Blood Supply of Myocardial Cells + Conducting System Flashcards
what artery supplies the myocardial cells?
coronary arteries
where does the coronary artery exit from?
from behind the aortic valve cusps in the first part of the aorta
where do the coronary arteries drain into?
most drain into a vein called the coronary sinus, which empties into the right atrium
what is the purpose of the conducting system?
to initiate heartbeat and spread the action potential rapidly throughout the cell
what is the purpose of gap junctions?
gap junctions inerconnect myocardial cells and allow action potential to be spread from one cell to another
how is he action potential spread from cell to cell?
The action potential spreads over cell membranes. The positive charge from the Na+ affects adjacent cells, resulting in depolarisation, the newly depolarised cells can cause further depolarisation, and the gap junctions enable ions to travel directly to other cells. The initial excitation of one cardiac cell allows eventually results in the excitation of all cardiac cells.
where does initial depolarisation occur?
in a group of conducting system cells called the sinoatrial node (SAN). The action potential spreads from SAN –> atria –> ventricles
what is the resting membrane potential of the SAN?
-55 - -60 mV –> closer to the threshold of depolarisation and hence depolarises first
what is a unique feature of SAN not found anywhere else in the body?
slow inflow of NA+ (results in resting membrane potential)
what is the pacemaker potential?
The SA node does not have a steady resting potential, instead it undergoes SLOW DEPOLARISATION - this is known as the pacemaker potential; it brings the membrane potential to a threshold, at which point an action potential occurs.
which three ion channel mechanisms contribute to the pacemaker potential?
- progressive reduction in K+ permeability
- F-type channels (‘funny’ channels in pacemaker cells that open when membrane potential is negative, allowing Na+ inflow)
- T-type Ca2+ channel opens very briefly to allow inflow of Ca2+, contributes to final depolarising boost to pacemaker potential
SA and AV are similar in shape. So why does SAN initiate action potential?
because the pacemaker currents in SA bring them more rapidly to threshold
what happens once pacemaker mechanisms have brought a nodal cell to its threshold?
- an action potential occurs
- depolarising phase is caused by influx of Ca2+ via L-type Ca2+ channels
- Ca2+ currents depolarise membrane more slowly than Na+
- action potential propagates more slowly in nodal cells than other cardiac cells (explains slow transmission of cardiact excitation through thr AV node)
how do contractions occur in the left and right atria?
simultaneously, as conduction is rapid enough
how is the action potential conducted from SAN to AVN
through internodal pathways
