3.6.2.1 Nervous impulse

Question 1

the structure of a myelinated motor neurone

Answer 1

Cell body (dendrites, nucleus)
Axon
Myelin sheath
node of ranvier
Schuwann cell (rich in lipid, wrap around)
axon terminal

Question 2

Myelinated vs non- myelinated
- how is it slower

Answer 2

saltatory conductin
electronic insulation

• In non-myelinated, the next section of the axon membrane is depolarised / the whole axon membrane depolarise
• In myelinated, depolarisation / ion movement only occurs at the nodes of Ranvier;
• The impulse jumps from node to node / saltatory conduction;

Question 3

AP might be slow down by

Answer 3

auto immune disease - damage by immune system

Question 4

2 reason that causes the resting membrane potential to be -70mV

Answer 4

• The sodium-potassium pump actively transports 3 sodium ions out and 2 potassium ions in.
• Then makes the inside more negative than the outside of the axon/outside more positive.
• There are more potassium leak channels (always open) than sodium leak channels (always open) in the axon membrane.
• More potassium ions leak out than sodium ions leak in, adding to the negative resting potential on the inside. (facilitated diffusion)

Na+ higher outside

Question 5

defintion of nerve impulse

Answer 5

A nerve impulse is an action potential that moves along a neurone in one direction

Question 6

definition of action potential

Answer 6

An action potential is a change in the membrane potential that spreads along an axon

Question 7

Whole process of all or nothing principle in generating action potential

Answer 7

• The arriving stimulus disturbs the resting membrane potential (of -65 /-70 mV), causing voltage-gated sodium (Na+) channels to open.
• Na+ diffuses into the axon, down its concentration gradient, causing the membrane to depolarise (the inside now becomes more positively charged than the outside).
• When the membrane potential reaches +40 mV the voltage-gated Na+ channels close and the voltage-gated potassium (K+) channels open.
• K+ diffuses out of the axon into the extracellular fluid, causing the membrane to repolarise (becoming more negatively charged on the inside again compared to the outside).
• The membrane potential overshoots the resting membrane potential to about -70 / -75 mV, called hyperpolarisation.
• At this point, the voltage-gated K+ channels close.
• Leak channels and the sodium potassium pumps work to restore the resting potential

Question 8

how an action potential passes along an unmyelinated axon?

Answer 8

• The axon membrane is at rest - it is polarised. A stimulus then arrives which disturbs the membrane potential, causing voltage-gated Na+ channels open and for Na+ to diffuse in.
• If the threshold is reached, an action potential will be initiated and depolarisation of the membrane will occur (becomes more positive inside).
• Local ion movements occur, where Na+ ions diffuse through the channel down a concentration gradient. This movement of Na+ ions creates local electrical currents.
• The local electrical currents disturb the membrane potential a little further along the axon (the next adjacent section) - causing Na+ channels to open and Na+ to diffuse in, generating a new action potential. As this next section of the membrane depolarises, the previous section repolarises.
• The process repeats and a wave of depolarisation spreads along the whole length of the axon of the neurone

Question 9

why action potential is passive

Answer 9

only happen due to concentration gradient in resting potential

Question 10

Factor affecting the speed of conductance

Answer 10

myelination + saltatory conduction
Impulses travel faster by jumping between the nodes of Ranvier (saltatory conduction)

axon diameter
Impulses travel faster in larger axons as there is less resistance to ion flow

temperature
Enzymes (Na+K+ATPase pump) and ion diffusion are affected by temperature.
Low temperatures can slow down the transmission in organisms that do not regulate their body temperature (cold-blooded).

Question 11

The meaning of all or nothing principle

Answer 11

• A specific level of stimulus, called the threshold value, is required to trigger an action potential.
• Below the threshold, no APs are generated.
• All APs are more or less the same size - a bigger stimulus will not generate a bigger AP.

Question 12

How can the organism detect the size of the stimulus under all or nothing principle

Answer 12

The larger the stimulus, the higher the frequency of APs

OR

Different neurones have different threshold values (and the incoming information can be interpreted by the brain)

Question 13

the importance of refractory period

Answer 13

• producing dicrete impulses
• limiting the frequency of impulse transmission

Question 14

Refractory period

Answer 14

During the refractory period (for about 5 ms in the figure) the voltage gated sodium channels remain closed and cannot be stimulated to open.

This means that an action potential during this time cannot occur.

This period corresponds to when the membrane is depolarised, repolarised and hyperpolarised

During the later stage of the refractory period (hyperpolarisation), the sodium potassium pump works to redistribute the ions for subsequent impulses to be propagated

Another stimulus won’t trigger an AP until normal resting excitability is restored.
This means that the highest frequency achieved would be one every 6 ms.