Module 4

Question 1

What types of ion channels are required for the conduction of an action potential?

Answer 1

Voltage gated Na+ and K+ channels, as well as the leak channels that help set the resting membrane potential.

Question 2

What are voltage gated Na+ and K+ channels activated by?

Answer 2

Depolarization

Question 3

What occurs during the rising phase (depolarization) of an action potential?

Answer 3

• Depolarizing stimuli open voltage gated Na+ channels at -55mV which allow Na+ to travel down the electrochemical gradient.
• At +30mV, the Na+ channels inactivate.

Question 4

What occurs during the falling phase (repolarization) of an action potential?

Answer 4

Voltage gated K+ also open in response to depolarization, but do so more slowly than Na+ channels, causing delayed efflux.

Question 5

What occurs during the after-hyperpolarization phase (undershoot) of an action potential?

Answer 5

• Voltage gated K+ channels do not immediately close when reaching -70mV, causing membrane potential to dip below the resting membrane potential.
• leak channels bring the membrane potential back to -70mV.

Question 6

How do voltage gated Na+ channels suddenly close at the peak of an action potential?

Answer 6

Na+ channels contain two gates: an activation gate and an inactivation gate.

Question 7

Absolute refractory period vs relative refractory period

Answer 7

Absolute refractory period: a second action potential cannot be initiated (1-2ms)
Relative refractory period: a second action potential can be initiated but requires a larger than normal stimulus (2-5ms)

Question 8

What is the purpose of a refractory period?

Answer 8

It ensures that an action potential travels in one direction and limits the rate at which signals can be transmitted down a neuron.

Question 9

How are action potentials conducted? (6 steps)

Answer 9

1. graded potential enters the trigger zone
2. voltage gated Na+ channels open, and Na+ enters the axon
3. positive charge spreads along the adjacent sections of the axon by local current flow
4. local current flow causes a new section of the membrane to depolarize
5. loss of K+ repolarizes the membrane
6. the refractory period prevents backward conduction

Question 10

What two physical parameters determine the velocity of action potentials in mammalian neurons?

Answer 10

1. the diameter of the axon: larger diameter will offer less internal resistance to current flow, and more ions will flow in a given time, bringing adjacent regions to threshold faster.
2. the resistance of the axon membrane to ion leakage: current will spread to adjacent sections more rapidly if it is not lost via leak channels.

Question 11

Where do neurons communicate?

Answer 11

Neurons communicate at synapses: presynaptic cell (neuron) to postsynaptic cell (neuron, muscle, target cell)

Question 12

How many synapses can a postsynaptic neuron contain?

Answer 12

Up to 150 000 synapses on its dendrites, soma, and even on the axon and axon terminals.

Question 13

What do the majority of neurons in the nervous system use for cell-to-cell communication?

Answer 13

Chemical signals.

Question 14

What is a neurocrine? What are the three types?

Answer 14

A neurocrine is a chemical substance released from neurons and used for cell-to-cell communication.
The three types are: neurotransmitters, neuromodulators, and neurohormones.

Question 15

What is a neurotransmitter? What does it do?

Answer 15

A neurotransmitter is a chemical that is released, acts on a postsynaptic cell in close vicinity, and causes a rapid response in the postsynaptic cell.

Question 16

What is a neuromodulator? What does it do?

Answer 16

A neuromodulator is a chemical that is released, acts on a postsynaptic cell in close vicinity, and causes a slow response in the postsynaptic cell.

Question 17

Where are neurohormones secreted? What do they do?

Answer 17

Neurohormones are secreted into the bloodstream and act on targets throughout the body.

Question 18

What are the two categories of neurocrine receptors?

Answer 18

Ionotropic receptors (ligand gated ion channels) and metabotropic receptors (G-protein coupled receptors).

Question 19

What happens when ligands bind to ionotropic receptors?

Answer 19

Ligand binding to ionotropic receptors causes conformational changes leading to the opening of a channel.
Mediates a fast postsynaptic responses.

Question 20

What happens when ligands bind to metabotropic receptors?

Answer 20

Ligand binding to metabotropic receptors leads to a g-protein mediated cellular response: interacting directly with ion channels or activate membrane bound enzymes.

Question 21

Where are large peptide neurotransmitters produced and packaged?

Answer 21

Produced and packaged into vesicles at the soma and then transported (fast axonal transport).

Question 22

Where are small neurotransmitters synthesized and packaged?

Answer 22

Synthesized and packaged at the axon terminal (empty vesicles transported from the soma or recycled vesicles).

Question 23

How are enzymes for synthesis brought to the terminal?

Answer 23

Via slow axonal transport.

Question 24

How are neurotransmitters released?

Answer 24

Release occurs via Ca^2+ mediated exocytosis.

Question 25

What does neurotransmitter binding depend on?

Answer 25

Size and concentration in the cleft.

Question 26

Why is neural signalling of short duration?

Answer 26

Neural signalling is of short duration due to the rapid removal or inactivation of neurotransmitters in the synaptic cleft.

Question 27

What is convergence?

Answer 27

Multiple presynaptic neurons providing input to a smaller number of postsynaptic neurons.

Question 28

What is divergence?

Answer 28

A small number of presynaptic neurons branching to affect a larger number of postsynaptic neurons.

Question 29

When does spatial summation occur?

Answer 29

Occurs when the currents from multiple nearly simultaneous graded potentials combine.
It can be inhibitory.

Question 30

What is temporal summation?

Answer 30

The summation of graded potentials in a postsynaptic neuron from the same presynaptic neuron.

Question 31

What does presynaptic inhibition do?

Answer 31

Inhibits neurotransmitter release.

Question 32

What does presynaptic facilitation do?

Answer 32

Increases neurotransmitter release.

Question 33

Global presynaptic inhibition vs selective presynaptic inhibition

Answer 33

Global presynaptic inhibition: all targets of the postsynaptic neuron are inhibited equally.
Selective presynaptic inhibition: an inhibitory neuron synapses on one collateral of the presynaptic neuron and selectively inhibits one target.

Question 34

How can synaptic activity be altered?

Answer 34

By changing the target cell’s responsiveness to neurotransmitter