Transmission Of Nerve Impulses

Question 1

What is the “resting potential” of a neurone?

Answer 1

• Neurone is not transmitting nerve impulses.
• Charge difference across membrane.
• Outside is more positive than inside.
• membrane = polarised.
• Na+/K+ ions pumped through protein channels.

Question 2

How is resisting potential achieved?

Answer 2

1- The Na+/K+ pump uses ATP energy to actively pump 3NA+ OUT for every 2K+ IN.
2- The gated Na+ channels are closed so Na+ cannot diffuse back in.
3- The gated K+ channels are open so k+ can move back out by facilitated diffusion.
NET RESULT - more + charges on outside of membrane than inside, so membrane is polarised.

Question 3

What is action potential?

Answer 3

• If generator potential in receptor is big enough, AP will be triggered along sensory neurone.
• Axon membrane becomes more permeable to Na+, which diffuse in to reverse charge difference across membrane.

Question 4

How is an action potential achieved?

Answer 4

1- membrane starts off in resting polarised state (pd across membrane is -70mv).
2- when stimulated, gated Na+ channels open and Na+ diffuse in.
3- when enough Na+ diffuse in, p.d changes to -55mv = threshold potential and causes voltage gated Na+ channels to open so even more Na+ diffuse in
- THIS IS POSITIVE FEEDBACK : AS MORE Na+ ENTER, THEY BRING ABOUT CHARGE CHANGE TO ALLOW EVEN MORE Na+ TO ENTER.
4- p.d across membrane increases to +40m, even more positive on outside, this is action potential, which is transmitted along whole neurone as N.I Na+ channels close, K+ channels open.
5- Membrane is no longer permeable to Na+, but is to K+, which diffuse out, repolarisation occurs (more + on outside again).
6- Hyperpolarisation -> too many K+ diffuse out
Voltage gated K+ channels close, resting potential is restore by Na+ and K+ pump (which is always working).

Question 5

What is the refractory period?

Answer 5

• Until resting potential is restored, you cannot stimulate the neurone again.
• Allows neurone to recover after A.P.
• Ensures A.P don’t overlap.
• Ensures impulse only moves in one direction along neurone (can’t go back as it’s resting).

Question 6

What is a nerve impulse?

Answer 6

A.P that starts at 1 end of a neurone and is propagated to other end.

Question 7

How does A.P travel along neurone?

Answer 7

1- After Na+ diffuse in when membrane is depolarised, some diffuse sideways, creating a localised electrical current.
2- Changing the charge further along the axon membrane, opening more Na+ channels.
3- More Na+ diffuse in, some sideways - more depolarisation,

Question 8

What size is the A.P depending on the stimulus?

Answer 8

A.P all same size (+40mv) regardless of stimulus.

Question 9

How does the frequency of A.P along a neurone change?

Answer 9

• The stronger the stimulus = the more frequent action potential to the brain.

Question 10

What is myelin sheath?
- what does it do?
- what secretes it?

Answer 10

• Lipid.
• Electrical insulator (doesn’t let ions in/out).
• Secreted by Schwann cells.

Question 11

Where can ion movement occur in myelin sheath?

Answer 11

• Ion movement can only occur at gaps in myelin sheath, so N.I jumps from one node of Ranvier to the other to pass along neurone, so impulse transmission is faster. This is known as saltatory conduction.

Question 12

Factors that affect the speed of N.I along neurone.

Answer 12

1) axon diameter
Wider diameter = faster N.I = less resistance to movement of ions in/out of memb.
2) Temperature
Diffusion varies with temp = ions move through membrane via diffusion to trigger A.P. Increase of temp increases rate of diffusion.
3) mylein sheath
Non-myelinated neurones -> A.P moves along neurone in wave
-Slows N.I - ion channels have to open and close all the way along the length of the neurone
- longer to reach end of axon