Resting + Action Potentials and Receptors (Chapter 15)

Question 1

What do neurones transmit?

Answer 1

Electrical impulses, which travel very rapidly along the CSM from one end of the cell to the other

Question 2

What are electrical impulses (signals)?

Answer 2

Brief changes in the distribution of electrical charge across the CSM called action potentials, caused by the very rapid movement of Na+ and K+ into and out of the axon

Question 3

Describe the resting potential

Answer 3

• In a resting axon, the inside of the axon always has a slightly negative electrical potential compared with the outside
• ∴ the resting potential is the potential difference (the difference between the potential of the outside and inside)

Question 4

What is normally the value of the resting potential?

Answer 4

-70 to -60 mV - ∴ the electrical potential of the inside of the axon is between 60 and 70 mV lower than the outside

Question 5

What is the resting potential produced and maintained by and how do these work?

Answer 5

Sodium-potassium pumps (membrane proteins) in the CSM which constantly move Na+ out of the axon and K+ into the axon against their conc gradients, using energy form the hydrolysis of ATP

Question 6

How do the sodium-potassium pumps work?

Answer 6

1) 3 Na+ are removed from the axon for every 2 K+ brought back in
2) the membrane has protein channels for K+ and Na+ which are open all the time - there are far more of these for K+ than for Na+
3) ∴ some K+ diffuses back out again much faster than Na+ diffuses back in
4) however, there are many large, negatively charged molecules inside the cell that attract K+, reducing the chance that they will diffuse out
5) the result of these effects is an overall excess of negative ions inside the membrane compared with the outside

Question 7

Why does Na+ move into the axon during an action potential, despite the fact that the membrane is relatively impermeable to Na+?

Answer 7

The electrochemical gradient - a double gradient consisting of 1) a steep concentration gradient and 2) the inside of the membrane being negatively charged, which attracts positively charged ions

Question 8

What happens when the axon is stimulated? (summary of action potential)

Answer 8

1) the p.d. across the CSM of the axon suddenly switches from -70mV to +30mV
2) it then swiftly returns to normal after a brief ‘overshoot’

Question 9

What is an action potential and how long does it take?

Answer 9

• A rapid, fleeting change in p.d. across the membrane

- The whole process takes roughly 3s

Question 10

What is an action potential caused by?

Answer 10

Changes in the permeability of the CSM to Na+ and K+

Question 11

What is a voltage-gated channel?

Answer 11

• Channels that allow Na+ and K+ to pass through
• They open and close depending on the electrical potential (voltage) across the membrane
• When the membrane is at its resting potential, these channels are closed

Question 12

How does depolarisation occur in an action potential?

Answer 12

1) a stimulus causes the opening of a few voltage-gated channels in the CSM which allow Na+ to pass through
2) bc there is a much greater [Na+] outside the axon than inside, Na+ begins to enter through the open channels
3) this changes the p.d. across the membrane, which becomes less negative on the inside
4) this depolarisation triggers more channels to open so that more Na+ enter ∴ there is more depolarisation
5) if the p.d. reaches between -60mV and -50mV (threshold potential), many more Na+ channels open and the inside reaches a potential of +30mV

Question 13

How does repolarisation occur in an action potential?

Answer 13

1) After roughly 1ms, all the Na+ voltage-gated channels close (∴ Na+ stop diffusing into the axon) and K+ channels open
2) K+ diffuse out of the axon, down their conc gradient
3) the outward movement of K+ removes positive charge from inside the axon to the outside ∴ making the p.d. briefly more negative than normal (hyperpolarisation) and then returning the p.d. to normal (-70mV) - this is repolarisation
4) the K+ channels then close and the Na+ channels become responsive to depolarisation again

Question 14

Why does the Na-K pump continuously pump Na+ out and K+ in when the axon is at rest?

Answer 14

To help maintain the distribution of Na+ and K+ across the membranes so that many more action potentials occur

Question 15

How do action potentials help to transmit information along a neurone when a stimulus is applied somewhere along an axon experimentally?

Answer 15

1) an action potential at any point in an axon’s CSM triggers the production of an action potential in the membrane on either side of it
2) the temporary depolarisation of the membranes at the site of the action potential causes a local circuit to be set up between the depolarised region and the resting regions on either side of it
3) these local circuits depolarise the adjoining regions ∴ generating action potentials in them too

Question 16

Why, in reality, do action potentials begin at one end and ‘new’ action potentials are generated ahead, not behind?

Answer 16

Because the region behind it will still be recovering from the action potential it has just had and the Na+ voltage-gated channels are hut tight and cannot be stimulated to open, however great the stimulus

Question 17

What is the period of recovery where the axon is unresponsive called?

Answer 17

The refractory period

Question 18

What does the existence of refractory periods mean?

Answer 18

1) action potentials are discrete events - they do not merge into one another
2) there is a minimum time between action potentials occurring at any one place on a nerve
3) the length of the refractory period determines the maximum frequency at which impulses are transmitted along neurones

Question 19

What can you say about the size of the action potential?

Answer 19

1) action potentials do not change in size as they travel or according to the intensity of the stimulus
2) ∴ the action potential will continue to reach a peak value of +30mV inside all the way along

Question 20

What can you say about the speed at which action potentials travel?

