Nerve impulses (BIOL5)

Question 1

Features of neurones

Answer 1

• In resting state, outside membrane +vely charged compared to inside - more +ve ions outside
  
  • Membrane polarised - diff in charge (voltage)
  • Voltage during resting -70mV
• Resting potential maintained by Na-K pumps and K+ channels
  
  • Na-K pumps move Na out, but membrane impermeable to Na, so can’t diffuse back
    
    • Creates Na+ electrochemical gradient
  • Na-K pumps move K+ in, but membrane permeable to K+ so diffuse out by channels!

Question 2

Describe sodium-potassium pumps

Answer 2

• Active transport moves 3Na+ out per 2K+ in!
• ATP needed!

Question 3

Describe potassium ion channels

Answer 3

Allows facilitated diffusion of K+ out of neurone down conc gradient

Question 4

Features of action potentials

Answer 4

• Stimulus triggers Na+ channels open
• If stimulus big enough, triggers change in pot diff - an AP:

1. Stimulus
2. Depolarisation
3. Repolarisation
4. Hyperpolarisation
5. Resting potential

Question 5

(1) Stimulus in action potential

Answer 5

• Excites membrane, Na+ channels open
• Membrane more permeable to Na+ - diffuse into neurone down Na+ electochemical gradient

Question 6

(2) Depolarisation of action potential

Answer 6

• Potential diff reaches threshold, more Na+ channels open
• More Na+ diffuse into neurone

Question 7

(3) Repolarisation of action potential

Answer 7

• Na+ channels close
• K+ channels open
• Membrane more permeable to K+, diffuse out neurone down conc grad.
• Membrane begins to reach resting potential

Question 8

(4) Hyperpolarisation of action potential

Answer 8

• K+ channels slow to close, so slight ‘overshoot’
  
  • Too many K+ diffuse out
• Potential diff becomes more negative than resting potential (less than -70mV)

Question 9

(5) Resting potential of action potential

Answer 9

• Ion channels rest
• Na-K pump returns membrane to resting

Question 10

Why can’t neurones become excited again straight after an AP?

Answer 10

• Ion channels are recovering
• Can’t be made open
• Na+ channels closed during repolarisation, and K+ channels close during hyperpolarisation
• This is the refactory period

Question 11

Features refractory period

Answer 11

• Acts as time delay b/w AP
  
  • Ensures AP don’t overlap, but pass along as discrete (separate) impulses
  • Makes sure AP are unidirectional

Question 12

Describe the all-or-nothing principle

Answer 12

• AP always fire w/ same charge, no matter how big stimulus is!
• If threshold not reached, AP won’t fire
  
  • ALL-OR-NOTHING!
• Bigger stimulus won’t cause bigger AP, but will cause them to fire more freq

Question 13

What are the factors that affect AP speed?

Answer 13

• Myelination
• Axon diameter
• Temp

Question 14

Effect of myelination on AP speed

Answer 14

• Myelinated neurones have myelinated sheath
  
  • Electrical insulator
  • Made of schwann cell
    
    • B/w schwann cells are patches of bare membrane - nodes of Ranvier
      
      • Na+ channels conc here
• Depolarisation only happens at nodes of Ranvier (where Na+ can get through membrane!)
• Saltatory conduction:
  
  • Neurone’s cytoplasm conduct e- change to depolarise next node
  • So impulse ‘jumps’ from node to node!
• In non-myelinated neurone, impulse travels as wave along whole length of axon memb
  
  • Slow than saltatory!

Question 15

Effect of axon diameter on AP speed

Answer 15

• AP conducted much quicker along axons w/ bigger diameter
  
  • There’s less resistance
  • Depolarisation reaches other parts of neurone cell membrane quicker!

Question 16

Effect of temp on AP speed

Answer 16

• Conduction speed inc. as temp inc
  
  • Ions diffuse faster
  • Inc. kinetic energy
    
    • Speed only inc up to 40oC, after this, proteins denature