Skeletal Striated Muscle Tissue

• Muscle tissues are specialized for mechanical work and contraction.
• Muscle cells are called myocytes or muscle fibers, containing contractile proteins.
• Contractile proteins form myofilaments grouped into myofibrils.
• There are 3 types of muscle tissue: skeletal, cardiac and smooth.
• The types of muscle tissue differ in structure, location, function, and contraction mode.

Skeletal Striated Muscle Tissue

• The skeletal striated muscle cell (SSMC) is also known as a rhabdomyocyte.
• The SSMC is the functional unit of skeletal muscle.
• SSMC are long cylindrical cells with transverse striations.
• Most of the cytoplasm in SSMC is occupied by myofibrils, forming the myoplasm.
• SSMC contain many ovoid nuclei aligned at the cell's periphery.
• SSMC contain numerous and large mitochondria.
• The sarcoplasmic reticulum (SER) in SSMC is highly developed.
• The cytoskeleton in SSMC is formed by microtubules and desmin filaments.
• Lipid vacuoles and glycogen reserves exist near the mitochondria.
• Myoglobin red pigments fix and deliver oxygen to mitochondria.
• Sarcolemma is the plasma membrane, surrounded by a basal lamina.
• The SSMC is unable to divide.
• Skeletal striated muscle cells associate to form skeletal striated muscles.
• Epimysium is dense connective tissue surrounding the skeletal striated muscle.
• Perimysium extends from epimysium into the muscle, forming partitions called fascicles.
• Endomysium is invaginations of the perimysium, surrounding and separating neighboring rhabdomyocytes within fascicles.
• Actual skeletal muscle contains bundles of fascicles.
• Each fascicle contains a bunch of muscle cells or myofibers.
• Each muscle cell has a bunch of myofibrils.
• Connective tissue surrounds the cells, fascicles, and entire muscle.
• Muscle is composed of muscle cells, connective tissue, blood vessels, and nerves.
• Myoplasm in SSMC has hundreds or thousands of myofibrils.
• Myofibrils are cylindrical, parallel, and located throughout a cell.
• In longitudinal section, myofibrils show a transverse striation of alternating bands.
• Dark bands are A bands, light bands are I bands, with Z lines in the middle of I bands.
• The portion of the myofibril between two Z lines is a sarcomere.
• A bands are formed by thick myosin and thin actin filaments.
• I bands contain only thin actin filaments.
• The H zone is a clearer transverse zone in the middle of the A band.
• The H zone is formed of myosin, divided by the M line.
• The myosin molecule has a cylindrical tail and two spherical heads.
• The myosin heads interact with actin through binding sites.
• Myosin heads have ATPase activity for ATP hydrolysis.

Muscle Contraction: Filament Sliding Theory

• When a muscle is at rest, the muscular cells are relaxed.
• Thin and thick filaments overlap only slightly in a relaxed state.
• Ca++ ions are stored in the terminal cisternae of the SER.
• Muscle cell stimulation by a nerve impulse releases acetylcholine.
• Acetylcholine depolarizes the sarcolemma.
• The electrical impulse activates the sarcoplasmic reticulum, releasing Ca++ ions into the sarcoplasm.
• Myosin heads attach to the exposed actin filaments at binding sites.
• Myosin heads pull the actin filaments and Z lines toward the sarcomere's center.
• The I band and H zone decrease in length, shortening the sarcomere.

Innervation and Blood Supply

• Connective tissue provides pathways for blood vessels and nerves.
• Blood vessels enter via perimysium, branching and extending with capillaries between muscle cells within endomysium.
• Lymphatic vessels are limited to the thickest connective tissue septa.
• Each skeletal muscle fiber is innervated.
• Nerve endings contact the muscle fiber surface at the motor end plate.
• The motor end plate contains neurotransmitter receptors opposite axon terminals of a motor neuron.

