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5a ionic basis of action potential (1. hypopolarization (is the initial…
5a
ionic basis of
action potential
1. hypopolarization
is the initial incr
of the membrane potential
to the value of the threshold potential(RMP)
by K+ & Cl- influx
resting state
2.Depolarization
the threshold potential
that was reached
during hyperpolarization
-opens
voltage gated Na channels
causes a large
influx of Na ions
-this phase is called
depolarization
3.overshoot
during depolarization
the inside of the cell becomes
more & more electropositive,
until the potential
gets closer to the
electrochemical equilibrium
for Na of +61mV
this phase of
extreme positivity
is the overshoot phase
4.Repolarization
after the overshoot
the Na permeability
suddenly decreases
-due to the closing of its channels
the overshoot value
of the cell potential
opens voltage gated K channels
this opening causes
a large K efflux,
-decr the cell's electropositivity
this phase is the
repolarization phase
the purpose is to
restore RMP
-repolarization always
leads first to hyperpolarization
(where memb potential is more -ve than
the RMP.
this stage is follwed
by hyperpolarization phase again
or after-depolarization
refractory period
is the time after an AP
is generated,during which
the excitable cell
cannot produce another AP
absolute
refractiveness
overlaps the
depolarization &
abt 2/3 of repolarization phase
a new AP cannot be
generated during depolarization
because all the
voltage gated Na channels
are opened or being opened
at their max speed
during early repolarization
a new AP is impossible
since Na channels are inactive
& need the RMP to be
in a closed state,frm which
they can be in an open state again
absolute refractiveness
ends when enough Na channels
recover frm their inactive state
Relative
refractiveness
is the period
when the generation of a
new AP is possible,
but only upon a
suprathreshold stimulus
this period overlaps
the final 1/3 of
repolarization