Microanatomy of Muscle Tissue

Muscle cells are highly specialized structures capable of contraction to facilitate movement, including locomotion, propulsion, and pressure regulation.
The terms "myocytes," "myofibres," and "muscle cells" are often used interchangeably to describe the individual units of muscle tissue.
Sarcoplasm refers to the cytoplasm of a muscle cell, which contains significant stores of glycogen and myoglobin.
Sarcolemma is the specific name for the plasma membrane of a muscle cell.
Sarcoplasmic reticulum is the highly specialized smooth endoplasmic reticulum found within muscle cells; its primary function is to regulate the flow and storage of calcium ions ( $Ca^{2+}$ ), which is critical for muscle contraction.

Diameter: Range of $10 \text{ to } 110\,\mu m$ .
Length: Range of $40\,mm \text{ to } 50\,cm$ .
Structural Features: - Myofibrils are arranged in tubes. - Exhibit distinct cross-striations. - Characterized by peripheralized, multiple nuclei (multinucleated).

Diameter: Approximately $15\,\mu m$ .
Length: Approximately $100\,\mu m$ .
Structural Features: - Exhibit a branching arrangement. - Possess intercalated disks which provide strength and allow the cells to function as a syncytium. - Contain a single, centrally located nucleus. - Display cross-striations. - Rich in mitochondria, which can comprise up to $20\%$ of the cell volume to meet high oxygen demands.

Diameter: Range of $6 \text{ to } 8\,\mu m$ .
Length: Range of $20\,\mu m \text{ to } 1\,mm$ (the latter specifically in the uterus during pregnancy).
Structural Features: - Fusiform or spindle-shaped cells. - Possess an elongated nucleus with blunt ends. - Lack cross-striations (non-striated).

The majority of muscle tissue originates from the mesoderm germ layer.
Skeletal Muscle Development: - Undifferentiated mesenchymal cells, known as myoblasts, align and fuse together to form multinucleated tubes called myotubes. - These myotubes differentiate to form functional myofilaments. - During differentiation, the nuclei are displaced toward the periphery (against the plasma membrane). - Satellite cells are mesenchymal stem cells that do not differentiate and instead remain between the basal lamina and the sarcolemma; they retain mitotic potential and function in muscle repair.

Epimysium: A layer of dense irregular connective tissue (deep fascia) that surrounds the entire muscle organ.
Perimysium: Dense connective tissue that surrounds bundles of muscle cells known as fascicles.
Endomysium: A delicate layer composed of reticular fibers that surrounds each individual myocyte.
Hierarchy: Muscle $\rightarrow$ Fascicle $\rightarrow$ Myofiber (Myocyte) $\rightarrow$ Myofibril $\rightarrow$ Myofilament.

Myofibrils: Cylindrical bundles of contractile proteins called myofilaments. Their specific arrangement creates the striated appearance seen in skeletal and cardiac muscle.
Myofilaments: These are sub-microscopic structures (visible only by Electron Microscopy/EM) categorized into: - Contractile proteins: Actin and Myosin. - Regulatory proteins: Tropomyosin and Troponin.
The Sarcomere (Repeated assembly of filaments): - A Band: A dark segment corresponding to the area where thick (myosin) and thin (actin) filaments overlap. - I Band: A pale segment composed exclusively of thin actin filaments. - Z Disc: The boundary of a sarcomere. - H Zone: The center of the A band where only thick filaments are present. - M Line: The midline of the sarcomere.
Contraction Mechanism: Muscle motion is the conversion of chemical energy (ATP) into mechanical energy. During contraction, actin filaments slide over myosin filaments, resulting in the shortening of the I band.
Structural Support: Other proteins such as desmin, tropomyosin, and troponin help hold actin and myosin in position.

Cardiac myocytes are also referred to as cardiomyocytes or myocardiocytes.
Cardiac muscle functions as a syncytium, where cells act as a single unit due to intercalated disks.
Intercalated Disks: Contain gap junctions (for electrical coupling/communication) and desmosomes (for physical adhesion).
Lipofuscin pigments may be present within these cells.

Cells contain a single centrally located nucleus.
Although actin and myosin myofilaments are present, they are not ordered into myofibrils, hence the lack of striations.
The sarcoplasmic reticulum is poorly developed compared to striated muscle.
Innervation: Regulated by both parasympathetic and sympathetic nervous systems.
Function: Contraction is involuntary and involves: - Peristalsis: Wave-like contractions in the gastrointestinal tract. - Vascular dynamics: Contraction to alter blood flow and regulate blood pressure. - Propulsion: Found in the urinary bladder and uterus. - Minor role in secretion.
Tunica Muscularis: Generally organized into an inner circular layer and an outer longitudinal layer within the walls of luminal organs.

Myoepithelial Cells: - Origin: Ectoderm. - Features: Basket-like shape that encloses clusters of glandular cells in exocrine glands. - Contain actin and myosin; function similarly to smooth muscle.
Myofibroblasts: - Origin: Mesoderm. - Function: Responsible for wound contraction.

Skeletal Muscle: Has limited regeneration capacity thanks to satellite cells. When damage occurs, fibroblasts also form connective tissue (scar tissue).
Cardiac Muscle: Lacks the ability to regenerate.
Smooth Muscle: Regeneration is limited. Some repair is possible through mitotic activity and the differentiation of new smooth muscle cells from pericapillary mesenchymal cells.
General Rule: In all muscle types, repair is typically completed by scar tissue formation and requires an intact basal lamina.