5.3.3 nerve impulses: action potentials

Question 1

action potential

Answer 1

brief reversal of potential across membrane of neurone causing a peak of +40mV compared to resting potential of -60mV

Question 2

positive feedback

Answer 2

mechanism that increases a change taking it further away from optimum

Question 3

resting potential

Answer 3

potential difference across membrane while neurone is at rest

Question 4

describe actions of a neurone at rest

Answer 4

• actively pumping ions across cell surface membrane
  –> sodium/potassium ion pumps use ATP to pump 3 sodium ions out & 2 potassium ions in
• gated sodium ion channels closed
• some potassium ion channels open = membrane more permeable to potassium ions than sodium
  –> diffuse out of cell
• cytoplasm contains large organic anions (negatively charged ions) = negative potential compared to outside
  –> cell membrane = polarised

Question 5

what is the resting potential of the cell membrane

Answer 5

-60mV

Question 6

describe the 9 stages of an action potential

Answer 6

1. membrane starts in resting state (polarised & inside of cell -60mV)
   - higher concentration of sodium ions outside vs inside
   - higher concentration of potassium ions inside vs outside
2. sodium ion channels open & some sodium ions diffuse into cell down conc gradient
3. membrane DEPOLARISES & reaches threshold of -50mV
4. positive feedback causes nearby voltage-gated sodium ion channels to open & sodium ions flood in = cell becomes positively charge inside
5. potential difference across cell surface membrane reaches +40mV (inside of cell positive vs outside)
6. sodium ion channels close & potassium ion channels open
7. potassium ions diffuse out bringing potential difference more negative inside vs outside = REPOLARISATION
8. potential difference overshoots slightly = HYPERPOLARISATION
9. original potential difference restored = resting state

Question 7

generator potential

Answer 7

• in generator region of neurone, gated channels opened by action of synapse
• when few gated channels open, they allow few sodium ions into cell & produce small depolarisation

Question 8

‘all-or-nothing’ response

Answer 8

• action potential is self-perpetuating = once it starts at one point of neurone, it will continue along to end of neurone
• all action potentials are of same magnitude

Question 9

describe the refractory period

Answer 9

• after an action potential, the sodium/potassium ions are in wrong places
• concentrations of ions inside/outside cell must be restored
• short time after = impossible to stimulate cell membrane to reach another action potential