6.2.1 Nerve impulses

Question 1

Describe the general structure of a motor neuron

Answer 1

Cell body: contains organelles and high proportion of RER
Dendrons: branch into dendrites which carry impulses towards cell body
Axon: long, unbranched fibre carries nerve impulses away from cell body

Question 2

Describe the additional features of a myelinated motor neuron

Answer 2

Schwann cells: wrap around axon many times
Myelin sheath: made from myelin-rich membranes of Schwann cells
Nodes of Ranvier: very short gaps between neighboring Schwann cells where there is no myelin sheath

Question 3

Name 3 processes Schwann cells are involved in

Answer 3

1. electrical insulation
2. phagocytosis
3. nerve regeneration

Question 4

How does an action potential pass along an unmyelinated neurone?

Answer 4

1. Stimulus leads to influx of Na+ ions. First section of membrane depolarises
2. Local electrical currents cause sodium voltage-gated channels further along membrane to open. Meanwhile, the section behind begins to repolarise
3. Sequential wave of depolarisation

Question 5

Explain why myelinated axons conduct impulses faster than unmyelinated axons

Answer 5

Saltatory conduction: impulse ‘jumps’ from one node of Ranvier to another. Depolarisation cannot occur where myelin sheath acts as electrical insulator
So impulse does not travel along whole axon length

Question 6

What is resting potential?

Answer 6

Potential difference (voltage) across neuron membrane when not stimulated (-50 to -90 mV, usually about -70mV in humans)

Question 7

How is resting potential established?

Answer 7

1. Membrane is more permeable to K+ than Na+
2. Sodium-potassium pump actively transports 3Na+ out of cell and 2K+ into cell
   Establishes electrochemical gradient: cell contents more negative than extracellular environment

Question 8

Name the stages in generating an action potential

Answer 8

1. depolarisation
2. repolarisation
3. hyperpolarisation
   4 return to resting potential

Question 9

What happens during depolarisation?

Answer 9

1. Stimulus -> facilitated diffusion of Na+ ions into cell down electrochemical gradient
2. Potential difference across membrane becomes more positive
3. If membrane reaches threshold potential (-50mV), voltage-gated Na+ channels open
4. Significant influx of Na+ ions reverses potential difference to +40mV

Question 10

What happens during repolarisation?

Answer 10

1. Voltage-gated Na+ channels close and voltage-gated K+ channels open
2. Facilitated diffusion of K+ ions out of cell down their electrochemical gradient
3. Potential difference across membrane becomes more negative

Question 11

What happens during hyperpolarisation?

Answer 11

1. ‘Overshoot’ when K+ ions diffuse out = potential difference becomes more negative than resting potential
2. Refractory period: no stimulus is large enough to raise membrane potential to threshold
3. Voltage-gated K+ channels close and sodium-potassium pump re-establishes resting potential

Question 12

Explain the importance of the refractory period

Answer 12

No action potential can be generate in hyperpolarised sections of membrane:
- ensures unidirectional impulse
- ensures discrete impulses
- limits frequency of impulse trasnmission

Question 13

What is the ‘all or nothing’ principle?

Answer 13

Any stimulus that causes the membrane to reach threshold potential will generate an action potential
All action potentials have same magnitude

Question 14

Name the factors that affect the speed of conductance

Answer 14

• Myelin sheath
• Axon diameter
• Temperature

Question 15

How does axon diameter affect the speed of conductance?

Answer 15

Greater diameter = faster
- less resistance to flow of ions (depolarisation and repolarisation)
- less ‘leakage’ of ions (easier to maintain membrane potential)

Question 16

How does temperature affect speed conductance?

Answer 16

Higher temperature = faster
- faster rate of diffusion (depolarisation and repolarisation)
- faster rate of respiration (enzyme-controlled) = more ATP for active transport to re-establish resting potential
Temperature too high = membrane proteins denature

Question 17

Suggest an appropriate statistical test to determine whether a factor has a significant effect on the speed of conductance

Answer 17

Student’s t-test (comparing means of continuous data)

Question 18

Suggest appropriate units for the maximum frequency of impulse conduction

Answer 18

Hz

Question 19

How can an organism detect the strength of a stimulus?

Answer 19

Larger stimulus raises membrane to threshold potential more quickly after hyperpolarisation = greater frequency of impulses