cardiac AP 1: ventricular muscle Flashcards
what is cardiac muscle contraction triggered by
action potential at the membrane plasma (sarcolemma)
where does electrical excitation originate from
SAN and spreads throigh atria and ventricles
what separates electrical charge
plasma membrane of cells
what sets up a membrane potential
an uneven distribution of ions
what determines the uneven distribution of ion
fx of ions pumps
ion exchangers
ion channels
what are the main ions in the modulation of membrane potential
calcium
potassium
sodium
at rest what channels are closed
sodium
calcium
how does potassium leak out durin rest
through inward rectifier k+ channels
what does the inward rectifier k+ channels generate
it generates -90mV current
how long is an AP duration
~250-300msec
what occurs during upstroke of graph
sodium influx
what happens during plateau
calcium influx
what occurs doing repolarisation
potassium efflux
what is refractory period
electrically inexcitable period
- no summation of contractions
- no tetanic contractions
what occurs during refractory period
permits filling of the ventricles between contractions
ventricular AP has how many phases
5 phases
0-4
what happens at phase 0 (4)
action potential reaches adjacent myocyte
depolarisation begins
fast sodium channels open beyond their threshold of -65mV but inactivates rapidly
*channels are voltage dependent & tome dependent
sodium drive the mb potential in a positive direction, generates overshoot
when fast sodium channels are in inactivated confoguration, what occurs
refractory period
*AP cannot be triggered in inactive state
what occurs during phase 1
brief period of repolarisation caused by transient outward current
how are currrnt in phase 1 carried
thru voltage gated potassium chanels
they are activated by depolarisation and inactivate quickly
what occurs during phase 2
a plateau period with inward current thru voltage gated calcium channels called L type calcium channels
L= long opening
inward current counterbalanced the small K+ efflux that occurs throughout the AP
what occurs during phase 3
plateau period progresses K+ efflux increases due to opening of voltage dependent K+ chanels
known as delayed rectifier or slow K+ channels (Kv or Ks)
when repolarisation occurs Kir reopen to complete repolarisation and Kv (or Ks) are closed when resting Vm is reached
what occurs in phase 4
75-90% of rasied calcium in the cytosol is pumped to SR via calcium ATPase
remainder is extruded from the cel mainly by 3Na+/Ca2+ exchangers with a small contribution from sarcolemmal Ca2+ ATPase
inward background current reduces Vm
what does removal of ca2+ from ECF do
prevent contraction
why removal of ca2+ prevent contraction
because no calcium entry to trigger calcium-induced calcium release from the sarcoplasmic reticulum
no long plateau phase in AP
adrenergic stimulation of cardiac myocytes leads to increase in i(Ca). what does this do
increases contractility
L type calcium are blocked by antagonist such as
verapamil
diltiazem
what do L type ca2+ antagonist do
reduce plateau duration and cardiac contractility
restong membrane potential is established by
k+ efflux
small background of na+ influx
what determines the form of AP
magnitude
direction
time
of na+ ca2+ and k+
what does prolonged AP provide
extended refractory period
what does extended refractory period do
avoids tetanic contraction
AP is influenced bu
autonomic nerves
what does shorter duration of AP do
allows more AP to occur per minute
brief current of repolarisation in phase 1 has contribution from
Cl- influx thru Cl- channels
