Synaptic transmission

Question 1

What are the 2 types of synapses?

Answer 1

1. Electrical synapses: No chemical transduction involved and is common in
   - Escape reflex neurones
   - Cardiac muscle cells
   - Epithelial cells
   E.g. Gap junction - contains protein pores(Connexon - made of 6 subunits) that bridge the gap between two cells, it is large enough to allow substance with a molecular weight of up to 1000 to pass through
2. Chemical synapse: Chemical substance used as intermediate to convert electrical signals in the pre-synaptic cell intro electrical signals in the post-synaptic cells
   - Found in neurones

Question 2

Describe the properties of the following in electrical and chemical synapse

1. Distance between membranes
2. Cytoplasmic continuity
3. Structural unit (s)
4. Transmitter
5. Transmission delay
6. Transmission direction

Answer 2

Electrical vs Chemical

1. 3.5 nm - 20-40 nm
2. Yes - No
3. Gap junction channel - Many (vesicles, docking/fusion proteins, channels and postsynaptic receptors
4. Ionic current - Chemical transmitter
5. No - Yes (1-5msec)
6. Can be bidirectional - Unidirectional

Question 3

What is the criteria for identifying a neurotransmitter?

Answer 3

1. When applied experimentally to postsynaptic membrane -mimics normal transmission
2. Manufactured in the presynaptic cell and stored in its terminals
3. Released when presynaptic neurones is excited
4. Drugs that act post-synaptically to affect normal transmission should affect responses to externally applied candidate neurotransmitter in the same way
5. Should be a mechanism for removing substance from the synaptic cleft (enzyme or uptake) e.g. - removal of Ach in the NMJ

Question 4

Is the following excitatory or inhibitory?
- Ach
- NE
- Dopamine
- Serotonin
- GABA
- Glutamate
- Glycine
- Aspartate
- Substance P
Met-enkephalin

Answer 4

Ach: mostly excitatory to vertebrate skeletal muscles
NE: both
Dopamine: mostly excitatory
Serotonin: generally inhibitory
GABA: Inhibitory
Glutamate: Excitatory
Glycine: inhibitory 
Aspartate: Excitatory
Substance P: Excitatory
Met-enkephalins: generally inhibitory

Question 5

What are the 2 main types of neurotransmitters:

Answer 5

1. Ionotropic: Directly linked to an ion-channels, also called ligand-gated ion channels or
2. Metabotropic: G-protein coupled receptors,not directly liked to an ion channel, use a second messenger
   Ionotropic respond faster

Question 6

What is the active zone?

Answer 6

The pre-synaptic membrane only releases neurotransmitters in the active zone

Question 7

Describe the process of synaptic transmission

Answer 7

1. Action potential propagation in pre-synaptic neurone
2. Electrical depolarisation causes opening of Ca2+ ions
3. Ca2+ flow through the pre-synaptic membrane and therefore increasing the [Ca2+] inside the presynaptic neurone
4. High [Ca2+] activates a set of Ca2+ sensitive proteins attached to vehicles that contain a neurotransmitter
   - Packing a chemical into vesicles, vesicles have to be organised at the membrane therefore a set of proteins that trigger vesicle/membrane fusion have to be primed and triggered.
5. Proteins change shape and dock vesicles to membrane of pre-synaptic cell
   - V-SNARE (attached to vesicle) binds to syntaxin on T-SNARE (found on target membrane protein in pre-synaptic neurone) This all occurs on active zone - dense collection of SNARE proteins
6. Vesicles and protein are recycled through endocytosis ***

Question 8

What is IPSPs and EPSPs?

Answer 8

They are both post-synaptic potentials An excitatory postsynaptic potential (EPSP) is a postsynaptic potential that makes the postsynaptic neuron more likely to fire an action potential. This temporary DEPOLARISATION of postsynaptic membrane potential, caused by the flow of positively charged ions into the postsynaptic cell, is a result of opening ligand-gated ion channels. These are the opposite of inhibitory postsynaptic potentials (IPSPs), HYPER-POLARISATION of post-synaptic neurone which usually results from the flow of negative ions into the cell or positive ions out of the cell. This forms a synaptic potential that makes a postsynaptic neuron less likely to generate an action potential.

Question 9

What is temporal and spatial summation?

Answer 9

Action potentials are non-decremental - therefore do not decrease in size, where synaptic potentials are decremental

Temporal summation of EPSP: Adding together of EPSP’s generated by firing of the same pre-synaptic terminal at high frequency to generate an action potential in the post-synaptic neurone

Spatial summation: Adding together of EPSP’s generated by firing of two or more pre-synaptic neurones simultaneously to generate an action potential in the post-synaptic neurone

Question 10

Describe the following properties of AP, EPSP and IPSP

1. Direction
2. Magnitude
3. Conduction properties (decremental or not)
4. Duration

Answer 10

AP : EPSP : IPSP
More+ depolarisation : More+ depolarisation : More -be depolarisation
All or None with Vrest to +30 : Vrest to threshold : Vrest to Ek or Ecl
Non-decremental : Decremental : Decremental
2-3 msec : 15-20 msec : 15-20 msec

Question 11

Why are there IPSPs?

Answer 11

E.g. is to inhibit motor neurones of antagonistic muscles

Question 12

What is composite postsynaptic potential?

Answer 12

the postsynaptic potential representing the sum of all the postsynaptic potentials generated at all the active synaptic boutons: composite postsynaptic potential