Cardiac Muscle Structure and Function

Cardiac Muscle

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

Cardiac Muscle Cell Structure

• Cardiomyocytes are short, branched, and interconnected
• Interconnected by intercalated discs, which facilitate electrical communication and mechanical connection.

Cardiac Muscle Layers

• Epicardium: Outermost layer of the heart, protective layer; acts as a buffer against external impacts. Consists of thin connective tissue and mesothelium. Coronary arteries and nerves pass through this layer.
• Myocardium: Middle and thickest layer; the primary muscle layer that pumps blood. Composed of cardiomyocytes. Contraction function is performed by cardiomyocytes which are striated muscle cells. Intercalated discs connect cardiomyocytes.
• Endocardium: Innermost layer; lines the heart chambers and valves. This layer provides a smooth surface to allow blood to flow properly. Composed of thin connective tissue and endothelial cells.

Cardiac Muscle and Other Structures

• Intercalated Discs: Special junctions that connect cardiomyocytes. Include gap junctions (facilitate rapid electrical impulse passage) and desmosomes (provide mechanical strength). Also involved in the regular arrangement of actin and myosin filaments, creating striations.
• DIAD (Dyad): A structure unique to cardiac muscle, made of a T tubule and terminal cisternae of the sarcoplasmic reticulum. Plays a critical role in regulating calcium ions, which trigger contraction.

Cardiac Muscle Function

• Cardiac Action Potential: The electrical events that precede contraction. Resembles the typical action potential but with a unique plateau phase. This phase makes it longer than skeletal muscle action potential. The action potential is initiated by a pacemaker cell.
• Contraction: Begins with calcium ions entering the myocardial cell, which triggers the release of more calcium. This process activates actin-myosin filaments, leading to the physical contraction of the heart muscle.
• Regulation of Calcium Signal: Calcium-induced calcium release (CICR) is how calcium enters myocardium. The DIAD transmits the action potential to the sarcoplasmic reticulum for calcium release.
• Relationship between Electrical Stimulus and Contraction: The T tubule allows quick action potential spread to the depths of the cell. Calcium released from the terminal cisternae activates actin-myosin filaments, triggering contraction.

Cardiac Anatomy

• Location: Middle mediastinum at vertebrae T5–T8
• Chambers: Four chambers, two atria (upper), two ventricles (lower)
• Septum: Interatrial septum and interventricular septum separate right and left sides, and the latter is thicker for greater pressure development.
• Blood Flow: Blood from the superior and inferior vena cava flows into the right atrium. Then, it goes to the right ventricle. It goes to the pulmonary artery before it reaches the lungs. After the lungs, the blood goes back to the heart via the pulmonary veins. Left atrium, left ventricle, and aorta direct further blood flow.

Heart Valves

• Atrioventricular (AV) Valves: Tricuspid (right side) and bicuspid (left side), regulate blood flow between atria and ventricles.
• Semilunar Valves: Aortic and pulmonary, control blood flow out of the ventricles into the aorta and pulmonary trunk.

Heart Wall Layers

• Endocardium: Lines the heart chambers
• Myocardium: Middle layer, composed of cardiac muscle
• Epicardium: The outermost layer; a protective layer composed of thin connective tissue and mesothelium.

Cardiac Muscle Layers

• Heart Wall: The heart wall consists of three layers: endocardium, myocardium and epicardium
• Cardiomyocytes: Specialized muscle cells within the myocardium responsible for the heart's contraction.
• Intercalated Discs: Connective structures in the myocardium that allow for rapid transmission of electrical signals throughout the heart

Heart Rate and Pacemaker Cells

• Heart Rate: The frequency of cardiac contractions (beats per minute)
• Pacemaker Cells: Specialized cells that generate and transmit electrical impulses, initiating the heart's contraction, such as Sinoatrial (SA) node, Atrioventricular (AV) node. These cells establish the rhythm.
• SA Node: The primary pacemaker, located in the right atrium near the superior vena cava entrance, sets the heart rate (60 - 80 bpm).
• AV Node: Located in the interatrial septum; slows the impulse to allow atria to contract first and ventricles to fill completely.
• Conduction Pathway: Electrical signals spread through the heart activating cells through internodal pathways to the AV node (bundle of His to Purkinje fibers).

Cardiac Cycle

Phases:

• Diastole (relaxation): Atrial and ventricular filling phases
• Systole (contraction): Atrial and ventricular emptying phases

Coronary Circulation

• Coronary Circulation: The network of blood vessels supplying oxygen and nutrients to the myocardium (cardiac muscle).
• Left Coronary Artery (LCA): Branches into left anterior descending (LAD) and circumflex arteries. Supplies blood to the front and large part of interventricular septum of the left ventricle.
• Right Coronary Artery (RCA): Supplies blood to the right atrium, ventricle and part of the septum.

Heart Sounds

• S1: Closure of AV valves, "lub" sound.
• S2: Closure of semilunar valves, "dub" sound.

Venous Circulation

• Venous Valves and Venous Pump: Aid in blood flow toward the heart and prevent backflow to prevent accumulation.
• Veins as Capacitance Vessels: Capable of storing large blood volumes.

Autonomic Nervous System

• Sympathetic Division: Primarily involved in "fight or flight" - Increasing heart rate, blood pressure, and diverting blood flow to skeletal muscles.
• Parasympathetic Division: Primarily involved in "rest and digest" - Decreasing heart rate, blood pressure, and promoting digestion.

Blood Pressure Regulation

• Short-term regulation: Neural mechanisms (baroreceptor reflex) control blood pressure in response to acute changes.
• Long-term regulation: Hormonal mechanisms (renin-angiotensin-aldosterone system, and others) control blood pressure in response to chronic changes.

Other important Concepts

• Abnormal Rhythms: Tachycardia (fast heart rate), Bradycardia (slow heart rate), and Arrhythmias (irregular heart beats)
• Varicose Veins: Weakened or damaged valves in superficial veins that cause pooling of blood.
• Heart Failure: A condition where the heart cannot pump enough blood to meet the body's needs.
• Cardiac Output: The product of heart rate and stroke volume, indicating how much blood the heart pumps per unit of time (ml/min)
• Stroke Volume: The amount of blood pumped by the ventricle per contraction (ml/beat)

Discover the intricate anatomy and physiology of cardiac muscle in this quiz. Learn about the layers of the heart, the unique structure of cardiomyocytes, and how they facilitate heart function through intercalated discs. Test your knowledge on the critical components that make up the muscular heart.