neuroscience - chapter 6

Question 1

What is synaptic transmission?

Answer 1

-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

Question 2

Let’s revisit action potentials…
-where is it triggered?
-where is it initiated?
-where does it travel?
-what does it activate?

Answer 2

• Triggered by electrical inputs from dendrites/soma
• Initiated in axon hillock
• Travels down axon
• Activates synaptic terminals

Question 3

What are the two types of synapses?

Answer 3

-electrical
-chemical

Question 4

How are the neurites of two cells connected?

Answer 4

Gap junctions!

Question 5

Are gap junctions electrical synapses?

Answer 5

yes!

Question 6

Describe what a gap junction is made of.

Answer 6

-6 connexin make up one connexon
-2 connexon make up one gap junction channel
-many gap junction channels make up gap junction

Question 7

What can pass through these junction channels?

Answer 7

Ions and small molecules can pass in both directions through these channels.

Question 8

What do the gap junction channels bridge?

Answer 8

The cytoplasm of the two cells.

Question 9

How does current pass across the synapse (include chemical and electrical concepts to reason)

Answer 9

-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)

Question 10

Define an electrical synapse.

Answer 10

A gap junction interconnecting the dendrites of two neurons.

Question 11

Explain how action potential in one neuron can cause electrical PSP (post-synaptic potential in another neuron).

Answer 11

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.

Question 12

How do gap junctions allow synchronization (oscillations)?

Answer 12

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.

Question 13

Are most synapses electrical or chemical?

Answer 13

chemical!

Question 14

In chemical synapses, what do they use as a messenger and what/how causes a response?

Answer 14

• They use a neurotransmitter (NT) as a messenger
• NT binding to receptors causes a postsynaptic response

Question 15

Describe the synaptic cleft.

Answer 15

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.

Question 16

What does the terminal contain?

Answer 16

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).

Question 17

Name the three different synaptic arrangements in the CNS.

Answer 17

1. Axodendritic (axon-dendrite)
2. Axosomatic (axon-soma)
3. Axoaxonic (axon-hillock)

Question 18

Which arrangement has bigger effect since its closer?

Answer 18

Axoaxonic

Question 19

Name the two categories of CNS synaptic membrane differentiations.

Answer 19

1. Asymmetrical membrane differentiations - Gray’s type I synapse is asymmetrical and usually excitatory.
2. Symmetrical membrane differentiations - Gray’s type II synapse is symmetrical and usually inhibitory.

Question 20

Do larger synapses have more or less active zones.

Answer 20

They have more.

Question 21

What are the advantages of chemical communication?

Answer 21

1. 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.
2. 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

Question 22

What is the rule about neuromuscular junctions?

Answer 22

One neuron can target many muscle fibers but only on muscle fiber can be targeted by one neuron.

Question 23

Define the neuromuscular junction.

Answer 23

It is the synapse between a neuron and a muscle fibre. + very intensely studied (earliest studies)

Question 24

What do junctional folds do?

Answer 24

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.

Question 25

What was the first neurotransmitter to be discovered and that the NMJ uses?

Answer 25

Acetylcholine (ACh)

Question 26

How is muscle contraction caused?

Answer 26

• Synaptic vesicles contain ACh
• ACh is released into the cleft
• ACh binds with receptors
• This causes muscle contraction

Question 27

How is a neurotransmitter released?

Answer 27

• 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)

Question 28

What is the role of Ca2+ in this process?

Answer 28

-calcium is positive
-more outside than inside
-influx to flow in
-key allow vesicles to bind to specialized proteins

Question 29

What is the only purpose of action potentials in this process?

Answer 29

It is to trigger the opening of voltage-gated Ca2+ channels.

Question 30

What are the two mechanisms that regulate the amount of neurotransmitter released from a presynaptic neuron, influencing synaptic strength.

Answer 30

Inhibition and Facilitation.

Question 31

Describe the mechanism of pre-synaptic inhibition.

Answer 31

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.

Question 32

Describe the mechanism of pre-synaptic facilitation.

Answer 32

-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.

Question 33

What are the three ways to terminate the synaptic transmission?

Answer 33

-reuptake
-enzymatic metabolism
-uptake by glial cells (specifically astrocytes)

Question 34

Give an example.

Answer 34

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)

Question 35

Define endocytosis.

Answer 35

Endocytosis: reabsorbing excess membrane.

Exocytosis alone would increase the area of the presynaptic membrane
Endocytosis pinches off and recycles membrane