module 5 - 13.4 nervous transmission

Question 1

what is resting potential?

Answer 1

potential difference or voltage across the whole plasma membrane of a neurone

Question 2

why is the plasma membrane polarised?

Answer 2

• because the outside of the neurone is more positively charged than inside of the cell
• voltage is -70mV the less positive side on inside of the neurone

Question 3

how is resting potential generated?

Answer 3

by 3 transmembrane proteins:
- Na+/K+ ATPase (sodium potassium pump): a carrier protein/pump that required ATP as it does active transport and never stops working
- Na+ channel proteins: can open or close, called voltage gated channel proteins, does facilitated diffusion
- K+ channel proteins: can open or close, called voltage gated channel proteins, does facilitated diffusion

Question 4

what does the sodium ion potassium ion ATPase pump do?

Answer 4

pumps sodium from the cytoplasm of the neurone into the tissue fluid and then pumps pumps potassium ions from the tissue fluid into the cytoplasm
- for every 3 sodium ions out, 2 potassium ions are pumped into the cytoplasm

Question 5

what does the unequal pumping of the 2 ions create?

Answer 5

• a concentration/electrochemical gradient of each ion across the plasma membrane of the neurone
• this creates a voltage across the membrane but not the -70mV

Question 6

what does the opening and closing of the Na+ channel protein depend on?

Answer 6

depends on localised voltage across the membrane where each protein is

Question 7

what does the voltage created by the Na+/K+ ATPase pump cause the Na+ to do?

Answer 7

• causes Na+ channel proteins to keep their pores closed
• this then stops Na+ from diffusing down the concentration gradient and increases the concentration of positive ions outside the neurone

Question 8

what does the opening and closing of the K+ channel protein depend on?

Answer 8

depends on localised voltage across the membrane where each protein is

Question 9

what does the voltage created by the Na+/K+ ATPase pump cause the K+ to do?

Answer 9

• causes K+ channel proteins to keep their pores closed
• this then causes K+ to diffuse down the concentration gradient and increases the concentration of positive ions outside the neurone

Question 10

what can the generator potential be made by?

Answer 10

sensory receptor or another neurone

Question 11

what happens if the generator potential can be made by a sensory receptor or another neurone?

Answer 11

• then a loaclised change in plasma membrane voltage will have been created
• this is localised depolarisation of plasma membrane

Question 12

what is the localised depolarisation of the plasma membrane?

Answer 12

• transduction of the stimulus energy into the electrical energy of an action potential
• this causes depolarisation of plasma membrane to +40mV and will spread across the neurone as a wave of depolarisation

Question 13

at rest, are the nodes in the heart polarised or depolarlised?

Answer 13

polarised - a positive change builds up on the inside of the node, negative on the outside
- this is caused by ions building up across the cell membranes of the SAN

Question 14

what happens when a contraction occurs?

Answer 14

• the nodes are depolarised
• positively charged ions move out of the nodes stimulating electrical activity in the heart
• this spreads through the electrical conduction system of the heart as a wave of depolarisation by moving between myocytes through intercalated discs

Question 15

what is the resting/action potential figure?

Answer 15

-70/65mV

Question 16

how is the resting potential established?

Answer 16

established by the combined action of the 4 transmembrane proteins, with the voltage gated Na+ channel proteins closed and the open K+ channel proteins
- plasma membrane is POLARISED

Question 17

what does a localised generator potential create?

Answer 17

• creates localised voltage across neurones plasma membrane and reaches the voltage required to cause Na+ voltage gated channel proteins to open their pores
• sodium ions diffuse into neurone, so plasma membrane is -60mV

Question 18

what does the opening of Na+ voltage gated channel proteins cause in an action potential?

Answer 18

• causes plasma membrane to have more Na+ voltage gated channel proteins opening pores so more Na+ ions flood into neurone
• plasma membrane now rapidly depolarising
• inside of neurone becomes positive

Question 19

what happens after the inside of the neurone becomes positive and the plasma membrane rapidly depolarises?

Answer 19

• +40mV is reached
• plasma membrane now fully depolarised
• Na+ voltage gated channel proteins now close pores as plasma membrane is at wrong voltage to have them open
• K+ voltage gated channel proteins open their pores

Question 20

after the K+ voltage gated channel proteins open their pores, the sodium ions can no longer enter the neurone, why?

Answer 20

• potassium ions diffuse out neurone down their electrochemical gradient through 2 types of K+ channel proteins
• movement of K+ out the neurone causes plasma membrane to repolarlise
• inside neurone loses positive charge
• plasma membrane back to -70mV

Question 21

what happens when the potassium ions continue to leave the neurone during action potential?

Answer 21

• causes plasma membrane to become hypoerpolarised and voltage of -84mV
• inside of neurone more negative then resting potential
• triggers most K+ voltage gated channel proteins to close pores
• sodium-potassium pump re-establishes resting potential of -70mV

Question 22

what is the refractory period?

Answer 22

the short period of a few milliseconds when an action potential cannot be initiated (after an action potential has passed over a section of plasma membrane)

Question 23

what is the refractory period caused by?

Answer 23

Na+ voltage gated channel proteins cannot reopen for a few millisecondss once they have closed at +40mV

Question 24

why is the refractory period vital?

Answer 24

• because it makes it impossible for an action potential to travel backwards along an axon where the action potential has just been
• means action potentials cannot overlap

Question 25

what are action potentials travelling along an axon?

Answer 25

• unidirectional
• discrete (dont overlap)

Question 26

what is saltatory conduction?

Answer 26

when an electrical impulse jumps gtom one node to the next, increasing the speed of transmission of the impulse

Question 27

what happens when an action potential is initiated at the start of the myelinated axon?

Answer 27

• sodium ions flood into cytoplasm
• sodium ions then diffuse through cytoplasm towards next Node of Ranvier
• this is localised current

Question 28

what happens when the sodium ions arrive at the next Node of Raniver during saltatory conduction?

Answer 28

• they cause the membrane voltage to change and act as a localised generator potential (just like in the start of an action potential)
• then next node of Ranvier opens as they are now at the right voltage to
• new action potential now triggered in this node

Question 29

what happens as a result of the new action potential being triggered?

Answer 29

causes a flow of sodium ions to flow through the cytoplasm towards next node

Question 30

what is the transmission of an action potential along the axon affected by?

Answer 30

1. myelinated or non-myelinnated
2. axon diameter - lower resistance to ion flow in cytoplasm and m=bigger SA allowing more transmembrane proteins
3. temperature - increases rate of diffusion

Question 31

which animals have myelinated neurones?

Answer 31

mammals

Question 32

what is the threshold value within the ‘all or nothing principle’?

Answer 32

• an action potential wont be triggered if a threshold value is not reached by a stimulus
• the threshold value has to be reached by the voltage generated by localised generator potential

Question 33

what is the ‘all or nothing principle’?

Answer 33

• if threshold value is reached, an action potential is triggered and this will always be the same size of action potential
• different sized stimuli won’t generate different sized action potentials
• different sized stimuli will generate action potentials with different frequencies