1. Week 10: Synaptic transmission – Part 1
BIOL3833
Week 10: Synaptic transmission – Part 1
It took us 8 weeks to understand how a single neuron works.
We learned principles that apply broadly throughout the nervous system
Now we move on: How do systems and networks of neurons work?

2. What’s Next? Understanding the functional properties of Neural Systems
Examining the nature of communication between two neurons
Understanding the behavior of neural networks (Not as easy as many once believed)

3. These inputs occur at the synapse

4. Human brain has about 85 Billion neurons
Each cell sends outputs to 100s of other cells
Most cells receive inputs from other cells.
So, there are approximately 75 trillion connections in the human brain

5. Understanding the behavior of neural networks
(Not as easy as many once believed)

6. The idea of the Connectome

7. The STG network (connectome):
26-30 Neurons (depending on species) Connections are known Inputs are known
AB anterior burster PD pyloric dilator LPG lateral posterior gastric LP lateral pyloric
IC inferior cardiac LG lateral gastric MG medial gastric
GM gastric mill PY pyloric constrictor VD ventricular dilator
Int1 interneuron 1 AM anterior median DG dorsal gastric
Stein (2009) J. Comp. Physiol. A

9. Communication between neurons
The point of communication is the synapse
Initial theory (1890s) was that all communication was electrical transmission
1920s - evidence for chemical transmission emerged
By 1950s – acceptance of both types of synapse in the nervous system

10. Easiest way to transmit signal from one cell to another?
Let ions move directly between cells

11. “Gap Junction” or “Electrical” synapses
Advantages
Fast transmission
Disadvantages
Loss of signal strength
Cannot change polarity
Restricted to voltage changes only
Useful for
Synchronizing activity between neurons
Responses that require high speed: For example, escape

12. A more flexible way to transmit signal from one cell to another?
Chemical transmission

13. Presynaptic Events
Action potential invades axon terminal
Activates voltage gated calcium channels
Calcium influx increases probability of vesicle release
Vesicles are transferred to the plasma membrane at active zones
Vesicle fuses with membrane, releasing neurotransmitter
Neurotransmitter diffuses across the synaptic cleft
Transmitter binds to postsynaptic cell (more later)
Transmitter action is terminated by reuptake or enzymatic degradation

14. Postsynaptic events
Transmitter binds to receptor
The receptor
Directly opens ion channel (ionotropic) or
Activates 2nd messenger pathway (metabotropic)
The effects of any neurotransmitter are determined by the properties of the receptor, not the transmitter

16. Chemical synapses Advantages Disadvantages Useful for
Can amplify signals
Can transform signals (e.g., change polarity, duration)
Wide range of effects on postsynaptic cell (excite, inhibit, change metabolism, alter the function of existing ion channels, etc.)
Prolong the effects of the signal
Disadvantages
Slower than gap junctions
Useful for
All kinds of stuff

17. Glutamate receptors are excitatory
Most prevalent excitatory receptors in the CNS
Three general classes: AMPA, NMDA, and Kainate
Numerous variations within each class
AMPA receptors:
Fast, transient currents, permeable to sodium and potassium
NMDA receptors
Slow persistent currents, permeable to sodium, potassium and calcium
Require Glycine as co-transmitter
Magnesium block must be removed for activation

18. Glutamate receptor currents

19. What happens when receptors are permeable to more than one ion?

20. Return of the GHK Equation
The concept of reversal potential
Activating a particular type of receptor will move the membrane toward Ereceptor
So: activating a population of AMPA receptors will pull the membrane potential toward EAMPA
And - Just like with ion channels:
IAMPA = (VM – EAMPA)*gAMPA
What determines whether a receptor is inhibitory or excitatory is its reversal potential
If EREV is above threshold it is excitatory.
If EREV is below threshold it is inhibitory.

21. GABA receptors are inhibitory (usually)
GABAA
GABAB
Why is GABA-A Usually inhibitory? When might it be excitatory? When ECl is above threshold
Ionotropic
Fast
Chloride permeable
Usually inhibitory
2nd Messenger targets ion channels
Slow
Opens nearby K+ channels
Always inhibitory

22. GABA-A
GABA-B

23. Discuss, debate, conclude (10 minutes): How can a receptor that opens chloride channels be excitatory?

24. Discuss, debate, conclude (10 minutes): If activating a receptor depolarizes the membrane does this guarantee that the receptor is excitatory?
The answer is NO:

25. Synaptic simulations: Experimental arrangement
The answer is NO:

26. Your next STEP point: Each table prepares and signs a “Contract for Exam2”
The answer is NO: