Synapses Flashcards
Diagram showing how chemical transmission across synapse links two associated neurons
Three types of synaptic arrangement
Axodendritic synapse
Axosomatic synapse
Axoaxonic synapse
The neuromuscular junction - diagram
Diagram of synapses on dendritic spine
Pie cut into chemical synapse
The ___ contains vesicles that store and secrete neurotransmitters
Presynaptic axon terminal
Microscopy of presynaptic cell
What are the four steps of chemical synaptic transmission
- Synthesis and packaging of neurotransmitter
- Release of neurotransmitter
- Action on postsynaptic cell
- Termination of signaling
Where are neurotransmitters synthesized?
In the terminal button
The enzymes that are necessary to synthesize the neurotransmitter are found in the ___
Cytosol of the terminal button
What happens to the neurotransmitter after it’s synthesized?
It’s packaged into vesicles (against their concentration gradient)
Active zones
Sites of neurotransmitter release on presynaptic side
Postsynaptic density
Structure that contains receptors that turn the chemical signal (NT) into electrical signal (AP)
Different types of neurotransmitters
What are the three main families of neurotransmitters?
- Amino acids: glutamate, GABA, glycine
- Amines: Catecholamines (dopamine –> norepinephrine (NE) –> epinephrine; monoamines (serotonin, acetylcholine)
- Peptides: Enkephalins, endorphins, dynorphins, substance P, neuropeptide Y, CCK, VIP
Amino acid neurotransmitters
Catecholamines
- Dopamine
- Norepinephrine/noradrenaline
- Epinephrine/adrenaline
Monoamines
- Serotonin
- Acetylcholine
In the synthesis of catecholamines, the precursor is ___
Amino acid tyrosine
Monoamine synthesis:
1. Synthesis of ___ is multi-enzymatic (more than one enzyme)
2. Synthesis of ___ only requires one enzyme
- Seratonin
- Acetylcholine
Peptide neurotransmitters
Molecules that have an important function in the brain but aren’t classified as neurotransmitters
What are the criteria for classification as a neurotransmitter?
- Is biochemically synthesized
- The molecule is stored/sequestered in the cell
- Stimulation (electric) causes release of molecule
- Molecule causes cell to respond (change in Vm)
- System to terminate action of molecule
Tyrosine hydroxylase (TH) enzyme
- Facilitates the synthesis of L-dopa and is:
1. the rate-limiting step of catecholamine synthesis
2. the target of end-product inhibition (inhibited by dopa and norepinephrine)
Steps of catecholamine synthesis
Tyrosine –> L-dopa (dopa) –> dopamine (DA) –> norepinephrine (NE) –> epinephrine
Mnemonic for catecholamine synthesis:
TLDNE: Too Long Did Not Explain
Tyrosine L-dopa Dopamine Norepinephrine Epinephrine
Peptides and their synthesis
Peptides are tiny proteins that follow a different sequence of synthesis than NTs.
1. Ribosomes string amino acids together at rough ER
2. Golgi apparatus processes and splits apart peptide
3. Secretory granules transport peptide to axon terminal where it is then stored
Dense core vesicles/secretory granules
Peptides are transported into larger, darker vesicles called dense core vesicles/secretory granules
What molecule is the major source of neurotransmitters?
Amino acids (there are many exceptions)
Packaging of neurotransmitters (diagram shows GABA)
- There needs to be an energy source to package the neurotransmitters in vesicles at high concentrations (movement against conc gradient)
- A lot of neurotransmitters are packaged using a concentration gradient (H+ goes down its gradient, coupled with movement of neurotransmitter down vesicle transporter)
Dale’s principle
One neuron synthesizes and releases one specific type of neurotransmitter (serotonergic neurons = serotonin)
There are a few exceptions
Exceptions to Dale’s Principle
- Many peptide-containing neurons can release a neurotransmitter along with a peptide
- Certain axon terminals can release 2 or more neurotransmitters simultaneously. These neurotransmitters are called co-transmitters
What does NT release at the axon terminal depend on?
Voltage-gated calcium channels
What is the purpose of AP propagation to the axon terminal?
- To open VG Ca2+ channels –> Ca2+ influx is necessary and sufficient for NT release
- Like other VG channels, VG Ca 2+ channels open when the cell depolarizes → after opening, very high driving force on Ca 2+ → Ca2+ rushes into the cell
How is peptide release different from NT release?
