Muscle Tissue Organization and Histology

Types of Muscle Tissue:
- Skeletal Muscle
- Cardiac Muscle
- Smooth Muscle

Embryological Origin: Understand the embryological origins of the three different types of muscles.
Components of Muscle Tissue: Identify and describe components of muscle tissue.
Histological Organization: Explain the characteristics and histological organization of all different muscle types.
Sliding Filament Model: Describe the sliding filament model of contraction for skeletal muscle as studied in class.
Repair Processes: Describe and compare the repair processes for the three types of muscle.

Key Developmental Structures:
- Somites: Precursor to muscle tissue formed during embryonic development.
- Intraembryonic Coelom: Space within the embryo that contributes to muscle development.
- Mesodermal Layers:
- Somatic Mesoderm: Contributes to skeletal muscle and dermis.
- Splanchnic Mesoderm: Involved in the formation of visceral structures, including cardiac and smooth muscle.

Key Layers:
- Ectoderm: Contributes to nervous tissue.
- Mesoderm: Forms muscle and connective tissue.
- Endoderm: Contributes to epithelial lining and internal organs.

Skeletal Muscle Development:
- Originates from somatic mesoderm.
Cardiac Muscle Development:
- Also from splanchnic mesoderm, develops into myocardium.
Smooth Muscle Development:
- Originates from splanchnic mesoderm, involved in visceral organ walls.

Differences Between Skeletal, Cardiac, and Smooth Muscle:

Characteristic	Skeletal Muscle	Cardiac Muscle	Smooth Muscle
Body Location	Attached to bones or skin	Walls of the heart	Hallow visceral organs (including blood vessels)
Cell Shape & Appearance	Long, cylindrical, striated	Branching, striated with intercalated discs	Fusiform, non-striated
Control	Voluntary	Involuntary	Involuntary
Contractile Characteristics	Quick to contract, tire easily	Sustained contractions, resistant to fatigue	Slow contractions

Skeletal Muscle

Properties:
- Packaged into skeletal muscles that are attached to bones and skin.
- Fibers are the longest of all muscle types and exhibit striations (stripes).
- Known as voluntary muscle because they can be consciously controlled.
- Key characteristics:
- Rapid contraction capability.
- Easily fatigued.
- Powerful performance.

Muscle Fiber: Refers to a muscle cell.
Syncytium: Multinucleated mass produced by the fusion of multiple individual muscle cells.
Sarcoplasmic Terms:
- Sarcoplasm: Cytoplasm of muscle fiber.
- Sarcoplasmic reticulum: Smooth endoplasmic reticulum associated with muscle cells.
- Sarcolemma: Plasma membrane of muscle fiber.

Structure:
- Muscle and muscle fibers are enveloped in connective tissue sheaths for support and reinforcement.
- Types of Sheaths:
- Epimysium: Dense irregular connective tissue surrounding the entire muscle; may blend with fascia.
- Perimysium: Fibrous connective tissue surrounding fascicles (groups of muscle fibers).
- Endomysium: Fine areolar connective tissue surrounding each muscle fiber.

Myofibers: Surrounded by endomysium consisting of reticular connective tissue with basal lamina.
Striations: Longitudinal stripes formed due to the arrangement of myofibrils within the muscle fiber.
- A Bands: Dark bands, containing thick (myosin) and thin (actin) filaments.
- I Bands: Light bands, containing thin (actin) filaments exclusively.
- M line: Midpoint of A band, anchoring myosin filaments.
- Z Disc: Coin-shaped proteins anchoring actin filaments, marking the end of the sarcomere.

Composition & Features:
- Myofibrils are rod-like elements densely packed within a fiber, accounting for approximately 80% of muscle cell volume.
- Organize into structural units known as sarcomeres, which function as the smallest contractile unit of muscle.

Mechanism:
- Thin actin filaments slide over thick myosin filaments to cause contraction.
- Key Observations During Contraction:
- Length of the sarcomere decreases.
- Z discs move closer together.
- I band length decreases or disappears at maximal contraction.
- A band length remains unchanged.

Sarcoplasmic Reticulum (SR):
- Network of smooth endoplasmic reticulum tubules surrounding myofibrils, regulating intracellular calcium levels.
- Contains terminal cisternae that form perpendicular cross channels at A-I junctions.

Protrusions of the sarcolemma into the cell’s interior, allowing for rapid transmission of electrical signals throughout the muscle fiber.
Each T-tubule forms a triad with two terminal cisternae from adjacent sarcomeres.

Skeletal Muscle Repair:
- Skeletal muscle fibers are post-mitotic, meaning they do not regenerate through normal cell division.
- Satellite Cells: Reside within the basal lamina and can differentiate into myoblasts and fuse to repair muscle fibers.
- Limitations exist when numerous satellite cells are depleted through extensive or recurrent injuries.
Cardiac Muscle Repair:
- Derived from splanchnic mesoderm, cardiac muscle does not typically regenerate; scar tissue replaces damaged areas.

Structure:
- Striated and involuntary, forming the myocardium of the heart.
- Cells have intercalated discs that connect adjacent cells for synchronous contraction.
- Single T-tubule and terminal cisterna contribute to the unique diads formation.

Properties:
- Nonstriated and involuntary, composed of small tapered cells with centrally located nuclei.
- SR is reduced or absent, and myofilaments are arranged in a crisscross pattern with dense bodies instead of Z discs.
- Capable of both hyperplasia (increase in number of cells) and hypertrophy (increase in size of cells).