D.4 The heart Flashcards
(112 cards)
What is cardiac muscle?
A type of striated muscle found in the wall of the heart.
Describe the structure of cardiac muscle cells
-This muscle has thick and thin muscle fibres with myofibrils containing myofilaments similar to those found in skeletal muscle.
-The nucleus of cardiac muscle cells is found in the centre of the cell.
-The cells are rich in mitochondria and glycogen granules that are found adjacent to the myofibrils.
-Unlike skeletal muscle, which has multinucleate cells, the cardiac muscle has numerous short, cylindrical cells arranged end-to-end, resulting in long, branched fibres giving a characteristic Y-shape.
What does the structure of cardiac muscle cells allow?
The propagation of stimuli through the heart wall.
Diagram of cardiac muscle cells
What is an intercalated disc?
The attachment site between cardiac muscle cells. It appears as a linear structure transverse to the muscle fibre.
Explain how intercalated discs work
-Cardiac muscle contains transverse cross-bands called intercalated discs.
-These discs represent the attachment site between adjacent cells.
-Intercalated discs contain adhering junctions between cells that hold cells together and gap junctions, which allow communication between cells.
What are gap junctions?
-Arrays of densely packed protein channels that permit intercellular passage of ions and small molecules.
-Electrical activation of the heart requires cell-cell transfer of current via gap junctions.
What do the intercaled discs and gap junctions in cardiac muscle allow to happen?
This characteristic structure of cardiac muscle allows electrical impulses to pass rapidly from cell to cell, so the linked cells contract almost simultaneously.
Explain how contraction works in cardiac muscle cells
-These cells have an “all-or-none” effect: once a cell is activated, it produces maximum contraction.
-These cells also have a long refractory period, during which the cells cannot contract for a second time.
-This ensures that the contraction or systole of the heart is separated by a resting period or diastole.
In what way does a cardiac muscle cell differ from a skeletal muscle cell?
-Cardiac cells have intercalated discs while skeletal cells do not.
-This allows communication between cardiac cells, as the electrical activation needs to propagate from one cell to the other.
What is the refractory period?
Time for cardiac muscle cell membrane to be ready for a second stimulus.
What causes the heartbeat?
The spontaneous rhythmic contraction of cardiac muscle.
Explain how the heartbeat is created and regulated
-This contraction starts in embryonic tissues by specialized cells forming the pacemaker.
-Nodes and bundles that transmit the impulse to various parts of the cardiac muscle tissue regulate the heartbeat.
Diagram of cardiac conduction
Where is the heartbeat initiated?
In the sinoatrial node (SA), a group of specialised cardiac muscle cells found where the superior vena cava joins the right atrium.
Explain what happens after a heartbeat is initiated (until atrial systole occurs)
-Signals from the sinoatrial node that cause contraction cannot pass directly from atria to ventricles.
- Instead, the impulse spreads along the atria to the atrioventricular node (AV), from where it spreads to the ventricle.
-There is a delay between the arrival and passing on of a stimulus at the atrioventricular node.
-This delay allows time for atrial systole before the atrioventricular (tricuspid and bicuspid or mitral) valves close.
-The blood, therefore, leaves the atria to the ventricles when the atrial systole occurs.
The delay between the arrival and passing of a stimulus at the AV node gives time for ___
The atria to contract before the AV valves close.
Explain what happens after the blood enters the ventricles
-The blood is now in the ventricles and the atrioventricular valves are closed.
-In the interventricular walls, there are specialised fibres grouped to form a bundle called Bundle of His.
-This bundle splits into two branches that go to the left and right ventricles, leading to other specialised cells called Purkinje fibres.
-These fibres carry the impulse at a very high speed.
-Conducting fibres ensure coordinated contraction of the entire ventricle wall.
-The ventricular contraction is called the systole.
-Blood then flows out of the heart as the semilunar (pulmonary and aortic) valves open and give access to the aorta and pulmonary artery.
-The heart is now relaxed; this is called the diastole.
How are heart sounds generated?
-By the beating of the heart and the closing of the heart valves.
-Normal heart sounds are caused by the atrioventricular valves and semilunar valves closing causing changes in blood flow.
Describe the heart sounds of a healthy person
-When listening to the heart of a healthy person, a rhythmic lub-dub sound can be heard.
-The closing of the atrioventricular valves at the beginning of the ventricular contraction (systole) produces the first sound, the lub.
-Immediately afterwards, the closing of the semilunar valves just after the ventricular systole and beginning of the diastole causes the second sound, the dub sound.
What is a stethoscope?
-A tool used to listen to the heart sounds.
-This is an acoustic device that has a resonator that is placed on the patient’s chest and two tubes connected to earpieces.
What is the order of the cardiac conduction?
SA node, AV node, bundle of His, Purkinje fibres.
Why is there a delay between the arrival and passing on of a stimulus at the atrioventricular node?
It allows time for atrial systole before the atrioventricular valves close.
What is an action potential?
An electrical event associated with different concentrations of ions across membranes.
