Nerves

Question 1

2 ways in which an axon may be adapted to allow for faster speed of impulse?

Answer 1

Myelinated- allows for saltatory conduction
Longer axon diameter- less resistance to the flow of ions

Question 2

What is the resting potential?

Answer 2

-70mV
The inside of an axon has a charge that is 70mV more negative than the outside

Question 3

Establishing a resting potential?

Answer 3

Na+/K+ pump- involving active transport and ATP
Pump moves 2K+ ions in and 3Na+ ions out
Creates an electrochemical gradient and results in K+ diffusing out through open K+ channels and Na+ diffusing in
Cell membrane is more permeable to K+ ions- more K+ ion channels open than Na+ ion channels. Na+ ion channels are closed- high Na+ concentration outside of cell.

Question 4

When does depolarisation occur?

Answer 4

An increased voltage above the resting potential- due to the neurone membrane becoming more permeable to Na+ ions

Question 5

How is an action potential generated?

Answer 5

Stimulus- Na+ ion channels open.
Membrane becomes more permeable to sodium, so Na+ ions diffuse into the neurone down the Na+ ion electrochemical gradient. Making the inside of the neurone less negative- from -70 to -55.
If the threshold potential is reached (-55mV) more gated voltage Na+ channels open- more Na+ ions diffuse into the membrane
At a potential difference of around +40mV, the Na+ ion channels close and K+ ion channels open. The membrane is more permeable to K+ so K+ ions diffuse out the neurone down the K+ ion concentration gradient- begins to get the membrane back to resting potential.

Question 6

All or nothing principle?

Answer 6

If the depolarisation does not exceed -55mV, an action potential and the impulse are not produced (nothing)

Question 7

Does every stimulus that triggers depolarisation to -55mV always peak at the same maximum voltage?

Answer 7

Yes- bigger stimuli increase the frequency of action potentials- not the size
Dont get a larger peak, just a larger frequency
Important as it makes sure that animals only respond to large enough stimuli, rather than responding to every slight change in the environment, which would overwhelm them

Question 8

What is the refractory period?

Answer 8

-80mV
After an action potential has been generated the membrane enters a refractory period when it cant be stimulated because Na+ channels are recovering and cant be opened
This is important: discrete impulses can be produced- meaning that each action potential cant be generated immediately after one another
Ensures that action potentials travel in one direction- stops the action potential from spreading in 2 directions, which would prevent a response
Na+ ion channels are closed during hyperpolarisation- depolarisation cant occur in that region until the resting potential has been restored