Microscopic Anatomy and Physiology of the Heart and Myocardium

The Cardiovascular System Components: The system consists of three primary components that work in tandem to maintain homeostasis:
- Blood: The fluid medium for transport.
- The Heart: The central pump.
- The Vessels: The conduit system for blood distribution.
Primary Function of the Heart: The heart serves as a rhythmic pump essential for maintaining a vital life cycle. It pumps blood throughout the vascular network to ensure:
- Delivery of oxygen to the cells.
- Removal of metabolic wastes from the cells via the bloodstream.
Tissue Composition: The specific muscle tissue found within the heart is known as the myocardium (the prefix "myo-" denotes muscle).

Autolithmicity: A unique physiological property of the myocardium that allows the heart to generate its own electrical impulses independently.
- While the heart generates its own rhythm, this rhythm can be modulated, varied, or changed by external inputs from the Nervous System and the Endocrine System.
Major Cell Populations: There are two distinct types of cells within the heart:
- Myocardial Contractile Cells: These comprise the vast majority of the heart, accounting for approximately $99\%$ of the total cells. They contain contractile proteins that facilitate the physical squeezing and pumping of blood.
- Myocardial Conducting Cells: These make up the remaining $1\%$ of the heart cells. They form a specialized conduction system and function similarly to neurons, though they are technically highly specialized cardiac muscle cells.
- Purkinje Cells: These are a sub-category of specialized conducting cells responsible for carrying electrical impulses specifically through the ventricles.

Physical Characteristics: Cardiomyocytes (contractile cells) differ from skeletal muscle cells in several ways:
- Size: They are shorter in length than skeletal muscle cells.
- Shape: They possess a unique branched structure, unlike the linear structure of skeletal muscle.
Internal Organization:
- Myofibrils: These are long threads or bundles found inside the muscle cell that run in parallel to one another.
- Sarcomeres: The myofibrils are organized into repeating functional units called sarcomeres.
- Z-Lines: These are the boundaries of the sarcomere, appearing as pink, Z-shaped vertical lines. A single sarcomere is defined as the region from one Z-line to another Z-line.

Intercalated Discs: These are unique features of cardiac muscle tissue that help bind cells together to ensure the heart acts as one unified functional unit. They contain two critical structures:
- Desmosomes: Described as acting like "buttons," these structures lock the cells tightly together to prevent them from pulling apart during contraction.
- Gap Junctions: Channels that allow for the passage of ions between cells, facilitating rapid communication and electrical coupling between cardiomyocytes.
The Sarcolemma and T-Tubules:
- Sarcolemma: The specialized plasma membrane of the muscle cell.
- T-Tubules (Transverse Tubules): These are deep extensions of the sarcolemma that penetrate into the interior of the cell. Their primary purpose is to ensure that an action potential reaches all parts of the cell interior, not just the surface, to trigger synchronous contraction.

T-Tubule Distribution:
- Cardiac Muscle: T-tubules are found only at the Z-lines.
- Skeletal Muscle: T-tubules are found at the junction of A-bands (dark bands containing myosin and actin) and I-bands (light bands containing only actin).
- Relative Density: Skeletal muscle contains more T-tubules than cardiac muscle.
Sarcoplasmic Reticulum (SR):
- Definition: A specialized version of the endoplasmic reticulum found in muscle cells used for calcium storage.
- Storage Capacity: The SR in cardiac muscle stores relatively low amounts of calcium ions ( $Ca^{2+}$ ), whereas the SR in skeletal muscle contains a large storage of calcium ions.
Calcium Sources for Contraction:
- Cardiac Muscle: Because internal storage is limited, most of the $Ca^{2+}\!$ required for contraction comes from the extracellular environment (outside the cell).
- Skeletal Muscle: The $Ca^{2+}\!$ required for contraction is sourced internally from the SR.
Contraction Onset:
- Cardiac Muscle: Exhibits a slower onset of contraction.
- Skeletal Muscle: Exhibits a much faster onset of contraction.
Mitochondrial Volume:
- Function: Mitochondria serve as the "powerhouse" of the cell to generate Adenosine Triphosphate (ATP).
- Density: Cardiac muscle cells have a significantly higher density of mitochondria to support continuous activity, accounting for approximately $25\%$ of the total cell volume.