nerve impulses

Question 1

Describe the structure of a myelinated motor neurone

Answer 1

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 channels)

Question 4

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

Answer 4

Question 5

Draw / label a graph showing an action potential

Answer 5

Question 6

Describe the all-or-nothing principle

Answer 6

● 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 7

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

Answer 7

Question 8

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

Answer 8

● 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 9

Describe the nature of the refractory period

Answer 9

● 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 10

Explain the importance of the refractory period

Answer 10

● 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 – can’t be propagated in a refractory region

In the second half of the refractory period an action potential can be produced but requires greater stimulation
to reach threshold

Question 11

Describe the factors that affect speed of conductance

Answer 11

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