- At the axon terminal
- This is because dense-core vesicles are far away, so more calcium is needed to trigger release
- This release requires more AP
What are vSNAREs?
Proteins in the vesicle involved in docking to the presynaptic membrane at the synapse
tSNAREs
On the presynaptic membrane (axon terminal), this interacts with vSNAREs on the vesicle to help anchor the vesicle to the presynaptic membrane
What is synaptotagmin?
- A protein found in the SNARE complex
- Calcium sensing protein (calcium bindding to synaptotagmin initiates the process of vesicle fusion)
Process of NT release at the axon terminal
- AP depolarizes terminal membrane
- VG calcium channels open
- Calcium enters and binds to synaptotagmin
- Synaptotagmin triggers the SNARE complex to change
conformation, allowing for vesicle fusion and release of
NT
Action of nicotine at synapse
Binds to nicotinic acetylcholine receptors
What is a gap junction?
Specialized connection between two adjacent cells that allows for direct passage of ions
Diagram of gap junction
Connexons
- In an electrical synapse, 6 connexins form up a connexon, and 2 connexons form a gap junction channel
- Electrical synapse allows electrical current to pass between neurons through gap junctions
Features of gap junctions
With gap junctions, cells:
- Can be said to be electrically coupled
- Have faster transmission
One neurotransmitter can act on multiple receptors, like glutamate can affect ___, ___, and ___
AMPA, NMDA, and kainate receptors
Neuromuscular junction
The place at which the motor neuron terminates onto the motor end-plate of the muscle fiber
Otto Loewi confirmed the ___
Existence of chemical synaptic transmission (through NTs)
Describe Otto Loewi’s experiment and what it revealed
- Used a frog’s heart
- Electrical stimulation of the vagus nerve causes the heart rate to slow
- Connected two frog hearts via solution
- Stimulation of the vagus nerve of the first heart → beating of the second heart also slows
- Revealed that some chemical substance mediated change in beating!
***This chemical substance was identified to be acetylcholine
Describe two features of ionotropic receptors/ligand-gated channels
- Have a ligand (a neurotransmitter) that opens the receptor
- Are faster but have a more transient effect on the postsynaptic cell than G-protein-coupled receptors
Which is faster: ligand-gated channel or G-protein coupled receptor?
Ligand-gated channel
Unlike chemical synapses, electrical synapses are ___
Bidirectional
Of the following neurotransmitters, which pair is most likely to be found together in the same neurons?
a) dopamine and epinephrine
b) acetylcholine and serotonin
c) serotonin and GABA
d) GABA and norepinephrine
Dopamine and epinephrine (both catecholamines)
Stimulation of the vagus nerve causes the release of acetylcholine (ACh) in the heart, which slows down the heart rate through activation of ___ ACh receptors
Muscarinic
IPSPs can be caused by opening ___ channels
Potassium
NMDA is an ___ for glutamate receptors
Agonist
A miniature postsynaptic potential is the postsynaptic response to ___
A single vesicle of neurotransmitter
GABA is an inhibitory neurotransmitter because it causes ___
Entry of Cl- into neurons (causes hyperpolarization)
In which of the following neurotransmitter pairs is the second NT made from the first by a single enzyme?
a) dopamine, epinephrine
b) Glutamate, GABA
c) Acetylcholine, serotonin
b) Glutamate, GABA
Peptide NTs - examples
Enkephalins, endorphins, dynorphins, substance P, neuropeptide Y, CCK, VIP
Peptide NTs are stored in
Secretory/dense-core vesicles
Where are amine and amino acid neurotransmitters synthesized?
In the axon terminal
Where are peptide NTs synthesized?
In the soma
A miniature postsynaptic potential is the postsynaptic response to
a) a single presynaptic potential
b) a single action potential in the postsynaptic neuron
c) a single vesicle of neurotransmitter
d) the opening of a single transmitter-gated ion channel
c) a single vesicle of neurotransmitter
What property do gap junctions and chemical synapses have in common?
a) an action potential in one neuron can cause an action potential in a postsynaptic neuron
b) ions can flow directly from one neuron to another
c) communication across the synapse is bidirectional
a) an action potential in one neuron can cause an action potential in a postsynaptic neuron
Which neurotransmitter is synthesized from glutamate?
a) ACh
b) GABA
c) Glycine
b) GABA
Most excitatory synapses have a postsynaptic site at a ___
Dendritic spine
