13.4 Nervous transmission

Question 1

what is the normal resting potential

Answer 1

-70mV

Question 2

what results in the creation of a resting potential

Answer 2

• Na+ actively transported out of the axon and K+ actively transported into the axon by the sodium-potassium pump (3 Na+ out=2K+ in)
• more Na+ outside and more K+ inside the axon so Na+ diffuse back into the axon down an electrochemical gradient and K+ diffuse out
• most ‘gated’ sodium channels are close but potassium ones are open so Na+ don’t move but K+ diffuse out
• more positively charged ions outside the axon so resting potential is created across the membrane

Question 3

what is the sodium-potassium pump

Answer 3

a specific intrinsic protein that actively transports Na+ out of and K+ into the axon

Question 4

what causes depolarisation

Answer 4

• if some Na+ channels are opened, then Na+ will quickly diffuse down its concentration gradient into the cell from the tissue fluid - therefore potential difference becomes +40mV

Question 5

when does an action potential occur

Answer 5

voltage-gated ion channels in the axon membrane change shape as a result of the change of voltage across its membrane

Question 6

describe the stages of an action potential

Answer 6

1. membrane is in its resting potential (higher conc of Na+ outside and higher conc of K+ inside)
2. Na+ channels open and some Na+ diffuse into the cell
3. membrane depolarises
4. positive feedback causes voltage-gated Na+ channels to open and many Na+ flood in (the cell starts to become more positively charged inside)
5. potential difference reachers +40mV and the inside is positive compared to the outside
6. Na+ channels close and K+ channels open
7. repolarisation - K+ diffuse out and make the inside negative
8. potential difference overshoots slightly making the cell hyperpolarised
9. original potential difference is restored and cell returns to resting state

Question 7

what is the refractory period

Answer 7

• the short period of time after an action potential where the axon can’t be excited again
• voltage-gated Na+ channels remain closed

Question 8

why is the refractory period important

Answer 8

• prevents propagation of an action potential backwards along the axon
• makes sure axon potentials are unidirectional
• ensures that action potentials do not overlap and occur as discrete impulses

Question 9

what is saltatory conduction

Answer 9

• action potential ‘jumps’ from one node to another because depolarisation of the axon membrane can only occur at the nodes of Ranvier in myelinated axons
• longer localised circuits therefore arise between adjacent nodes

Question 10

what are the advantages of saltatory conduction

Answer 10

• speeds up the action potential transmission as channels opening and ions moving happens in less places
• repolarisation uses ATP in the sodium pump, so reducing amount of depolarisation needed makes it more energy efficient over a long period of time

Question 11

what are the effects of axon diameter on the speed at which an action potential travels

Answer 11

• the bigger the axon diameter, the faster the impulse is transmitted
• less resistance to the flow of ions in the cytoplasm in larger axons

Question 12

what are the effects of temperature on the speed at which an action potential travels

Answer 12

• the higher the temperature, the faster the nerve impulse
• ions diffuse faster at higher temperatures
• however at over 40 degrees, proteins get denatured

Question 13

explain the all-or-nothing principle

Answer 13

• threshold value always triggers a response
• no matter how large the stimulus, the same sized action potential will always be triggered
• size of stimulus does affect number of action potentials generated in given time
• the larger the stimulus, the more frequently action potentials are generated