Action Potentials

Question 1

When does a voltage-gated ion channel open?

Answer 1

When the membrane depolarises - due to a change in voltage

Question 2

What are all voltage-gated ion channels selective for?

Answer 2

Cations (K+, Na+ and Ca2+)

Question 3

What do the rings of charge around the mouth of voltage gated channels do?

Answer 3

help filter out desired ions from undesired. Ring of negative charge would attract cations and repel anions

Question 4

What is a hydration shell?

Answer 4

ring of water molecules that a positively or negatively charged ion attracts whilst in solution

Question 5

What is the knock of mechanism?

Answer 5

• Selectivity filter strips off the hydration shell from our ion
• Our ion moves into the channel and knocks forward ions already in the channel
• In the Kv channel the potassium ion interacts with the oxygen molecules in the channel – this means that the hydration shell is striped from the ion
• If the ion is too small it won’t be able to interact with the oxygen and will keep it’s hydration shell
• If the ion is too big it won’t be able to fit through the channel

Question 6

How does the sodium channel open?

Answer 6

• Open rapidly but inactivated after 1ms
• Sodium channel exists in:
   Resting state
   Open state
   Inactivated state
• Transitions between the states are dependent on voltage
• Changes between states are more probable as the membrane depolarises
• Voltage sensor has charges in it – if you put a charged substance in an electrical field and then change the electrical field the charged particle will move – this happens with the voltage sensor – as we depolarise the membrane the voltage sensor moves and opens the gate of the channel
• Sodium ions can now cross the membrane – as they cross it will increase the membrane potential even more – as it gets even more positive the inactivation gate will swing up and block the channel

Question 7

How does the voltage sensor work?

Answer 7

Voltage sensor has charges in it – if you put a charged substance in an electrical field and then change the electrical field the charged particle will move – this happens with the voltage sensor – as we depolarise the membrane the voltage sensor moves and opens the gate of the channel

Question 8

What are excitable cells key to and what do they do?

Answer 8

• key to communication in the nervous system
• allow sensing of the environment and response to it
• use action potentials to communicate

Question 9

What is the action potential?

Answer 9

• Transient fast reversal of the membrane potential

- Duration: few ms (nerve skeletal muscle) to a few hundred ms (heart)

Question 10

What does it mean that action potentials are all or none?

Answer 10

• Small (sub threshold) stimulus – no action potential

- Larger stimulus – fixed size of action potential

Question 11

How does the body code stimulus intensity?

Answer 11

by changes of frequency of the action potential – not the size of action potentials

Question 12

what is the sequence of events in an action potential?

Answer 12

• Beginning: resting membrane potential
• Apply stimulus: rising phase depolarisation (if over threshold then causes action potential)
• Peak
• Falling phase – undershoots resting potential (after hyperpolarisation)

Question 13

How does the probability of sodium channels being opened work?

Answer 13

Voltage gated sodium channel have a much greater probability of them closing than the probability of them being opened – probability of them opening increases during below threshold stimulus however probability of closing is still greater. If an above stimulus threshold is applied probability of opening&raquo_space; probability closing because the membrane is depolarised further

Question 14

How fast are the neuronal, skeletal muscle and cardiac muscle pacemaker action potentials?

Answer 14

• Neurone – 2ms
• Skeletal muscle – 5ms
• Cardiac muscle pacemaker – 200ms

Question 15

In different tissues what can we have?

Answer 15

• Different duration action potentials
• Different shape action potentials
• Action potentials can depend on different ionic currents

Question 16

What is the anatomy of an action potential?

Answer 16

• Resting membrane potential is depolarised past threshold: rapid opening of sodium channels
• Starts to head towards equilibrium potential for sodium
• When the peak is reached the sodium channels close (inactivate)
• At the peak potassium channels open
• Then we start to head back down to the resting membrane potential for potassium through repolarisation
• Go past the resting potential
• Potassium channels close
• Resting membrane potential is restored

Question 17

What happens with sodium channels opening during an actions potential?

Answer 17

• Start at resting membrane potential, then depolarise past threshold and sodium channels start to open due to voltage channels in sodium channels moving pulling open activation gate
• Sodium ions enter the voltage channel and further depolarisation takes place heading towards positive potential
• With positive potentials the inactivation gate swings up and blocks inner mouth of channel

Question 18

Do potassium channels have a faster or slower activation and inactivation than sodium?

Answer 18

Slower

Question 19

What is the sodium channel positive feedback loop?

Answer 19

- Sodium channel positive feedback loop
 Depolarization opens Na+ channels 
 Membrane depolarises further 
 Open more Na+ channels
 Membrane depolarises further 
 Opens more Na+ channels 
 Membrane depolarises further ect.

Question 20

What controls the sodium channel positive feedback loop?

Answer 20

Sodium channel inactivation

Question 21

What is the potassium channel negative feedback loop?

Answer 21

 Depolarization opens K+ channels
 Membrane repolarizes 
 Open fewer K+ channels 
 Membrane repolarizes
 Opens fewer K+ channels 
 Eventually leads to self-termination of negative feedback loop

Question 22

What does the term ‘Refractory mean’?

Answer 22

Can’t stimulate easily

Question 23

What happens in the absolute refractory period?

Answer 23

Can’t produce another action potential

Question 24

What happens in the relative refractory period?

Answer 24

the cell is less excitable – a larger stimulus is needed

Question 25

What is the refractive period due to?

Answer 25

inactivation of Na current and activation of K current

Question 26

What doesn’t really change during an action potential in terms of ions?

Answer 26

The concentration of the ions

Question 27

What is Dravet syndrome?

Answer 27

• a rare and devastating form of epilepsy
  • It starts in early childhood and typically involves prolonged generalized seizures that can be provoked by elevated temperatures

Question 28

what problems do people with Dravet syndrome have?

Answer 28

• People with Dravert syndrome suffer motor problems and cognitive issues and their condition typically worsens as they get older
• Many also have ADHD and autism.
• Up to 20% of people with Dravet die during their childhood

Question 29

What causes Dravet syndrome?

Answer 29

It is usually caused by a mutation in a gene called SCN1A, which codes for a sodium channel. It is thought that mutations in this sodium channel prevents action potentials in inhibitory neurons. This leads to a lack of inhibition in some brain regions, causing them to become overactive.

Question 30

Why is only the brain affected by Dravet syndrome?

Answer 30

SCN1A codes for NaV1.1 so only one of the 9 types of sodium channel is mutated. However, Nav1.1 is the main type of sodium channel expressed in the inhibitory neurons that malfunction in Dravet syndrome

Question 31

What may be effective in treating Dravet syndrome?

Answer 31

• Certain research has suggested that cannabinoids may be effective in treating Dravet syndrome:

• Most of the interest has centered on cannabidiol (CBD), which is a non-psychoactive component of cannabis
• Some patients have found extracts containing tetrahydrocannabinol (THC) to be more effective