Original and Conduction of Cardiac Impulse Flashcards
Where does excitation of the heart normally originates?
pacemaker cells in the SA node
Do the cells the SA node have a stable resting membrane potential?
No
What is the theshold resting membrane potential?
-40mV
In the pacemaker cells, does the permeability to K+ remain constant between action potentials?
No
What is the pacemaker potential?
. the slow depolarization of membrane potential to a threshold
What is the pacemaker potential due to?
DECREASE in K+ EFFLUX superimposed on a SLOW Na+ INFLUX (the funny current)
What happens once threshold is reached?
The RISING PHASE OF ACTION POTENTIAL (i.e. DEPOLARIZATION) is caused by activation of VOLTAGE-GATED Ca++ CHANNELS
Resulting in Ca++ INFLUX
What is the falling phase of action potential (repolarization) caused by?
activation of K+ CHANNELS
Resulting in K+ EFFLUX
What does cell to cell spread of excitation occur via?
gap junctions
What is the AV node and where is it?
-small bundle of specialized cardiac cells
ONLY point of electrical contact between atria and ventricles
-located at the base of the right atrium; just above the junction of atria and ventricles
Are AV node cells small or large in diameter?
small
Do AV node cells have fast or slow conduction?
slow
What does AV node conduction delay allow?
allows atrial systole (contraction) to precede ventricular systole
What allow rapid spread of action potential to the ventricles?
Bundle of His and network of Purkinje fibres
Is the action potential in contractile cardiac muscle cells different or similar from the action potential in pacemaker cells?
differs considerably
In the ventricular muscle action potential, what does the resting membrane potential remain at until the cell is excited?
-90
What is the rising phase of action potential (depolarization) caused by?
by FAST Na+ INFLUX
What does the fast sodium ion influx cause the action potential to rapidly become?
+30
What is the depolarization stage known as?
Phase 0
What is phase 1 of the Ventricular Muscle Action Potential?
Closure of Na+ channels and Transient K+ efflux
What is phase 2 of the Ventricular Muscle Action Potential?
Mainly Ca++ influx
What is phase 3 of the Ventricular Muscle Action Potential?
Closure of Ca++ channels and K+ efflux
What is phase 4 of the Ventricular Muscle Action Potential?
Resting membrane potential
What is the plateau phase and what is it due to?
When the membrane potential is maintained near the peak of action potential for a few hundred milliseconds (a unique characteristic of ventricular muscle action potential) and it is mainly due to INFLUX of Ca++ through voltage-gated Ca++ channels
