Module 3: Lecture 4

Question 1

what is the refractory period?

Answer 1

• concept in action potential
• refers to the fact that you are unable to generate a second action potential until the first one has completed
• there is two general phases

Question 2

what are the two general phases of the refractory period?

Answer 2

1. absolute refractory period: you cannot generate a second action potential
2. relative refractory period: you can generate a second action potential because sodium channels are closed and capable to opening- they have been reset

Question 3

why can a second action potential not happen until the first one is complete?

Answer 3

because the first action potential is already using all the voltage gated sodium ion channels so if you receive that input during the upward depolarization stage, there is nothing more you can do
- they are all fully open or just recently inactivated so they cannot open more or no longer allow sodium to enter the cell because the inactivation gate is blocking the pore and it will not open again

Question 4

why is it known as the relative refractory period?

Answer 4

because you can generate an action potential but requires a strong input stimulus
- occurs during hyperpolarization

Question 5

why does the stimulus need to be stronger in order to generate an action potential in the relative refractory period?

Answer 5

because relative reflection period generates an action potential in the hyperpolarization phase. the hyperpolarization phase is at about -80mV of membrane potential and a stimulus generally causes a change of -15mV which will only bring the mV to -65mV when the threshold potential is -55mV

Question 6

what is “one-way propagation”?

Answer 6

prevents the signals from going backwards

Question 7

what is critical to ensure one way propogation?

Answer 7

the structure and absolute refractory period of action potential

Question 8

what directions does nerves communicate?

Answer 8

unilateral directions
- this is because of the absolute refractory period

Question 9

when the action potential is completed, the resting membrane potential is restored through _________ _________.

Answer 9

leak channels
- they establish your resting membrane potential by allowing sodium and potassium ions to re equilibrate

Question 10

how fast is the restoration of concentration gradients after an action potential?

Answer 10

rapid
- almost the same amount of time as the depolarization period of the action potential

Question 11

how do we restore the original concentrations of Na+ and K+ after they are altered across the membrane from action potentials?

Answer 11

through Na+/K+ ATPase pump

Question 12

what counterbalances the rate of passive leakage from the channels?

Answer 12

the sodium-potassium pump

Question 13

how many sodium ions are pumped out of a cell for every 2 potassium ions pumped into the cell from the sodium potassium pump?

Answer 13

3 sodium ions

Question 14

what type of transport does the sodium potassium pump use?

Answer 14

• primary active transport
• requires energy (ATP) for providing the energy required for the conformational change of the transporter (‘ATPase’ pump)

Question 15

why is the transport from the sodium potassium pump NOT electrically neutral?

Answer 15

there is a net transfer of 1 positive charge OUT of the cell for each molecule of ATP hydrolyzed (makes the inside of the cell more negative)
- pump is therefore critical for restoring the normal chemical concentration

Question 16

how do the sodium potassium pump work?

Answer 16

• transmembrane proteins starts by facing its active site which is intracellularly
• have 3 binding sits for sodium(high-affinity) and two binding sites for potassium ions(low affinity)
• phosphorylation of the protein pump is causes a change to face it extracellularly and decreases its affinity for sodium and increases its affinity for potassium then reverts back to its original form when phosphate it removed

Question 17

what is the mechanism for the long distance cellular communication?

Answer 17

action potential

Question 18

how should a signal be transmitted?

Answer 18

from one cell to another, along a specific pathway

Question 19

what are the four zones of a nerve cell?

Answer 19

1. input zone
2. trigger zone
3. conducting zone
4. output zone

Question 20

what is the input zone of our nerve cells?

Answer 20

where the cell receives all of its communication from other nerves
- includes dendrites (mostly) AND cell body

Question 21

where are all of your other nerves going to contact with that specific nerve and communicate with it?

Answer 21

the input zone

Question 22

what is the trigger zone of our nerve cells?

Answer 22

where action potentials are initiated because they have a high concentration of voltage gated sodium and potassium channels
- includes the axon hillock

Question 23

where are the graded potentials occuring?

Answer 23

in the input zone

Question 24

in the nerve cell, where does the action potential get initiated?

Answer 24

the axon hillock in the trigger zone

Question 25

where does the action potential go in the nerve if it is generated?

Answer 25

it will travel down the conducting zone which is essential the cell or the nerves axon

Question 26

what is the conducting zone in the nerve?

Answer 26

part that conducts action potential often over long distances
- includes the axon

Question 27

where does the action potential terminate?

Answer 27

the output zone

Question 28

what is the output zone of a cell?

Answer 28

part that releases a neurotransmitter that influences other cells
- includes the axon terminals

Question 29

once generated at the axon hillock, will the action potential be conducted with further stimulation?

Answer 29

no. no more further input and it will be able to be at the same magnitude throughout
- NOT decremental unlike graded potentials

Question 30

what are the two methods that action potentials travel down the axon?

Answer 30

1. contiguous conduction
2. saltatory conduction

Question 31

what is contiguous conduction?

Answer 31

propagation of the action potential along every patch of membrane down the length of the axon

Question 32

how does an action potential travel across an axon through contiguous conduction?

Answer 32

the voltage gated channels are letting tons of sodium into the cell into an area and causes it to disperse across the cell. once enough sodium has dispersed into a new part of the axon and has reached threshold potential, it causes the voltage sodium ion gated channels to open in this new area as well and cause action potential. this continues the whole axon down. the previous active area now returns to resting potential

Question 33

once an action potential is initiated in one part of a nerve cell membrane, a ________________________ is initiated so that the action potential is propagated along the rest of the fibre __________.

Answer 33

1. a self-perpetuating cycle is initiated
2. automatically

Question 34

does an action potential travel along the membrane?

Answer 34

no. instead it triggers an identical new action potential in the adjacent area of the membrane

Question 35

what prevents action potential from being reinitiated in the previously inactive area since sodium travels in both directions in contiguous conduction?

Answer 35

the inactivation gate of sodium channels not being able to respond again due to the absolute refractory period so that there is only “one-way propagation” and sodium travels one way

Question 36

what is preventing backwards propagation?

Answer 36

the absolute refractory period

Question 37

what is saltatory conduction relevant to?

Answer 37

myelinated fibres

Question 38

what is a myelinated axon?

Answer 38

it has an insulating sheath running down the entire length of the cell axon

Question 39

how does saltatory conduction travel across an axon?

Answer 39

jumps from node of ranvier to node of ranvier

Question 40

what is a node of ranvier?

Answer 40

a gap in the insulating sheath of an axon that allow the cell membrane to rather than have to excite the entire axon, it only initiated the action potential at very key areas at these regular intervals along this myelin sheath

Question 41

is myelin sheath hydrophillic?

Answer 41

no, hydrophobic
- has a lot of phospholipids so it is a really strong insulator

Question 42

what are the components of a myelin sheath?

Answer 42

• 80% phospholipids
• 20% proteins

Question 43

what does myelin sheath help with?

Answer 43

increasing the conduction velocity of signals in a more efficient manner

Question 44

in the peripheral nervous system, myelination occurs due to?

Answer 44

Schwann cells in which the individual cells wrap around the axon creating that insulating sheath, and it relieves these very regular intervals known as the nodes of ranvier

Question 45

what is the length of the nodes of ranvier?

Answer 45

2 microns and occur about every 1mm

Question 46

at what location is the axon exposed to the extracellular fluid?

Answer 46

nodes of ranvier
- this is where all the voltage gated channels are going to concentrate