15.2+.3+.4 The nerve impulses

Question 1

Resting potential of axon

Answer 1

1. 3 Na+ ions actively transported out of axon by sodium-potassium pumps
2. 2 K+ ions actively transported into axon by sodium-potassium pumps
3. K+ ions begin to diffuse out of axon through open voltage gated channels
4. Na+ voltage channel gates are closed so can’t diffuse in
5. electrochemical gradient is created(axon is negatively charged relative to outside- polarised)

Question 1

depolarisation

Answer 1

1. energy of stimulus reaches over threshold value
2. causes Na+ voltage-gated channels in axon membrane to open so Na+ diffuse into the axon along electrochemical gradient
3. A reversal in potential difference in membrane is triggered
4. As Na+ ions diffuse into axon, more Na+ channels open(more permeable), causing greater influx
5. The axon is now more positive than the external environment- having reached +40mV

Question 2

all or nothing principle

Answer 2

once energy reaches over threshold value, all action potentials reach the same charge

Question 3

saltatory conduction

Answer 3

fatty myelin sheath is electrical insulator, preventing action potentials from forming

localised circuits arise between nodes of Ranvier

actions potentials jump from node to node

Question 4

myelinated axon vs unmyelinated axon

Answer 4

action potential is faster in myelinated

unmyelinated events of depolarisation takes place all along the axon

Question 5

3 factors affecting speed of action potential

Answer 5

1. myelin sheath
2. diameter of axon
3. temperature

Question 6

diameter of axon action potential speed

Answer 6

greater the diameter, the faster the speed of conductance
- less leakage of ions from larger axon
-leakage makes membrane potentials harder to maintain

Question 7

temperature action potential speed

Answer 7

• the higher the temp, the faster the rate of diffusion, the faster the nerve impulse

-energy for active transport comes from respiration, which is controlled by enzymes
= too high temperature denatures plasma membrane proteins and enzymes and impulses dont conduct

Question 8

Threshold value

Answer 8

certain level of stimulus that triggers an action potential
- below it, no action potential and no impulse

Question 9

2 ways to tell size of stimlus

Answer 9

1. number of impulses passing in a given time(larger stimulus, more impulses)
2. different neurones have different threshold values. the brain interprets number and type of neurons that pass impulses as result of stimulus and so can determine its size

Question 10

refractory period

Answer 10

period after an action potential has been created in any region of an axon

• inward movement of sodium ions are prevented because voltage-gated channels are closed
• impossible for further action potential to be generated

Question 11

3 functions refractory period

Answer 11

1. ensures action potentials are propagated in one direction only
2. produces discrete impulses
3. limits number of action potentials

Question 12

repolarisation

Answer 12

1. Once action potential has reached +40mV, voltage gates on Na+ channels close and K+ voltage channels open
2. electrical gradient is now reversed, so more K+ ions voltage channels open so more K+ ions diffuse out- start repolarisation of axon

Question 13

Hyperpolarisation

Answer 13

outward diffusion of K+ ions cause temporary overshoot of electrical gradient(inside of axon more negative than usual)