07/03/19
Dr Maurice Elphick
Synaptic Transmission
Types of Synapse
1) Electrical synapses
a. Rapid propagation between neurons – almost no delay
b. Gap junctions connect neuron membrane to adjacent cell membrane
i. E.g. in cardiomyocytes in the heart
ii. Gap junction = 2 connexon rings
iii. Connexon = 6 connexins
2) Chemical synapses
a. Use exocytosis of neurotransmitters
i. Vesicles
b. Mechanism:
i. Terminal at rest
ii. AP arrives, vesicle fuses with terminal membrane – exocytosis
iii. Transmitter binds to receptor channels
1. Ion channels open
iv. Transmitter removed from cleft
1. Fused membrane is recycled
Stages of Chemical Synapses
1) Action potentials invade axon terminals
2) Depolarisation of axon terminal
3) Activation of Ca2+ voltage gated channels
4) Influx of Ca2+ down a concentration gradient
a. Increase in cytosolic Ca2+ concentration
5) Ca2+ causes fusion of neurotransmitter vesicles to presynaptic membrane
a. Excitation-secretion coupling
6) Vesicles release neurotransmitter into cleft
7) Neurotransmitter binds to post synaptic membrane receptors
8) Activation of receptors causes electrophysiological response in next cell
Nature of response is determined by the type of transmitter and receptor
Types of Synapse Messages
• Excitatory – transmitters cause depolarisation of postsynaptic cell
o Excitatory post-synaptic potential (EPSP) – D on the graph
• Inhibitory – transmitters cause hyperpolarisation of next cell
o Inhibitory post-synaptic potential (IPSP) – H on the graph
Dr Maurice Elphick
Synaptic Transmission
Types of Synapse
1) Electrical synapses
a. Rapid propagation between neurons – almost no delay
b. Gap junctions connect neuron membrane to adjacent cell membrane
i. E.g. in cardiomyocytes in the heart
ii. Gap junction = 2 connexon rings
iii. Connexon = 6 connexins
2) Chemical synapses
a. Use exocytosis of neurotransmitters
i. Vesicles
b. Mechanism:
i. Terminal at rest
ii. AP arrives, vesicle fuses with terminal membrane – exocytosis
iii. Transmitter binds to receptor channels
1. Ion channels open
iv. Transmitter removed from cleft
1. Fused membrane is recycled
Stages of Chemical Synapses
1) Action potentials invade axon terminals
2) Depolarisation of axon terminal
3) Activation of Ca2+ voltage gated channels
4) Influx of Ca2+ down a concentration gradient
a. Increase in cytosolic Ca2+ concentration
5) Ca2+ causes fusion of neurotransmitter vesicles to presynaptic membrane
a. Excitation-secretion coupling
6) Vesicles release neurotransmitter into cleft
7) Neurotransmitter binds to post synaptic membrane receptors
8) Activation of receptors causes electrophysiological response in next cell
Nature of response is determined by the type of transmitter and receptor
Types of Synapse Messages
• Excitatory – transmitters cause depolarisation of postsynaptic cell
o Excitatory post-synaptic potential (EPSP) – D on the graph
• Inhibitory – transmitters cause hyperpolarisation of next cell
o Inhibitory post-synaptic potential (IPSP) – H on the graph
