13.4 Nervous Transmission

Question 1

What is the definition of resting potential?

Answer 1

potential difference across axon membrane

of neurone at rest

≈ -70mV

Question 2

How is resting potential maintained/restored?

Answer 2

FORMATION:

Sodium-potassium pump actively transports 3Na⁺ out for every 2K⁺ in

More positive charge outside than in

∴ inner membrane negative compared to outer membrane - polarised

MAINTAINANCE:

leakage - some Na⁺ diffuses back in, some K⁺ diffuses back out

through potassium/sodium ion channels

down electrochemical gradient

Question 3

How does a stimulus induce an action potential?

How does the axon membrane repolarise?

Answer 3

(resting potential, voltage-gated channels are closed)

Stimulus causes stretch >> Na⁺ stretch-channels open

Na⁺ diffuses in down electrochemical gradient, begins depolarisation

Once membrane depolarises to generator potential, opens Na⁺ VGC

greater influx of Na⁺ into axon - positive feedback

Membrane depolarises to action potential >> closes Na⁺ VGC, opens K⁺ VGC

K⁺ diffuse out, no diffusion of Na⁺

(delay in closing K⁺ channels can cause hyperpolarisation)

Sodium-potassium pump begins to restore resting potential by rebalancing ions

Question 4

Fill in the gaps:

Answer 4

1. resting potential
2. generator potential
3. action potential

Question 5

What is the approximate value of resting potential?

Answer 5

-70mV

Question 6

What is the approximate value of action potential?

Answer 6

+40mV

Question 7

What is an electrochemical gradient?

Answer 7

2 areas

which have a difference in charge + chemical concentration

Question 8

What is the “all or nothing” principle?

Answer 8

magnitude of each action potential

is not proportional to strength of stimulus.

So long as threshold value is reached, membrane always completely depolarises to +40mV.

e.g a very strong stimulus and a weak stimulus will both cause the membrane to depolarise to +40mV, so long as they both reach the threshold value.

Question 9

How are action potentials propagated along an axon?

Answer 9

• At resting potential:
  
  • [Na⁺] outside > inside
  • [K⁺] inside > outside
    
    • overall greater charge outside, so membrane polarised.
• Stimulus >> Na⁺ stretch channels open >> Na⁺ VGC open
  
  • Na⁺ diffuses in down electrochemical gradient
  • charges on membrane flip
• High [Na⁺] in axon, low [Na⁺] in area adjacent to Na⁺ influx
  
  • Na⁺ diffuses along axon
  • causes depolarisation in adjacent membrane region
  • localised circuits formed
• Propagating action potentials carries on continuously along axon
• In earliest region of depolarisation, repolarisation begins:
  
  • Na⁺ VGC close, K⁺ VGC open
  • Sodium-potassium pump begins repolarisation

Question 10

At resting potential, what is the concentration of Na⁺ & K⁺ outside compared to inside the axon?

Answer 10

Higher concentration of Na⁺ outside than inside

Equal concentrations of K⁺ inside & outside (no net movement)

Question 11

What is the definition of refractory period?

Answer 11

short period of time after excitation

when axon cannot be excited again.

(resting potential not yet restored, so no Na⁺ to diffuse in)

Question 12

What is saltatory conduction?

Answer 12

the way in which electrical impulses travel

through a myelinated neurone

Question 13

Outline how saltatory conduction works:

Answer 13

• Localised circuits cannot form in areas covered by myelin sheath
  
  • myelin sheath has high lipid content, ∴ good electrical insulator
• Can only form in adjacent Nodes of Ranvier
  
  • Longer localised circuits
• Action potential/impulse “jumps” along membrane
  
  • more energy + time efficient

Question 14

What are the benefits of saltatory conduction over conduction in unmyelinated neurones?

Answer 14

• more time efficient
  
  • less ion channels, so less time required to open + close
• more energy efficient
  
  • less ion channels, less ATP required to open + close

Question 15

Describe & explain the factors affecting rate of impulse transmission along an axon:

Answer 15

• AXON DIAMETER
  
  • wider diameter >> less resistance to ions diffusing along axon
• TEMPERATURE
  
  • higher E_k >> ions diffuse faster

Question 16

How does the body differentiate between a strong & weak stimulus?

Answer 16

the stronger the stimulus, the more frequent the action potentials

(the magnitude of each action potential always remains the same (+40mV), it is only the frequency which changes)

Question 17

How does hyperpolarisation occur?

Answer 17

during repolarisation:

K⁺ VCG open (to allow K⁺ to diffuse out & make inside axon more negative)

delay in closing K⁺ VGC can lead to too many K⁺ diffusing out

membrane reaches below -70mV - hyperpolarised

Question 18

Why is a refractory period important?

Answer 18

If axon can be immediately polarised after depolarisation,

could lead to action potentials moving backwards (since Na⁺ could diffuse in both directions)