3.6.2.1 Nerve Impulses (Unit 6 Responding to Environment)

Question 1

What is the voltage across the membrane at resting potential?

Answer 1

around -70mV

Question 2

How is resting potential maintained?

Answer 2

Sodium potassium pumps transports 3 sodium ions out of the neurone for every 2 potassium ions that are transported in. The membrane is impermeable to sodium and permeable to potassium.

Question 3

What happen in response to the stimulus to make the inside of the neurone less negative?

Answer 3

Sodium channels open. Sodium diffuses into the neurone down the electrochemical gradient.

Question 4

What happens if the threshold (around -40mV) is reached?

Answer 4

Depolarisation occurs. More sodium channels open, sodium diffuses rapidly into the neurone.

Question 5

At what potential difference do sodium channels close?

Answer 5

around +40mV

Question 6

After depolarisation, how does the neurone return to resting potential?

Answer 6

Sodium channels close and potassium channels open. Potassium diffuses out of the neurone.

Question 7

Why does hyperpolarisation occur?

Answer 7

The potassium ion channels are slow to close so there’s a slight overshoot where too many potassium ions diffuse out of the neurone.

Question 8

What is the refractory period?

Answer 8

The period of time after an action potential when the neurone can’t be excited.

Question 9

How does the nuerone respond to a bigger stimulus?

Answer 9

Action potentials will occur more frequently.

Question 10

During an action potential, the membrane potential rises to +40 mV and then falls. Use information from the graph to explain the fall in membrane potential.

Answer 10

Potassium channels open;

Potassium out;

Sodium channels close;

Question 11

Sodium and potassium ions can only cross the axon membrane through proteins.

Explain why.

Answer 11

can not pass through phospholipid bilayer;
because water soluble / not lipid soluble / charged / hydrophilic /
hydrated;

Question 12

Explain the importance of reflex actions.

Answer 12

1. automatic (adjustments to changes in environment) / involuntary;
2. reducing / avoiding damage to tissues / prevents injury / named injury
   e. g. burning;
3. role in homeostasis / example;
4. posture / balance;
5. finding / obtaining food / mate / suitable conditions;
6. escape from predators;

Question 13

Give two differences between a cholinergic synapse and a neuromuscular junction.

Answer 13

neurone to neurone and neurone to muscle;
action potential in neurone and no action potential in muscle /
sarcolemma;
no summation in muscle;
muscle response always excitatory (never inhibitory);
some neuromuscular junctions have different neurotransmitters;

Question 14

Describe how resting potential is maintained/restored

Answer 14

Na+/K+ pumps 3 Na+ out of the axon and 2 K+ into the axon using ATP
Membrane is more permeable to K+ so K+ moves out through channels and less permeable to Na+
Axon has a more negative potential difference inside

Question 15

Describe the refractory period

Answer 15

The period between firing of one action potential and the next
The axon is hyperpolarised (very negative) on the inside

Question 16

Why is the refractory period necessary

Answer 16

Refractory period ensures:
Action potentials occur in one direction
Limits number of action potentials
Keeps action potentials separated

Question 17

Describe action potential

Answer 17

Na+ ions enter the axon (e.g. Na+ receptors at synapses, stetch mediated-sodium chnnels)
The axon membrane is depolarised
Potential difference across axon becomes more positive
Potential difference will reach threshold triggering action potential
Voltage gated Na+ channels will open
Na+ moves into the axon by facilitated diffusion
Na+ channels close, K+ channels open
K+ leaves the axon by facilitated diffusion – axon is repolarised
Axon membrane becomes hyperpolarised in the refractory period
Na+/K+ pump restores resting potential

Question 18

Describe the all or nothing principle

Answer 18

There must be a certain level of stimulus for a action potential to be triggered.
There must be enough depolarisation to pass the threshold

Regardless of how strong a stimulus is, the same action potential is produced.

A larger stumulus can be detected by having more frequent action potentials.

Question 19

What is the myelin sheath?

Answer 19

Axons may be wrapped in Schwann Cells

Many layers of wrapping

Schwann cells contain lipid Myelin…
…hence, ‘Myelin Sheath’

Insulates the axon (ions cannot go in / out in these areas.

Question 20

What is saltatory conduction?

Answer 20

The myelin sheath insulates the axon (ions cannot go in / out in these areas).

The impulse must ‘jump’ from ‘node to node’ (node of ranvier)

This speeds up the transmission of AP’s

This is called Saltatory conduction only occurs in axons with a myelin sheath

Question 21

How does temperature affect nervous transmission?

Answer 21

Temp – speeds it up if increased, more diffusion and kinetic energy, ATP required for Na+/K+ pump and enzymes are sped up

Question 22

How does the diameter of the axon affect nervous transmission?

Answer 22

Increases the speed as there is less leakage from the axon and the potential difference/voltage can be maintained