Membrane Potentials

Question 1

state what it is meant by the key term ‘electrical potential’

Answer 1

separated electrical charges of opposite sine have potential to do work if they are allowed to come together - this potential is called electrical potential

Question 2

state what it is meant by the key term ‘current’

Answer 2

a current is the movement of an electrical charge

Question 3

state what it is meant by the key term ‘resistance’

Answer 3

1. resistance is the hindrance to electrical charge movement
2. if resistance is high, the current will be low (and visa versa)

Question 4

what is Ohm’s law

Answer 4

current = voltage / resistance

Question 5

state what it is meant by the key term ‘insulators’

Answer 5

insulators are materials that have a high electrical resistance and reduce current flow

Question 6

state what it is meant by the key term ‘conductors’

Answer 6

conductors have a low resistance and allow a rapid flow of electrical current

Question 7

state what it is meant by the key term ‘resting potential’

Answer 7

all cells under resting potentials have a potential difference across their plasma membranes, with the inside of the cell negatively charged with respect to the outside of the cell

Question 8

what 2 factors does the magnitude of the resting potential depend on

Answer 8

1. differences in specific ion concentrations in the intracellular and extracellular fluids
2. differences in the membrane permeabilities to the different ions

Question 9

state what it is meant by the key term ‘equilibrium potential’

Answer 9

the equilibrium potential is the voltage difference across the membrane that produces a flux of a given ion species that is equal but opposite to the flux to to the conc^ grad of that same ion species

Question 10

state what it is meant by the key term ‘graded potential’

Answer 10

graded potentials are changes in membrane potentials that are confined to a relatively small region of the plasma membrane

Question 11

state what it is meant by the key term ‘action potential’

Answer 11

a brief all-or-none depolarisation of the membrane which reverses the polarity in neurons. it has a threshold and a refactor period

Question 12

state what it is meant by the key term ‘synaptic potential’

Answer 12

a graded potential change produced in post-synaptic neurone in response to release of neurotransmitter by presynaptic terminal that may be depolarising or hyper polarising

Question 13

state what it is meant by the key term ‘receptor potential’

Answer 13

a graded potential produced at the peripheral endings of afferent neurones in response to a stimulus

Question 14

state what it is meant by the key term ‘pacemaker potentrial’

Answer 14

a spontaneously occurring graded potential change that occurs in certain specialised cells

Question 15

state what it is meant by the key term ‘threshold potential’

Answer 15

the membrane potential at which an action potential is initiated

Question 16

state what it is meant by the key term ‘excitability’

Answer 16

ion channels give cells the ability to produce electrical signals that can transmit information between different regions of the membrane - such membranes are called excitable membranes

Question 17

state what it is meant by the key term ‘depolarised’

Answer 17

depolarised is when the membrane becomes less negative (closer to zero)

Question 18

state what it is meant by the key term ‘overshoot’

Answer 18

overshoot refers to a reversal of the membrane potential polarity, that is, when the inside of the membrane becomes positive in relation to the outside of the membrane

Question 19

state what it is meant by the key term ‘repolarisation’

Answer 19

depolarisation is when a depolarised membrane returns back to it’s resting potential

Question 20

state what it is meant by the key term ‘hyperpolarised’

Answer 20

hyperpolarised is where the membrane becomes more negatively charged than it is at rest

Question 21

state 3 examples of graded potentials

Answer 21

1. receptor potential
2. pacemaker potential
3. synaptic potential

Question 22

graded potentials an be defined as decremental, state what decremental means

Answer 22

decremental states that the flow of charge decreases as the distance from the side of origin of the graded potential increases

Question 23

graded potentials can be described as having ‘summation’, state what summation means

Answer 23

summation - if additional stimuli occur before the graded potential has died away, these can add unto the depolarisation to the last stimulus

Question 24

what are the two sorts of graded potentials and what do they mean

Answer 24

1. depolarisation - membrane becomes less negative than at rest
2. hyperpolarisation - membrane becomes more negative than at rest

Question 25

state 3 characteristics about action potentials

Answer 25

1. membrane potential may change as much as 100mV from -70 to +30
2. very rapid, approx 1-4 milliseconds
3. can have frequencies of 700 per second

Question 26

state what it is meant by the key term ‘threshold stimuli’

Answer 26

a threshold stimuli is categorised as a stimulus that is strong enough to depolarise the membrane to the action potential threshold

Question 27

state what it is meant by the key term ‘sub-threshold potentials’

Answer 27

sub-threshold potentials are weak stimuli cause depolarisation of the membrane but not enough ti reach the action potential threshold - these stimuli are referred to as sub-threshold stimuli

Question 28

state what it is meant by the ‘all-or-none law’

Answer 28

the all-or-none law states that if a stimuli is strong enough to depolarise the membrane to the action potential threshold, then the action potential occurs maximally. if the threshold is not reached, because the stimuli was too week, then the action potential does not happen at all

