neuroscience - chapter 6 Flashcards
What is synaptic transmission?
-process by which nerve cells (neurons) communicate with each other or with other types of cells, (muscle or gland cells)
-occurs at specialized junctions called synapses
Let’s revisit action potentials…
-where is it triggered?
-where is it initiated?
-where does it travel?
-what does it activate?
- Triggered by electrical inputs from dendrites/soma
- Initiated in axon hillock
- Travels down axon
- Activates synaptic terminals
What are the two types of synapses?
-electrical
-chemical
How are the neurites of two cells connected?
Gap junctions!
Are gap junctions electrical synapses?
yes!
Describe what a gap junction is made of.
-6 connexin make up one connexon
-2 connexon make up one gap junction channel
-many gap junction channels make up gap junction
What can pass through these junction channels?
Ions and small molecules can pass in both directions through these channels.
What do the gap junction channels bridge?
The cytoplasm of the two cells.
How does current pass across the synapse (include chemical and electrical concepts to reason)
-Current does not pass across a chemical synapse (using neurotransmitter)
-Current passes directly across gap junctions
(But, the effect is rather small if the postsynaptic cell is large)
Define an electrical synapse.
A gap junction interconnecting the dendrites of two neurons.
Explain how action potential in one neuron can cause electrical PSP (post-synaptic potential in another neuron).
An action potential generated in one neuron causes a small amount of ionic current to flow through gap junction channels into a second neuron, inducing an electrical post-synaptic potential.
How do gap junctions allow synchronization (oscillations)?
Electrical synapses can help neurons to synchronize their activity. Certain brain stem neurons generate small, regular oscillations of Vm and occasional action potentials. (a) When two neurons are connected by gap junctions (cells 1 and
2), their oscillations and action potentials are well synchronized. (b) Similar neurons with no gap junctions (cells 3 and 4) generate oscillations and action potentials that are entirely uncoordinated.
Are most synapses electrical or chemical?
chemical!
In chemical synapses, what do they use as a messenger and what/how causes a response?
- They use a neurotransmitter (NT) as a messenger
- NT binding to receptors causes a postsynaptic response
Describe the synaptic cleft.
The synaptic cleft is 20–50 nm and filled with a matrix of fibrous extracellular protein which serves as a “glue” that binds the pre- and postsynaptic membranes together.
What does the terminal contain?
The terminal typically contains dozens of small membrane-enclosed spheres (50 nm in diameter) called synaptic vesicles, which store neurotransmitters.
Many axon terminals also contain larger vesicles (100 nm in diameter) called secretory granules, aka dense-core vesicles because they look dark in the electron microscope (contain peptides).
Name the three different synaptic arrangements in the CNS.
- Axodendritic (axon-dendrite)
- Axosomatic (axon-soma)
- Axoaxonic (axon-hillock)
Which arrangement has bigger effect since its closer?
Axoaxonic
Name the two categories of CNS synaptic membrane differentiations.
- Asymmetrical membrane differentiations - Gray’s type I synapse is asymmetrical and usually excitatory.
- Symmetrical membrane differentiations - Gray’s type II synapse is symmetrical and usually inhibitory.
Do larger synapses have more or less active zones.
They have more.
What are the advantages of chemical communication?
- Amplification of the signal
A brief presynaptic action potential can trigger a large postsynaptic response (gap junctions can’t do this) - the effect can only be similar to what pre-synaptic is doing at that time. - Diversity (nuanced communication)
There are many types of transmitters and receptors
- Responses can be excitatory (fire), inhibitory (no fire), or modulatory (regulate efficacy of synapse)
-Responses can be very fast or very slow
-This adds greatly to the complexity of information processing
What is the rule about neuromuscular junctions?
One neuron can target many muscle fibers but only on muscle fiber can be targeted by one neuron.
Define the neuromuscular junction.
It is the synapse between a neuron and a muscle fibre. + very intensely studied (earliest studies)
What do junctional folds do?
The junctional folds contain receptors.
-Allows for N.T. to fall into holes.
-Allows N.T. to have continued and concentrated effect on muscle fibre.
What was the first neurotransmitter to be discovered and that the NMJ uses?
Acetylcholine (ACh)
How is muscle contraction caused?
- Synaptic vesicles contain ACh
- ACh is released into the cleft
- ACh binds with receptors
- This causes muscle contraction
How is a neurotransmitter released?
- The presynaptic AP opens voltage-gated calcium channels at the terminal
- Calcium (Ca2+) enters the terminal
- Ca2+ interacts with fusion proteins that fuse the vesicle with the membrane and release the vesicle contents (exocytosis)
What is the role of Ca2+ in this process?
-calcium is positive
-more outside than inside
-influx to flow in
-key allow vesicles to bind to specialized proteins
What is the only purpose of action potentials in this process?
It is to trigger the opening of voltage-gated Ca2+ channels.
What are the two mechanisms that regulate the amount of neurotransmitter released from a presynaptic neuron, influencing synaptic strength.
Inhibition and Facilitation.
Describe the mechanism of pre-synaptic inhibition.
Presynaptic Inhibition:
-Axo-axonic synapses: These occur when one neuron forms a synapse on the axon terminal of another neuron, modulating its activity.
-Mechanism: The inhibitory terminal reduces the influx of Ca²⁺ into the presynaptic terminal.
-Effect: Less Ca²⁺ influx leads to reduced release of neurotransmitter from the presynaptic neuron.
Describe the mechanism of pre-synaptic facilitation.
-Axo-axonic synapses: These can also mediate facilitation, where an excitatory neuron acts on the presynaptic terminal.
-Mechanism: The excitatory terminal increases Ca²⁺ influx into the presynaptic terminal.
-Effect: More Ca²⁺ influx results in increased release of neurotransmitter, enhancing synaptic transmission.
What are the three ways to terminate the synaptic transmission?
-reuptake
-enzymatic metabolism
-uptake by glial cells (specifically astrocytes)
Give an example.
Example: ACh (Acetylcholine)
● ACh binds to the receptor for only a short time
● ACh can then encounter an acetylcholinesterase molecule
● Acetylcholinesterase degrades ACh (very efficiently)
Define endocytosis.
Endocytosis: reabsorbing excess membrane.
Exocytosis alone would increase the area of the presynaptic membrane
Endocytosis pinches off and recycles membrane
