Synaptic Transmission Lecture 12

Synapses n Communication b/n neurons n Electrical l Electrotonic conduction n Chemical l Ligand / receptor ~

Electrical Synapses n Tight jucnctions l Connexons l Passive current l & larger molecules (e.g., ATP) n Synchronous activity l Breathing l Hormone release from hypothalamus ~

Synaptic Transmission n Nature of signal across synapse? n Sherrington l electrotonic transmission n Most neural conduction too slow l Delay in spinal reflexes ~

Synaptic Transmission n Otto Loewi had a dream l Chemical Signal? l How can it be demonstrated? n Frog Heart Preparation ~

n Stimulate Vagus l Heart rate slows ~ electrical stimulation -

n Stimulate A l Collect fluid ~ A B -

A B n Perfuse heart B n Heart B slows l chemical = vagusstoff ~ -

B n Repeat w/ accelerator nerve l HR increases l Acceleransstoff ~ A +

The Chase Is On n Search for chemical neurotransmitters (NT) n Criteria n Synaptic Model ~

Synaptic Events n Action Potential reaches axon terminal n Chemical substance released l Neurotransmitter (NT) n Diffuses across synapse n Binds to receptor protein l EPSP or IPSP ~

1. Precursor transport 2. NT synthesis 3. Storage 4. Release 5. Activation 6. Termination ~ Synaptic Transmission Model

Presynaptic Axon Terminal Postsynaptic Membrane Terminal Button Dendritic Spine

1. Precursor Transport

E 2. Synthesis Enzymes & cofactors ~

3. Storage E

Synapse Terminal Button Dendritic Spine 3. Storage In vesicles

Ca++ 4. Release diffusion AP

5. Activation l NT binds to receptor

Activation n NT binds to postsynaptic receptor l opens ion channel l EPSP u Na+ into cell l or IPSP u K+ out of cell ~

6. Termination n 4 Basic methods l Diffusion l Enzymatic degradation l Presynaptic reuptake u active transport l Autoreceptors ~

6. Termination l autoreceptors A

Autoreceptors n On presynaptic terminal n Binds NT l same as postsynaptic receptors l different receptor subtype n Decreases NT release & synthesis ~

Termination n Must have discrete signal n What if ion channels continually open? l ions move toward equilibrium l membrane no longer polarized l no EPSPs or IPSPs n Disrupts neural communication n Termination ---> neuron repolarizes ~