Lecture 7 - conduction

Question 1

Why does neuronal conduction speed matter?

Answer 1

• an inactivation gate causes the unidirectional movement of action potential.
• the speed of action potential propagation is determined by how fast the sect segment of membrane gets depolarised to threshold

Question 2

What determines how fast the sect segment of membrane gets depolarised to threshold?

Answer 2

Determined by 2 factors:
- space constant (aka length constant) - how far electrical charges can spread passively
- time constant

Question 3

What is the space constant (length constant)?

Answer 3

how far can current spread passively along the axon

• when positive current enters the axon, it spreads along the axon passively, like current in a wire
• ‘passively’ meant - electrical charges moving according to laws of electricity, not opening or closing of ion channels
• these ideas come from cable theory, first worked out for undersea telegraph cables

SPACE CONSTANT IS A NUMBER THAT DESCRIBES FAR CHARGES WILL PASS PASSIVELY ALONG AN AXON

Question 4

What is the electric current?

Answer 4

the low itself

Question 5

What is voltage?

Answer 5

the force pushing the charge

Question 6

How is an axon like a leaky flow itself?

Answer 6

membrane surrounding axon made of lipids which aren’t conducting, however it does have ion channels?

• resistance, inside & outside cell (not high) - resistance across (high)

Y = square root (rm/ri)

rm = membrane resistance
ri = internal resistance (axial resistance)

Question 7

What is membrane resistance?

Answer 7

current (flowing water) spreads further if the membrane (hose wall) is less leaky

Question 8

What is internal resistance?

Answer 8

current (flowing water) spreads further if there is little resistance to its moving down the axon.

Question 9

What is the space constant equation?

Answer 9

space constant = square root (rm/ri)

Question 10

How does the size of the axon influence the space constant?

Answer 10

• membrane resistance is inversely proportional to the surface area of the membrane (more area, more leaks)
• internal resistance is inversely proportional to the cross-sectional area of the axon (wider hose = leaks)
• membrane resistance depends on the axon’s circumference (circumference = 2piradius)

Upside down Y is dependent of square root (radius)

Question 11

What is time constant?

Answer 11

a cell membrane is both a resistor (current can pass through but not very well) and a capacitor (charge can build up on one side)

Time is described by the time constant

T = rm x cm

Membrane resistance = how unleash is the hose wall

Membrane capacitance = how stretchy is the hose walls

Question 12

What effect does myelin have on membrane resistance & membrane capacitance?

Answer 12

myelin increases membrane resistance & decreases membrane capacitance

Question 13

How is myelin created?

Answer 13

oligodendrocytes/Schwaan cells wrap around axons many times, surround them with many layers of membrane - like a Swiss roll

this creates myelin

• this is like wrapping a wire with insulating tape
• it also increases the distance between the extracellular & intracellular solution. Myelin moves the 2 ‘capacitor’ plates further apart, decreasing capacitance

Question 14

What happens as a result of myelin?

Answer 14

• space constant increases
• time constant stays the same

Question 15

What is the result of an increase in membrane resistance due to myelin?

Answer 15

• longer space constant
• current can spread further down the axons

Question 16

What is the result of a decrease in membrane capacitance due to myelin?

Answer 16

• counteracts the effect of increased membrane resistance on the time constant
• membrane can still charge up as quickly as normal

Question 17

What is the main effect of myelin?

Answer 17

myelinated axons can conduct faster than unmyelinated axons

Question 18

What is the advantage of vertebrates having myelin?

Answer 18

they are larger as they have myelin which allows the nervous system to work faster, as the action potential can travel faster

Question 19

Where are sodium channels put?

Answer 19

in the gaps between myelin

Question 20

Describe the process of saltatory conduction

Answer 20

1. current enters through Na+ channels at a node of Ranvier
2. then depolarisation spreads passively down the axon (this is sped up by longer space constant)
3. at the next node of Ranvier, depolarisation triggers voltage-gated Na+ channels to regenerate the action potential

This is only occurs as the space constant allows passive flow to occur over long distances

Question 21

How does saltatory conduction save energy?

Answer 21

saltatory conduction means Na+ only enters at nodes of Ranvier (instead of all along the axon) –> less work for Na+/K+ to pump to restore Na+ gradient

Question 22

How does myelination save space?

Answer 22

without myelin, you can only increase conduction speed by widening the axon following (upside down y is dependent on the square root of radius)

• to increase speed 10x, have to increase radius by 100x –> axon volume increases by 10,000 times.

Question 23

Why do different axons conduct at different speeds

Answer 23

• myelin
• wide axons are also costly (e.g. they take up space & have proteins)

only myelinate and widen axons that need to carry information very quickly
- squid giant axons (escape reflex)
-myelinated axons: proprioception (our body’s sense of where our limbs are), motor axons

Question 24

What type of axons are unmyelinated?

Answer 24

pain & temperature

Question 25

What is the result of de-myelinating diseases?

Answer 25

impaired neuronal conduction

• the distribution of ion channels is designed with myelin in mind (e.g. voltage-gated Na+ channels are only at the nodes of Ranvier), so if myelin disappears, signals will not travel correctly
• ectopic spokes because of maladaptive homeostatic compensation. This is where the neurone is trying to fix the problem in a way that causes other problems. This could include putting sodium channels in the wrong place

Question 26

What is multiple sclerosis?

Answer 26

• auto-immune disorder, immune system attacks myelin
• episodic: symptoms get worse, then better, then worse etc.
• diverse neurological symptoms, e.g. vision problems, numbness/tingling, muscle spasms/weakness, many others
• symptoms might be worse when under stress or at high temperatures - neuronal conduction is ‘safer’ at low temperatures, because Na+ channels inactivate more slowly

Question 27

What is Guillain-Barre syndrome?

Answer 27

• auto-immune disorder affecting PNS myelin
• symptoms: numbness, tingling, weakness
• patients usually recover because PNS myelin can regenerate (unlike CNS myelin)

Question 28

What does signal propagation depend on?

Answer 28

• internal vs membrane resistance
• membrane capacitance
• thus on axon diameter & myelination (myelination solve space-speed trade-off)

Question 29

What neurons are typically very myelinated?

Answer 29

vertebrate neurones, and only the ones that need to transmit information quickly

Question 30

What makes signal propagation more energy-efficient?

Answer 30

saltatory conduction

Question 31

What does a loss of myelin lead to?

Answer 31

neurological problems our axons aren’t made to work without myelin