Neuronal conduction

Question 1

What channels open for the downstroke of an action potential?

Answer 1

Voltage gate K+ channels

Question 2

What is the speed of action potential determined by?

Answer 2

How fast the next segment of the membrane takes to get depolarised to the threshold
- Determined by space constant and time constant

Question 3

What is the space constant?

Answer 3

How far a current can passively spread along the axon
- How far a current spreads before it decays to a certain fraction of initial value

Question 4

What is the space constant equation?

Answer 4

Space constant = membrane resistance / internal axial resistance

Question 5

Define internal resistance

Answer 5

Resistance to current flowing along the length of an axon

Question 6

Define membrane resistance

Answer 6

Indication of how much current leaks out of the membrane

Question 7

What is the relationship between membrane resistance and surface area of the membrane?

Answer 7

Membrane resistance is inversely proportional to the surface area of the membrane

Question 8

What is the relationship between internal resistance and cross sectional area?

Answer 8

Internal resistance is inversely proportional to the cross sectional area of the axon

Question 9

What do internal and membrane resistance depend on?

Answer 9

Internal - area
Membrane - circumference

Question 10

If an axon is narrow, what will the space constant be?

Answer 10

Short

Question 11

What is a capacitor?

Answer 11

Two plates with a gap between them
- Charge can build up on one side and create a voltage

Question 12

What is capacitance?

Answer 12

The store of how much charge can push against the membrane

Question 13

What is time constant?

Answer 13

A measure of how long it takes the membrane to “charge” up

Question 14

What is the effect of myelin on membrane resistance and membrane capacitance?

Answer 14

Increases resistance
Decreases capacitance

Question 15

How does myelin decrease capacitance?

Answer 15

It increases the distance between the extracellular and intracellular fluid

Question 16

What is the equation for time constant?

Answer 16

T = rm x Cm
T = time constant
rm = membrane resistance
Cm= membrane capacitance

Question 17

How does myelin increase the time constant?

Answer 17

Myelin increases membrane resistance so longer space constant so current is able to spread further down the axon

Question 18

What type of animals have myelin?

Answer 18

Vertebrates

Question 19

Describe saltatory conduction.

Answer 19

Myelin doesn’t wrap the entire axon ->
Non myelinated zones are called nodes of ranvier ->
A current enters through Na+ channels at the node of ranvier ->
Depolarisation spreads passively down the axon ->
At the next node of ranvier, depolarisation triggers voltage gated Na+ channels to regenerate the action potential.

Question 20

Why does saltatory conduction save energy?

Answer 20

Sodium only enters at the nodes of ranvier, therefore there is less work for the Na+/K+ pump to restore the Na+ gradient

Question 21

What sense (and the axons used) requires myelinated axons?

Answer 21

Proprioception
Motor axons

Question 22

What sensations don’t require myelinated axons to form a response?

Answer 22

Pain
Temperature

Question 23

Why do demyelinating diseases impair neuronal conduction?

Answer 23

Myelin removal reduces the propagation of the action potential
- less membrane resistance and less space constant

Question 24

Name a demyelinating disease

Answer 24

Multiple sclerosis

Question 25

Name a disease that affects PNS myelin

Answer 25

Guillain Barre syndrome

Question 26

Name the symptoms of multiple sclerosis

Answer 26

Vision problems
Numbness, tingling
Muscle spasms/weakness

Question 27

When may symptoms worsen in multiple sclerosis? Why?

Answer 27

Under high stress or at high temperatures
- Neuronal conduction is safer at lower temps due to Na+ channels inactivating more slowly

Question 28

What are the symptoms of Guillain Barre syndrome?

Answer 28

Numbness
Tingling
Weakness

Question 29

Which type of myelin can be regenerated?

Answer 29

PNS myelin