Presented by : Miss A. Harris
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Describe with diagram the structure of a synapse
Describe the mechanism of synaptic transmission
List the name of some common
neurotransmitters and briefly state how they
work
Name the three types of synapses and describe
them and their function
List some common drugs which affect the
synaptic mechanism. Briefly state how they are
able to do this
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A synapse is the “gap” between the axon of one
nerve and the dendrite of the next one.
A synapse consists of a swelling at the end of a
nerve fibre called a Synaptic knob lying in
close proximity to the membrane of a dendrite
 The cytoplasm of the synaptic knob contains
numerous mitochondria and small synaptic
vesicles. Each vesicle contains a chemical
called a neurotransmitter which is
responsible for the transmission of nerve
impulse across the synapse.
 The membrane of the synaptic knob nearest
the synapse is thickened and is called the presynaptic membrane..
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The membrane of the dendrite is also
thickened and called the post-synaptic
membrane.
 These membranes are separated by a gap of
about 20nm, the Synaptic cleft.
 The pre-synaptic membrane is modified for
the attachment of Synaptic vesicles and the
release of transmitter substance into the
synaptic cleft
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The post-synaptic membrane contains large
protein molecules which acts as receptor sites
for the transmitter substances and numerous
channels and pores, normally closed, for the
movement of ions into the post-synaptic
neuron
Step 1. The neurotransmitter is
manufactured by the neuron and stored
in vesicles at the axon terminal
Step 2. When the action potential
reaches the axon terminal, it causes the
vesicles to release the neurotransmitter
molecules into the synaptic cleft
Step 3. The neurotransmitter diffuses
across the cleft and binds to receptors
on the post-synaptic cell
Step 4. The activated receptors cause
changes in the activity of the postsynaptic neuron
Step 5. The neurotransmitter
molecules are released from the
receptors and diffuse back into the
synaptic cleft
Step 6. The Neurotransmitter is reabsorbed by the post synaptic neuron.
This process is known as Reuptake
 Impulse from action potential opens ion
channels for Ca++
 The increased Ca++ concentration in the
axon terminal initiates the release of the
neurotransmitter
 neurotransmitter is released from its
vesicle and crosses the “gap” or synaptic
cleft and attaches to a protein receptor on
the dendrite
Interaction of neurotransmitter and protein
receptor open post-synaptic membrane ion
channel for Na+
 After transmission the neurotransmitter is
either degraded by an enzyme or taken back
into the pre-synaptic membrane by a
transporter or reuptake pump
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Neurotransmitters are chemicals used by a
neuron to transmit an impulse across a
synapse to another cell
 There are dozens of different
neurotransmitters in the neurons of the body
 Each neuron generally synthesizes and
releases a single type of neurotransmitter
 A neurotransmitter can either be Excitory or
Inhibitory
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A neurotransmitter is called excitory if
activation of the receptor causes
depolarisation of the membrane and
promotes action potential generation
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A neurotransmitter is called inhibitory if the
activation of the receptor causes
hyperpolarisation and depresses action
potential generation.
Neurotransmitter
Role in the body
Acetylcholine
A neurotransmitter used by the spinal cord neurons to
control muscles and by many neurons in the brain to regulate
memory. In most instances, acetylcholine is excitatory
Dopamine
The neurotransmitter that produces feelings of pleasure when
released by the brain reward system. Dopamine has multiple
functions depending on where in the brain it acts. It is
usually inhibitory.
Gamma-aminobutvric
acid (GABA)
The major inhibitory neurotransmitter in the brain
Glutamate
The most common excitatory neurotransmitter in the brain
Glycine
A neurotransmitter used mainly by neurons in the spinal
cord. It probably always acts as an inhibitory
neurotransmitter
Norepinephrine
Norepinephrine acts as a neurotransmitter and a hormone. In
the peripheral nervous system, it is part of the flight-or-flight
response. In the brain, it acts as a neurotransmitter
regulating normal brain processes. Norepinephrine is
usually excitatory, but is inhibitory in a few brain areas
Neurotransmitters
Role in the body
Serotonin
A neurotransmitter involved in many
functions including mood, appetite, and
sensory perception. In the spinal cord,
serotonin is inhibitory in pain pathways.
There are three types of synapses:
Axodendritic Synapse
 Synaptic transmission from axon to dendrite
of a nerve cell
 It is also excitory
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Axosomatic Synapse
 Synaptic transmission from Axon to cell body
 It is also Inhibitory
Axoaxonic Synapse
 Synaptic transmission from Axon to Terminal
ending
 It is a Pre-synaptic Transmission
 It reduces polarity
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Drugs can affect synapses at a variety of sites
and in a variety of ways, including:
Increasing number of impulses
 Release neurotransmitters with or without
impulses
 Block reuptake or block receptors
 Produce more or less neurotransmitters
 Prevent vesicles from releasing
neurotransmitters
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Methamphetamine
Nicotine
Alcohol
Methamphetamine alters Dopamine
transmission in two ways
1. Enters Dopamine vesicles in the axon terminal
causing release of neurotransmitters
2. Blocks dopamine transporters from pumping
dopamine back into the transmitting neuron
This results in more dopamine in the
synaptic cleft
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This causes neurons to fire more often than
normal resulting in a euphoric feeling
After the drug wears off, dopamine levels
drop, and the user “crashes”. The euphoric
feeling will not return until the user takes
more methamphetamine
 Long-term use of methamphetamine causes
dopamine axons to wither and die
 Note that cocaine also blocks dopamine
transporters, thus it works in a similar manner
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Similar to methamphetamine and cocaine,
nicotine increases dopamine release in a
synapse
 Nicotine binds to receptors on the
presynaptic neuron
 Nicotine binds to the presynaptic receptors
exciting the neuron to fire more action
potentials causing an increase in dopamine
release
 Nicotine also affects neurons by increasing the
number of synaptic vesicles released.
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Alcohol has multiple effects on neurons. It
alters neuron membranes, ion channels,
enzymes, and receptors
 It binds directly to receptors for
acetylcholine, serotonin, and gamma
aminobutyric acid (GABA), and glutamate
 GABA is a neurotransmitter that has an
inhibitory effect on neurons
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When GABA attaches to its receptor on the
postsynaptic membrane, it allows Cl- ions to
pass into the neuron
 This hyperpolarizes the postsynaptic neuron to
inhibit transmission of an impulse
 When alcohol enters the brain, it binds to GABA
receptors and amplifies the hyperpolarization
effect of GABA
 The neuron activity is further diminished
 This accounts for some
of the sedative affects
of alcohol
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