Heart Beat/Heart Rhythm

Heart Beat/ Heart Rhythm71-89

  • The adult heart beats around 70 to 80 times a minute, at rest.
  • During a normal, healthy heartbeat, or what we call a cardiac cycle, the atria contract simultaneously.
  • Then, as they relax, the ventricles contract. This explains what happens during a cardiac cycle, but what it doesn't tell us is how the chambers rightly know their time to contract. In other words, what's controlling the heart beat?
  • The cells of the heart muscle (cardiac cells or cardiomyocytes), have a unique ability to contract on their own. This ability encompasses an intrinsic conduction system, which with its nervous and muscular characteristics allows conduction of impulses throughout the heart muscle.
  • The intrinsic conduction system sets the basic rhythm of the beating heart. It consists of autorhythmic cardiac cells that initiate and distribute impulses (action potentials) throughout the heart.

How is electrical impulse generated & transmitted in the heart?

Orderly contractions of the atria and ventricles are regulated by the transmission of electrical impulses that pass through an intricate network of specialised cardiac muscle cells. In the cardiac cycle, the independent contraction and relaxation of heart cells are coordinated through the activity of the heart's intrinsic conduction system and by cell-to-cell communication via the gap junctions in the myocardial cells themselves.The intrinsic conduction system of the heart consists of nodal tissue (localized in specific regions of the heart), whose specialized cells have both nervous and muscular characteristics.

The intrinsic conduction system includes-

  • Sinoatrial Node/ SA Node
  • Internodal Pathway
  • Atrioventricular Node/ AV Node
  • Atrioventricular Bundle/ AV Bundle
  • Bundle Branches
  • Purkinje Fibers
  • The starting point of the intrinsic conduction system is the Sinoatrial Node or SA node.
  • A node is a mass of cells, somewhat like a knot, and the SA node is a mass of cells that set the pace of the heartbeat.
  • The SA node located on the upper right atrium is very important, and it's often referred to as the pacemaker of the heart. In fact, if something goes wrong with this node, a person might need to have an artificial pacemaker implanted to keep their heart's steady beat.
  • Cells of the SA node can depolarize on their own, without any external influence.
  • Depolarization is a change in the cell's membrane potential, making it more positive on the inside, and it's this switch to a more positive state that sparks the electrical impulse needed to start each heartbeat.
  • Once the impulse is generated, it travels throughout the heart, kind of like an electrical current through a wire when you flip on a light switch.
  • The SA node initiates the depolarization impulse which, in turn, generates an action potential that spreads throughout the atria, and to the AV node.
This is located in the walls of the atria, and links the SA node to the AV node. It distributes the action potential to the contractile cells of the atria.
  • This is located in the inferior interatrial septum, and functions to transmit the electrical impulse from the atria to the ventricles.
  • The action potential is delayed here briefly, while the atria contract, before being transmitted to the AV bundle.
  • This delay is very brief, it's only about a tenth of a second, but it's enough time to ensure that the atria have expelled their blood into the ventricles before the ventricles contract.
  • This delay is very important, because if the atria and ventricles contracted at the same time, they would be pushing against each other and blood flow through the heart would lack coordination.
This transmits electrical impulses between the atria and the ventricles, and allows the action potential to move from the interatrial septum to the interventricular septum, connecting the AV node to the Bundle Branches.
Convey the action potential down the interventricular septum.
These extend from the bundle branches to the ventricular myocardium and form the last part of the cardiac conduction system. The Purkinje fibers convey the action potential to the contractile cells of the ventricle. Action potentials, which spread from the autorhythmic cells of the intrinsic conduction system to the contractile cells are electrical events. Subsequent contraction of the contractile cells is a mechanical event that causes a heartbeat.