Topic 6- Resting potentials, nerve impuleses and refactory period Flashcards
What is the function of ‘normal’ ion channels (pore proteins)?
-Let relatively small amounts of ions through slowly
Do potassium ion channels remain open all the time?
Yes
Why are sodium ion channels usually ignored? What does this cause the membrane to be?
-Because there are so few of them
-Impermeable to sodium ions
What are the functions of the voltage-gated channels?
-Allow large amounts of sodium/potassium ions to pass through rapidly
What do voltage-gated channels cause?
-A rapid change in the potential across the membrane
How do sodium- potassium pumps move ions? What does it require?
-Active transport
-ATP
How does the sodium potassium pump move ions compared to voltage gated channels (in terms of speed)?
-Moves ions slowly
Explain which proteins (S-P pump, ion channels, VGC) are open/ closed/working while establishing a resting potential
-Pump is working
-All voltage gated channles are closed
What ratio does the NA/K pump actively transport sodium out of and potassium in?
3:2
Explain what happens to establish a resting potential and during the refactory period
-Some K ions diffuse out of the ion channels but not very many as they are repelled by the positively charged NA ions on the outside
-This results in a high concentration of NA outside the axon and a high concentration of K inside the axon
-Overall, there are more negative ions than positive ions inside the axon (as there are other negatively charged ions in the axon, so they overide the positive charges of NA and K) therefore,e the inside is negatively charged (membrane is polarised)
Explain what happens when depolarisation occurs.
-Stimulus causes a few voltage-gated NA channels to open
-NA diffuses very rapidly (to pass the threshold of 50)
-As NA diffuses in more voltage-gated NA channels open (positive feedback)
-Inside of axon becomes positively charged
What triggers depolarisation
A stimulus
Why does NA diffuse very rapidly?
To produce an action potential, enough NA must enter to pass the threshold of 50
What effect do the pump and regular potassium ion channels have during depolarisation and repolarisation?
None- their effects are insignificant
Explain what happens during repolarisation
-NA gated channels close and voltage-gated K channels open
-K diffuses out very quickly
-This causes the inside of the axon to become negatively charged again (so it is repolarised)
What are the three factors which impact the speed of conduction in action potentials?
-Meylination
-Increase in temprature
-Increase in axon diameter (in unmyelinated axons only)
Explain why myelination increases the speed of conductance.
-Where myelin is present, ions cannot move through the axon membrane
-Therefore, the action potential ‘jumps’ from node to nod,e which is known as saltatory conduction
-So the impulses move faster
Explain why an increase in temprature increases speed of conductance
-Because the impulse depends on diffusion of ions, increasing temperature increases the speed of diffusion
Does an increase in temperature increase the speed of conductance in all animals?
-No, humans, mammals and birds have a constant body temperature as they are warm-blooded, so the temperature of the environment does not affect internal body temp
-Yes, cold-blooded animals
Explain how an increase in axon diameter increases speed of conductance (in unmyelinated axons only)
-Larger axons have a smaller surface area to volume ratio
-therefore have relatively less ‘leakage of ions’ (they maintain resting potentials better)
-The myelin sheath reduces ‘leakage’
What is meant by the electrochemical gradient?
Sodium ions pass into the axon down the electrochemical gradient- there is a greater concentration of sodium ions outside the axon than inside.
What is the all-or-nothing principle?
-An impulse is only transmitted if the initial stimulus is sufficient to increase the membrane potential above a threshold potential
-Rather than staying constant, threshold levels in receptors often increase with continued stimulation, so that a greater stimulus is required before impulses are sent along sensory neurones
Why is the refactory period important?
-It ensures that action potentials are discrete events, stopping them from merging into one another
-It ensures that ‘new’ action potentials are generated ahead (further along the axon), rather than behind the original action potential, as the region behind is ‘recovering’ from the action potential that has just occurred
-This means that the impulse can only travel in one direction, which is essential for the successful and efficient transmission of nerve impulses along neurones
-This also means there is a minimum time between action potentials occurring at any one place along a neurone
-The length of the refractory period is key in determining the maximum frequency at which impulses can be transmitted along neurones (between 500 and 1000 per second)
