Physiology of the Heart

Question 1

Cardiac muscle

Answer 1

• Striated, contractions occur via the sliding filament mechanism
• Short, fat with each fibre contains one or two centrally located nuclei
• Loose connective tissue connects the muscle to the cardiac skeleton giving the cells something to pull against

Question 2

Cardiac muscle cells

Answer 2

• Connected by intercalated discs with desmosomes for strength and gap junctions to allow ion flow between cells
• This allows the heart to beat as a functional syncytium = a bunch of individual cells acting as a single coordinated unit

Question 3

Cardiac vs. skeletal muscle

Answer 3

• Cardiac: many large mitochondria, myofibrils arranged in sarcomeres, less extensive sarcoplasmic reticulum than skeletal muscle

Question 4

Mitochondria in cardiac muscle

Answer 4

• There are a lot of mitochondria in cardiac muscle indicating how much it relies on aerobic respiration for its energy
• The mitochondria is capable of switching nutrient pathways to use whatever nutrient supply is available eg. lactic acid

Question 5

Cardiac vs. skeletal muscle

Answer 5

• Both muscles have their sarcomeres aligned giving them striations
• heart fibres contract as a unit by gap junctions electrically tieing them all together, skeletal does not
• The heart’s absolute refractory period is Lon get than skeletal muscles which prevent tetanic contractions

Question 6

Pacemaker cells

Answer 6

• Autorhythmic muscle fibres
• Make up 1% of cardiac muscles
• Self-excitable, initiate their own depolarization and set the rhythm for depolarizing the rest of the cardiac muscle

Question 7

Sacroplasmic reticulum

Answer 7

• Releases Ca2+, this release is triggered by an influx of Ca2+ from the ECF or through gap junctions
  _ The release of Ca2+ is not triggered by the depolarization wave that causes contraction in skeletal muscle

Question 8

Electrical events of the Heart

Answer 8

• The heart depolarizes and contracts without nervous system stimulation
• The rhythm can be altered by the autonomic nervous system (ANS)

Question 9

Intrinsic conduction system (ICS)

Answer 9

• Sets the basic rhythm of the heart
• The heart relies on gap junctions to conduct impulses throughout it and the ICS
• Has cardiac pacemaker cells with unstable resting potential

Question 10

Cardiac pacemaker cells

Answer 10

• Produce pacemaker potentials that continuously depolarize
• This initiates the action potentials conducted throughout the heart

Question 11

Pacemaker Potentials

Answer 11

They cross the membrane potential threshold and initiate action potentials depolarization and repolarization

Question 12

Intrinsic conduction sequence

Answer 12

Impulses pass through the cardiac pacemaker cells in this order:
1. The sinoatrial (SA) node
2. The atrioventricular (AV) node
3. The AV bundle (Bundle of His)
4. Right and left bundle branches
5. Subendocardial conducting network (Purkinje fibres)

Question 13

The sinoatrial (SA) node

Answer 13

• Located in the right atrium
• Primary pacemaker

Question 14

The atrioventricular (AV) node

Answer 14

• In the interatrial septum
• Slows signal down slightly to allow the atria to finish contracting before the ventricles contract

Question 15

The AV bundle (Bundle of His)

Answer 15

• The only electrical connection between the atria and ventricles.
• Conducts impulses into the ventricles from the AV node

Question 16

Right and left bundle branches

Answer 16

• From the bottom of the heart and branches to each side to meet the ventricles.
• Conducts down the interventricular septum

Question 17

Subendocardial conducting network (Purkinje fibres)

Answer 17

• Penetrates throughout the ventricles and distributes impulses
• Runs through the cardiac muscle so it distributes the impulses through the heart

Question 18

Defects in the intrinsic conduction system

Answer 18

• Uncoordinated atrial and ventricular contractions
• Arrhythmias are irregular heart rhythms
• Fibrillation is rapid, irregular, infecting contractions

Question 19

Fibrillation

Answer 19

• During ventricular fibrillation even though the heart is moving, it becomes useless for pumping blood, causing circulation to cease
• Treatment is defibrillation: disrupting the chaotic twitching and resetting the heart to regular, normal depolarizations

Question 20

Autonomic regulation of the heart

Answer 20

• The ANS modifies the heartbeat through cardiac centres in the medulla oblongata

Question 21

Caridoacceleratory centre

Answer 21

• Projects tot he sympathic neurons throughopout the heart, increasing both heart rate and concractile force

Question 22

Cardioinhibitroy centre

Answer 22

• Sends impulses to the parasympathetic dorsal vagus nucleus in the medulla oblongata
• Stimulates the vagus nerve to the heart.
• Decreases heart rate

Question 23

Action Potentials in Cardiac Muscle

Answer 23

• Stimulated cardiac muscle cells have fast voltage-gated Na+ channels that allow Na+ to enter the cell = rapid depolarization.
• Depolarization opens slow Ca2+ channels, allowing Ca2+ to enter the cell, even as K+ exits = plateau phase of AP that delays reploarization
• After 0.2s, Ca2+ channels are inactivated, additional K+ channels open, and the cell rapidly depolarizes back to the resting membrane potential

Question 24

Electrocardiography

Answer 24

Monitors and amplifies the electrical signals of the hear and records it as in aelectrocardiogram
P wave - Atria depolarization
QRS complex - ventricular depolarization
T wave - ventricular depolarization