Cardiac Muscle Tissue

• The heart wall (myocardium) is muscular tissue with connective tissue, blood and lymphatic vessels, and nerve fibers.
• The myocardium is largely made of striated muscular cells (cardiomyocytes).
• Cardiomyocytes allow spontaneous, involuntary contraction, blood propulsion, and the emptying of cardiac cavities.
• Myocardium is made of muscle fibers separated by septa of loose connective tissue with capillaries.
• Cardiac muscle cells are long cylindrical cells with ramified extremities.
• Cardiomyocytes have one elongated, centrally placed nucleus.
• The structure and ultrastructure of myofibrils are like those in skeletal striated muscle cells.
• Sarcolemma is limited by a basal lamina.
• Sarcolemma has no motor end plates or neuromuscular junctions.
• Cardiomyocytes are linked through cellular junctions called intercalated discs.
• Intercalated discs attach adjacent cells and contain gap junctions.
• Gap junctions allow the rapid spread of excitation between cells.
• Cardiomyocytes are characterized by oxidative metabolism.
• Cardiomyocytes have a slow contraction, and a dependence on extracellular Ca++ levels.

Smooth Muscular Tissue

• Smooth muscular tissue is involuntary and non-striated.
• It composes of smooth muscle cells, connective, vascular and nervous elements
• Smooth muscle cells (leiomyocytes) participate to major vital functions
• They are in walls of vessels and hollow organs (digestive, respiratory, urinary, genital tract) and in the eye
• Smooth muscle fibers are elongated, tapering, and nonstriated.
• Each cell has a single nucleus in the center.
• Sarcoplasm has two regions.
• One region confines major organelles (Golgi, REG, mitochondria).
• The second region is peripheral cytoplasm with myofilaments, whose organization is different from that of SSMC.
• It does not form sarcomeres.
• Thin myofilaments are actin filaments.
• Thick myosin myofilaments are less numerous than thin filaments.
• Contractile proteins (actin and myosin) bundles intertwine and attach to dense bodies.
• Dense bodies contain α-actinin, dispersed on the sarcolemma and in the sarcoplasm.
• Sarcolemma is rich in caveolae invaginations.
• Caveolae have regular distribution between dense bodies and contain essentially Ca++ ions.
• Caveolae may function like T tubules.

Innervation of Smooth Muscle Cells

• Innervation allows observation of 2 types of smooth muscle: single-unit and multi-unit.
• Single-unit smooth muscle or unitary smooth muscle is innervated by few nerve fibers per bundle.
• The presence of gap junctions allows the electrical signal to spread to adjacent cells.
• Single-unit smooth muscle is found primarily in hollow organs or guts, and is called visceral smooth muscle.
• Multi-unit smooth muscle contains fewer or no gap junctions.
• Each cell requires its own electrical impulse.
• Multi-unit smooth muscle is found in the skin, eyes, blood vessels, and so on.

Heterogeneity of Smooth Muscle Cells

• Visceral smooth muscle cells corresponds to the previously described type of smooth muscle.
• Vascular smooth muscle cells are in the tunica media of large elastic arteries.
• Myoepithelial cells of epithelial origin are smooth muscle cells localized between basement membrane cells and some exocrine glands.
• Myofibroblasts are cells having an intermediate morphology between smooth muscle cells and fibroblasts.
• Myofibroblasts are contractile and play an important role in healing and tissue repair.

Summary of Muscle Tissue Types

• Skeletal Muscle: Features striated, tubular, multinucleated fibers, voluntary control, usually attached to the skeleton.
• Smooth Muscle: features non-striated spindle-shaped, uninucleated fibers, involuntary control, usually covering walls of internal organs.
• Cardiac Muscle: features striated, branched and uninucleated, involuntary control, and only covering walls of the heart.
• Skeletal muscle has many flat peripheral nuclei.
• Cardiac has 1-2 plum central, nuclei per cell.
• Smooth has one central nucleus per cell.
• Skeletal has largest cell diameter.
• Cardiac has intermediate cell diameter.
• Cardiac has smallest cell diameter.
• Skeletal, cardiac muscles have striations, smooth does not.
• Skeletal, cardiac muscles have sarcoplasmic reticulum, smooth does not.
• T tubules are at the junction of A-I bands in skeletal muscle forming triads.
• T tubules are at the Z lines in cardiac muscles forming diads.
• Smooth Muscle forms no T tubules
• Skeletal Muscle has voluntary motor control and quick strong contractions.
• Cardiac Muscle has involuntary control and quick strong rhythmic contractions.
• Smooth Muscle has involuntary control and slow contractions in waves.
• Skeletal muscle has moderate blood supply.
• Cardiac muscle has extensive blood supply.
• Smooth muscle has less abundant blood supply.
• Main feature of skeletal muscle are prominent fascicles.
• Main feature of cardiac muscle are intercalated disks and branching cells.
• A main feature of smooth muscle is cells that overlap that can synthesize collagen and elastin.