Question 29

what two things can action potentials be blocked by

Answer 29

the local anaesthetics called: procaine (novocaine) and lidocaine (xylocaine)

Question 30

what is the effect of using the local anaesthetics procaine (novocaine) or lidocaine (xylocaine) - (2 things)

Answer 30

1. these drugs block voltage-gated Na+ channels and prevent them from opening via depolarisation
2. without AP’s, graded signals generated in sensory neurones (eg - injury) cannot send signals capable or reaching the brain (eg - cause sensation of pain)

Question 31

what can also interfere with action potentials, but that are not local anaesthetics

Answer 31

1. some animals produce toxins (potions) which block voltage-gated sodium ion channels
2. eg - pufferfish produces tetrodotoxin

Question 32

state what it is meant by the key term ‘absolute refractory period’

Answer 32

the refractory period is where, during an action potential, a second stimulus, no matter how strong, will not cause a second action potential

Question 33

what does the absolute refractory period occur (2 points)

Answer 33

1. occurs when the voltage gated Na+ channels are either already open or have proceeded to the inactive state during the first AP
2. inactivation gate must be removed via repolarisation of the membrane and closing Na+ channel before they can open to a 2nd stimulus

Question 34

state what it is meant by the key term ‘relative refractory period’ (2 points)

Answer 34

1. the relative refractory period is an interval during which a second AP can be produced but only if the stimulus strength is much greater than usual
2. can last 1-15 msec longer than the absolute refractory period and is during the period after hyperpolarisation

Question 35

how can an action potential be generated within the relative refractory period (2 points)

Answer 35

1. some voltage gated Na+ channels have returned to resting state and K+ channels that repolarised the membrane are still open
2. only if stimulus is large enough or has outlasted the absolute refractory period

Question 36

state the functions of a refractory period

Answer 36

1. ensure AP’s are unidirectional
2. limit frequencies of AP’s (usually 100 AP/s)
3. contribute to separation of AP’s so can pass down axon individually

Question 37

state what it is meant by the key term ‘action potential propagation’

Answer 37

action potentials produce local currents of it’s own which depolarises the region next to it, producing another action potential in the next site, and so on to cause action potential propagation along the length of the membrane

Question 38

what determines how quickly an action potential propagates along a membrane (2 things)

Answer 38

1. depends upon fibre diameter and whether or not it is myelinated
2. the larger the diameter, the faster the propagation as fibre has less internal resistance to the current bringing adjacent regions in faster

Question 39

state what it is meant by the key term ‘saltatory conduction’

Answer 39

saltatory conduction is the process by which action potentials propagate only within the nodes of ranvier and regenerate themselves at each non of ranvier

Question 40

why is propagation via saltatory conduction faster than at non-myelinated neurones (2 things)

Answer 40

1. myelin less leaky so less ions lose

2. pumps need to restore less ions at nodes of ranvier so is more metabolically efficient

Question 41

what are the three benefits of myelination (3 things)

Answer 41

1. adds speed to nerve impulses
2. reduces metabolic cost
3. saves room in nervous system as axons can be thinner

Question 42

state what it is meant by the key term ‘receptor potential’

Answer 42

in afferent neurones, the initial depolarisation to threshold is achieved by a graded potential called a receptor potential

Question 43

excluding receptors, how are all action potentials formed

Answer 43

in all other neurones (excluding receptors), depolarisation to threshold is due to a graded potential generated by a synaptic input to the neurone (synaptic potential), or a spontaneous change in the neurones membrane potential (pacemaker potential)

Question 44

what does the Nernst equation describe

Answer 44

the Nernst equation describes the equilibrium potential for a given ion species - that is, the electrical potential necessary to balance a given ionic concentration gradient across a membrane so that the net flux of an ion is zero

Question 45

what is the Nernst equation

Answer 45

Eion = 61/Z log x (Cout/Cin)

Question 46

what do the parts of the Nernst equation mean

Answer 46

1. Eion = equilibrium potential for specific ion, in mV
2. Cin = intracellular conc^ of ion
3. Cout = extracellular conc^ of ion
4. Z = the valance of the ion
5. 61 = constant value considering temp (37’c) and the Faraday electrical constant

Question 47

state what it is meant by the key term ‘electrogenic pump’

Answer 47

an electrogenic pump is when a pump (eg - Na+/K+ pump) moves net charge across a membrane and contributed directly to the membrane potential

Question 48

Explain, in 7 steps, the action potential mechanism

Answer 48

1. Resting membrane potential close to K+ equilibrium as normal K+ channels are leaky
2. AP begins when stimulus binds to a specific ion channel allowing Na+ to enter
3. Voltage gated Na+ channels now open: positive feedback
4. Inactivation gate blocks Na+ channels and K+ channels open
5. K+ flux rapid out of membrane repolarizing membrane
6. Return of resting potential closes Na+ channels but K+ channels are slow to close causing hyperpolarization
7. K+ channels close + resting potential is returned