Download Synaptic Transmisson

Synaptic Transmission
Syllabus 3.5.2
Toole page 174-179
Aims:
1.
Label the following structures:
–
–
Synapse
Neuromuscular junction
2.
Explain the sequence of events involved in
transmission across a cholinergic synapse and across
a neuromuscular junction.
3.
Explain the following terms:
–
–
–
4.
Unidirectionality
Temporal & spatial summation
Inhibition
Describe the effects of drugs on a synapse.
The structure of
the synapse &
neuromuscular
junction.
•
Read the comprehension and label the following diagram and determine the role
/ function of each structure.
The structure
of the synapse
A. Neuron (Presynaptic)
B. Neuron (Postsynaptic)
1. Mitochondria
2. Synaptic vesicle full of
neurotransmitter
3. Autoreceptor
4. Synaptic cleft
5. Neurotransmitter receptor
6. Calcium Channel
7. Fused vesicle releasing
neurotransmitter
8. Neurotransmitter re-uptake pump
The structure of a neuromuscular
junction.
1. Presynaptic terminal
motor neurone
2. Postsynaptic muscle
membrane
(Sarcolemma)
3. Synaptic vesicle
4. Receptor
5. Mitochondria
A cholinergic synapse
•
These types of synapse are common in vertebrates.
•
They occur in the CNS and at neuromuscular junctions.
•
In cholinergic synapse the neurotransmitter is
acetylcholine.
•
Acetylcholine is made up of two parts:
1. Acetyl (ethanoic acid)
2. Choline
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.
• The neurotransmitter
acetylcholine is
released into the
synaptic cleft.
What happens at a cholinergic
synapse? Stage 3
• Acetylcholine
diffuses across the
synaptic cleft.
• It binds to specific
receptor sites on the
sodium ion channel
in the postsynaptic
neurone membrane.
What happens at a cholinergic
synapse? Stage 4
• Sodium channels open.
• Sodium ions diffuse into
the postsynaptic
membrane causing
depolarisation.
• This may initiate an
action potential.
What happens at a cholinergic
synapse? Stage 5
• Acetylcholinesterase breaks
down acetylcholine into choline
and ethanoic acid.
• The removal of acetylcholine
from the receptors cause
sodium ion channels to close in
the membrane of the
postsynaptic neurone.
• The products diffuse back
across the synaptic cleft into
the presynaptic neurone.
What happens at a cholinergic
synapse? Stage 6
• ATP released by the
mitochondria is used
to recombine choline
and ethanoic acid into
acetylcholine.
• This is stored in
vesicles for future use.
Transmission across a synapse
1.
When an impulse arrives at a synaptic knob it causes calcium ion channels to open.
2.
This results in calcium ions diffusing into it from the surrounding fluid.
3.
The calcium ions cause some of the synaptic vesicles to move towards the presynaptic membrane.
4.
The vesicles fuse to the presynaptic membrane and discharge a neurotransmitter (acetylcholine) into
the synaptic cleft.
5.
The neurotransmitter (acetylcholine) diffuses across the cleft to the postsynaptic membrane.
6.
The neurotransmitter binds to receptor sites on the sodium ion channel in the membrane of the
postsynaptic neurone.
7.
This causes the sodium ion channels to open and sodium ions enter the post synaptic neurone
diffusing rapidly along a concentration gradient.
8.
The influx of sodium ions generates a new action potential in the postsynaptic neurone.
9.
Acetylcholinesterase hydrolyses acetylcholine into choline and ethanoic acid, which diffuses back
across the synaptic cleft into the presynaptic neurone.
10.
Sodium ion channels close in the absence of acetylcholine in the receptor sites.
11.
ATP released by mitochondria is used to recombine choline and ethanoic acid into acetylcholine.
•
Same stages as cholinergic synapses, however, the postsynaptic membrane is
the muscle fibre membrane (Sarcolemma).
•
Depolarisation of the sarcolemma leads to contraction of muscle fibre.
Features of synapses
1.
Unidirectionality
–
2.
Synapses can only pass impulses in one direction, from the
presynaptic neurone to the postsynaptic neurone.
Summation
–
Low frequency action potentials often produce insufficient amounts of
neurotransmitter to trigger a new action potential in the postsynaptic
neurone. They, can be made to do so by a process called summation
where neurotransmitter builds up in the synapse by one of two
methods:
a)
b)
3.
Spatial summation ~ Many different presynaptic neurones release
neurotransmitter.
Temporal summation ~ A single presynaptic neurones releases
neurotransmitter many times over a short period.
Inhibition
–
On the postsynaptic membrane of some synapses, the protein
channels carrying chloride ions can be made to open. Thus leads to
an influx of chloride ions, making the inside of the postsynaptic
membrane even more negative than when it is at resting potential.
Neurotransmitters
• There are dozens of different neurotransmitters in the
neurons of the body.
• Neurotransmitters can be either excitatory or inhibitory.
• Each neuron generally synthesises and releases a single
type of neurotransmitter.
• The major neurotransmitters are indicated on the next
slide.
Major Neurotransmitters in the Body
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.
GABA
(gamma-aminobutyric acid)
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.
Serotonin
A neurotransmitter involved in many functions including mood, appetite, and
sensory perception. In the spinal cord, serotonin is inhibitory in pain
pathways.
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.
Drugs Interfere with
Neurotransmission
•
Drugs can affect synapses at a variety of sites and in a
variety of ways, including:
1. Increasing number of impulses
2. Release neurotransmitter from vesicles with or
without impulses
3. Block reuptake or block receptors
4. Produce more or less neurotransmitter
5. Prevent vesicles from releasing neurotransmitter
Drugs That Influence Neurotransmitters
Change in Neurotransmission
Effect on Neurotransmitter
release or availability
Drug that acts this way
increase the number of impulses
increased neurotransmitter release
nicotine, alcohol, opiates
release neurotransmitter from
vesicles with or without impulses
increased neurotransmitter
release
amphetamines
methamphetamines
release more neurotransmitter in
response to an impulse
increased neurotransmitter
release
nicotine
block reuptake
more neurotransmitter present in
synaptic cleft
cocaine
amphetamine
produce less neurotransmitter
less neurotransmitter in synaptic
cleft
probably does not work this way
prevent vesicles from releasing
neurotransmitter
less neurotransmitter released
No drug example
block receptor with another
molecule
no change in the amount of
neurotransmitter released, or
neurotransmitter cannot bind to its
receptor on postsynaptic neuron
LSD
caffeine
Homework
•
In groups of 3 you will produce a leaflet that consists of 3 typed A4
information sheets.
•
Each information sheet should provide information to explain how
the following neurotransmitters effect the body:
1.
2.
3.
Endorphins
Serotonin
Gamma aminobutyric acid (GABA)
•
The information sheet should also explain how drugs can effect
the role of each specific neurotransmitter.
•
It is essential that diagrams of presynaptic neurones, postsynaptic
neurones and receptor sites are included to enhance the detail of
the information sheet.