Ion Channels of the heart Flashcards
(20 cards)
Where does the electrical journey of the heart begin, and what type of cells initiate it?
The electrical journey begins in the sinoatrial (SA) node, which is a cluster of excitable, spontaneously active cells that initiate depolarisation.
What is the role of the Bachmann’s bundle in the heart’s electrical conduction?
The Bachmann’s bundle is an important electrical tract in the left atrium that helps conduct the electrical impulse across from the right atrium.
What structures carry the electrical signal after the AV node?
After the AV node, the electrical impulse travels down the bundle of His, through the Purkinje fibres, and finally to the apex of the ventricles.
What is a functional syncytium in cardiac muscle, and what connects the cells within it?
A functional syncytium refers to the interconnected network of cardiac cells that allow coordinated electrical activity. The cells are connected by connexons, which are made up of two connexins, forming low-resistance pathways for electrical conduction.
What is conduction velocity in the context of cardiac tissue?
Conduction velocity is the speed at which an electrical excitation travels from one side of a region of the heart to the other.
How do nodal cells differ from ventricular myocytes in terms of contractile proteins and action potential development?
Nodal cells lack contractile proteins and have slow action potential development.
Myocytes have contractile proteins and develop action potentials rapidly.
What is the typical resting membrane potential of a ventricular myocyte?
The resting membrane potential of a ventricular myocyte is approximately -80 to -90 mV.
Which phase of the ventricular action potential is known as the plateau phase and what occurs during it?
Phase 2 is the plateau depolarisation phase, where calcium channels slowly open, allowing a continued influx of calcium, while sodium channels remain inactive and potassium channels are slightly inactive.
What ion channel is responsible for maintaining the resting membrane potential in ventricular myocytes?
The Kir2.1 potassium channel maintains the resting membrane potential in ventricular myocytes by allowing potassium efflux.
Which sodium channel is involved in Phase 0 of the ventricular action potential, and what gene encodes it?
The sodium channel involved is NaV1.5, which is encoded by the SCN5A gene. It allows a rapid influx of sodium during fast depolarisation.
What occurs during Phase 1 of the ventricular action potential?
During Phase 1 (notch or transient repolarisation), the NaV1.5 sodium channels inactivate, and voltage-gated potassium channels open briefly, causing a temporary repolarisation.
Which potassium channels are responsible for complete repolarisation in Phase 3, and what genes encode them?
Kv7.1 (encoded by KCNQ1) and Kv11.1 (encoded by KCNH2, also known as ERG) are responsible for repolarisation. These channels slowly open to return the membrane potential to negative values.
Why is Kv11.1 clinically significant?
Kv11.1 is significant because it is susceptible to blockage by many drugs, which can potentially lead to dangerous changes in cardiac repolarisation.
What is the difference in sodium channel expression between nodal cells and ventricular myocytes?
Ventricular myocytes express a high amount of SCN5A, leading to prominent voltage-gated sodium channels. Nodal cells, however, have low SCN5A expression and do not have functional voltage-gated sodium channels.
What is the “funny current” and which type of cardiac cell expresses it?
The funny current is a depolarising current that is activated by hyperpolarisation. It is found in nodal cells due to their high HCN channel expression. It is negligible in myocytes.
What allows some sodium and calcium channels to reopen during the refractory period?
During the refractory period, partial recovery of sodium and calcium channels can occur, which may allow reactivation of the membrane potential if the action potential is prolonged.
What causes the initial depolarisation in ventricular myocytes during Phase 0 of the action potential?
The initial depolarisation during Phase 0 is caused by the opening of voltage-sensitive sodium channels (NaV1.5), encoded by the SCN5A gene, leading to a rapid influx of sodium ions.
Why do nodal cells have an unstable membrane potential compared to myocytes?
Nodal cells have an unstable membrane potential because of their low resting potassium permeability and the presence of the funny current (due to high HCN expression), which leads to spontaneous depolarisation.
What is the role of the sodium-potassium ATPase pump in ventricular myocytes at rest?
At rest, the sodium-potassium ATPase pump helps maintain ionic balance by actively pumping potassium ions into the cell and sodium ions out, supporting the resting membrane potential of approximately -80 mV.
What allows the electrical impulse to spread efficiently through the heart muscle cells?
The electrical impulse spreads efficiently through functional syncytia, which are networks of heart muscle cells connected by connexons. These connexons form low-resistance pathways that allow rapid and coordinated depolarisation across the heart tissue.
