Nerve Impulses (15)

Question 1

What are the general features of a motor neuron’s structure?

Answer 1

cell body
dendrons
axon

Question 2

Describe the cell body.

Answer 2

contains organelles and a high proportion of RER

Question 3

Describe dendrons.

Answer 3

branch into dendrites which carry impulses towards cell body

Question 4

Describe the axon.

Answer 4

long, unbranched fibre which carries nerve impulses away from cell body

Question 5

What are the general additional features of a motor neuron’s structure when it is myelinated?

Answer 5

schwann cells
myelin sheath
nodes of ranvier

Question 6

Describe schwann cells.

Answer 6

made out of lipids and wrap around axon many times

Question 7

Describe myelin sheath.

Answer 7

made of schwann cells

Question 8

Describe nodes of ranvier.

Answer 8

very short gaps between neighbouring schwann cells where there’s no myelin sheath

Question 9

What 3 processes are schwann cells involved in?

Answer 9

1) electrical insulation
2) phagocytosis
3) nerve regeneration

Question 10

How does an action potential pass along an unmyelinated neuron?

Answer 10

1) stimulus leads to influx of sodium ions and the 1st section of membrane depolarises
2) local electrical currents cause sodium voltage-gated channels further along membrane to open
3) sequential wave of depolarisation

Question 11

Why do myelinated axons conduct impulses faster than unmyelinated axons?

Answer 11

• saltatory conduction
• depolarisation only occurs at the nodes
• impulse does not travel along the whole length

Question 12

What is saltatory conduction?

Answer 12

impulse jumps from 1 node of ranvier to another

Question 13

What is resting potential?

Answer 13

the potential difference across neuron membrane when not stimulated

Question 14

What is the resting potential in humans?

Answer 14

usually about -70mV

Question 15

How is resting potential established?

Answer 15

• membrane is more permeable to potassium ions than sodium ions
• sodium-potassium pump actively transports 3 sodium ions out of axon and 2 potassium ions into axon
• this established an electrochemical gradient

Question 16

What is meant by an electrochemical gradient?

Answer 16

when cell contents is more negative than extracellular environment

Question 17

What are the 4 stages in generating an action potential?

Answer 17

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

Question 18

What happens in depolarisation?

Answer 18

1) stimulus causes facilitated diffusion of sodium ions into cell down concentration gradient
2) potential difference across membrane becomes more positive
3) if membrane reaches threshold potential (-50mV), voltage-gated sodium ion channels open
4) significant influx of sodium ions reverses potential difference to +40mV

Question 19

What happens in repolarisation?

Answer 19

1) voltage-gated sodium ion channels close and voltage-gated potassium ion channels open
2) facilitated diffusion of potassium ions out of cell down their electrochemical gradient
3) potential difference across membrane becomes more negative

Question 20

What happens in hyperpolarisation?

Answer 20

1) there is an ‘overshoot’ when potassium ions diffuse out so potential difference becomes more negative than resting potential
2) causes refractory period
3) voltage-gated potassium ion channels close and sodium-potassium pump re-establishes resting potential

Question 21

What is the refractory period?

Answer 21

where no stimulus is large enough to raise membrane potential to threshold so no action potential can be generated in hyperpolarised sections of membrane

Question 22

What is the importance of the refractory period?

Answer 22

1) ensures unidirectional impulses
2) ensures discrete impulses
3) limits frequency of impulse transmission

Question 23

What are the units for max frequency of impulse conduction?

Answer 23

Hz

Question 24

What is meant by the ‘all or nothing’ principle?

Answer 24

any stimulus that causes the membrane to reach threshold potential will generate an action potential and all action potentials have the same magnitude

Question 25

What 3 factors affect the speed of conduction?

Answer 25

myelination
axon diameter
temperature (don’t mention if only 2 factors are asked for)

Question 26

How does the axon diameter affect speed of conduction?

Answer 26

• greater diameter = faster
• less resistance to flow of ions in depolarisation and repolarisation
• less ‘leakage’ of ions so easier to maintain membrane potential

Question 27

How does temperature affect speed of conduction?

Answer 27

• higher = faster
• too high = membrane proteins become denatured
• faster rate of diffusion for depolarisation and repolarisation
• faster rate of respiration in enzyme-controlled steps, so more ATP for active transport to re-establish resting potential

Question 28

How do organisms detect the strength of a stimulus?

Answer 28

larger stimulus raises membrane to threshold potential more quickly after hyperpolarisation so there’s a greater frequency of impulses