What is a synapse?

A synapse is the junction between 2
neurones.
 There is a very narrow gap of about 20nm
between neurones called the synaptic
cleft.
 An action potential cannot cross the
synaptic cleft, so nerve impulses are
carried by chemicals called
neurotransmitters.
A Synapse


Pre-synaptic neurone = neurone sending impulse
Post-synaptic neurone = neurone receiving impulse
Neurotransmitter
Neurotransmitter is made by
the pre-synaptic neurone and
is stored in synaptic vessels
at the end of the axon.
 The membrane of the postsynaptic neurone has
chemical-gated ion channels
called neuroreceptors.
These have specific binding
sites for neurotransmitters.

Cholinergic Synapses

Acetylcholine is a
common transmitter.
 Synapses that have
acetylcholine
transmitter are
called cholinergic
synapses.

 Some neurones
form more than 1
synapse.
This is an electron
micrograph of
synapses between
nerve fibres and a
neurone cell body.
What happens at a cholinergic
synapse? Stage 1

An action potential
arrives at presynaptic
membrane. Voltage
gated calcium
channels in the
presynaptic
membrane open,
calcium ions enter the
presynaptic neurone.
What happens at a cholinergic
synapse? Stage 2

Calcium ions cause
synaptic vesicles to
fuse with the
presynaptic
membrane,
releasing
acetylcholine into
the synaptic cleft.
What happens at a cholinergic
synapse? Stage 3

Acetylcholine
diffuses cross the
synaptic cleft and
binds to specific
neuroreceptor sites
in the post synaptic
membrane.
What happens at a cholinergic
synapse? Stage 4

Sodium channels
open. Sodium ions
diffuse into the
postsynaptic
membrane causing
depolarisation,
which may initiate
an action potential.
What happens at a cholinergic
synapse? Stage 5

Acetylcholinesterase
breaks down
acetylcholine. The
products diffuse back
into the presynaptic
neurone where
acetycholine is
resynthesised using
ATP from the
mitochondria.
Neuromuscular Junctions

Same stages as
cholinergic synapses,
but in this case the
postsynaptic
membrane is the
muscle fibre
membrane,
(Sarcolemma).
Depolarisation of the
sarcolemma leads to
contraction of muscle
fibre.
Drugs

Drugs which have molecules of
similar shape to transmitter
substances can affect protein
receptors in postsynaptic
membranes.
 Drugs that stimulate a nervous
system are called AGONISTS
 Drugs that inhibit a nervous
system are called
ANTAGONISTS.
Various effects of drugs on
synapses:
DRUG ACTION
EFFECT
Mimic a neurotransmitter
Switch on a synapse
Stimulate the release of a
neurotransmitter
Switch on a synapse
Open
a
channel
neuroreceptor
Switch on a synapse
Block
a
channel
neuroreceptor
Switch off a synapse
breakdown
Switch on a synapse
Inhibit
the
enzyme
Inhibit
pump
the
Na+K+ATPase
Block the Na+ or K+
channels
Stop action potentials
Stop action potentials
Effect of nicotine and atropine
Summary

A synapse is the point where 2 nerve cells
meet. Tiny gap = synaptic cleft.
 Chemical transmitter released from
presynaptic neurone diffuses across synaptic
cleft & fits into receptors on postsynaptic
membrane. May cause postsynaptic neurone
to depolarise & set up action potential.
 Neuromuscular junction = motor neurone
connects with muscle fibre – similar to a
synapse.
 Many drugs affect synapses.