3.6.2.1 nerve impulse

Question 1

what is a nerve impulse

Answer 1

movement of an action potential along a neurone

Question 2

what is resting potential

Answer 2

when voltage is maintained across the membrane of the axon when at rest. more K+ inside the neurone, more Na+ outside the neurone. at -70mV

Question 3

how is resting potential maintained

Answer 3

sodium/potassium pump actively transports 3Na+ out of the axon and 2 K+ inside the axon
the membrane is more permeable to K+ as there are more channels. K+ moves out via facilitated diffusion.

Question 4

what is an action potential

Answer 4

when the voltage of the neurone increases beyond the threshold beyond -55-40 mV

Question 5

depolarisation

Answer 5

once threshold is reached, voltage gated sodium ion channels open, Na+ diffuse into the axon making it less negative until +40mV

Question 6

repolarisation

Answer 6

at +40mV Na+ channels close and voltage gates K+ channels open. K+ diffuse out of the axon making the neurone more negative

Question 7

hyperpolarisation

Answer 7

K+ channels are slow to close causing an overshoot in voltage.
too many K+ diffuse out of the axon, causing potential difference to decrease lower than resting potential
the sodium/potassium pump returns the membrane to resting potential

Question 8

refractory period

Answer 8

Na+ channels are recovering and can not open
this allows nerve impulses to be discrete, (not overlap) unidirectional and limit the frequency

Question 9

all or nothing principle

Answer 9

if depolarisation does not exceed the threshold an action potential will not occur
any depolarisation exceeding threshold will peak at the same maximum voltage

Question 10

factors which affect speed of nerve impulses

Answer 10

myelination, axon diameter and temperature

Question 11

how does myelination affect speed

Answer 11

the myelin sheath acts as an electrical insulator allowing saltatory conduction which is when the nerve impulse jumps one node of ranvier to the next. without a myelin sheath continuous conduction occurs, where depolarisation occurs across the entire length of the axon which is slower

Question 12

how axon diameter affects speed

Answer 12

larger diameter, action potential moves faster due to less resistance and less leakage of ions

Question 13

how temperature affects speed

Answer 13

as temp increases towards optimum temp, kinetic energy of ions and channels increases, increasing rate of diffusion and therefore depolarisation. higher temp will also increase rate of respiration so more ATP produced for active transport. if temp increases past optimum temp, channels will denture as well as enzymes used in respiration