Mcb 80 - Lecture 6 Quiz Questions

Front 1

What is the relation between volts, amps and ohms?

Back 1

Volts = amps * ohms

Front 2

In neurobiology what does “conductance” mean?

Back 2

Conductance is the inverse of resistance; causes a rise in the conductance of Na (increase in gNa), meaning more Na channels are open, allowing it to flow into the cell

Front 3

Define action potential threshold.

Back 3

Depolarization of the membrane beyond a certain point needs to occur in order for the action potential to fire; usually about 10-15 mV away from resting potential

Front 4

Define overshoot and undershoot.

Back 4

Overshoot: The part of the action potential where the inside of the neuron is positively charged with respect to the outside; result of increased conductance for Na+ during rising potential
Undershoot: Falling phase (after-hyperpolarization); result of conductance for K+ increasing and Na+ decreasing during falling potential

Front 5

Define absolute and relative refractory periods.

Back 5

Absolute refractory period: from end of falling phase to lowest point of undershoot
Relative refractory period: from lower point of undershoot back to resting potential

Front 6

What kinds of stimuli initiate action potentials?

Back 6

Generator potential resulting form the entry of Na+ depolarizing the membrane

Front 7

How does positive feedback play a role in the rising phase of an action potential?

Back 7

Regenerative; depolarization (as a result from an external stimulus via synapse/receptor potentials/local potentials) opens Na channels which causes greater depolarization which, in turn, opens more Na channels and etc.

Front 8

What is a delayed rectifier channel and how does it participate in the action potential?

Back 8

K+ channel because it brings the membrane back to resting potential; Potassium channels require more depolarization
(increase in membrane potential) before they move from an open state to a
closed state, so that is why potassium channels are delayed with respect to
sodium.

Front 9

Briefly describe the structure of a voltage gated Na+ channel.

Back 9

Contains 4 domains (I-IV) & each membrane domain has 6 transmembrane alpha-helixes (S1-S6); - S4 has a lot of positively charged amino acids; acts as the voltage sensor (when inside is less negative it causes the positive charges to move outward, causing the unfolding of the channel); Also contain pore loop which forms the walls of the channel itself
Once the channel is opened allowing Na to enter, the channel then shuts off because it is inactivated by a plug

Front 10

What is the difference between a closed Na+ channel and an inactivated Na+ channel?

Back 10

Inactivated is just plugged; The only way to remove the plug is to bring the membrane potential back down to very negative values; Repolarization to Vrest de-inactivates the channel & channel is closed

Front 11

How do TTX and lidocaine affect the function of the nervous system? (Be specific.)

Back 11

The binding site of this toxin is located at the pore opening of the voltage-gated Na+ channel.
Lidocaine blocks the Na pores from inside

Front 12

Why are there so many kinds of voltage-sensitive channels?

Back 12

Many different kinds of voltage-sensitive channels mean many different ways to modulate a signal - amplify it, attenuate it, delay it, change it from local signal to far-traveling action potential, add multiple signals, inhibit signals in a brief or lasting way, etc; having many different ways to modulate a signal is useful for doing many computations

Front 13

How does an action potential alter the membrane potential along an axon (voltage versus distance)?

Back 13

The area just behind the action potential (the part of the axon where the action potential has just been) is hyperpolarized (very negative) because the action potential has just occurred and now those locations are probably in their refractory period. The area where the action potential is right now is very depolarized (very positive) because that is what happens
during an action potential - a big depolarization. The area in front of the action potential is at resting potential

Front 14

What are the passive cable properties of an axon?

Back 14

Capacitance (difference in charge between the inside and outside), resistance (channels open or closed), and axial resistance

Front 15

How do the axon’s cable properties attenuate signals?

Back 15

A big capacitance along the membrane reduces the speed at which an action potential can travel down the axon because the attraction of ions on either side of the membrane to each other, preventing them from moving quickly down the axon. Low membrane resistance means a leaky membrane - this is caused by many open ion channels that draw current away from the action potential and reduce the speed and distance the action potential can travel down the axon. Even if no current left the action potential through the membrane, there would still be resitance "axially" (as the action potential moves down the long axis of the axon) because current can't move through the axon infinitely fast... the axon is of a limited diameter, there are fluids and other matter there to bump into, etc.

Front 16

How and why does axon caliber affect conduction velocity?

Back 16

If the axon is wide and there are few open membrane pores, most of the current will flow down the inside of the axon rather than excaping across the membrane. The farther the current goes do n the axon, the farther ahead of the action potential the membrane will be depolarized, and the faster the action potential will propagate

Front 17

How does myelin affect cable properties?

Back 17

Myelin reduces membrane capacitance (there is a thick insulating wall between the inside and outside of the cell, reducing the interactions of ions on either side of the membrane) and also increases the membrane resistance (there are no sodium channels in myelinated areas for ions to leak out of).

Front 18

How and why does myelin affect conduction velocity?

Back 18

Allows action potential to skip from node to node (salutatory conduction) increasing the velocity; myelin acts as capacitors in series so very low membrane capacitance and resistors in series so very low membrane resistance

Front 19

What are nodes of Ranvier and why are they necessary?

Back 19

Breaks in the myelin sheath; voltage-gated sodium channels are concentrated in the membrane of the nodes