Genesisof Action Potential Flashcards
Electrotonic potential
Non propagated local potential due to change in ionic conductance due to local change in conductance.
Action potential
Probated impulse
Local responses
Potential changes observed near the site of current passage
Sub threshold current pulses
The size of the potential change observed depends on the distance of the recording electrode from the point of current passage.
The closer the recording electrode to the site of current passage, the larger the potential change
Size of the potential change
Decreases exponentially with the distance to 1/e (37%) of its maximum value.
What is the length of a typical mammalian nerve or muscles cell?
1 to 2 mm
What is the importance of an action potential?
Communication between nerve cells depends on an electrical disturbance.
Action potential
A transient change in the membrane potential that conveys information within excitable cells and the nervous system when excitation occurs. W
When is an action potential triggered?
When the depolarization is sufficient for the membrane potential to reach a threshold value.
What is threshold ?
The critical point in which a membrane must be depolarised in order to initiate a membrane response.
How does the action potential differs from the subthreshold and passive responses?
It is much larger response in which the polarity of the membrane potential actually overshoots.
The action potential is propagated down the entire length of the nerve fibres
The action potential is propagated without decrement( maintains it shape and size.
All or none response
The stimulus either produces a full sized action potential or fails to do so.
Facts about all or none
Stronger stimuli does not increase either size or speed of the action potential
A stimulus of stronger intensity occurs when the frequency of firing increases
involves the flow of ions across the plasma membrane
have a characteristic form ( amplitude, time course and duration)
Stages of action potential
Upstroke: rapid depolarization of membrane
Overshoot: membrane potential is positive
Downstroke: repolarisation phase; rapid return of membrane to its resting potential
Undershoot: membrane becomes more negative that it resting value
Which cells has the fastest action potential?
Cardiac muscle, motor , skeleton
Characteristics of action potential
Threshold, amplitude, duration
Types of subthreshold potential
Local, somatic , receptor
does a subthreshold stimuli elicit an action potential?
NO
does a suprathreshold change the size of an action potential
no
factors affecting the action
1) the gating and permeability of properties of specific types of ion channels. these properties depend on both Vm and time
2) membrane properties that don not depend on Vm and time such as resistance, and the geometry.
3) the intracellular and extra cellular concentrations that pass through these channels.
regulation of Na+ channels is regulated by two gating particles
m and h gates
which gate is the activated channel ?
m is the activated gate while h is the inactivating gate. for na+ to pass through the Na+ channel both gates must be opened
ij the resting potential the activation gates are__________ while the inactivation gates are _______
closed ……. open
describe Na+ channels
its intramembrane domain is known to consist of a number of α helices that span the membrane and surround the ion channel.
The Na+ channel has both an activation gate and an inactivation gate that account for the changes in gNa during an action potential. Groups of charged amino acid residues that form these gates have been tentatively identified. To enter the channel’s narrowest part, known as the selectivity filter, K+ and Na+, it is believed, must shed most of their waters of hydration.
To strip K+ and Na+ of their associated water molecules, negative amino acid residues that line the pore of the channel must have a particular geometric shape; the precise geometry differs for K+ and for Na+. This requirement is believed to confer the specificity of ion channels.
regulation of the K+ channels
The K+ channel has one gating potential that covers the extra cellular side of the K+ channels. the “n” gate must be opened for K+ to flow through the K+ channel.
structure of potassium channel
made up of 4 alpha subunits as well as a cytoplasmic beta subunit.
what is the resting potentiali of a cell?
-70 to 90mV
Na+ channels are closed by
“n” gate but the h gate is open
Depolization
When the membrane is depolarized to threshold, a positive feedback (regenerative) process produces the all-or-none upstroke of the action potential.
The response of the membrane to the stimulus (opening of “m” gates) causes an effect (membrane depolarization) that produces an even greater response.
Repolization
Inactivation of the Na+ channels & flow of Na+ into the cell stops
Activation the K+ channels, & K+ flows out of the cell, causing the membrane to repolarize
hyperpolization
“n” gates close slowly & membrane repolarizes
K+ conductance (gK) is higher at the end of the AP than during the resting state
The high gK causes the membrane to hyperpolarize (i.e., become more negative than the resting membrane potential).
Eventually, the “n” gates close & the membrane potential returns to its resting level
the role of Na/ K pump
it pumps 3 Na outside the cell and 2 K inside the cell using atp .
Local anaesthetics
lidocaine, novocaine, benzocaine - prevent action potential from being progated.
Na channels blockers
Tetrodotoxin- potent poisin. its binds to the extracellular side of its channel.
Saxitoxin - produced by reddish dinoflagellates that are responisible for the so call red tide
K+ channel blockers
Tetraeythylammonium - It enters the K+ channel from the cytoplasmic side and blocks the channel because it cannot pass through the channel.
absolute refractory period
Period from initiation & extends into the down-stroke
2nd AP absolutely cannot be initiated during this period.
“m” gates in Na+ channels are opening rapidly during the up-stroke
“h” gates produce inactivation of Na+ channels during early part of the down-stroke
Relative refractory period
No. of inactivated Na+ channels is reduced by repolarization
Voltage-gated K+ channels are open.
2nd AP can be initiated
Membrane is hyperpolarized (-80mV).
AP are more difficult to generate during this period relative to the resting potential (-70mV).
increase potassium in the ECF does what to the mebrane?
depolarises the membrane.
decrease calcium in the ECF does what to the membrane
allows the activation of the sodium channels at the more negative potential
