Neurons II- Lecture 5

Question 1

Describe the structures in a chemical synapse

Answer 1

Presynaptic neuron:
o Axon terminal with presynaptic membrane;
o Vesicles, filled with neurotransmitter (NT) molecules;
o Ca++ ion channels

Synaptic cleft between pre- & post-synaptic neuron

Postsynaptic neuron:
o Postsynaptic membrane (e.g., at dendritic spine)
o Various ion channels

Question 2

Describe the processes in a chemical synapse

Answer 2

Presynaptic neuron:
1. Action Potential arrives → Ca++ channel open;
2. Ca++ ions enter axon terminal → vesicles fuse with presynaptic membrane → releasing NT molecules into synaptic cleft

Synaptic cleft:
NT-molecules
diffuse across

Postsynaptic neuron:
NT bind with ion channels at receptor site, → ion channels open; ions enter postsynaptic neuron
-> generating post-synaptic potential (PSP)

NT molecules removed from receptor site → channels close

Question 3

What is Depolarization?

Answer 3

1. Voltage-gated ion channels open in response to a change in membrane potential.

Examples:
K+-channel and Na+-channels in axon hillock & axon, Ca++ channels in membrane of axon terminal;

All of these respond to a positive
charge on the inside of the membrane (depolarisation)

Question 4

Why do different channels respond to different neurotransmitters?

Answer 4

All ion channels in the post-synaptic membrane are transmitter-gated

-But different channels respond to different neurotransmitters
-Because they have differently-shaped receptor sites Like a key into a lock, NT molecules only fit into specific receptor sites, therefore:

Type of neurotransmitter determines which type of channel opens
Type of channel determines which type of ion can enter/leave the cell
Type of ion (positive or negative) determines how the cell behaves next

Question 5

Describe Excitation, inhibition and generator potential

Answer 5

Part 3. Closing the Loop – Excitation, Inhibition, & Generator Potential
1. Excitation and Inhibition:

Excitatory synapse: positive ions enter (depolarisation)
-new AP becomes more likely -> Inhibitory synapse: negative ions enter (hyperpolarisation) -> new AP becomes less likely

Whether a synapse is excitatory or inhibitory depends on the type of
o NT released by the axon terminal
o ion channel & receptor present in the post-synaptic membrane

Remember: For an AP to be triggered, the membrane potential at the axon hillock must
depolarise beyond -50 mV

Question 6

Describe Post-Synaptic summation

Answer 6

Post-synaptic summation:

Electrochemical changes in post-synaptic neurons triggered by a single AP insufficient to
generate new AP

Generator potential build-up in post-synaptic cell is slow and graded (unlike AP!)
GP integrates (‘sums’) changes caused by several APs at one post-synaptic neuron:
-Temporal summation: Combines PSPs occurring in rapid succession;
-Spatial summation: Combines PSPs from different synapses (of one post-synaptic neuron)

Question 7

Explain the Addendum: Neurotransmitter Removal (degradation and re-uptake)

Answer 7

 Neurotransmitters do not change when they bind to a receptor;
 have to be actively removed to stop their influence on post-synaptic cell.
 Two types of NT removal:

Degradation:
- Special enzymes in the synaptic cleft break down (inactivate) NTs
- Components are (partly) ‘recycled’ to make new NTs

Re-uptake:
- Receptor molecules at pre-synaptic axon terminal take up NTs
- return them into pre-synaptic cell