Synaptic Transmission

Lecture 12

Synapses

Communication b/n neurons Electrical

Electrotonic conduction Chemical

Ligand / receptor ~

Electrical Synapses

Tight junctions Connexons Passive current & larger molecules (e.g., ATP)

Synchronous activity Breathing Hormone release from

hypothalamus ~

Synaptic Transmission

Nature of signal across synapse?

Sherrington electrotonic

transmission Most neural

conduction too slow Delay in spinal

reflexes ~

Synaptic Transmission

Otto Loewi had a dream Chemical Signal? How can it be

demonstrated? Frog Heart

Preparation ~

Stimulate Vagus Heart rate slows ~

electrical stimulation

-

Stimulate A Collect fluid ~

A B-

A B

Perfuse heart B Heart B slows

chemical = vagusstoff ~

-

B

Repeat w/ accelerator nerve HR increases Acceleransstoff ~

A+

The Chase Is On

Search for chemical neurotransmitters (NT)

Criteria Synaptic Model ~

Synaptic Events

Action Potential reaches axon terminal Chemical substance released

Neurotransmitter (NT) Diffuses across synapse Binds to receptor protein

EPSP or IPSP ~

1. Precursor transport

2. NT synthesis

3. Storage

4. Release

5. Activation

6. Termination ~

Synaptic Transmission Model

PresynapticAxon Terminal

PostsynapticMembrane

Terminal Button

Dendritic Spine

1. Precursor Transport

E

2. Synthesis

Enzymes & cofactors ~

3. Storage

E

Synapse

Terminal Button Dendritic

Spine

3. StorageIn vesicles

Synapse

Ca++

AP

4. Release • via exocytosis• diffusion

Exocytosis: Membrane Fusion

Ca2+ influx2d messengerCa2+ -Calmodulin

SNAPs soluble-NSF-attachment proteins

SNAREsSNAP receptors ~

5. Activation NT binds to receptor

Activation

NT binds to postsynaptic receptor opens ion channel EPSP

Na+ into cell or IPSP

K+ out of cell ~

6. Termination

4 Basic methods Diffusion Enzymatic degradation Presynaptic reuptake

active transport Autoreceptors ~

6. Termination autoreceptors

A

Autoreceptors

On presynaptic terminal Binds NT

same as postsynaptic receptors different receptor subtype

Decreases NT release & synthesis ~

Termination

Must have discrete signal What if ion channels continually open?

ions move toward equilibrium membrane no longer polarized no EPSPs or IPSPs

Disrupts neural communication Termination ---> neuron repolarizes ~

Exocytosis: Membrane Fusion

Ca2+ influx2d messengerCa2+ -Calmodulin

SNAPs soluble-NSF-attachment proteins

SNAREsSNAP receptors ~