Announcements
• Slides used at tutorial posted to webpage
• Last lecture:
– Finished Action potentials
• Today
– Start synaptic transmission
Neural Signalling
Within
neurons
electrical
Between
neurons
chemical &
electrical
Definitions
1. Synapse : the functional contacts
between neurons and other cells
2. Synaptic Transmission: the process used
by neurons to relay information from one
cell to the next
Definitions
Presynaptic cell
Postsynaptic cell
Synapse
Types of Synapses
1. Electrical
 Direct flow of electrical current from one cell
to the next
2. Chemical
 Secrete neurotransmitter molecules that
activate receptors
Electrical synapses
• Physical link between two neurons called
a gap junction
Protein is called connexon
Electrical Synapses
Action potential in
one cell can cause
ions to flow through
the gap junction
into the next cell
Inject current
Record
Reduced
depolarization
Electrical Synapses
Electrical synapses :
1. Can be unidirectional or
bidirectional
2. Are very fast
3. Uses
A. Electrical synapses first discovered in crayfish
neurons involved in escape response
B. Synchronizing neural activity, e.g. hormone secreting
neurons in the mammalian hypothalamus
Chemical synapse
Presynaptic nerve
terminal
Synaptic vesicles
Synaptic cleft
Neurotransmitter
receptors
Postsynaptic nerve
terminal
Chemical Synapses
Features:
1. Use chemical neurotransmitters
2. There is a space between the pre- and
postsynaptic neuron, called the synaptic
cleft
3. Neurotransmitter is stored in synaptic
vesicles
Chemical Synapses
Features:
4. Can be excitatory or inhibitory
•
Depends on neurotransmitter and receptor
5. Can activate ion-channels or second
messenger pathways
6. The amount of transmitter released is
variable and can be modulated
Real synapse
Drosophila neuromuscular junction
Synaptic vesicles
Synaptic contacts
Active Zone
Na+
Ca++
Depolarization
Sequence of Events at a Chemical
Synapse
1.
2.
3.
4.
5.
6.
7.
8.
Action potential depolarizes nerve terminal
Voltage-gated Ca++ channels open & Ca++ flows into
the nerve terminal
Ca++ causes synaptic vesicles to fuse with the
plasma membrane
Neurotransmitters are released into synaptic cleft
Neurotransmitter binds to receptors
Opens ion channels and positive current flows into
postsynaptic cell
Current flow gives postsynaptic potential
If postsynaptic potential = threshold  Action Potential
Na+
Ca++
Depolarization
Measuring Synaptic Transmission
• Frog Neuromuscular Junction
Record from muscle
Stimulate nerve
Postsynaptic potential
mV
Muscle cell
Time (ms)
Stimulus
Chemical Synaptic Transmission
some key questions
1. What is the nature if vesicular release?
2. What is the role of Ca++?
3. What happens to the neurotransmitter
after it is secreted?
4. What are the electrical effects on the
postsynaptic cell?
The Synaptic Vesicle Cycle
Endosome
Budding
Exocytosis
Endocytosis
Docking
Priming
Fusion
Evidence for Vesicle Cycle
Marker found in endosome
Marker found in coated vesicles
Marker found in vesicles
Extracellular Marker
(HRP)
Evidence for Vesicle Cycle
1. Stimulate synaptic transmission in the
presence of extracellular marker (HRP)
2. Wash marker away
3. Examine the intracellular distribution of
the marker by electron microscope
4. Marker first seen in coated vesicles
5. After 5-20 minutes, found in endosome
6. After 1 hour found in synaptic vesicles
Summary & Concepts
1. Two types of synaptic communication
1. Electrical through gap junctions
2. Chemical  neurotransmitters
2. Depolarization, Ca++ influx, and vesicle fusion
release neurotransmitter
3. Neurotransmitter opens ion channels changing
the electrical potential of the postsynaptic cell
4. Synaptic vesicles go through a cycle of
exocytosis and endocytosis