Nervous Coordination

Question 1

Describe resting potential

Answer 1

Inside of axon has a negative charge relative to outside

Question 2

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

Answer 2

● sodium ions/potassium ions pump actively transports:
○ (3) sodium ions out of axon AND (2) potassium ions into axon
● Creating an electrochemical gradient:
○ Higher potassium conc. inside AND higher sodium
conc. outside
● Differential membrane permeability:
○ More permeable to potassium ions→ move out by facilitated diffusion
○ Less permeable to sodium ions (closed channels)

Question 3

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

Answer 3

1. Stimulus
   ● sodium ion channels open; membrane permeability to sodium ions increases
   sodium ions diffuse into axon down electrochemical gradient (causing depolarisation)
2. Depolarisation
   ● If threshold potential reached, an action potential is generated
   As more voltage-gated Na+ channels open (positive feedback effect)
   So more sodium ions diffuse in rapidly
3. Repolarisation
   ● Voltage-gated Na+channels close
   Voltage-gated K+ channels open; K+ diffuse out of axon
4. Hyperpolarisation
   ● potassium ion channels slow to close so there’s a slight overshoot – too many potassium diffuse
5. Resting potential
   ● for Restored by Na+/K+ pump

Question 4

Describe the all-or-nothing principle

Answer 4

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

Question 5

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

Answer 5

● Action potential passes as a wave of depolarisation
● Influx of sodium ion
in one region increases
permeability of adjoining region to Na+ by
causing voltage-gated Na+ channels to open
so adjoining region depolarises

Question 6

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

Answer 6

● 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 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 neurones
○ 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 sodium ion channels are closed and inactive

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 transmission at a certain intensity (prevents over reaction 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

Question 10

Describe how myelination affects the speed of conductance

Answer 10

● Depolarisation at Nodes of Ranvier only → saltatory conduction
● Impulse doesn’t travel whole length of axon

Question 11

Describe how axon diameter affects speed of conductance

Answer 11

● Bigger diameter means less resistance to flow of ions in cytoplasm

Question 12

Describe how temperature affects speed of conductance

Answer 12

● Increases rate of diffusion of Na+and K+ as more kinetic energy
● But proteins / enzymes could denature at a certain temperature