*Physiology 1 (lecture 1) Flashcards
Where does excitation of the heart normally originate?
In the pacemaker cells in the sino-atrial node
Where is the SA node located?
In the upper right atrium close to where the SVC enters the right atrium
What is the phrase used to describe a heart controlled by the SA node?
Sinus rhythm
What do pacemaker cells in the SA node exhibit which leads to the membrane potential depolarising?
What does this do
Spontaneous pacemaker potential
The spontaneous pacemaker potential is a gradual drift of the membrane potential towards depolarisation
This takes the membrane potential to threshold where an action potential is generated in the SA nodal cells
What happens to the membrane potential of pacemaker cells?
They gradually drift (pacemaker potential) until it reaches threshold value causing the cell to fire
The firing is followed by repolarisation before another action potential occurs
What causes the pacemaker potential in pacemaker cells (the slow depolarisation of the membrane potential to a threshold)?
Decrease in K+ efflux
Slow Na+ influx (the funny current)
(this creates a net movement of positively charged ions into the cell which happens spontaneously)
What causes the rising phase of the action potential once the threshold has been reached? (pacemaker cells)
Activation of voltage-gated Ca2+ channels resulting in Ca+ influx
What is the falling phase of the action potential (depolarisation) caused by? (pacemaker cells)
Activation of K+ channels resulting in K+ efflux
Anatomy of the conducing system of the heart?
Sino-atrial (SA) node - Atrioventricular node - bundle of His - left and right branches - purkinje fibres
What does cell-to-cell spread of excitation occur between?
Adjacent myocytes through gap junctions within intercalated discs (intercalated discs are made up of gap junctions, fascia adherents and desmosomes)
Where is the AV node located?
It is a small bundle of specialised cardiac cells located at the base of the right atrium just above the junction of atria and ventricles
How is excitation spread across the atria?
Mainly cell-to-cell conduction via gap junctions
How is excitation spread from the SA node to the AV node?
Mainly cell-to-cell conduction via gap junctions but there are also some internal pathways
Why is the conduction delayed by the AV node?
To allow atrial systole to precede ventricular systole
Does the action potential in pacemaker cells or contractile muscle cells have a plateau phase?
Contractile muscle cells
