Cardiac excitation

Question 1

what does the sino-atrial node do

Answer 1

heart rhythm
trigger for cardiac contraction
electrical activity from the SA node passes across the atria, and to the AV node
this creates a delay between the passage of electrical activity
this is why there is a delay between the atrial contraction and the ventricular contraction

Question 2

give a summary of the excitation pathway

Answer 2

heart rate is controlled by SA node
action potential activates atria
atrial action potential activates the AV node
AV node has a slow conduction velocity which introduces a delay of 0.1s
Av node activates bundle of HIs/purkinje fibres
purkinje fibres activate ventricles

Question 3

why doesn’t the electrical activity pass to the ventricles

Answer 3

connective tissue separates the atria from the ventricles which is non-conducting
bundle of His and AV node controls the passage of electrical activity from the atria to the ventricles
this can cause bundle brnach block

Question 4

what does mygenic mean

Answer 4

cardiac muscle generates its own action potentials
these develop spontaneously at the SA node
action potential is conducted from cells to cell via intercalated discs which have gap junctions

Question 5

what are gap junctions

Answer 5

each cell provides one half of a gap junction
they’re channels that conduct electrical activity

Question 6

summarise SA node action potential

Answer 6

pacemaker potential due to increase in calcium and sodium ions, and a decrease in potassium ions
action potential upstroke is due to an increase in calcium
repolarisation is due to an increase in potassium and a decrease in calcium
noradrenaline increases sodium and calcium
acetyl choline increases potassium and decreases calcium

Question 7

describe the long cardiac action potential

Answer 7

action potential determines the duration of contraction (only in the heart)
the calcium influx determines the duration of the plateau, which initiates contraction of the muscle

Question 8

difference between atrial and ventricular action potential

Answer 8

atrial is shorter, this is because the atria are smaller and the contraction occurs over a shorter period of time
you also dont want the atrial contraction to overlap with the ventricular

Question 9

source of calcium for contraction

Answer 9

calcium is released from the sarcoplasmic reticulum, but for heart cells calcium entry from outside is needed (calcium induced calcium release)
calcium influx is essential for initiation of contraction