Muscle Tissue Types and Characteristics

• The cells in muscle tissue optimize contractility, and originate from the mesoderm.

Muscle Tissue Types

• Skeletal muscle consists of large, elongated, multinucleated fibers for strong, quick, voluntary contractions through neuromuscular junctions.
• Cardiac muscle has irregular branched cells connected by intercalated discs. They enable strong, involuntary contractions.
• Smooth muscle has grouped, fusiform cells for weak, involuntary contractions.

Hypertrophy and Hyperplasia:

• Hypertrophy is the increase in diameter of muscle fibers, particularly in skeletal muscle, due to its inability to divide.
• Hyperplasia, on the other hand, allows for cell multiplication and an increase in cell numbers, exemplified by the uterus.

Skeletal Muscle:

• Skeletal muscle fibers are long, cylindrical, and multinucleated, with striated patterns and elongated nuclei. They contain muscle satellite cells.
• Muscle contraction results from sliding thick myosin filaments along thin actin filaments.
• Muscle fiber diameter varies based on specific muscle, age, gender, nutritional status, and physical training.

Muscle cell structure:

• The cytoplasm in muscle cells is called sarcoplasm.
• The smooth endoplasmic reticulum of muscle cells is a sarcoplasmic reticulum.
• The cell membrane and external lamina of muscle cells are sarcolemma.

Skeletal Muscle Organization

• The epimysium is a dense connective tissue sheath surrounding the entire muscle. Septa from it carries nerves and vessels.
• Perimysium is a connective tissue layer that surrounds bundles of muscle fibers called fascicles.
• The endomysium is a thin layer that surrounds individual muscle fibers. It is composed of reticular fibers and fibroblasts.

Muscle Fiber Organization:

• Myofibrils are long cylindrical filament bundles found within muscle fibers.
• Dark bands (A bands) are anisotropic and birefringent in polarized light.
• Light bands (I bands) are isotropic and do not alter polarized light. They are bisected by the Z disc.

Myofilaments:

• Myofibrils consist of thick and thin filaments organized into sarcomeres.
• Thick filaments contain 200-500 myosin molecules.
• Thin filaments contain F-actin, tropomyosin, and troponin.

Muscle Contraction:

• Tropomyosin features two polypeptide chains in the actin strands groove.
• Troponin is a complex of three subunits: TnT attaches to tropomyosin, TnC binds calcium ions, and TnI regulates interaction with actin.
• Titin is the largest protein, supports myofilaments, and connects them to Z-disks.

Creatine Kinase:

• Creatine Kinase (CK) is an enzyme that catalyzes phosphate group transfer from phosphocreatine to ADP to supply ATP. It indicates acute myocardial infarction.
• The isoenzymes of CK comprises: CK-MM (skeletal muscle), CK-MB (heart), and CK-BB (brain).

Sarcoplasmic Reticulum & T-tubules:

• The sarcoplasmic reticulum is a membranous smooth ER that pumps calcium ions to surround myofibrils.
• Transverse or T-tubules are invaginations of the sarcolemma. They trigger Calcium ion release and transmit action potential.
• Triads, composed of one T-tubule plus two terminal cisternae, affect sarcoplasmic reticulum, and allow sarcolemma depoalarization.

Rigor Mortis:

• Rigor mortis occurs from a lack of ATP stabilizes actin-myosin cross bridges once mitochondrial activity stops.

Innervation:

• Contraction requires innervation
• Motor end plates are axonal branches.
• Acetylcholine is the main contracting muscle neurotransmitter .
• The junctional folds hold acetylcholine receptor in the synaptic cleft.
• Acetylcholinesterase breaks down acetylcholine.
• A motor unit is one axon and all the muscle fiber it touches.

Skeletal Muscle Fiber Types differ by:

• Maximal contraction rate (fast or slow fibers)
• Major ATP synthesis pathway (oxidative phosphorylation or glycolysis)

Muscle Fiber Characteristics:

• Myoglobin is a globular sarcoplasmic protein similar to hemoglobin containing iron for oxygen storage.
• Fast glycolytic fibers rely mostly on anaerobic glucose metabolism from stored glycogen, and has low mitochondria and capillaries counts.
• Fast oxidative-glycolytic fibers have both physiological and histological traits.
• Slow oxidative muscle fibers contracts slowly for prolonged periods, has lots of mitochondria and capillaries.

Cardiac Muscle:

• Cardiac muscle has striated banding pattern, a central nucleus per cell, and rich capillary network endomysium.
• The perimysium separates fiber bundles and forms connective tissue masses, which constitutes “cardiac skeleton”.

Cardiac Muscle Contraction:

• Cardiac muscle fiber contraction is intrinsic and spontaneous.
• Impulses comes from myocardial fibers nodes generates conduction.
• Cells are connected at regions called intercalated discs. Transverse regions here have desmosomes for communication / adhesion, and longitudinal regions in the the discs have gap junctions.

Smooth Muscle:

• Smooth muscle specializes in slow, steady contraction.
• Fibers are elongated, tapering, and nonstriated. they are enclosed by basal lamina/reticular networks, the endomysium.

Smooth Muscle Contraction:

• Smooth muscle isn't controlled voluntary motor functions, and lacks well-defined neuromuscular junctions. Contraction here are stimulated by autonomic nerves.
• The gastrointestinal tract is controlled by paraqueen secretions.
• The uterus contracts from oxytocin from the pituitary.

Location of Smooth Muscle:

• Smooth muscle can be found inside the eye, walls of vessels, respiratory tubes, digestive tubes, urinary/reproductive organs.

Other Notes:

• Leiomyoma is a benign tumor that appears as fibroids on the walls of the uterus with abnormal cell growth.