Membrane Potentials Flashcards
state what it is meant by the key term ‘electrical potential’
separated electrical charges of opposite sine have potential to do work if they are allowed to come together - this potential is called electrical potential
state what it is meant by the key term ‘current’
a current is the movement of an electrical charge
state what it is meant by the key term ‘resistance’
- resistance is the hindrance to electrical charge movement
- if resistance is high, the current will be low (and visa versa)
what is Ohm’s law
current = voltage / resistance
state what it is meant by the key term ‘insulators’
insulators are materials that have a high electrical resistance and reduce current flow
state what it is meant by the key term ‘conductors’
conductors have a low resistance and allow a rapid flow of electrical current
state what it is meant by the key term ‘resting potential’
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
what 2 factors does the magnitude of the resting potential depend on
- differences in specific ion concentrations in the intracellular and extracellular fluids
- differences in the membrane permeabilities to the different ions
state what it is meant by the key term ‘equilibrium potential’
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
state what it is meant by the key term ‘graded potential’
graded potentials are changes in membrane potentials that are confined to a relatively small region of the plasma membrane
state what it is meant by the key term ‘action potential’
a brief all-or-none depolarisation of the membrane which reverses the polarity in neurons. it has a threshold and a refactor period
state what it is meant by the key term ‘synaptic potential’
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
state what it is meant by the key term ‘receptor potential’
a graded potential produced at the peripheral endings of afferent neurones in response to a stimulus
state what it is meant by the key term ‘pacemaker potentrial’
a spontaneously occurring graded potential change that occurs in certain specialised cells
state what it is meant by the key term ‘threshold potential’
the membrane potential at which an action potential is initiated
state what it is meant by the key term ‘excitability’
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
state what it is meant by the key term ‘depolarised’
depolarised is when the membrane becomes less negative (closer to zero)
state what it is meant by the key term ‘overshoot’
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
state what it is meant by the key term ‘repolarisation’
depolarisation is when a depolarised membrane returns back to it’s resting potential
state what it is meant by the key term ‘hyperpolarised’
hyperpolarised is where the membrane becomes more negatively charged than it is at rest
state 3 examples of graded potentials
- receptor potential
- pacemaker potential
- synaptic potential
graded potentials an be defined as decremental, state what decremental means
decremental states that the flow of charge decreases as the distance from the side of origin of the graded potential increases
graded potentials can be described as having ‘summation’, state what summation means
summation - if additional stimuli occur before the graded potential has died away, these can add unto the depolarisation to the last stimulus
what are the two sorts of graded potentials and what do they mean
- depolarisation - membrane becomes less negative than at rest
- hyperpolarisation - membrane becomes more negative than at rest
state 3 characteristics about action potentials
- membrane potential may change as much as 100mV from -70 to +30
- very rapid, approx 1-4 milliseconds
- can have frequencies of 700 per second
state what it is meant by the key term ‘threshold stimuli’
a threshold stimuli is categorised as a stimulus that is strong enough to depolarise the membrane to the action potential threshold
state what it is meant by the key term ‘sub-threshold potentials’
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
state what it is meant by the ‘all-or-none law’
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
what two things can action potentials be blocked by
the local anaesthetics called: procaine (novocaine) and lidocaine (xylocaine)
what is the effect of using the local anaesthetics procaine (novocaine) or lidocaine (xylocaine) - (2 things)
- these drugs block voltage-gated Na+ channels and prevent them from opening via depolarisation
- without AP’s, graded signals generated in sensory neurones (eg - injury) cannot send signals capable or reaching the brain (eg - cause sensation of pain)
what can also interfere with action potentials, but that are not local anaesthetics
- some animals produce toxins (potions) which block voltage-gated sodium ion channels
- eg - pufferfish produces tetrodotoxin
state what it is meant by the key term ‘absolute refractory period’
the refractory period is where, during an action potential, a second stimulus, no matter how strong, will not cause a second action potential
what does the absolute refractory period occur (2 points)
- occurs when the voltage gated Na+ channels are either already open or have proceeded to the inactive state during the first AP
- inactivation gate must be removed via repolarisation of the membrane and closing Na+ channel before they can open to a 2nd stimulus
state what it is meant by the key term ‘relative refractory period’ (2 points)
- 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
- can last 1-15 msec longer than the absolute refractory period and is during the period after hyperpolarisation
how can an action potential be generated within the relative refractory period (2 points)
- some voltage gated Na+ channels have returned to resting state and K+ channels that repolarised the membrane are still open
- only if stimulus is large enough or has outlasted the absolute refractory period
state the functions of a refractory period
- ensure AP’s are unidirectional
- limit frequencies of AP’s (usually 100 AP/s)
- contribute to separation of AP’s so can pass down axon individually
state what it is meant by the key term ‘action potential propagation’
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
what determines how quickly an action potential propagates along a membrane (2 things)
- depends upon fibre diameter and whether or not it is myelinated
- the larger the diameter, the faster the propagation as fibre has less internal resistance to the current bringing adjacent regions in faster
state what it is meant by the key term ‘saltatory conduction’
saltatory conduction is the process by which action potentials propagate only within the nodes of ranvier and regenerate themselves at each non of ranvier
why is propagation via saltatory conduction faster than at non-myelinated neurones (2 things)
- myelin less leaky so less ions lose
2. pumps need to restore less ions at nodes of ranvier so is more metabolically efficient
what are the three benefits of myelination (3 things)
- adds speed to nerve impulses
- reduces metabolic cost
- saves room in nervous system as axons can be thinner
state what it is meant by the key term ‘receptor potential’
in afferent neurones, the initial depolarisation to threshold is achieved by a graded potential called a receptor potential
excluding receptors, how are all action potentials formed
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)
what does the Nernst equation describe
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
what is the Nernst equation
Eion = 61/Z log x (Cout/Cin)
what do the parts of the Nernst equation mean
- Eion = equilibrium potential for specific ion, in mV
- Cin = intracellular conc^ of ion
- Cout = extracellular conc^ of ion
- Z = the valance of the ion
- 61 = constant value considering temp (37’c) and the Faraday electrical constant
state what it is meant by the key term ‘electrogenic pump’
an electrogenic pump is when a pump (eg - Na+/K+ pump) moves net charge across a membrane and contributed directly to the membrane potential
Explain, in 7 steps, the action potential mechanism
- Resting membrane potential close to K+ equilibrium as normal K+ channels are leaky
- AP begins when stimulus binds to a specific ion channel allowing Na+ to enter
- Voltage gated Na+ channels now open: positive feedback
- Inactivation gate blocks Na+ channels and K+ channels open
- K+ flux rapid out of membrane repolarizing membrane
- Return of resting potential closes Na+ channels but K+ channels are slow to close causing hyperpolarization
- K+ channels close + resting potential is returned
