Announcements

• Slides used at tutorial posted to webpage

• Last lecture:– Finished Action potentials

• Today– Start synaptic transmission

Within neurons

Between neurons

electrical chemical & electrical

Neural Signalling

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. UsesA. Electrical synapses first discovered in crayfish

neurons involved in escape response

B. Synchronizing neural activity, e.g. hormone secreting neurons in the mammalian hypothalamus

Presynaptic nerveterminal

Postsynaptic nerveterminal

Synaptic vesicles

Neurotransmitterreceptors

Synaptic cleft

Chemical synapse

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 synapseDrosophila neuromuscular junction

Synaptic contacts

Active Zone

Synaptic vesicles

Ca++Na+

Depolarization

Sequence of Events at a Chemical Synapse

1. Action potential depolarizes nerve terminal2. Voltage-gated Ca++ channels open & Ca++ flows into

the nerve terminal3. Ca++ causes synaptic vesicles to fuse with the

plasma membrane4. Neurotransmitters are released into synaptic cleft5. Neurotransmitter binds to receptors6. Opens ion channels and positive current flows into

postsynaptic cell7. Current flow gives postsynaptic potential8. If postsynaptic potential = threshold Action Potential

Ca++Na+

Depolarization

Measuring Synaptic Transmission• Frog Neuromuscular Junction

mV

Time (ms)

Postsynaptic potential

Stimulus

Stimulate nerveRecord from muscle

Muscle cell

Chemical Synaptic Transmissionsome 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

Docking

Priming Fusion

Endosome

Budding

Exocytosis Endocytosis

Evidence for Vesicle Cycle

Extracellular Marker(HRP)

Marker found in coated vesicles

Marker found in endosome

Marker found in vesicles

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 communication1. 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