[15.4] speed of the nerve impulses

Question 1

what 3 factors affect the speed at which an AP travels?

Answer 1

• myelin sheath
• diameter of the axon
• temperature

Question 2

how does the myelin sheath affect the speed AP travels?

Answer 2

• myelin insulates axon so ions are prevented from going in / out
• saltatory conduction means nerve impulse can jump from one NOR to another so speed of conductance is faster in myelinated than unmyelinated

Question 3

how does the diameter of the axon affect the speed AP travels?

Answer 3

• greater diameter = faster speed of conductance
• less SA relative to volume so less leakage of ions
• leakage makes membrane potentials harder to maintain, thus affecting AP

Question 4

how does temperature affect the speed AP travels?

Answer 4

• temp affects rate of diffusion of ions, high temp = faster nerve impulse
• energy for active transport comes from ATP, which is produced in respiration, which is controlled by enzymes
• enzymes function more rapidly at higher temps to a certain point, after which protein channels denature so impulses fail to be conducted

Question 5

why are nerve impulses described as ‘all-or-nothing’ responses?

Answer 5

• stimulus needs to be of a certain magnitude to establish an AP
• anything below TV will not generate AP so no impulse
• increasing stength of stimulus does not increase strength of AP

Question 6

how can an organism perceive the size of a stimulus if there is an AONR?

Answer 6

• more frequent APs
• having different neurones with different TVs

> brain intereprets number and type of neurones that pass impulse due to stimulus and thereby determines its size

Question 7

what is the refractory period?

Answer 7

• a brief period when no new APs can be generated in that particular part of the axon
• potential is below RP so too much Na+ needs to be brought in so VGSC close

Question 8

how do you bring down charge from +40 mV peak?

Answer 8

• close Na+ channels
• open K+ so it can leave via FD
• Na-K pump needs to keep working

Question 9

what are the 3 purposes of the refractory period?

Answer 9

• ensures that APs are propagated in one direction only
• produces discrete impulses
• limits number of APs

Question 10

what is the benefit of ensuring that APs are only propagated in 1D?

Answer 10

• APs can only pass from an active region to a resting region
• APs cannot be propagated in a region that is refractory, so it can only move in a forward direction
• if we don’t hyperpolarise, which causes refractory period, sodium channels wouldn’t close so SWPOD would go both ways

Question 11

what is the benefit of producing discrete impulses?

Answer 11

• ensures that APs are separated from one other
• brain has enough time to work out where impulse comes from

Question 12

what is the benefit of limiting the number of APs?

Answer 12

• limits the strength of stimulus that can be detected
• stops brain from being overstimulated, spaces out impulses sent to brain

Question 13

describe an axon in its different polarity states

Answer 13

1. re-polarised
2. de-polarised section:
   - hyperpolarised
   - re-polarised
   - depolarised
3. polarised

imagine this horizontal left to right pls

Question 14

describe a graph showing the charge of a nerve impulse

Answer 14

1. -70 mV: polarised, Na+ pumps are working, VGSC closed
2. VGSC starts to open, charge starts to increase, when it crosses TV, axon is depolarised (AP)
3. peak of +40 mV, VGSC close so charge doesn’t increase too much
4. repolarising, charge starts to decrease, Na+ channels closed & K+ channels opened
5. hyperpolarised, below -70 mV
6. levels off back to -70 mV, same as 1 but state is re-polarised rather than polarised