Synaptic Transmission

1
Synaptic Transmission

• Syllabus 3.5.2
• Toole page 174-179

2
Aims

• Label the following structures
• Synapse
• Neuromuscular junction
• Explain the sequence of events involved in
  transmission across a cholinergic synapse and
  across a neuromuscular junction.
• Explain the following terms
• Unidirectionality
• Temporal spatial summation
• Inhibition
• Describe the effects of drugs on a synapse.

3
The structure of the synapse neuromuscular
junction.

• Read the comprehension and label the following
  diagram and determine the role / function of each
  structure.

4
The structure of the synapse

• Neuron (Presynaptic)
• Neuron (Postsynaptic)
• Mitochondria
• Synaptic vesicle full of neurotransmitter
• Autoreceptor
• Synaptic cleft
• Neurotransmitter receptor
• Calcium Channel
• Fused vesicle releasing neurotransmitter
• Neurotransmitter re-uptake pump

5
The structure of a neuromuscular junction.

1. Presynaptic terminal motor neurone
2. Postsynaptic muscle membrane (Sarcolemma)
3. Synaptic vesicle
4. Receptor
5. Mitochondria

6
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
• Acetyl (ethanoic acid)
• Choline

7
(No Transcript)
8
(No Transcript)
9
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.

10
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.

11
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.

12
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.

13
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.

14
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.

15
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.

16

• 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.

17
Features of synapses

• Unidirectionality
• 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
• 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.

18
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.

19
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.
20
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.

21
Drugs Interfere with Neurotransmission

• Drugs can affect synapses at a variety of sites
  and in a variety of ways, including
• Increasing number of impulses
• Release neurotransmitter from vesicles with or
  without impulses
• Block reuptake or block receptors
• Produce more or less neurotransmitter
• Prevent vesicles from releasing neurotransmitter

22
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
23
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
• 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.