Physiology Flashcards
The heart can beat rhythmically in the absence of external stimuli. what is this called?
autorhythmicity
What is the heart?
an electrically controlled muscular pump which sucks and pumps blood.
Where are the electrical signals which control the heart generated?
within the heart.
Where does excitation of the heart normally originate?
in the pacemaker cells in the Sino-atrial (SA) node
Where is the SA node located?
upper right atrium close to where the superior vena cave enters the right atrium.
what does the cluster of specialised pacemaker cells in the SA node do?
initiate the heart beat.
What does the SA node normally do?
sets the pace for the entire heart.
a heart controlled by the sino-atrial node is said to be in?
sinus rhythm
what do the cells in the SA Node not have?
no stable resting membrane potential.
what do the cells in the SA node generate?
regular spontaneous pacemaker potentials
where does the cardiac impulse originate?
Sino-atrial (SA) node
what do the spontaneous pacemaker potential in the SA node do?
slowly depolarises the membrane to a threshold to generate an action potential.
how is the action potential spread?
by cell-to-cell conduction from the SA node to the atrio-ventricular (AV) node where it is delayed. From the AV node it is then spread to the ventricles via Bundle of his and its branches and the purkinje fibres and by cell-to-cell conduction within the ventricles.
The AV node is the only point of what?
point of electrical contact between the atria and the ventricles.
what is the pacemaker potential (slow depolarisation of membrane potential to a threshold) due to?
- decrease in K+ efflux
- Na+ and K+ influx (the funny current)
- transient Ca++ influx (T-type Ca++ channels)
What is the rising phase of action potential caused by? what does it result in?
activation of long lasting L-type Ca++ channels and results in Ca++ influx.
what is the falling phase of action potential caused by? what does it result in?
inactivation of L-type Ca++ channels and activation of K+ channels resulting in K+ efflux.
Cell-to-cell spread of excitation is spread via?
gap junctions
what is the AV node?
small bundle of specialised cardiac cells.
where is the AV node located?
at the base of the right atrium - just above the junction of atria and ventricles.
why is the conduction delayed in the AV node?
allows atrial systole (contraction) to precede ventricular systole.
what is the resting membrane potential of cardiac muscle cells?
-90mV
what is the rising phase of action potential in cardiac muscle cells caused by? what happens to membrane potential when this happens? what is this known as?
fast Na+ influx
rapidly reverses it to +20mV
phase 0 of action potential in contractile cardiac muscle cells.
what are the phases of ventricular muscle action potential?
phase 0 (rapid depolarisation) - fast Na+ influx phase 1 (early repolarisation)- closure of Na+ channels and transient K+ efflux phase 2 (plateau)- slow Ca2+ influx phase 3 (final repolarisation) - closure of Ca2+ channels and K+ efflux phase 4 - resting membrane potential returned by Na/K+ ATPase.