Chapter 8

Synapse

• - Presynaptic Neuron
• neuron 1
• "sending neuron
• prior to synapse in question
• * Action Potential moves down axon and reaches threshold*
• -Postsynaptic Neuron
• neuron 2
• receives information from presynaptic neuron
• "receiving neuron"

Electrical Synapses

• - tunnels between two cells, allow ion transmission from one cell to the other
• - allow direct ion sharing therefore.... allow direct transmission of graded potentials
• - also known as gap junctions
• -potential to activate multiple neurons
• - ions shared and graded potentials generated in one neuron and shared with neighboring neurons
• - "internal synchronization"- multiple action potentials by only starting with activating one cell
• - rapid transmission

Chemical Synapses

• - slower communication process than electrical synapses
• - no direct connection (gap)
• - uses neurotransmitters (to cross the gap and to communicate with other cells)
• - 1) presynaptic region
• voltage- gated calcium ion channels
• synaptic vesicles
• SNAREs
• - 2) neurotransmitters
• - 3) postsynaptic region
• receptors/ion channels
• 4) mechanisms of neurotransmitter removal from synaptic cleft

Chemical Synapses: Pre-synaptic region

• -Voltage-gated calcium ion channels
• calcium starts to activate systems within the axon terminal
• once action potential reaches axon terminal switch from voltage-gated sodium to voltage gated calcium --> calcium flows in to power intracellular communication
• calcium not used from electrical stand point but used form a chemical standpoint
• - Synaptic Vesicles
• packets of neurotransmitters
• constant amount of neurotransmitters in each packet
• vesicles prepackage neurotransmitters
• start to migrate toward axon terminal as calcium flows in

Chemical Synapses: Pre-synaptic region

• -SNAREs
• little proteins embedded within the protein at the end of the axon terminal
• capture neurotransmitter vesicles
• release of neurotransmitter into the synapse when calcium flows in
• increase the efficiency of neurotransmitter release
• - botox injections
• work by destroying SNAREs in motor neurons
• new SNAREs are made
• inhibits contraction

Chemical Synapses: Neurotransmitters

• - Function
• messengers
• bind to receptor
• never enter postsynaptic cell (not inhibitory or excitatory)
• - release
• need Ca2+ flowing, decrease gradient (flux) to opprn channel
• *voltage- gated Ca2+ channel
• -types

Chemical Synapses: Post-synaptic Region

• - Receptors/Ion Channels
• ligand-gated ion channels
• - What is the result of opening ion channels on the post-synaptic membrane?
• causes graded potentials

Chemical Synapses: Mechanisms

• - Mechanisms of neurotransmitters removal from synaptic cleft
• - after neurotransmitters have been released and receptor binding has occurred, the neurotransmitters must be rapidly removed from the synapse
• 1) re-uptake by pre-synaptic neuron (most common
• - release then vacuum back up
• 2) enzymatic degradation
• 3) removal by support/glial cells
• 4) diffusion out of synapses
• ^^ must happen/ neurotransmitters must be removed quickly

The Big Picture of a Chemical Synapse

• 1) neurotransmitter molecules are synthesized and packaged in vesicles
• 2) An action potential arrives at the presynaptic terminal
• 3) Voltage-gated Ca2+ channel open. Ca2+ enter.
• 4) A rise in Ca2+ triggers fusion of synaptic vesicles with the presynaptic membrane
• 5) Transmitter molecules diffuse across the synaptic cleft and bind to specific receptors on the postsynaptic cell.
• 6) Bound receptors activate the postsynaptic cell.
• 7) A neurotransmitter breaks down, is taken up by the presynaptic terminal or other cells, or diffuses away from the synapse.

Synaptic Contacts

• 1) Divergence
• increase signal strength
• does NOT decrease intensity of action potentials
• 2) Convergence
• post-synaptic neuron receives signals from multiple sources
• 6000- 8000 sources contacting neuron (ALOT)
• reaching -55mv threshold easy with multiple contacts

Slide 11

• - spatial summation= multiple pre-synaptic neurons covering
• - temporal summation= pre-synaptic neuron rapidly signals post-synaptic neuron
• - one pre-synaptic neuron --> multiple graded potentials