1. Functional Anatomy Excitatory Synapses Inhibitory Synapses

2. Synaptic Transmission
Synapse
Point of impulse transmission between neurons
Impulses are transmitted from pre-synaptic neuron to:
Cell body, dendrite or axon of another neuron
A muscle
Gland
19-Apr-19
Synaptic Transmission

3. Synaptic Transmission
Functional Anatomy
Considerable variations in structure
The ends of pre synaptic axons
Enlarged to form synaptic knobs (terminal bouttons, end bulb)
Within the end bulbs
Synaptic vesicles with transmitter
Mitochondria
19-Apr-19
Synaptic Transmission

4. Synaptic Transmission
Functional Anatomy
Each synaptic terminal is separated from postsynaptic structure by synaptic cleft (20–40 nm)
On post synaptic membrane
Many receptors for the transmitter
19-Apr-19
Synaptic Transmission

5. Synaptic Transmission
Neurotransmitter Vesicles
Voltage gated Ca++ channels
Ca++
Ca++
Ca++
Pre synaptic membrane
Ca++
Post synaptic membrane
Synaptic cleft
Na+
Receptors on postsynaptic membrane
Na+
19-Apr-19
Synaptic Transmission

6. Synaptic Transmission
Synaptic Endings
Axo-axonal
Axo-dentritic
Axo-somatic
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Synaptic Transmission

7. Synaptic Transmission
Chemical Synapses
Transmission at most synapses is chemical
Impulses in pre-synaptic axon causes release of chemical neurotransmitter
Chemical transmitter binds to receptors on post synaptic membrane
19-Apr-19
Synaptic Transmission

8. Synaptic Transmission
Chemical Synapses
Triggers opening or closing of ionic channels on post synaptic neuron leads to:
Excitation
Inhibition
Modification of sensitivity
19-Apr-19
Synaptic Transmission

9. Synaptic Transmission
Electrical Synapses
At some synapses
Impulse transmission is by electrical means
The membrane of
Pre-synaptic
Post-synaptic neurons
Come closer together
19-Apr-19
Synaptic Transmission

10. Synaptic Transmission
Electrical Synapses
Gap junctions form between the cells
These form low resistance bridges where ions pass with relative ease
Found in
Lateral vestibular nucleus
Smooth muscle fibres
Cardiac muscle cells
19-Apr-19
Synaptic Transmission

11. Synaptic Transmission
Excitatory Synapse
Arrival of AP at pre-synaptic nerve terminal
Opening of Ca++ channels
Ca++ enter the nerve terminal
Release of neurotransmitter into synaptic cleft AP
Ca++
Ca++
Na+
Na+ AP
Threshold
19-Apr-19
RMP
Synaptic Transmission
EPSP

12. Synaptic Transmission
Excitatory Synapse
Neurotransmitter diffuses
Binds on receptors on post synaptic membrane
Opening of ionic gates (Na+, K+) AP
Ca++
Ca++
Na+
Na+ AP
Threshold
19-Apr-19
RMP
Synaptic Transmission
EPSP

13. Synaptic Transmission
Excitatory Synapse
Entry of Na+ into post synaptic cell
Local depolarization (excitatory postsynaptic potential; EPSP)
Conducted AP if threshold is attained
Ca++
Ca++
Na+
Na+ AP
Threshold
19-Apr-19
RMP
Synaptic Transmission
EPSP

14. Synaptic Transmission
Temporal Summation
Excitation of single pre synaptic terminal
Is not enough to excite post synaptic cell
Single EPSP is about 0.5 to 1 mv
EPSP AP
19-Apr-19
Synaptic Transmission

15. Synaptic Transmission
Temporal Summation
You require about 10 to 20 mv change to reach threshold for conducted AP
If you have a neuron firing repetitively in close succession
EPSP AP
19-Apr-19
Synaptic Transmission

16. Synaptic Transmission
Temporal Summation
It is possible to have EPSP summate
Reach threshold value for the post synaptic neuron to fire an AP
This is temporal summation
EPSP AP
19-Apr-19
Synaptic Transmission

17. Synaptic Transmission
Spatial Summation
If you have a group of neurons
Firing one after another in close succession
EPSP summate and can reach threshold value 2 1 3 AP AP
Threshold
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RMP
Synaptic Transmission 1 2 3

18. Synaptic Transmission
Spatial Summation
Resulting into postsynaptic neuron firing AP
This is known as Spatial Summation 2 1 3 AP AP
Threshold
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RMP
Synaptic Transmission 1 2 3

19. Synaptic Transmission
Inhibitory Synapse
Two types of synaptic inhibition
Post synaptic inhibition
Pre synaptic inhibition
19-Apr-19
Synaptic Transmission

20. Post Synaptic Inhibition
Events that occur
Are similar to those at excitatory synapse except
Influx of Ca++ in axon terminal
Leads to release of an Inhibitory transmitter
Ca++ K+
Ca++
Cl-
Cl- K+
Threshold
- 45mv
-70mv
RMP
-85mv
19-Apr-19
Synaptic Transmission
IPSP
Hyper-polarization

21. Post Synaptic Inhibition
Opens K+, Cl- channels
K+ Move out of postsynaptic cell
While Cl- move into the post synaptic cell
Ca++ K+
Ca++
Cl-
Cl- K+
Threshold
- 45mv
-70mv
RMP
-85mv
19-Apr-19
Synaptic Transmission
IPSP
Hyper-polarization

