerve gases work this way
as do flea collars and sheep dips…read more

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