Action potentials Flashcards
what is the advantage of passive conduction
direct coding
what is direct coding
coded strength directly indicates the signal strength
what are the two disadvantages of passive conduction
attenuation
noise
what is attentuation
coded signal decreases with distance
what are three advantages of active conduction
high amplitude
noise resistant
quite fast
what is the basis of an action potential
changing the relative na and k conductances the membrane potential can change from -75 to +55
what are 4 points about leakage channels
always open
relatively low conductance
channels not affected by voltage or ligand
what are the two examples of voltage dependent channels for an action potential
na and k
what activates voltage dependent channels for an action potential?
depolarisation
at resting potential (low conductance) channels tend to be closed
open (high conductance) when membrane depolarises- gets more positive
describe opening and closing of na and k channels during action potential
na open (active) rapdily but then shut automatically (inactive) even if membrane stays depolarised - self limiting k channels open (active) more slowly and do not shut while membrane stays depolarised
what is it that determines the action potential slope on a graph
feedback loops
what are the two feedback loops of an action potential
positive and negative
describe the positive feedback loop of an action potentual
depolarisation
increased sodium conductance
increase inflow of sodium
describe the negative feedback loop of an action potential
depolarisation
increased potassium conductance
k outflow
leads to hyperpolarisation
what does hyperpolarisation do
counteracts the depolarisation
what is an action potential threshold
all or none factor
if stimulus is not big enough, increased sodium conductance cant overcome leakage of potassium
what is a refractory period
second stimulus elicited shortly after a first spike fails to initatiate a response
what does refractory period set
an upper limit to spike frequency
what does the refractory period ensure
uni directional propagation
what are the two types of refractory periods
absolute
relative
what is the absolute refractory period
cannot get another spike with second stimulus
after na active, takes them a while to deactive
until fully deactiveted, they cannot open the channel
what is the relative refractory period
needs bigger than normal stimulus to get another spike
after membrane repolarises, k channels close slowly
residual excess k conductance after spike
makes membrane more difficult to depolarise
how do spikes propagate
through passive conduction
what reduces the space constant for propagation
excess potassium conductance
what is important for travelling far distances (AP)
conduction velocity
how can we increase conduction velocity
increase axon diameter
what are two disadvantages of fat axons
expensive
cant fit too many into small nerve bundles
what is myelination
myelin wrap around neurons
what does myelination do
blocks membrane leaks increasing the time constant and reduces capacitance meaning membranes charge faster
what is saltatory conduction
action potential jumps from node to node (nodes of ranvier)
what is mutiple sclerosis
immune system destroys central myelin
reduces space constant
when will current flow
if there is a driving force and a conducting path
what was the main aim of hodgkin and huxleys work
to explain the ionic basis of action potentials
what are two useful drugs for HH experiments
tetrodotoxin (TTX) tetraethyl ammonium (TEA)
what does TTX do
blocks na channels
what does TEA do
blocks k channels