22. Post Synaptic Inhibition
Leads to hyper polarization (IPSP)
This drives the membrane potential away from threshold
Making it difficult to excite post synaptic cell
Ca++ K+
Ca++
Cl-
Cl- K+
Threshold
- 45mv
-70mv
RMP
-85mv
19-Apr-19
Synaptic Transmission
IPSP
Hyper-polarization

23. Pre Synaptic Inhibition
Excitatory cell
Inhibitory cell
The inhibitory neuron axon
Terminate on the axon of the excitatory neuron
Transmitter causing partial depolarization
Excitatory transmitter
Post synaptic cell
19-Apr-19
Synaptic Transmission

24. Pre Synaptic Inhibition
Excitatory cell
Inhibitory cell
The inhibitory neuron axon terminal release a
Transmitter that depresses the voltage of AP on excitatory neuron
Partial depolarization
Transmitter causing partial depolarization
Excitatory transmitter
Post synaptic cell
19-Apr-19
Synaptic Transmission

25. Pre Synaptic Inhibition
Excitatory cell
Inhibitory cell
This reduces
The amount of neurotransmitter released by the excitatory neuron
Causing inhibition of
The post synaptic neuron
Transmitter causing partial depolarization
Excitatory transmitter
Post synaptic cell
19-Apr-19
Synaptic Transmission

26. Synaptic Transmission
Neurotransmitters
Excitatory
Acetylcholine
Noradrenalin, Adrenalin
Glutamate, Aspartate
Inhibitory
GABA (gamma amino butyric acid)
Glycine, Histamine
19-Apr-19
Synaptic Transmission

27. Synaptic Transmission
Neuromuscular Junction (NMJ)
Single motor neuron
Myelin Synaptic buttons
End - plate
Axon 2 m
Each nerve terminal has
~ 300,000 small synaptic vesicles
Dense bar active zone
Actylcholine receptors (nicotinic)
10,000 m2
voltage - gated channels (permeable to both Na+ and K+)
Synaptic cleft
Junctional fold
19-Apr-19
Synaptic Transmission

28. Neuromuscular Transmission
Junction between nerve and muscle
Muscle membrane that lie directly under the axon terminal
Is known as end plate (postsynaptic membrane)
Axon terminal
Sarcolema
Motor end plate
19-Apr-19
Synaptic Transmission

29. Synaptic Transmission
NMJ Transmission
The axon terminal of the motor neuron
Contain vesicles like those found at nerve to nerve synaptic junctions
19-Apr-19
Synaptic Transmission

30. Synaptic Transmission
Neurotransmitter Vesicles
Voltage gated Ca++ channels
Ca++
Ca++
Ca++
Pre synaptic membrane
Ca++
Post synaptic membrane
Synaptic cleft
Na+
Receptors on postsynaptic membrane
Na+
19-Apr-19
Synaptic Transmission

31. Synaptic Transmission
NMJ Transmission
Arrival of AP at the axon terminal causes
Depolarization of nerve plasma membrane
Open voltage –sensitive Ca++ channel
Ca++ diffuse into axon terminal
Ca++ triggers release of Acetylcholine (Ach) into synaptic cleft
19-Apr-19
Synaptic Transmission

32. Synaptic Transmission
NMJ Transmission
Ach binds to receptors on postsynaptic membrane
Opening of ionic channels (Na+, K+)
Na+ moves in more than K+ move out
Produces local depolarization
Endplate potential (EPP) [cf EPSP]
19-Apr-19
Synaptic Transmission

33. Synaptic Transmission
NMJ Transmission
The magnitude of this EPP
Much larger than the EPSP
Larger amount of transmitter released
Cause much more channels to open
Hence an EPP from one AP is sufficient to cause
AP to develop on the postsynaptic membrane
In actual fact
EPP elicited by one nerve AP is larger than that required to to elicit muscle AP
19-Apr-19
Synaptic Transmission

34. Synaptic Transmission
Break Down of Ach
At the endplate there is also present
Acetylcholinesterase
Enzyme responsible for the breakdown of Acetylcholine to choline & acetate
This causes the closure of the ionic channels to enable normal repolarization of post synaptic membrane
19-Apr-19
Synaptic Transmission

35. Interruption of NMJ Transmission
Several substances can interfere with NMJ transmission
Curare
Binds strongly to Ach receptor
Does not open ionic channels
Is not destroyed by acetylcholinesterase
Hence no EPP
Muscle paralysis
Curare is also used during anaesthesia
Cause muscle relaxation during surgery
19-Apr-19
Synaptic Transmission

36. Interruption of NMJ Transmission
Inhibition of acetylcholinesterase
Organophosphates
Part of pesiticides & nerve gas
Initial binding of Ach on the receptor is not inactivated
Muscle is held In a permanent depolarized state
Cannot initiate another AP
19-Apr-19
Synaptic Transmission

37. Interruption of NMJ Transmission
Bacterial toxins
Clostridia botulinum bacteria
Botulism
Prevent release of ach from the axon terminal
Prevent transmission
19-Apr-19
Synaptic Transmission

38. Synaptic Transmission
Myasthenia Gravis
Condition characterized by muscle weakness
Can be serious depending on the severity
Due to development of antibodies against
Ach receptors which become destroyed
19-Apr-19
Synaptic Transmission