Answer 20

It does not vary according to the size of the stimulus

Question 21

What is the effect of strong/weak stimuli on action potentials?

Answer 21

Action potentials have different frequencies resulting from strong and weak stimuli

Question 22

What does a weak stimulus result in?

Answer 22

1) fewer action potentials per second

2) action potentials passing along just one or two neurones

Question 23

What is the effect of a strong stimulus?

Answer 23

1) produces a rapid succession of action potentials, each one following along the axon just behind its predecessor
2) likely to stimulate more neurones than a weak stimulus
3) can produce action potentials in many neurones (more than one or two)

Question 24

What can the brain interpret about action potentials and stimuli?

Answer 24

1) the frequency of action potentials arriving along the axon of a sensory neurone
2) the number of neurones carrying action potentials, to get information about the strength of the stimulus being detected
3) the nature of the stimulus is deduced from the position of the sensory neurone bringing the information

Question 25

How much slower is the speed of conduction in unmyelinated neurones?

Answer 25

0.5 m/s vs 100 m/s

Question 26

How does myelin speed up the rate at which action potentials travel?

Answer 26

By insulating the axon membrane

Question 27

Explain how myelin speeds up the rate at which action potentials travel

Answer 27

1) Na+ and K+ cannot flow through the myelin sheath ∴ it is not possible for depolarisation or action potentials to occur in parts of the axon which are surrounded by the myelin sheath
2) ∴ action potentials can only occur at the nodes of Ranvier, where all the channel and pump proteins are concentrated
3) so the local circuits exist from one node to the next ∴ action potentials jump from one node to next (saltatory conduction)
4) this can increase the speed of transmission in a myelinated axon by up to 50 times that in an unmyelinated axon

Question 28

What is saltatory conduction?

Answer 28

When action potentials jump from one node to the next

Question 29

What affects the speed of transmission?

Answer 29

Diameter - thick axons transmit impulses faster than thin ones bc their resistance is much less

Question 30

What are examples of stimuli that generate action potentials?

Answer 30

Light, pressure, sound, temp, chemicals

Question 31

What is a receptor cell?

Answer 31

• A cell that responds to a stimulus by initiating an action potential
• Transducers ∴ they convert energy in one form into energy in an electrical impulse in a neurone

Question 32

Where are receptor cells often found?

Answer 32

In sense organs

Question 33

What are chemoreceptors?

Answer 33

Specialised cells which detect a specific type of stimulus and influence the electrical activity of a sensory neurone e.g. those in the taste buds

Question 34

What are (some) touch receptors?

Answer 34

The ends of sensory neurones

Question 35

Describe the structure of the tongue

Answer 35

1) it is covered in many papillae (small bumps)
2) each papilla has many taste buds over its surface
3) within each taste bud are chemoreceptors
4) each chemoreceptor is covered with receptor proteins that detect the different chemicals from food or drink that dissolve in saliva
5) each receptor detects a different type of chemical, giving us a different sensation - five tastes: sweet, sour, salt, bitter and umami (savoury)

Question 36

Describe how chemoreceptors in taste buds work

Answer 36

1) A chemical stimulus binds to a chemoreceptor, triggering Na+ to diffuse through highly selective channel proteins in the CSM of the microvilli, leading to depolarisation of the membrane - this increase in positive charge inside the cell is the receptor potential
2) if there is sufficient stimulation by Na+ inside the mouth, then the receptor potential becomes large enough to stimulate the opening of voltage-gated Ca2+ channels
3) Ca2+ enter the cytoplasm and stimulates the exocytosis of vesicles containing neurotransmitter from the basal membrane
4) the neurotransmitter stimulates an action potential in the sensory neurone that transmit impulses to the taste centre in the cerebral cortex of the brain

Question 37

What are chemoreceptors in the tongue directly influenced by?

Answer 37

Na+

Question 38

What happens when receptors are stimulated?

Answer 38

They are depolarised

Question 39

What happens when there is a very weak stimulus for receptor cells?

Answer 39

The cells are not depolarised very much and the sensory neurone is not activated to send impulses

Question 40

What happens where there is a stronger stimulus for receptor cells?

Answer 40

The sensory neurone is activated and transmits impulses to the CNS

Question 41

What is the all-or-nothing law?

Answer 41

Neurones either transmit impulses from one end to the other or they do not

Question 42

What happens if the receptor potential is below a certain threshold?

Answer 42

The stimulus only causes local depolarisation of the receptor cell

Question 43

What happens if the receptor potential is above a certain threshold?

Answer 43

The receptor cell stimulates the sensory neurone to send impulses and action potentials are initiated in the sensory neurone

Question 44

What do action potentials always have the same of and what does this mean?

Answer 44

• Action potentials always have the same amplitude
• ∴ if the intensity of the stimulus increases, the action potentials are produced more frequently, they do NOT become bigger

Question 45

What is true about threshold levels in receptors?

Answer 45

They rarely stay constant all the time bc with continued stimulation, they often increase so that it requires a greater stimulus before receptors send impulses along sensory neurones