Muscle Tissue Overview

Muscle Tissue

• Muscle tissue is of mesodermal origin
• Muscle tissue is composed of specialized cells called myocytes that contract
• Sarcomere is the cytoplasm of a myocyte
• Sarcolemma is the cell membrane of a myocyte
• Sarcoplasmic reticulum surrounds muscle fibers
• Myocytes are composed of contractile protein filaments (myofilaments), including myosin and actin
• Muscle tissue has three types: skeletal, cardiac, and smooth

Skeletal Muscle

• Skeletal muscle cells are elongated, multinucleated, and have peripherally located nuclei
• They have large, very long, unbranched, cylindrical fibers
• Skeletal muscle has multiple subsarcolemmal nuclei
• It exhibits strong, quick, intermittent contractions
• Skeletal muscle is voluntary and controlled by the somatic nervous system
• Skeletal muscle can hypertrophy by growth of new myofibrils increasing the diameter of the muscle fibers
• Skeletal muscle cells contain ordered, arranged contractile sarcomeres forming cross-striations
• Two types of skeletal muscle fibers: Type I and Type II

Type I Muscle Fibers (Slow Oxidative)

• Rich capillary supply
• Numerous mitochondria
• Aerobic respiration
• High myoglobin content (red fibers)
• Less prone to fatigue
• Do not produce high tension
• Function: maintaining posture, stabilizing bones, and joints

Type II Muscle Fibers (Fast Oxidative/Intermediate)

• Produce ATP relatively quickly
• Produce high amounts of tension
• High amount of mitochondria
• Aerobic respiration
• Myoglobin content less than Type I
• Less prone to fatigue
• Function: walking

Type IIb/IIx Muscle Fibers (Fast Glycolytic)

• Anaerobic glycolysis
• Less mitochondria (white fibers)
• Less myoglobin
• Rapid, forceful contractions
• Quick, powerful movements
• Prone to fatigue
• Function: weight lifting

Sarcomere

• I-band: actin filaments
• A-band: myosin filaments, may overlap with actin filaments
• H-band: zone of myosin filaments within the A-band
• Z-line: zone of apposition of actin filaments belonging to neighboring sarcomeres (mediated by alpha-actinin)
• M-line: band of connections between myosin filaments (mediated by proteins, e.g., creatine phosphate myomesin, M-protein).
• T-tubule
• Sarcoplasmic reticulum
• Triad
• External lamina

Smooth Muscle

• Found in walls of hollow viscera and blood vessels (erector pili)
• Small, spindle-shaped fibers (30-200 µm in relaxed state)
• No striations
• Single central nucleus
• Weak, slow rhythmic contractions
• Controlled by the autonomic nervous system
• Mode of contraction: Ca release initiates a cascade that leads to sliding filaments, requiring calmodulin, and cAMP
• Progesterone relaxes smooth muscles

Bone

• Bone is a hard, rigid connective tissue, with an extracellular matrix impregnated with inorganic salts (e.g., calcium phosphate, carbonate).
• Features: sclerous connective tissue, specialized in providing support due to infiltration with inorganic salts. It is characterized by rigidity, hardness, and power of regeneration and repair. It constantly undergoes remodeling to adapt to changing physical, biochemical, and hormonal influences
• Functions: support, protection, movement, mineral storage, hematopoiesis.
• Two types of bone: compact and spongy.
• The structural unit of compact bone is called an osteon or haversian system; it's a cylinder with longitudinal orientation and 5-20 concentrically arranged lamellae. Lamellae have a crisscross pattern.
• Spongy (trabecular) bone is a lattice-like network structure with trabeculae (plates) made of lamellae arranged in response to stress (Wolff's law).

Bone Cells

• Osteoblasts: responsible for forming new bone. They synthesize and secrete collagen matrix and calcium salts
• Osteocytes: primary cell of mature bone tissue, maintaining bone tissue. They lack mitotic activity and communicate via cytoplasmic processes extending through canaliculi
• Osteogenic cells: mesenchymal stem cell line, source and precursors for osteoblasts.
• Osteoclasts: responsible for bone resorption. They are multinucleated cells that originate from monocytes and macrophages

Bone Matrix

• The extracellular matrix comprises the non-cellular component of bone tissue. It is highly specialized and contains collagen and associated proteins along with mineralized components, particularly calcium hydroxyapatite crystals organized into thin layers known as lamellae.

Bone Health and Diseases

•  Essential nutrients for bone health: calcium, Vitamin D, Vitamin K, magnesium, fluoride, omega-3 fatty acids.
• Diseases related to skeletal system:
  • Osteoporosis
  • Rickets
  • Osteomalacia
  • Paget's Disease
  • Rheumatoid arthritis