L8 Synapses and Neurotransmitters - Pt 1 Flashcards
What is a synapse?
A synapse is a junction between two neurons allowing signals to pass from one to the other.
What is the name of the process of signaling via synapses?
Synaptic transmission
How many synapses and neurons does your brain have?
Your brain has ~100 trillion (10^14) synapses, compared to ~100 billion (10^11) neurons
How are electrical synapses formed and what does this allow them to do?
They are formed of gap junction the allows passage of current directly between neurons.
What is the approximate diameter of a gap junction?
Gap junction diameter - 1 to 2 nm (Approx.)
What is connexin/ connexon?
They are building blocks of gap junctions
What is a gap junction?
A gap junction is essentially a tiny tunnel that directly connects the inside of two cells. These tunnels are formed by proteins called connexins, and they allow for the rapid exchange of ions, small molecules, and signaling molecules between the cells.
What are electrical synapses good for? (2)
- Fast Communication
- Synchronising neurons
How did Otto Loewi unravel the presence of chemical synpases?
He demonstrated using two frog hearts that nerves release a chemical which slows the heartbeat.
1. Stimulate vagus in donor heart
2. Heart rate slows in donor heart
3. Remove fluid sample in donor heart
4. Add the fluid sample in recipient heart.
5. Heart rate slows in recepients heart.
Chemical synapse affected heart rate
What is a prototypical chemical synpase?
A prototypical chemical synapse is a specialized junction through which neurons communicate with each other or with non-neuronal cells (like muscle cells or gland cells) using chemical messengers called neurotransmitters.
What could be a post-synaptic cell?
The post-synaptic cell could be another neuron, or a non-neuronal cell:
* motor neuron -> skeletal muscle
* autonomic neuron -> hormonal gland, smooth muscle, or heart
What are the steps in chemical synaptic transmission? (5)
Slide 12
1)Package neurotransmitters in vesicles, put them at the pre-synaptic terminal
2) Action potential arrives voltage-gated Ca2+ channels open
3) Ca2+ influx vesicles fuse to membrane, neurotransmitters released
4) Neurotransmitters diffuse across the synaptic cleft, activate receptors on the postsynaptic cell further signaling
5) Neurotransmitters are removed from the cleft
What are dense-core secretory granules?
Dense-core secretory granules are small, membrane-bound sacs within cells that store and release a variety of molecules, including hormones, neuropeptides, and neurotransmitters.
What are the 4 differences between Synaptic vesicles and dense-core secretory granules?
Synapti vesicles -
1. Clear , small 40 - 50nm
2. Small moleclue transmitters
3. Filled by transporter proteins at the presynaptic terminal.
4. Recycled by endocytosis.
Dense core secretory granules:
1. Dense, large 100nm
2. Peptide neurotransmitters
3. Created and filled by the ER/Golgi secretory apparatus.
4. One and done.
How was the influx of calcium at presynaptic terminals recorded?
It was recorded using a calcium - sensitive fluorescent protein. The brother it is, the high concentration of calcium influx
What happens to the vesicles after fusing to the memebrane?
Vesicles is recycled through endocytosis, absorbed in pre synapse.
What is SNARE?
They are a large proteins that are essential for vesicle fusion, a process by which membrane-bound vesicles (containing neurotransmitters or hormones) fuse with another membrane (like the cell membrane or another organelle membrane) to release their contents.
What happens when Ca2+ binds to synaptotagmin?
A conformational change makes the SNAREs ‘zipper’ together, forcing the vesicle to fuse to the plasma membrane
How do neurotransmitters affect the post synaptic neuron?
Neurotransmitters affect the postsynaptic neuron by binding
SNAREs are targets for….?
SNAREs are targets for toxins (botulinum toxin, tetanus toxin)
What other cellular processes require vesicle fusion?
Insulin secretion
What happens when the neurotransmitters bind to the ligand - gated ion channels (ionotropic receptors)?
When neurotransmitter binds to ligand- gated ion channels, it directly depolarises or hyper-polarises the postsynaptic cell
Does the neurotransmitter itself go into the post synaptic cell?
NO IT DOES NOT
How are neurotransmitters removed?
1) They diffuse away
2) They are actively taken up by transporters for recycling (into the presynaptic neuron or glia)
3) They are destroyed in the synaptic cleft by enzymes
What are the differences between electrical vs chemical synapses?
- Electrical - Signals pass in both directions but C passes in one direction
- E: Signals are passed directly, can only be attenuated. C: Signals can be radically transformed (inverted, amplified, modulated)
- : Fast (<0.3 ms , Chemical: Slower (0.3-5ms)
What does NMJ stand for?
Neuromuscular junction
What is a neuromuscular junction?
It’s a specialized synapse, meaning it’s a site where a nerve cell communicates with another cell (in this case, a muscle cell).
Its primary function is to transmit signals from the nervous system to muscle fibers, triggering muscle contraction.
Give me 3 features of neuromuscular junction
- Fast and reliable neurotransmission
- Motor neuron action potentials always cause muscle cell action potentials
- Uses the neurotransmitter acetylcholine
How does the NMJ achieve such efficient transmission?
Presynaptic:
Large number of active zones
Postsynaptic (motor end-plate):
Contains junctional folds, densely filled with neurotransmitter receptors
How did we figure out that neurotransmitters are released from vesicles?
Bernard Katz and his colleagues performed electrophysiological studies at the neuromuscular junction.
They discovered that neurotransmitter release occurred in discrete “packets” or quanta. This quantal release strongly suggested that neurotransmitters were released from pre-packaged units, which aligned with the vesicle hypothesis.
What does each quantum represent?
Each ‘quantum’ is one vesicle full of neurotransmitter
Name the 5 varieties of CNS synapses.
- Dendrosomatic synapses
- Dendrodentritic synapses
- Axoaxonic synapses
- Axodendritic synapses
- Axosomatic synapses.
What is the primary role of junctional folds?
The primary role of junctional folds is to maximize the surface area available for acetylcholine receptors (AChRs). This ensures that a large number of AChRs are present to receive the neurotransmitter acetylcholine.
