Action Potentials Flashcards
When does a voltage-gated ion channel open?
When the membrane depolarises - due to a change in voltage
What are all voltage-gated ion channels selective for?
Cations (K+, Na+ and Ca2+)
What do the rings of charge around the mouth of voltage gated channels do?
help filter out desired ions from undesired. Ring of negative charge would attract cations and repel anions
What is a hydration shell?
ring of water molecules that a positively or negatively charged ion attracts whilst in solution
What is the knock of mechanism?
- 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
How does the sodium channel open?
- 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
How does the voltage sensor work?
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
What are excitable cells key to and what do they do?
- key to communication in the nervous system
- allow sensing of the environment and response to it
- use action potentials to communicate
What is the action potential?
- Transient fast reversal of the membrane potential
- Duration: few ms (nerve skeletal muscle) to a few hundred ms (heart)
What does it mean that action potentials are all or none?
- Small (sub threshold) stimulus – no action potential
- Larger stimulus – fixed size of action potential
How does the body code stimulus intensity?
by changes of frequency of the action potential – not the size of action potentials
what is the sequence of events in an action potential?
- Beginning: resting membrane potential
- Apply stimulus: rising phase depolarisation (if over threshold then causes action potential)
- Peak
- Falling phase – undershoots resting potential (after hyperpolarisation)
How does the probability of sodium channels being opened work?
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»_space; probability closing because the membrane is depolarised further
How fast are the neuronal, skeletal muscle and cardiac muscle pacemaker action potentials?
- Neurone – 2ms
- Skeletal muscle – 5ms
- Cardiac muscle pacemaker – 200ms
In different tissues what can we have?
- Different duration action potentials
- Different shape action potentials
- Action potentials can depend on different ionic currents
What is the anatomy of an action potential?
- 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
What happens with sodium channels opening during an actions potential?
- 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
Do potassium channels have a faster or slower activation and inactivation than sodium?
Slower
What is the sodium channel positive feedback loop?
- 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.
What controls the sodium channel positive feedback loop?
Sodium channel inactivation
What is the potassium channel negative feedback loop?
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
What does the term ‘Refractory mean’?
Can’t stimulate easily
What happens in the absolute refractory period?
Can’t produce another action potential
What happens in the relative refractory period?
the cell is less excitable – a larger stimulus is needed
What is the refractive period due to?
inactivation of Na current and activation of K current
What doesn’t really change during an action potential in terms of ions?
The concentration of the ions
What is Dravet syndrome?
- 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
what problems do people with Dravet syndrome have?
- 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
What causes Dravet syndrome?
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.
Why is only the brain affected by Dravet syndrome?
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
What may be effective in treating Dravet syndrome?
• 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
