cell excitability Flashcards
compare action and graded potentials
action potential = all or nothing, goes longer distances whereas graded potential = variable in size, local
action potential = goes one way along the axon, graded potential can pass both ways
action potential codes stimulus intensity by frequency as size is uniform, graded potentials coded by size as they’re not all or nothing
focusing on potassium as an example, explain what equilibrium potential is
+ve and -ve ions tend to pair in solution
a nerve cell has potassium ion channels that allow potassium to move out of the cell
this means however that only the cation (K+) can move from the inside to the outside of the cell, driven by the concentration gradient
this leaves behind anions inside the cell, causing the inside to become -ve and the outside to become +ve
the negative charges inside exert a force to keep the +ve ions inside as well, this is the electrical force opposing the concentration gradient
the equilibrium potential is when the electrical force equals the force of the concentration gradient, so there is no net movement of ions
what are the two most important pumps in nerve cells?
Na+/K+ ATPase keeps intracellular potassium and extracellular Na+ high, the 2:3 keeps inside cell more -ve than outside
Ca2+ pump is essential in keeping intracellular Ca2+ low
what are 4 key points to remember about equilibrium potential?
only takes a tiny change in ionic concentration to cause a large change in membrane potential
difference in electrical charge actually occurs right at the surface of the membrane because the ions attract through the bilayer
an ion will be driven across the membrane at a much greater rate if the membrane potential is very far from equilibrium potential
if a membrane is only permeable to one kind of ion it will be set at the equilibrium potential of that ion
what is the Nernst equation?
calculates the equilibrium potential of an ion using its intra and extracellular concentrations
E= 2.303 RT/zF log([ion]o/[ion]i)
the whole bit before the log is usually around 61.54 (unless ofc it’s a negative ion, then -61.54)
what is the ion driving force?
the membrane potential subtract the equilibrium potential for that ion, gives you the magnitude of the force with which that ion moves
describe the phases of an action potential
- voltage gated Na+ channels open so permeability Na+»K+
- rising phase - rapid depolarisation caused by Na+ moving very quickly into the neuron due to strong ionic driving force
3.overshoots at the peak a bit, Vm approaches ENa+ - falling phase - Na+ ion channels inactivate, K+ channels open, K+ driven out of the neuron
- refractory overshoot - voltage gated K+ take too long to close, so delayed rectifiers open - K+ channels allowing K+ in the cell to get back up to resting potential after going too far
what are the properties of an action potential?
transient, rapid and reversible changes in membrane potential from -ve to +ve
typically triggered by Na+ permeability increase
all of the same size and duration
do not decrease as conducted down the axon
describe the structure of a voltage gated Na+ channel
6 transmembrane domains
4th one detects changes in membrane potential
lots of positively charged amino acids
quickly inactivated
what are three ‘useful’ poisons that effect neurons and now do they do so?
TEA - tetraethylammonium blocks K+ channels
Lidocaine - blocks Na+ channels
TTX - from a puffer fish, blocks all Na+ channels
what are three key factors effecting conduction ?(anton)
diameter - the greater it is the lower the resistance to flow the quicker the speed of conduction
leaky membrane - membrane permeability or how ‘leaky’ the membrane is to ions like Na+
myelination - prevents current loss along axon, allows for quicker saltatory conduction
how does the giant squid demonstrate the benefits of myelination?
has the largest axon of around 1mm in diameter
conduction is 25m/s, which isn’t actually faster than a lot of smaller but myelinated axons, so diameter does increase speed of conduction, but not as much as myelin
compare activation of axons vs dendrites
an AP is usually generated at the spike initiation zone - the axon hillock
dendrites usually deal in graded potentials
describe the refractory periods of action potentials
absolute refractory period = 1ms no matter what you do the neuron cannot generate another action potential
relative refractory period - a few extra ms during which another action potential is possible but would require a stronger stimulus
what are EPSPs and how can they reach an AP?
its a graded potential essentially, so not quite reaching the threshold for an action potential (can also have inhibitory postsynaptic potentials too)
can reach an AP using temporal or spatial summation
