Module 3 Lecture 6 - Synapses

Question 1

What is a synpase(s)?

Answer 1

A junction between axon terminals of a pre-synaptic neuron and the dendrites of a post-synaptic neuron

Axon terminal of a pre-synaptic cell sends chemicals via synapses to the dendrites of a post-synaptic cell

Dendrites and cell body can receive up to 100k signal inputs

Question 2

What are the different types of synapses? What’s an example?

Answer 2

1. Electrical (pre and post-synaptic cells joined by gap junctions)
2. Chemical (Neurotransmitters)

Question 3

What are components of a chemical synapse?

Answer 3

• Pre-synaptic neuron
• Axon terminal
• Synaptic cleft
• Post-synaptic neuron
• Dendrites, cell body

**Synaptic Knob: Location of the synaptic vesicles that contain neurotransmitters ** (act as chemical messengers)

Question 4

Can an action potential spread from a pre-synaptic neuron to a post-synaptic neuron?

Answer 4

No because of the synaptic cleft; physical gap that prevents the direct propagation of an action potential.
- However this means a chemical signal (neurotransmitter) can jump the gap

Hence why at the end of a pre-synaptic neuron, a new graded potential is required for a new action potential to occur in the post-synaptic neuron

Question 5

How are neurotransmitters released into the synaptic cleft?

Answer 5

1. Neurotransmitters are stored in small vesicles encased in lipid bilayer membrane, but before being activated they (vesicles) are stored in the synaptic knob
   -Neurotransmitters synthesized are waiting for action potential to let calcium in and therefore themselves get released
2. Neurotransmitter release is initiated when an action potential reaches the presynaptic axon terminal

=> Calcium voltage gated channels on the synaptic knob region
=> Calcium influx triggers fusion of synaptic vesicles to the presynaptic docking site and release of NT by exocytosis

Question 6

Describe what is happening in the picture?

Answer 6

• Action potential is received by the axon terminal which activates calcium voltage gated channels to allow calcium to enter the pre-synaptic knob
• This causes the release of NT via exocytosis into the synaptic cleft

Not necessarily the electrical gradient pushing Ca2+ in, rather its the overwhelming concentration gradient that pushes the calcium in.

Question 7

Before calcium is activated, what are the vesicles holding the NT doing?

Answer 7

Before calcium is activated, the vesicles are loosely docked to the active zones (pre-synaptic knob region) of the docking site by interacting with SNARE Proteins

Snare proteins are anchored on both the vesicle and the membrane of the axon terminal

Question 8

How do the vesicles in the docking site release their NT?

Answer 8

Once calcium is activated it binds to Synaptotagmin which causes a conformational change of the SNAREs that were loosely binded to the vesicle
* Vesicles will be bound by the SNARE proteins which causes the membrane to fuse and release the NT

Pre-synaptic membrane is fusing to the axon terminals membrane

Question 9

What are subsynaptic receptors?

Answer 9

Receptors found on the dendrites or cell body of the next neuron

NT bind to these receptors in order to allow ions to move in or even move out of the cell

Question 10

What can subsynaptic receptors be?

Answer 10

• Ionotropic receptors (NT binds to receptor and allows ions to move in) - A.K.A. Ligand gated ion channels
• Metabotropic receptors = where the receptor can indirectly act on an ion channel via secondary messengers

Question 11

Are subsynpatic receptors the same as voltage gated channels?

Answer 11

No; these ion channels are distinct from voltage gated ion channels and are called ligand gated ion channels