Chapter 5: Synaptic Transmission Flashcards
Goes with Action potential so study that too!
Overall Process
What is the overall process of synaptic transmission?
- Nuerotransmitter releases across synaptic cleft
- Bind to receptors on the post -synaptic side on the dendrites & soma
- When it binds, a ligand- gated ion channel will either open or close
- EPSPs and IPSPs constantly go into the cell or neuron and then it will be summed to see if we can initiate an action potential once again!
Chemical —> Electrical —-> Chemical
A Brief History
Compare and describe the differences between the reticular theory and the neuron doctrine in the sense of electrical and chemical transmission…
**Reticular Theory (Golgi)
*** Flow of signals was purely electrical and there was a continuos flow
Neuron Doctrine (Cajal)
* ****Current would flow out into the extracellular spce
* A chemical signal (neurotransmitter) is relseased by the cell sending the signal to the (presynaptic cell)
* Does something to the cell recieveing the signal.
Both were right :)
A Brief History
What are synapses and does it have to be between two neurons?
Synapses is the functional connection between two cells and it can be between 2 neurons or even between a neuron and a muscle cell.
A Brief History
In what way does the information flow?
Unidirectional. It goes from pre synaptic to post synaptic.
There are exceptions to this but for now this is okay.
A Brief History
Describe the Loewi Experiement
Gave us the first evidence of pure synaptic chemical transmission.
* He put two frogs hearts into seperate solutions but the first still had the vagus nerve attached. He used a tube to connect both solutions with the hearts in it.
* The heart rate of heart one slowed and then as the solution was transferred to heart 2, the heart rate also slowed.
Electrical Synapses
Describe electrical synapses
- Electrical synapses allows direct transfer of ionic current from one cell to the next.
- Occurs at the neuronal gap junctions which are found in several brain regions.
- There are connexons on both pre-synapctic and post-synapctic sides and they bring the cells very close in contact.
Electrical Synapses
What are gap junctions channels made up of?
- Formed by 2 connexons
- A connexon is formed by 6 connexins
Electrical Synapses
What exactly do gap junctions do?
* They allow the flow of ions from cytoplasm of one cell to cytoplasm of another cell
* Allows bi-directional flow of ionic current
* Very Fast transmission
Ex. flies are really fast b/c of this.
Chemical Synapses
Describe the pre-synaptic & post-synaptic sides
Presynaptic Side
* Axon terminal
* Neurotransmitter vesicles & secretory glands
* Active zones: sites of neurotransmitter release
**Postsynaptic Side
* **Post Synaptic Denstiy: Contains Neurotransmitter receptors
Chemical Synapses
How wide is a synaptic cleft?
20-50 nm wide
Chemical Synapses
Compare electrical vs. chemical synapses
Electrical
* Gap junctions connect cytoplasm of pre & postsynaptic cell
* Bidirectional
*** Ionic current flows through gap junctions channnels that bridge pre and post synaptic cells **
* If post synaptic cells is depolarized then it causes an action potential
Chemical
* Neurons seperated by synaptic cleft
* No current crosses from pre to post
* **Action Potential leads to release of chemical transmitter that diffuses across synaptic cleft **
* Transmitter interacts with receptors on postsynaptic cell»_space;> depolarization or hyperpolarization.
Chemical Synapses
You are recording from a pre-and post synaptic neuron. You measure a post-synaptic potential (PSP) in the presynaptic neuron and almost instantly see an PSP in the post-synaptic neuron . This is an example of a chemical synapse.
T/F
False
Has to be an electrical synapse because a chemical synapse isn’t that fast.
Properties of Chemical Synapses
What are the types of CNS synapses?
- Axodendritic: Axon to dendrite
- Axospinous: Axon to dentritic spine
- Axosomatic: Axon to axon
- Axoaxonic: Axon to Axon <— can cause more or less neurotransmitters to be released b/c nerve terminals of neurons are touching each other.
Properties of Chemical Synapses
What are autoreceptors and what are their purpose?
Autoreceptors are presynaptic receptors sensitive to neurotransmitter released by the presynaptic terminal.
* **located outside active zone and far from post synaptic density so if a neurotransmitter gets close to it then there is too many being released. **
* When too many neurotransmitters are released, they inhibit it and functions as a safety valve (negative feedback) by informing neuroterminal to stop releasing neurotransmitters.
Two different types:
* Terminal (located on the nerve terminal)
* Somatodendritic (located on soma or dendrites)
Properties of Chemical Synapses
Compare and contrast Gray’s type 1 to Gray’s type 2
Gray’s type 1: Asymmetrical ; usually excitatory
Gray’s type 2: Symmetrical; usually inhibitory
Properties of Chemical Synapses
What are archetypal synapse: Nueromuscular Junctions (NMJ)
- Chemical synapses exist between motor neurons of the spinal cord and skeletal muscle - the NMJ
- Established the principles of synaptic transmission
- One of the largest synapses in the body
- Not all synaptic junctions exists in the CNS**
The Process of Chemical Synaptic Transmission
Give a brief overview of Chemical Synaptic Transmission
- Neurotransmitter/peptide synthesis
- Packaging into synaptic vesicles
- Vesicles fusions
- Neurotransmitter release into synaptic cleft
- Binding to post synaptic receptors
- Biochemical/electrical response elicited in postsynaptic cell
- Removal of neurotransmiter from synaptic cleft
Neurotransmitters
What is the classical criteria to determine whether a chemical is a neurotransmitter?
- The substance must be present in the presynaptic terminal
- The substance must be released in response to presynaptic depolarization and the release must be calcium-dependent.
- Specific receptors for the substance must be present on the postsynaptic cell.
Neurotransmitters: Synthesis & Storage
What must neurotransmitters be in order to be ready for release?
Synthesized and readied.
* Nuerons have many neurotransmitters that are abundant.
* Each neuron contains specialized enzymes required to synthesize the NT in the nerve terminal.
Neurotransmitters: Synthesis & Storage
What are vesicular transporters?
**They are in the vesicular membrane and concentrate NTs into synaptic vesicles **
* Small, clear-core vesicles
* Packing into synapctic vesicles protects NT from breakdown by enzymes in presynaptic terminal.
Neuropeptides: Synthesis & Storage
What are neuropeptides?
Synthesized in the ER, split in the Golgi, and bud off into a secretory granule.
* Large, dense-core vesicles
* Precursor proteins ar broken down by enzymes during shipment to axon terminals - peptide is “liberated”
* No replenished very quickly
Neuropeptides: Synthesis & Storage
Neurotransmitters vs. Neuropeptides
- NP Release: Greater depolarization needed
- Vesicle Size: Small clear vs large dense core vesicles.
Neurotransmitters: Release
What is Ca2+ mediated exocytosis and how does it work?
- When a wave of depolarizatio reaches the axon terminal»>voltage -gated Ca2+ channels open.
- Neurotransmitter release is depended on the influx of calcium into pre- synaptic terminal to allow synaptic vesicles or secretory granules to fuse to pre-synaptic membrane and release their contents into the synaptic cleft.
* Ca2+ sensitive proteins causes the vesicles to fuse with the cell membrane, releasing neurotransmitters
Neurotransmitters: Release
What are SNARE Proteins and what do they do?
SNARE Proteins bring two membranes close together.
* Synaptobrevin in synaptic vesicle membrane
* Syntaxin & SNAP -25 in plasma membrane
