neuronal conduction and myelin

Question 1

2 determinants of speed of conduction

Answer 1

how fast next segment of membrane is depolarised to the threshold

1. space constant (length constant)
2. time constant

Question 2

space constant - what does it represent

Answer 2

how far current spreads passively along axon before it decays to a certain fraction of initial value

Question 3

space constant - equation

Answer 3

λ = sqrt [rm/ri]
space constant = sqrt of membrane resistance / internal resistance (axial resistance)

Question 4

space constant - impact of membrane resistance (rm) and internal resistance (ri)

Answer 4

membrane resistance = current spreads further if membrane is less “leaky” - less channels to leak ions out e.g. potassium

internal resistance = current spreads further if there is little resistance to it moving down the axon

Question 5

space constant - rm and ri relation to surface area

Answer 5

membrane resistance is inversely proportional to SA of membrane — more area = more channels for “leaks” of ions

internal resistance is inversely proportional to SA of axon — wider axon = current travels more easily

membrane resistance depends on axons CIRCUMFERENCE

internal resistance depends on axons AREA

Question 6

space constant - relationship with axon size

Answer 6

space constant is proportional to sqrt of axons radius

therefore, wider axons have longer space constant

Question 7

time constant - what does it represent

Answer 7

how long it takes the membrane to “charge up”

Question 8

time constant - equation

Answer 8

τ = (rm)(cm)
time constant = membrane resistance x membrane capacitance

Question 9

time constant - resistance and capacitance of membranes

Answer 9

resistance = current can pass through membrane but not very well - resistance due to “leaky” membrane with channels
more resistance = less “leaky”

capacitance = charge can build up on one side of the membrane, creating voltage (potential difference) - due to “stretchy” membranes

Question 10

myelination - what cells?

Answer 10

oligodendrocytes (CNS) and Schwann cells (PNS)

Question 11

myelination - effect on resistance

Answer 11

blocks channels so membrane is less “leaky” and therefore increases membrane resistance

Question 12

myelination - effect on capacitance

Answer 12

increases distance between intra and extra cellular solutions and therefore decreases capacitance

Question 13

myelination - effect on time constant

Answer 13

T = (rm)(cm)
myelin increases rm and decreases cm so time constant stays the same = takes the same time to “charge up” the membrane

Question 14

myelination - effect on space constant

Answer 14

increases - current can spread further along the axon passively

Question 15

saltatory conduction

Answer 15

gaps in myelin = nodes of Ranvier
saltatory = conduction jumps between nodes as Na+ enters for depolarisation which spreads passively down axon (sped up by longer space constant)

Question 16

why is myelin and saltatory conduction is beneficial

Answer 16

faster
saves energy = Na+ only enters at nodes so less work for sodium potassium pump to restore the Na+ gradient
saves space = only other way to increase speed is widening axon - space constant is proportional to sqrt of radius - to increase speed 10x means 100x bigger radius meaning a 10,000x increase in volume

however:
myelin is costly for energy - therefore only myelinate and widen axons where info needs to go super fast (e.g. squids giant axons due to escape reflexes)
myelinated = proprioception - location and movement of body parts - motor axon
unmyelinated = pain, temperature

Question 17

demyelinating diseases - impact of them

Answer 17

impair neuronal conduction
distribution of ion channels is made for myelination so without it signals cannot travel correctly - may decay before next channels at node; may travel more slowly; or maybe only strong signals may get through
ectopic spikes - maladaptive homeostatic compensation

Question 18

demyelinating diseases - multiple sclerosis (ms)

Answer 18

autoimmune disorder which attacks myelin
episodic - symptoms in waves
diverse symptoms - vision, numbness, muscle spasms - can very due to stress, high temperatures (neuronal conduction is “safer” at low temps due to Na+ channels inactivating more slowly so signal has more time to propagate)

Question 19

demyelinating diseases - Guillain-Barre syndrome

Answer 19

autoimmune disorder which attacks myelin in PNS
symptoms = numbness, tingling, weakness
can usually recover as PNS myelin can regenerate unlike CNS myelin