lecture 9 - the excitable heart Flashcards
What are the 2 types of cells that make up the heart?
Electrical and contractile
What is the alternative name for cardiac electrical cells?
Conduction cells
What percentage of heart tissue is made up of electrical cells?
1%
What is the appearance of cardiac electrical/conduction cells?
Pale striations - due to low actin and myosin as contraction is redundant
Purkinje cells and AV nodal cells are examples of what type of cardiac cell?
Electrical/conduction cells
What is the appearance of cardiac contractile cells?
Striated, due to actin and myosin filaments forming sarcomeres.
Cardiomyocytes are what type of cardiac cell?
Contractile cells
How do action potentials propagate along and between cardiac cells?
They propagate along the surface membrane of both electrical and contractile cells.
Where do depolarisations that initiate contraction originate in the heart?
The sinoatrial (SA) node
How do electrical signals spread between neighbouring cells in cardiac tissue?
Via gap junctions
How do gap junctions facilitate the transmission of an electrical signal between cardiac cells?
They are pores with low resistance to ionic current, allowing it to flow to the surrounding cells.
How are conduction cells connected in terms of signal transmission?
By gap junctions
How are contractile cells connected in terms of signal transmission?
Gap junctions
How are conduction cells and contractile cells connected in terms of signal transmission?
Gap junctions
Why is the heart described as a ‘functional syncytium?
Because all of the cardiac cells behave and contract as one
What are are the key components (in order) of the heart’s conduction pathway?
SA node, internodal bundles, AV node, AV bundle branches (left and right), purkinje fibres
What connects the SA node and the AV node?
Internodal bundles
Where is the SA node located?
At the superior of the right atrium
Where is the AV node located?
At the inferior of the right atrium
Where do the AV bundle branches run?
From the AV node, down the surface of the interventricular septum to the apex of the heart, then branch into perkinje fibres.
What are the 2 AV bundle branches?
Left and Right
Where do purkinje fibres originate?
At the end of the AV bundle branches at the apex of the heart
Where do purkinje fibres run?
Up the left and right ventricles from the apex of the heart.
What is the function of the AV node?
Helps to transmit electrical signals from the atria to the ventricles. Provides a short delay in transmission to allow the atria to finish contracting before the ventricles start contracting.
What are the steps of relaxation, depolarisation and repolarisation during heart contraction?
Initially at quiescence (all 4 chambers relaxed), excitation spreads from SA node, atria depolarise, atria repolarise and AV node sends excitation to ventricles, ventricles depolarise and contract, ventricles repolarise, heart fully relaxes again
What is the term used to describe the point when the entire heart is relaxed/fully repolarised?
Quiescence
What are the key features of an ECG trace?
P wave, QRS complex, T wave
What causes the P wave of an ECG?
Atrial depolarisation
What causes the QRS complex of an ECG trace?
Atrial repolarisation & Ventricular depolarisation
What causes the T wave of an ECG trace?
Ventricular repolarisation
What is the cause of the gap between the P wave and QRS complex?
The delay between atrial and ventricular depolarisation, caused by the AV node
Why is the QRS complex the largest wave on an ECG trace?
The ventricles contain the greatest amount of muscle so create a greater depolarisation
What are the 2 heart sounds?
Lubb and Dupp
What part of an ECG trace is the lubb heart sound associated with?
The S wave - end of the QRS complex - ventricular depolarisation complete and isovolumetric phase begins so the atrioventricular valves close
What part of an ECG trace is the dupp heart sound associated with?
At the end of the T wave - ventricular repolarisation complete and the semilunar valves close
