Blood Supply of Myocardial Cells + Conducting System

Question 1

what artery supplies the myocardial cells?

Answer 1

coronary arteries

Question 2

where does the coronary artery exit from?

Answer 2

from behind the aortic valve cusps in the first part of the aorta

Question 3

where do the coronary arteries drain into?

Answer 3

most drain into a vein called the coronary sinus, which empties into the right atrium

Question 4

what is the purpose of the conducting system?

Answer 4

to initiate heartbeat and spread the action potential rapidly throughout the cell

Question 5

what is the purpose of gap junctions?

Answer 5

gap junctions inerconnect myocardial cells and allow action potential to be spread from one cell to another

Question 6

how is he action potential spread from cell to cell?

Answer 6

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.

Question 7

where does initial depolarisation occur?

Answer 7

in a group of conducting system cells called the sinoatrial node (SAN). The action potential spreads from SAN –> atria –> ventricles

Question 8

what is the resting membrane potential of the SAN?

Answer 8

-55 - -60 mV –> closer to the threshold of depolarisation and hence depolarises first

Question 9

what is a unique feature of SAN not found anywhere else in the body?

Answer 9

slow inflow of NA+ (results in resting membrane potential)

Question 10

what is the pacemaker potential?

Answer 10

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.

Question 11

which three ion channel mechanisms contribute to the pacemaker potential?

Answer 11

1. progressive reduction in K+ permeability
2. F-type channels (‘funny’ channels in pacemaker cells that open when membrane potential is negative, allowing Na+ inflow)
3. T-type Ca2+ channel opens very briefly to allow inflow of Ca2+, contributes to final depolarising boost to pacemaker potential

Question 12

SA and AV are similar in shape. So why does SAN initiate action potential?

Answer 12

because the pacemaker currents in SA bring them more rapidly to threshold

Question 13

what happens once pacemaker mechanisms have brought a nodal cell to its threshold?

Answer 13

• 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)

Question 14

how do contractions occur in the left and right atria?

Answer 14

simultaneously, as conduction is rapid enough

Question 15

how is the action potential conducted from SAN to AVN

Answer 15

through internodal pathways

Question 16

where is the AVN located and what purpose does it serve?

Answer 16

it is located at the base of the right atrium

it transmits cardiac impulse from the atria to the ventricle

Question 17

what is the purpose of the elongated structure of the AVN?

Answer 17

it slows down the propagation of action potential through the AV node (by approximately 0.1s) –> this enables blood to drain from the atria into the ventricles –> enables atrial contraction to be complete before ventrical excitation

Question 18

what is the bundle of His?

Answer 18

the pathway along which, after the AV node has been excited, the action potential progresses down the interventricular septum

Question 19

what are Purkinje fibres?

Answer 19

conducting cells that rapidly distribute the impulse throughout much of the ventricle

the Purkinje fibres receive the impulse from the conducting fibres that diverge from the bundle of His

they transmit the impulse to the myocardial cells in the ventricles, which spread the action potential throughout the rest of the ventricles