L8-Neurons

Question 1

What does the refractory period prevent?

Answer 1

The backflow of AP.

Ensures UNIDIRECTIONAL propagation of the AP. Limits the frequency of APs.

Question 2

2 types of refractory periods:

Answer 2

absolute and relative

Question 3

Why does backward current flow not re-excite previously the active area of a membrane during an action potential?

Answer 3

Because the previously active area is now in its refractory period. No backflow.

Question 4

During which time is the particular membrane section undergoing and AP in its absolute refractory period?

Answer 4

The absolute refractory period lasts the entire time from opening of the voltage-gated Na+ channel’s activation gates at THRESHOLD, through closure of the inactivation gates at PEAK of AP, until the return to RESTING POTENTIAL when activation gate closes and inactivation gate opens once again.

Question 5

Why is there a relative refractory period following an action potential? Aka, why is a stronger than normal stimulus required to generate a second action potential?

Answer 5

The relative refractory period occurs due to 2 lingering effects:

1) lingering inactivation of Na+ channels
2) slowness to close of the K+ channels

During this time, fewer than normal Na+ channels are in a position to be jolted open by a triggering event. Simultaneously, K+ is still leaving the cell in a hyperpolarization event.

Question 6

Does a stronger stimulus generate a larger action potential?

Answer 6

No, but it does generate a greater number of action potentials

Question 7

Myelination ______ the speed of conduction of action potentials

Answer 7

myelination INCREASES the speed of conduction of action potentials

Question 8

Myelin consists primarily of ______ molecules

Answer 8

Lipid

Question 9

Is the myelin coating an insulator or a conductor?

Answer 9

Insulator

Question 10

Myelin-forming cells in:

a. CNS
b. PNS

Answer 10

a. oligodendrocytes

b. Schwann cells

Question 11

Tetrodoxin and saxitoxin do what?

Answer 11

Tetrodoxin and saxitoxin are neural toxins which inhibit the function of voltage-gated Na+ channels and thus block APs

Question 12

Dendrodoxin

Answer 12

Dendrodoxin is a neural toxin which inhibits voltage-gated K+ channels and thus blocks APs

Question 13

3 basic parts of a neuron

Answer 13

a. cell body
b. dendrites
c. axon

Question 14

Define “nerve”

Answer 14

a bundle of AXONS outside of the CNS

Question 15

Define “fiber tract”

Answer 15

a bundle of AXONS inside the CNS

Question 16

Define “axon hillock”

Answer 16

• Neuron’s trigger zone

- first portion of axon + region of the cell body where axon originates

Question 17

3 functional classes of neurons:

Answer 17

1. efferent neuron
2. interneuron
3. sensory neuron

Question 18

3 structural classes of neurons

Answer 18

1. multipolar neuron:
   - single axon, multiple dendrites.
   - integrates lots of information
   - majority of neurons in CNS
2. bipolar neuron:
   - 2 extensions.
   - specialized sensory neurons (ex: retina nerves, vestibular nerve)
3. unipolar neuron:
   - only has one axon or dendrite.
   - common in insects.

Question 19

2 methods of propagation:

Answer 19

1. contiguous conduction

2. saltatory conduction

Question 20

How does AP spread in contiguous conduction?

Answer 20

• AP spreads along every patch of membrane down the length of the UNMYELINATED axon.
• AP waveform triggers new APs in adjacent areas

Question 21

Limitations of contiguous conduction?

Answer 21

• AP only affects membrane for short distances (lost through leaky channels)
• Depolarization to threshold takes time
• AP must be regenerated at each point along axon, which takes time

Question 22

Where are the Na+ and K+ channels located on the axon?

Answer 22

Nodes of Ranvier

Question 23

Does saltatory conduction occur on unmyelinated or myelinated fibres?

Answer 23

MYELINATED

BEESH

Question 24

T/F: saltatory conduction is 50X faster than conduction on unmyeinated fibres? Why is this true/false?

Answer 24

True. Because myelin consists of lipids and acts as an insulator, preventing outward movement of electricity.

Question 25

Which type of conduction carries the most urgent types of info?

Answer 25

Saltatory conduction, because it’s faster.

Question 26

Why is it important to have fast nerve impulses?

Answer 26

• life-or-death escape from predators
• quick reaction to injury (ex: burn)
• mating dances/intricate animal behaviours

Question 27

What determines speed of conduction?

Answer 27

1. How easily electric charge can move down axon
2. how well the axon can hold on to the charge to reduce recharge time

INCREASE CONDUCTION SPEED EITHER BY DECREASING INTERIOR RESISTANCE OR BY DECREASING TRANS-FIBER CAPACITANCE, OR BOTH.

Question 28

Axonal gigantism effect on conduction

Answer 28

• resistance drops with the square of the diameter
• increases conduction speed in proportion to the squared root of the interior diameter
• easy solution, but takes up space

Question 29

Myelin effect on conduction

Answer 29

insulation by lipid sheath INCREASES TRANSVERSE RESISTANCE and REDUCES THE TRANSVERSE CAPACITANCE of immediately adjacent internodal membrane.
ie. reduces current flowing through internodal surfaces and speeds internode charging