The Nerve Impulse

Question 1

what is the resting potential?

Answer 1

-the potential difference that neurones have across their cell surface membrane

Question 2

why is there a resting potential what causes this?

Answer 2

-the neurones are polarised due to the electrochemical gradient across the membrane

Question 3

what does polarised mean?

Answer 3

-it is when the outside of the neurone is more positive relative to the inside

Question 4

how are neurones polarised during the resting potential?

Answer 4

-this potential difference is because of there being an excess of positively charged sodium ions outside the membrane

Question 5

how many millivolts is the polarised potential difference in neurones during the resting potential? (the outside is more positive to inside)

Answer 5

-70mV

Question 6

what is one adaptation of the cell surface membrane of neurones to ensure the outside is more positive to the inside during the resting potential?

Answer 6

-the cell surface membrane is largely impermeable to the flow of sodium ions when not conduction an impulse

Question 7

when a neurone is stimulated what happens to the cell surface membrane and what does this cause to happen?

Answer 7

-the membrane becomes permeable to sodium ions
-because the outside is more positive to the inside the ions move down the concentration gradient and into the neurone

Question 8

how does depolarisation of the neurone occur during the action potential and what does this cause to open?

Answer 8

-as the membrane becomes permeable to the ions the potential difference across the membrane decreases
-when a threshold is reached a number of gated protein channels open which rapidly causes more ions to enter which leads to this depolaristation

Question 9

what does the action potential result in?

Answer 9

-the inside of the neurone being more positive to the outside

Question 10

at the peak of the action potential what is the membrane potential in millivolts? why?

Answer 10

+40 mV
-because the inside is more positive compared to the outside

Question 11

what is the name of the phase that begins immediately after a peak of action potential is reached?

Answer 11

-the recovery phase

Question 12

what happens during the recovery phase?

Answer 12

-positive ions both diffuse and are pumped out of the neurone which rapidly restores the resting potential

Question 13

what happens to the neurone membrane during the recovery phase?

Answer 13

-the membrane becomes largely impermeable to sodium ions again

Question 14

why can another impulse not occur during the recovery phase?

Answer 14

-gated ion channels are closed
-resting potential has not been fully restored

Question 15

what is another name for the recovery phase?

Answer 15

-refractory period

Question 16

what is hyperpolarisation?

Answer 16

-at the end of the recovery phase/ refractory period there is a slight overshoot meaning the membrane becomes slightly more negative than normal resting potential

Question 17

what are the 2 functions of the refractory period?

Answer 17

-ensures nerves are propagated in one direction (unidirectional) which is important as axons can transmit impulses in both directions
-limits the number of action potentials to prevent overstimulation of nerves

Question 18

what are two important principles of the refractory period?

Answer 18

-the threshold stimulus
-the all of nothing law

Question 19

what does the threshold stimulus refer too?

Answer 19

• the level of stimulation a neurone requires before an action potential is reached e.g. a critical point of depolarisation must be reached

Question 20

what does the all or nothing law refer to?

Answer 20

-when the threshold stimulus is reached the action potential either occur or it doesn’t

Question 21

how do nerve impulses move along neurones?

Answer 21

-a wave of depolarisation moves rapidly along the neurone
-immediately behind this depolarised zone the neurone becomes repolarised

Question 22

what is the purpose of the local circuits being set up in the neurone?

Answer 22

-when one part of the membrane becomes depolarised local circuits are set up with the area immediately adjacent to the depolarised zone

Question 23

how does nerve propagation actually occur?

Answer 23

-positive ions from the depolarised zone pass along directly infront to the polarised zone
-immediately before the depolarised zone the region behind this become repolarised
-outside the membrane (where it is more positive to the inside) the ions move into the depolarised zone
-causing a wave of depolarisation which causes the impulse so move along the nerve fibers

Question 24

what are the 2 factors that influence the speed of a nerve impulse?

Answer 24

-the presence/ absence of myelin sheath
-thickness of axon

Question 25

what does the myelin sheath act as and what does it prevent?

Answer 25

-acts as an electrical insulator
-prevents depolarisation

Question 26

what is saltatory conduction and why does the myelin sheath cause this? does this speed up or slow down impulse propagation?

Answer 26

-every 1-2mm the sheath is disrupted by gaps called nodes of Ranvier
-at these gaps depolarisation can occur
-this means that the local circuits can only form at the nodes which causes sections of the neurone to be bypassed
-so the impulses jump from gap to gap which causes the speed to incease?

Question 27

in what way does the diameter affect the speed of an impulse? why is this?

Answer 27

-the thicker the axon the faster the impulse
-less leakage will occur of ions due to the thicker axon

Question 28

why is leakage of ions due to a thin axon a problem?

Answer 28

-more leakage will occur
-which makes it more difficult to maintain the potential gradients needed for resting and action potentials to form

Question 29

how does temperature affect the speed of an impulse?

Answer 29

-temperature affects the rate of diffusion of ions involved and so affects the speed of impulse propagation
-high temp = faster impulses