Nervous coordination 3.6.2 ( Organisms respond to changes in their internal and external environments 3.6)

Question 1

Describe the structure of a myelinated motor neurone

Answer 1

• dendrite =branches coming out of neurone
• Cell body = part containing the dendrites
• Axon = long tail
• Myelin sheath = made of Schwann cells creating a layer of insulation
• Node = gaps between myelin sheath
• Axon terminal = end branches of a neuron
Direction of nerve impulses is Dendrite => axon terminal

Question 2

Describe resting potential

Answer 2

Inside of axon has a negative charge relative to outside ( as more positive ions outside compared to inside )

Question 3

Explain how a resting potential is established across the axon membrane in a neurone

Answer 3

• Na+ / K+ pump actively transports :
  • 3 Na+ out of axon AND 2 K+ into axon
• Creating an electrochemical gradient
  • Higher K+ conc inside AND higher Na+ conc outside
• Differential membrane permeability
  • More permeable to K+ -> move out by facilitated diffusion
  • Less permeable to Na+ (closed channel)

Question 4

Explain how changes in membrane permeability lead to depolarisation and the generation of an action potential

Answer 4

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

Question 5

Describe the all-or-nothing principle

Answer 5

• For an action potential to be produced , depolarisation must exceed threshold potential
• Action potentials produced are always the same magnitude / size / peak at same potential
- Bigger stimuli instead increase frequency of action potentials

Question 6

Explain how the passage of an action potential along non-myelinated and myelinated axons results in nerve impulses

Answer 6

Non-myelinated axon :
• Action potential passed as a wave of depolarisation
• Influx of Na* in one region increases permeability of adjoining region to Na+ by causing voltage -gated Na+ channels to open so adjoining region depolarises

Myelinated axon :
• Myelination provides electrical insulation
• Depolarisation of axon at nodes of Ranvier only
• Resulting in saltatory conduction
• So there is no need for depolarisation along whole length of axon

Question 7

Suggest how damage to the myelin sheath can lead to slow responses and / or jerky movement

Answer 7

• Less / no saltatory conduction ; depolarisation occurs along whole length of axon
- So nerve impulses take longer to reach neuromuscular junction ; delay in muscle contraction
• Ions / depolarisation may pass / leak to other neurons
- causing wrong muscle fibres to contract

Question 8

Describe the nature of the refractory period

Answer 8

• Time taken to restore axon to resting potential when no further action potential can be generated
• As Na+ channels are closed / inactive / will not open

Question 9

Explain the importance of the refractory period

Answer 9

• Ensures discrete impulses are produced ( action potentials don’t overlap )
• Limits frequency of impulse transmissions at a certain intensity ( prevents overreaction to stimulus )
- Higher intensity stimulus causes higher frequency of action potentials
- But only up to certain intensity
• Also ensures action potentials travel in one direction - can’t be propagated in a refractory region

Question 10

Describe the factors that affect speed of conductance

Answer 10

Myelination :
• depolarisation at nodes of ranvier only - saltatory conduction
• Impulse doesn’t travel / depolarise whole length of axon
Axon Diameter :
• Bigger diameter means less resistance to flow of ions in cytoplasm
-Temperature :
• Incresses rate of diffusion of Na* and K+ as more kinetic energy
• But proteins / enzymes could denature at a certain temperature

Question 11

what is the stimulus part of an action potential

Answer 11

•Na+ channels open ; membrane permeability to Na+ increases
• Na+ diffuse into axon down electrochemical gradient causing depolarisation

Question 12

what is the depolarisation part of an action potential

Answer 12

• If threshold potential reached , an action potential is generated
• As more voltage - gated Na+ channels open
• So more Na+ diffuse in rapidly

Question 13

what is the repolarisation part of an action potential

Answer 13

• Voltage - gated Na+ channels close
• Voltage - gated K+ channels open ; K+ diffuse out of axon

Question 14

what is the hyperpolarisation part of an action potential

Answer 14

• K+ channels slow to close so there’s a slight overshoot - too many K+ diffuse out

Question 15

what is the resting potential of an action potential

Answer 15

Restored by sodium potassium pump