The mechanism of cardiac automaticity

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2.

3. The mechanism of cardiac automaticity

4. Learning objective

11.1.3.3 to explain the mechanism of
cardiac automaticity

5. Success criteria

1. Describe structure of the heart and
indicate the link between structure of the
heart muscle and its ability to automatism.
2. Explain the mechanism of heart
automatism.

6. Terminology

• Right and left atrium
• Right and left ventricle
• Cardiac muscles
• Sinoatrial node (SAN)
• Pacemaker
• Atrioventricular node (AVN)
• Purkinje fibres
• Heart beat
• Electrical impulse/stimulus
• Atrial walls
• Relax/contract

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https://media.giphy.com/media/sLtgwHCm9pinu/giphy.gif
http://www.scientistcindy.com/muscle-tissue.html

10. Cardiac muscles

• Cardiac muscle tissue is made up of cells that are striated and have only one
nucleus per cell. These cells create branched structures within the tissue that
allows connections to be made between cells via gap junctions. Cardiac
muscle contraction is under the control of the autonomic nervous system
and, therefore, is involuntary. Image how your life would be if you had to
remember to tell your heart to beat every second or so!
• Cardiac muscle cells are called cardiomyocytes. Cardiomyocytes are
connected via gap junctions which allow electrochemical signals from one
cell to travel directly into the adjacent cells. The gap junction acts like a
tunnel connecting the cytoplasm of the two cells together. In cardiac muscle,
these junctions act to coordinate the spread of the action potential
(generated by the pacemaker cells) to the rest of the cardiomyocytes of the
heart, thereby creating a coordinated, rhythmic heart beat

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12. The heart structure

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14. How electrical impulses move through the heart

15. Parts of the Electrical System

• S-A node (sinoatrial node) — known as the heart’s natural pacemaker, the
S-A node has special cells that create the electricity that makes your heart
beat.
• A-V node (atrioventricular node) — the A-V node is the bridge between the
atria and ventricles. Electrical signals pass from the atria down to the
ventricles through the A-V node.
• His-Purkinje system — the His-Purkinje system carries the electrical signals
throughout the ventricles to make them contract. The parts of the HisPurkinje system include:
• o His Bundle (the start of the system)
• o Right bundle branch
• o Left bundle branch
• o Purkinje fibers (the end of the system)

16. SAN, AVN, His bundle and Purkinje fibres

17. Control of the heat beat: SAN or pacemaker

1. Each cardiac cycle begins in the right atrium.
There is a small patch of muscle tissue in the
right atrium wall, called the sinoatrial node
(SAN), which automatically contracts and relaxes
all the time. It doesn’t need a nerve impulse to
start it on, so it is said to be myogenic – that is,
‘started by the muscle’.
The pacemaker’s rate can be adjusted by nerves
transmitting impulses to the pacemaker from
the brain.

18. Control of the heat beat

2. As the muscle in the SAN
contracts, it produces an electrical
excitation wave which sweeps
through all of the muscle in the
atria of the heart. This excitation
wave makes the muscle in the atrial
walls contract.

19. Control of the heat beat: AVN

3. The excitation wave sweeps
onwards and reaches another patch
of cells, called the atrioventricular
node (AVN). This node is the only way
in which the electrical impulse can get
down to the ventricles. The AVN
delays the impulse for a fraction of a
second, before it travels down into the
ventricles. This delay means that the
ventricles receive the signal to
contract after the atria receive the
signal.

20. Control of the heat beat: Purkinje tissue

4. The excitation wave moves
swiftly down through the septum of
the heart, along fibres known as
Purkyne tissue. Once the excitation
wave arrives at the base of the
ventricles it sweeps upwards,
through the ventricle walls. The
ventricles contract.

21. Control of the heat beat: SAN

5. The ventricles then relax. Then
the muscle in the SAN contracts
again, and the whole sequence
runs through once more.

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24. Success criteria

1. Describe structure of the heart and
indicate the link between structure of the
heart muscle and its ability to automatism.
2. Explain the mechanism of heart
automatism.