1. Section 2 Interaction between neurons
Synapse: specialized zone of contact at which one neuron communicate with another.

2. 1. Chemical synapse transmission
Section Outline
Synaptic structure
The classification of synapse
Process of chemical synapse transmission
The synapse transmission features
Synapse transmission modulation
Synaptic plasticity

3. 1.1 Synaptic structure

4. 1.1 Synaptic structure

5. Vesicles

6. 1.2 Classification of synapse
On the basis of connecting area of synapses
Axo-somatic
Axo-dendritic
Axo-axonic
Dendro-dendritic
Dendro-axonic
Dendro-somatic

8. Axodendritic synapse

9. 1.2 Classification of synapse
On the basis of action on postsynaptic neuron
Excitatory synapse:
excitatory neurotransmitter→depolarization
Inhibitory synapse:
inhibitory neurotransmitter→hyperpolarization

10. Excited or inhibited?
Photographs using the electron microscope have shown that synapses can be either asymmetrical (red arrow) or symmetrical (green arrow). In the figure on the left, notice that the red arrow is pointing to a synapse that has one dark band and one lighter band. The green arrow is pointing to a synapse that has two dark bands. Asymmetrical synapses are thought to be excitatory synapses and symmetrical synapse are thought to be inhibitory synapses. The yellow line outlines the dendrite (D).

12. 1.2 Classification of synapse
On the basis of signal transmission mode between synapses
Chemical synapse
Electrical synapse (gap junction)

15. 1.3 Process of chemical synapse transmission
No structural continuity between pre- and postsynaptic neurons: nm.
Synaptic vesicles cluster at active zone.
Unidirection, synaptic delay (1 ms or longer)

16. 1.3 Process of chemical synapse transmission

17. 1.3 Process of chemical synapse transmission

18. 1.3 Process of chemical synapse transmission
Excitatory postsynaptic potential, EPSP
Inhibitory postsynaptic potential, IPSP

19. 1.3 Process of chemical synapse transmission
Excitatory postsynaptic potential, EPSP:
Excitatory neurotransmitter released by presynaptic terminal causes depolarization which leads to the excitation and activation of postsynaptic neuron.

20. Ionic mechanism of EPSP
Action potential arrives at presynaptic neurone (synaptic knob)
Action potential causes calcium channels to open (Ca 2+ enters )
Ca 2+ cause synaptic vesicle to move and release excitatory transmitter
Transmitter diffuses across synaptic cleft
Transmitter binds to receptor on postsynaptic membrane
Na+, K+ channels open (Na+>K+ )
Causes depolarisation of the postsynaptic membrane (EPSP)
Action potential is produced in the initial segment of axon

23. Electrical trigger
For communication between neurones to occur, an electrical impulse must travel down an axon to the synaptic terminal.

24. Causes hyperpolarisation of the postsynaptic membrane (IPSP)
Ionic mechanism of IPSP
Action potential arrives at presynaptic neurone (synaptic knob)
Action potential causes calcium channels to open (Ca 2+ enters )
Ca 2+ cause synaptic vesicle to move and release inhibitory transmitter
Transmitter diffuses across synaptic cleft
Transmitter binds to receptor on postsynaptic membrane
Cl-, K+ channels open
Causes hyperpolarisation of the postsynaptic membrane (IPSP)
IPSP moves the membrane potential away from the firing level of the cell →decreases excitability

28. 1.4 Properties of synaptic transmission
One-way transmission
Synaptic delay
Summation
Change of excitatory rhythm
After discharge
Susceptibility to internal environment and fatigue

30. rapid succesive action potentials
two neurones action potentials act together

32. 1.4 Properties of synaptic transmission
One-way transmission
Synaptic delay
Summation
Change of excitatory rhythm
After discharge
Susceptibility to internal environment and fatigue

33. After discharge

34. 1.4 Properties of synaptic transmission
One-way transmission
Synaptic delay
Summation
Change of excitatory rhythm
After discharge
Susceptibility to internal environment and fatigue

35. 1.5 Modulation of synaptic transmission
Presynaptic modulation
Ca2+
Autoreceptor
Uptake of neurotransmitter

36. 1.5 Modulation of synaptic transmission
Postsynaptic modulation
Amount of postsynaptic receptor
Affinity of ligand and receptor
Up regulation
Down regulation Internalization
Desensitization

37. 1.6 Synaptic plasticity Posttetanic potentiation
Habituation and sensitization
Long-term potentiation (LTP) and long-term depression (LTD)

38. Long-term potentiation
Field EPSPs
LTP is an electrophysiological measure of sustained increase in synaptic efficacy when given high-frequency stimulation
Cellular and behavioral studies suggest that learning and memory can be modeled by LTP
High-frequency stimulation
200 msec
12 bursts; 4 pulses; 100Hz
(TBS 12x)

39. 2. Non-synaptic chemical transmission
Varicosity
In PNS:
In CNS:
Epinephrinergic
Dopaminergic
Serotoninergic

40. 3. Electrical synapse transmission
Distance between Pre- and Postsynaptic neurons: 2-3nm.
Cytoplasmic continuity: gap junction.
Low resistance, short synaptic delay (<0.1ms), bidirectional current flow.

41. Six connexins
A channel called a
connexon
Diameter 2nm
Highly synchronized

42. Comparison of chemical and electrical synapses
Properties Electrical Chemical
Synaptic cleft nm nm
Cytoplasmic continuity yes no
Ultrastructure gap junction chemical synapse
Transmission ionic current neurotransmitter
Synaptic delay no yes
Direction bidirection unidirection

43. 4. Local neuronal circuit
Local circuit neuron
Local neuronal circuit

44. Summary - Neurotransmission

45. Summary Classification of synapse Process of synaptic transmission
Characteristics of synaptic transmission
Modulation of synaptic transmission

47. Summation of Postsynaptic Potentials
Temporal summation occurs when single synapse receives many EPSPs in a short period of time
Spatial summation occurs when single synapse receives many EPSPs from many presynaptic cells

48. Advantages of electrical synapses
More reliable than chemical
Faster (escape responses)
Synchronizing cells
Intracellular transfer of molecules (Ca++. ATP and cAMP)
Can be dynamic (Dopamine regulates some in retina)

49. 1.1 Synaptic structure

52. 1.3 Process of chemical synapse transmission
Slow postsynaptic potential
Autonomic ganglia, cardiac and smooth muscle, cortical neurons
Have a latency of ms
Last several seconds
Slow EPSP: decrease in K+ conductance
Slow IPSP: increase in K+ conductance