Module 3 - Lecture 2 - Synapses and Postsynaptic Cells - Review Questions

Question 1

Understand electrical synapses and how the AP propagates

Answer 1

Electrical synapses are a minority of synapses in the nervous system and the current flows through gap junctions, which are areas where the membranes of two communicating neurons come extremely close to one another and are linked together. Passive flow electrical current from one neuron to another occurs through unique channels called connexons.

Question 2

What are some of the differences between chemical and electrical synapses?

Answer 2

Chemical synapses require the manufacturing, storing and release of neurotransmitters which are released into the space between the two communicating neurons called the synaptic cleft. This process is slower than electric synapses, which are very fast.

Electrical synapses can have bidirectional transmission – allows electrical synapses to synchronize electrical activity among populations of neurons.

Question 3

What are the 3 key steps for a chemical synapse?

Answer 3

PRESYNAPTIC NEURON = In the presynaptic neuron, a neurotransmitter needs to be manufactured and released in response to an action potential.
POSTSYNAPTIC NEURON = The postsynaptic neuron needs to recognize and respond to the presence of the neurotransmitter. The chemical signal needs to be translated into an electrical signal.
NEUROTRANSMITTER = Neurotransmitter needs to be removed from the synaptic cleft and repackaged for later use.

Question 4

What are the 3 fundamental steps for chemical synapses on the presynaptic side?

Answer 4

1. Neurotransmitter needs to be produced using precursors
2. The neurotransmitters need to be packaged and stored
3. The vesicles then fuse(a process called exocytosis) to the cell membrane in response to the influx of calcium and are released into the synaptic cleft.

Question 5

What are the ingredients for neurotransmitter synthesis?

Answer 5

Precursors and enzymes need to be available in the presynaptic terminal to allow the synthesis of the neurotransmitter to occur.

Question 6

What are the major steps needed in releasing a neurotransmitter?

Answer 6

1. Action potential reaches the end plate of the presynaptic neuron.
2. Voltage gated calcium channels open
3. Calcium enters into the presynaptic terminal
4. Vesicles fuse to the cell membrane and release their neurotransmitters

Question 7

What is a quanta and what dictates the number of quanta released?

Answer 7

Quanta = the amount of neurotransmitter released by a single action potential. Dictated by:

1. The number of vesicles fused depends on ….
2. How much calcium enters the terminal, which depends on…
3. How many calcium channels open, which depends on…
4. The SIZE = frequency/duration of the depolarizing current.

Question 8

What is a typical response of the post-synaptic structure?

Answer 8

Following exocytosis, transmitters diffuse across the synaptic cleft and bind to specific receptors on the membrane of the post-synaptic structure. The binding of neurotransmitters to the receptors causes channels in the post-synaptic membrane to open, thus changing the ability of ions to flow across the membrane and change the polarity.

The post-synaptic structure needs to be able to recognize and respond to the presence of the neurotransmitter (i.e. have the right receptors (lock and key model)).

Question 9

What happens to the membrane potential of a post-synaptic structure what is it called in neurons and muscles?

Answer 9

The membrane becomes more positive and increases the likelihood an action potential is going to be elicited. This is called an end plate potential in a muscle cell and a post-synaptic potential in a neuron.

Question 10

What is the name of a post-synaptic potential that causes a neuron to become more positive and negative?

*Do these responses happen in a specific part of the nervous system?

Answer 10

In the CNS the response depends on the receptor:

Excitation = pushes the postsynaptic cell membrane positively towards initiating an action potential. * This is in response to receptor binding is depolarization.

Inhibition = pushes the postsynaptic cell negatively away from the threshold for producing an action potential. *This is in response to receptor binding is hyperpolarization.

Question 11

What is the ultimate goal of post-synaptic potential?

Answer 11

To change the membrane potential of the post-synaptic neuron to convey information.
To reach threshold and to propagate the action potential.

Question 12

How does the muscle membrane potentials differ from neurons (how is the NMJ different than other synapses)?

Answer 12

Muscle membrane potentials differ because their resting potential is higher than neurons. Also, muscle membrane potentials spread inward through the muscle fiber rather than just down the axon.

Question 13

What is meant by the “safety-factor” of the neuromuscular junction?

Answer 13

The safety factor of transmission refers to:

1. The abundance of acetylcholine released per nerve impulse
2. The excessive amount of acetylcholine receptors on the post synaptic muscle membrane

which results in an action potential almost all the time.

Question 14

What are the different ways a neuron stops chemical synaptic transmission?

Answer 14

Not all neurotransmitters are terminated in the same way:

1. Broken down with a specific enzyme (for example acetylcholine)
2. Re-uptake into the neuron or glial cells
3. Diffuse away from its concentration gradient and go into the bloodstream

*The neurotransmitter MUST be stopped in some way or it will continue to act on the postsynaptic neuron (or cell)