The contractions of the heart are…
MYOGENIC - meaning that the signal for cardiac compressions originates in the heart muscle tissues themselves
Where/what is the sinoatrial node?
in the right atrium, there is a cluster of heart muscle cells (cardiomyocytes) that direct the contraction of heart muscle tissues - this cluster of cells is called the sinoatrial node
Pacemaker role
controls the rate at which the heart beats
Primary, secondary, and tertiary pacemaker
Primary pacemaker = SA node triggers cardiac contractions at 60 to 100 bpm (NORMAL SINUS RHYTHM)
Secondary pacemaker = AV node maintains contractions at about 40-60bpm (if SA node fails)
Tertiary pacemaker = Bundle of His maintains contractions at about 30-40bpm (if AV node fails)
Fibrillation? Defibrillation?
Interference with pacemakers leads to the irregular and unregulated contraction of the heart muscle - this is called fibrillation
When fibrillation occurs, a normal sinus rhythm can be re-established with a CONTROLLED electrical current
The electrical conduction of a heart beat occurs according to the following events: (4)
- The sinoatrial node sends out an electrical impulse that stimulates contraction of the myocardium (heart muscle tissue)
- This impulse directly causes the atria to contract and stimulates another node at the junction between the atrium and ventricle
- This second node – the atrioventricular node (AV node) – sends signals down the septum via a nerve bundle (Bundle of His)
- The Bundle of His innervates nerve fibres (Purkinje fibres) in the ventricular wall, causing ventricular contraction
Why is there a delay between atrial and ventricular contractions (resulting in two heart sounds)?
Why is the separation of atrial and ventricular contraction is important?
This delay allows time for the ventricles to fill with blood following atrial contractions/systole so as to maximize blood flow (before AV valves close)
Cardiac conduction
- Cardiac muscle cells are not fused together but are instead connected via gap junctions at intercalated discs
- This means that while electrical signals can pass between cells, each cell is capable of independent contraction
- The coordinated contraction of cardiac muscle cells is controlled by specialized autorhythmic cells (‘pace makers’)
atrial contraction
- SA node sends out electrical signals which are propagated throughout the entire atria via gap junctions in the intercalated discs
- In response, the cardiac muscle within the atrial walls contract simultaneously (atrial systole)
- SEE IMAGE
more info - The atria and ventricles of the heart are separated by a fibrous cardiac skeleton composed of connective tissue
- This connective tissue functions to anchor the heart valves in place and cannot conduct electrical signals
- The signals from the sinoatrial node must instead be relayed through a second node located within this cardiac skeleton
- This second node is called the atrioventricular node (or AV node) and separates atrial and ventricular contractions
- The AV node propagates electrical signals more slowly than the SA node, creating a delay in the passing on of the signal
image!!
- SA node sends electrical signal via gap junctions that stimulates the contraction of the atrial myocardium
- The atria contracts and blood is pushed into the ventricles.
- The SA node passes on the signal to the AV node found in the cardiac skeleton.
ventricular contraction
- Ventricular contraction occurs following excitation of the atrioventricular node (located at the atrial and ventricular junction)
- The AV node sends signals down the septum via a nerve bundle (Bundle of His)
- The Bundle of His innervates nerve fibres (Purkinje fibres) in the ventricular wall, causing ventricular contraction
- This sequence of events ensures contractions begin at the apex (bottom), forcing blood up towards the arteries
diastole
- After every contraction of the heart, there is a period of insensitivity to stimulation (i.e. a refractory period)
- This recovery period (diastole) is relatively long, and allows the heart to passively refill with blood between beats
- Helps prevent heart tissue becoming fatigued, allowing contractions to continue for life
