Electrical activity of the heart Flashcards
Discuss the structure of the heart
Dual pump
4 chambers (right + left atria/ventricles)
Controlled by the nervous system - but can function w/out
99% cardiac cells = contractile (req. action potential to contract)
1% = capable of spontaneously firing ‘autorhythmic’ (spontaneously generate their own action potential)
Where are autorhythmic cells (spontaneously firing cells) located?
- Sino-atrial node
- Atrioventricular node
- Bundle of His, bundle branches, purkinje fibres
What is the natural pacemaker + why?
What is the the spontaneous discharge rates for each node?
SA node = 70-80 action potentials/min
AV node = 40-60 action potentials/min
(purkinje fibres = 20-40 action potentials/min)
- SA node = natural pacemaker*
- Because fastest firing cells drive the heart*
Draw a diagram of an action potential as would be recorded in a pacemaker cell in the sinoatrial node.
Describe the ionic basis of this action potential.
Describe the sequence of events by which excitation spreads from the sinoatrial node through the heart.
Relate this to the sequence of contraction of the heart chambers
Excitation spreads in a coordinated manner through specialised conducting tissue
SA node
rapidly through the atria
AV node
rapidly though bundle of His + down bundle branches + purkinje fibres
through ventricular muscle cells
How does excitation spread rapidly through the heart?
Presence of intercalated discs (gap junctions) between fibres
- provide low resistance pathways
- cardiac muscle consists of individual heart muscle cells (cardiomyocytes) connected by intercalated discs to work as a single functional organ or syncytium
- intercalated disc support synchronised contraction of entire cardiac tissue (all-or-nothing)
What helps atrial excitation + contraction to be complete before ventricular contraction?
AV delay
Ventricular excitation occurs synchronously. to enable ventricles to contract as co-ordinated unit + expel blood.
What part of the heart helps this to happen?
Rapid spread down septum through purkinje fibres
Draw a diagram of a “classical” ventricular cell action potential and describe its ionic basis
What is excitation coupling?
Explain the process.
An action potential in the skeletal muscle cell is what triggers muscle cell contraction. We have seen that calcium ions regulate whether or not contraction can occur. Thus, what is needed is a way to link muscle excitation (the depolarization of the action potential) to Ca++ release from the sarcoplasmic reticulum. This link is known as excitation-contraction coupling.
Read slide
How does sympathetic nervous system increase heart rate?
When Beta1-adrenoceptors are activated, the stimulate activation of sympathetic nerves, therefore, increasing heart rate
How does parasympathetic nervous system decrease heart rate?
When M2 muscarinic receptors in SA node are activated, they activate parasympathetic nerves, which decrease heart rate
