Cardiac Muscle and Heart Function Quiz

Cardiac Muscle

• Located in the middle layer of the heart, the myocardium
• Situated between the endocardium (inner layer) and the epicardium (outer layer)
• Muscle cells are called cardiomyocytes
• Cardiomyocytes are short, branched, and interconnected
• Interconnected via intercalated discs, facilitating electrical and mechanical communication

Cell Structure

• Cardiomyocytes are short, branched, and interconnected
• Connected by intercalated discs
• These discs provide electrical communication and mechanical connection between cells

Cardiac Muscle Layers

• Epicardium: The outermost layer of the heart, acting as a protective barrier and a passageway for coronary arteries and nerves
• Composed of thin connective tissue and mesothelium
• Myocardium: The middle and thickest layer; the main muscle layer responsible for pumping blood
• Contains striated muscle cells (cardiomyocytes) connected by intercalated disks
• Endocardium: The innermost layer; lines the heart chambers and valves, facilitating smooth blood flow
• Composed of thin connective tissue and endothelial cells

Cardiac Muscle Contraction Mechanism

• Contraction begins with Ca2+ ions entering the cell (calcium-induced calcium release).
• Intercalated discs transmit the action potential from the T tubule to the sarcoplasmic reticulum.
• This stimulates calcium release from the sarcoplasmic reticulum.
• Calcium released activates actin-myosin filaments, thereby initiating contraction.
• Cardiac muscle exhibits less regular and slower contraction dynamics compared to skeletal muscle, due to the difference in structure (DIAD vs. triad).

Cardiac Anatomy

• The heart is located in the middle mediastinum, at the level of thoracic vertebrae 15-18.
• It has four chambers (two atria and two ventricles)
• Right and left sides are separated by the interatrial and interventricular septa.
• The interventricular septum is thicker than the interatrial septum to generate more pressure during ventricular contraction.
• The right atrium receives blood from the superior and inferior vena cava.

Heart Valves

• Heart valves are one-way valves that allow blood flow in a single direction.
• Atrioventricular (AV) valves (tricuspid and bicuspid/mitral) control blood flow between atria and ventricles.
• Semilunar valves (aortic and pulmonary) control blood flow from the ventricles to the aorta and pulmonary arteries
• Their opening and closing depend passively on pressure gradients.

Heart Wall

• Three layers: endocardium, myocardium, and epicardium
• Myocardium is the heart muscle composed of cardiomyocytes with intercalated discs, facilitating rapid signal transmission.
• The rapid spread of this signal across the heart without damping is called a syncytium.

Heart Syncytia

• Two functional syncytia in the heart: atrial and ventricular
• Separated by a non-conductive fibrous tissue called the annulus fibrosus
• This separation allows the atria to contract before the ventricles

Heart Pacemaker Cells

• Sinoatrial (SA) node is the primary pacemaker (60-100 bpm) located near the entrance of the superior vena cava in the right atrium.
• Atrioventricular (AV) node is the secondary pacemaker (40-60 bpm) located in the interatrial septum behind the tricuspid valve.

Blood Pressure Regulation

• Maintaining blood pressure requires both short-term and long-term mechanisms.
• Short-term mechanisms involve neural control (e.g., baroreceptor reflex) adjusting blood vessel diameter (vasoconstriction/vasodilation).
• Long-term mechanisms involving hormonal pathways (e.g., Renin-angiotensin-aldosterone system) regulate blood volume and electrolyte balance.

Cardiac Output

• Cardiac output is the volume of blood pumped by the heart per minute (CO = SV x HR).
• Stroke Volume (SV): Volume of blood pumped per heartbeat.
• Heart Rate (HR): Number of heartbeats per minute.
• Normal resting adult human heart rate is 60-100 bpm.

Cardiac Action Potential

• Action potentials in cardiac cells have unique phases.
• Phase 4 is the resting potential (-90mV); only K+ channels are open
• Phase 0 is rapid depolarization caused by Na+ influx
• Phase 1 is early repolarization from K+ efflux
• Phase 2 is the plateau phase caused by Ca2+ influx and K+ efflux
• Phase 3 is repolarization from continued K+ efflux

SA Node Pacemaker Potential

• SA node cells are specialized cardiomyocytes with a resting membrane potential (-60 to -55mV).
• This is closer to the threshold potential, making them more excitable compared to other cardiomyocytes.

Funny Channels

• Funny channels (Na+ leak channels) in the SA node membrane contribute to maintaining a less negative resting membrane potential.
• They facilitate spontaneous depolarization, allowing the SA node to initiate the heart beat.

Venous Circulation

• Veins act as capacitance vessels, storing blood.
• Venous return is driven by the low pressure gradient towards the heart.
• Veins have valves to prevent backflow and skeletal muscle pumps aid venous return.
• Factors such as posture (standing) can affect venous pressure, leading to venous pooling or varicose veins.

Coronary Circulation

• Coronary arteries supply oxygen and nutrients to the myocardium.
• The left coronary artery (LCA) branches into the left anterior descending (LAD) and circumflex arteries, supplying the left side of the heart.
• The right coronary artery (RCA) supplies the right side of the heart.

Abnormal Heart Rates

• Tachycardia: Heart rate above 100 bpm.
• Bradycardia: Heart rate below 60 bpm.
• Arrhythmia: Irregular heart beat patterns.