THE SYNAPSE

1
THE SYNAPSE
2
The synapse

• A place where two neurones functionally meet.
• Where they meet the neurones do not touch.
• There is a small gap, about 20nm wide
• The gap is called the synaptic cleft
• The neurone that carries the impulse to the
  synapse is called the presynaptic neurone
• The neurone that carries the impulse away is
  called the postsynaptic membrane

3
(No Transcript)
4
(No Transcript)
5
Structure of the synapse

• Axons of neurones end in swellings called
  synaptic bulb
• The surface of the bulb is called the presynaptic
  membrane
• which is separated from the postsynaptic
  membrane by the synaptic cleft.
• The post synaptic membrane (SM) has channels
  through which specific ions can pass
• The post SM also has a large number of protein
  molecules on its surface which act as receptor
  sites for the neurotransmitter substance

6
Structure of the synapse

• In the synaptic bulb there are also many
  mitochondria
• (this suggests active transport is involved)
• Also present are synaptic vesicles
• These contain a neurotransmitter substance

7
Structure of the synapse

• Dopamine and serotonin are the active
  neurotransmitters in the braindont worry about
  them!
• Acetylcholine and noradrenaline are the two main
  neurotransmitters in the body

8
Structure of the synapse

• Neurones that release acetylcholine are said to
  have cholinergic synapses
• Neurones that release noradrenaline are said to
  have adrenergic synapses

9
Synaptic transmission

• When an AP arrives at a synaptic knob it causes
  calcium channels to open in the pre-synaptic
  membrane.
• Ca rushes into the synaptic bulb down its
  concentration gradient from the cleft.

10
Synaptic transmission

• This influx of calcium ions causes vesicles
  containing acetylcholine to move towards the
  presynaptic membrane.
• The vesicles fuse with the membrane releasing the
  neurotransmitter into the synaptic cleft.

11
Synaptic transmission

• The acetylcholine diffuses across the cleft and
  attaches to specific receptor sites on the
  postsynaptic membrane.
• The protein receptor sites have a complementary
  shape to that of acetylcholine but the binding is
  temporary

12
Synaptic transmission

• The binding of the neurotransmitter to the
  receptor site opens up sodium channels in the
  postsynaptic membrane.
• Sodium ions flood in depolarising the membrane
  and set of an AP.

13
Synaptic transmission

• The enzyme acetylcholinesterase which is present
  in the synaptic cleft splits the acetylcholine
  into acetate and choline.

14
Synaptic transmission

• The choline is taken back up by the synaptic knob
  and recombines with acetyl coenzyme A to reform
  acetylcholine.

15
Synaptic transmission

• Hence the need for mitochondria in the synaptic
  knob.
• If the acetylcholine was not removed from the
  postsynaptic membrane it would keep firing action
  potentials!

16
Functions of the synapse

• A number of APs have to arrive at the presynaptic
  membrane before there will be enough transmitter
  released (the threshold level) to initiate an
  action potential in the postsynaptic cell.
• This is called temporal summation.

17
Temporal summation
presynaptic cell
18
Temporal summation
low frequency of action potentials
presynaptic cell
19
Temporal summation
low frequency of action potentials
presynaptic cell
transmitter below threshold level
20
Temporal summation
low frequency of action potentials
presynaptic cell
transmitter below threshold level
no depolarisation of postsynaptic cell
21
Temporal summation
high frequency of action potentials
low frequency of action potentials
presynaptic cell
transmitter below threshold level
no depolarisation of postsynaptic cell
22
Temporal summation
high frequency of action potentials
low frequency of action potentials
presynaptic cell
transmitter below threshold level
transmitter reaches threshold level
no depolarisation of postsynaptic cell
23
Temporal summation
high frequency of action potentials
low frequency of action potentials
presynaptic cell
transmitter below threshold level
transmitter reaches threshold level
no depolarisation of postsynaptic cell
postsynaptic cell depolarises
24
Functions of the synapse

• A number of presynaptic neurones may form
  synapses with one postsynaptic neurone
• APs arriving in each presynaptic neurone will
  release transmitter, which builds up to the
  threshold level and triggers a postsynaptic
  impulse.
• This is called spatial summation

25
action potential only produced in one presynaptic
cell
26
action potential only produced in one presynaptic
cell
transmitter below threshold level
27
action potential only produced in one presynaptic
cell
transmitter below threshold level
no depolarisation of postsynaptic membrane
28
action potential only produced in one presynaptic
cell
action potential only produced in both
presynaptic cells
transmitter below threshold level
no depolarisation of postsynaptic membrane
29
action potential only produced in one presynaptic
cell
action potential only produced in both
presynaptic cells
transmitter reaches level
transmitter below threshold level
no depolarisation of postsynaptic membrane
30
action potential only produced in one presynaptic
cell
action potential only produced in both
presynaptic cells
transmitter reaches level
transmitter below threshold level
no depolarisation of postsynaptic membrane
postsynaptic cell depolarises
31
Functions of the synapse

• As synaptic vesicles are only present in the
  synaptic bulb then impulses can only pass in one
  direction.

32
Effects of drugs on the synapse

• Many drugs can affect the synapse

33
Effects of drugs on the synapse

• Nicotine very similar affect to acetylcholine,
  has an excitatory effect on the postsynaptic
  membrane. In large concentrations it can block
  synaptic transmission after an initial stimulation

34
Effects of drugs on the synapse

• Caffeine can cause an increase in the release of
  excitatory transmitter substances in the brain
  and has a mild stimulatory
• effect.

35
Effects of drugs on the synapse

• Opiates depress nerve transmission in sensory
  pathways of the spinal cord and brain that signal
  pain. This explains why opiates are such
  effective pain killers.

36
Effects of drugs on the synapse

• Both morphine and codeine are used as pain
  killer.
• The block the receptor sites in the post synaptic
  membrane.