nervous coordination

Question 1

1. Stimulus

Answer 1

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

Question 2

2. Depolarisation

Answer 2

● If threshold potential reached, an action potential is generated:
● As more voltage-gated Na+ channels open (positive feedback effect)
● More Na+ diffuses in rapidly

Question 3

3. Repolarisation

Answer 3

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

Question 4

4. Hyperpolarisation

Answer 4

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

Question 5

5. Resting potential

Answer 5

● Restored by Na+/K+ pump

Question 6

The all-or-nothing principle

Answer 6

● 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

Question 7

The passage of an action potential along non-myelinated axon

Answer 7

● Action potential passes 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

Question 8

The passage of an action potential along myelinated axon

Answer 8

● Myelination provides electrical insulation
● Depolarisation of axon at nodes of Ranvier only
● Resulting in saltatory conduction (local currents circuits)
● So there is no need for depolarisation along whole length of axon

Question 9

Application: damage to myelin sheath eg. multiple sclerosis → slow responses / jerky movement

Answer 9

● 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 neurones
○ Causing wrong muscle fibres to contract