Nerve impulses

Question 1

What is the overall charge inside the axon?

Answer 1

Negative.

Question 2

In the sodium-potassium pump, what number of each ion (Na+ and K+) are pumped, and in which direction?

Answer 2

3 Na+ OUT of the axon and 2 K+ IN to the axon.

Question 3

What is used to power active transport?

Answer 3

ATP.

Question 4

What does the sodium potassium pump do?

Answer 4

Hydrolyses ATP to pump 3 NA+ ions out of the neurone for 2 K+ ions in.

Question 5

Why do K+ ions diffuse out more rapidly than Na+ ions?

Answer 5

The nerve cell membrane is 25 times more permeable to potassium ions than sodium ions because it has more K+ ion channel proteins than Na+ ion channel proteins.

Question 6

What is passive movement of ions by concentration gradient?

Answer 6

The movement of ions as they diffuse down their concentration gradient - from high concentration to low concentration.

Question 7

What is passive movement of ions by electrical potential?

Answer 7

The movement of ions by attraction to the charge opposite to the one they have.

Question 8

What causes the resting potential?

Answer 8

The inside of the axon membrane is negatively charged compared to the outside. This causes a potential difference across the axon membrane.

Question 9

What is the resting potential of a nerve? (give units)

Answer 9

-70 mV.

Question 10

What word can be used to describe a neuron at resting potential (more negative inside than outside)?

Answer 10

Polarised.

Question 11

What word can be used to describe a neuron that is excited?

Answer 11

Depolarised.

Question 12

Name 3 ways in which depolarisation may be achieved:

Answer 12

• A stimulus arriving at a receptor cell (e.g. vibration of hair cell in the ear)
• A chemical fitting into a receptor site
  (e. g. a neurotransmitter)
• A nerve impulse travelling down a neuron.

Question 13

What word can be used to describe how a nerve impulse progresses along neurons?

Answer 13

Self-propagating.

Question 14

What is an action potential?

Answer 14

The action potential is the state of the neuron membrane when a nerve impulse passes by.

Question 15

What is the significance of voltage gated Na+ channels in an action potential?

Answer 15

• Small change in membrane voltage depolarises membrane enough to open some voltage-gated Na+ channels
• As more sodium ions enter the cell, more sodium channels open meaning yet more sodium ions enter
• This means a small change in the membrane permeability to Na+ means a big change in membrane potential.

Question 16

Why can action potentials be described as “all-or-nothing” responses?

Answer 16

Nerve impulses all look the same; there are no big ones and little ones. This is because membrane potential either reaches the threshold to cause an action potential or not.

Question 17

What are the stages of an action potential in chronological order? (5 stages)

Answer 17

1) Stimulus
2) Depolarisation
3) Repolarisation
4) Hyperpolarisation (refractory period)
5) Return to resting potential

Question 18

What are the respective voltages for: 1) Resting potential 2) Threshold 3) Repolarisation 4) Hyperpolarisation (refractory period)? (with units)

Answer 18

1) -70 mV
2) -55 mV
3) +35 mV
4) -90 mV

Question 19

At the peak of the action potential, after depolarisation, what happens to start repolarisation?

Answer 19

Na+ channels close and K+ voltage-gated channels open, K+ floods out of neuron.

Question 20

What happens during hyperpolarisation?

Answer 20

The membrane potential falls below the resting potential of –70mV. During this period no impulses can pass along that part of the membrane. This is called the refractory period.

Question 21

How is resting potential restored after hyperpolarisation?

Answer 21

Gradual active pumping of the ions (K+ in and Na+ out) restores the resting potential.

Question 22

What happens in unmyelinated neurons?

Answer 22

Localised electrical currents are set up and the action potential is propagated along the neuron.

Question 23

What makes up a myelin sheath? (2)

Answer 23

Schwann cells and nodes of Ranvier.

Question 24

What can be found at the nodes of Ranvier?

Answer 24

Voltage-gated sodium channels.

Question 25

What word describes how impulses are propagated in myelinated neurons?

Answer 25

Saltatory propagation.

Question 26

Describe saltatory propagation in myelinated neurons.

Answer 26

• The influx of sodium ions at one node creates just enough depolarisation to reach the threshold of the next node of Ranvier.
• In this way, the action potential jumps from one node to the next (1-3mm in length)
• The net effect is much faster propagation of the nerve impulse than is possible in unmyelinated neurons.

Question 27

What are the speeds of a myelinated and unmyelinated neuron? (include units)

Answer 27

Myelinated: 90 m/s
Unmyelinated: 30 m/s

Question 28

Name and briefly describe the 3 factors affecting the speed of an action potential.

Answer 28

1) Myelin sheath - acts as an electrical insulator, allows action potential to jump from one Node of Ranvier to the next
2) Diameter of the axon – greater diameter = faster conductance (due to less leakage).
3) Temperature - higher temp = faster nerve impulse (rate of diffusion is faster, rate of enzyme activity is higher)

Question 29

Name 3 reasons why there is a refractory period.

Answer 29

• Action potentials travel in one direction only.
• Produces discrete impulses.
• Limits the frequency of impulses.

Question 30

Briefly describe the 2 ways that we detect the size of a stimulus.

Answer 30

• The number of impulses in a given time – the larger the stimulus, the more impulses generated. Thus it is frequency of the impulses that is important not their size.
• By having neurons with different threshold values – the brain interprets the number and type of neurons and thereby determines its size.