Human Histology: Muscular Tissue

• Muscular Tissue, unit 3 of Human Histology (Lecture) MT120225 at UST General Santos for the Second Semester AY 2024-2025, aims to enable students to identify different types of muscular tissues and their respective functions.

Overview and Classification of Muscle

• Muscle tissue consists of contractile cells responsible for movement.
• Muscle cells originate from the mesoderm.

Muscle Classification by Appearance

• Striated muscle exhibits cross-striations under a light microscope.
• Smooth muscle lacks cross-striations.

Muscle Classification by Location

• Skeletal muscle facilitates the movement of the axial and appendicular skeleton.
• Visceral striated muscle is restricted to soft tissues and involved in speech, breathing, and swallowing.
• Cardiac muscle is found in the heart wall and the base of large veins entering the heart.

Muscle Tissue Terminology

• Muscle cells can be called fibers.
• Cytoplasm is referred to as sarcoplasm.
• Smooth endoplasmic reticulum is known as sarcoplasmic reticulum.
• The cell membrane is termed sarcolemma.
• Mitochondria are called sarcosomes.

Functions of Muscles

• Movement of the body
• Maintaining posture
• Respiration
• Production of body heat
• Communication
• Constriction of organs and vessels
• Contraction of the heart

Properties of Muscle Tissue

• Contractility enables muscles to shorten and generate force.
• Excitability allows muscles to respond to stimuli.
• Extensibility is the ability of muscles to be stretched.
• Elasticity allows muscles to return to their original length after stretching.

Skeletal Muscle

• Skeletal muscle comprises long, multinucleated muscle cells.
• Skeletal muscle is ensheathed in connective tissue coverings

Connective Tissue Coverings

• Epimysium: A thick, dense, irregular connective tissue that surrounds each skeletal muscle, containing large nerves, blood vessels, and lymphatics.
• Perimysium: Thin connective tissue surrounding bundles of muscle fibers called fascicles, also containing nerves, blood vessels, and lymphatics.
• Endomysium: Very thin reticular fibers surrounding individual muscle fibers, containing nerve fibers capillaries, and scattered fibroblast.

Skeletal Muscle Fiber

• Myofibrils consist of 5,000-10,000 units per muscle fiber and span the length of the muscle fiber.
• They have a diameter of 1-2 um and are arranged parallel to the long axis of the cell.
• Sarcomeres are repeating units which exhibit transverse striations of alternating light and dark bands.
• Sarcomeres are between two Z lines.
• They are 1.5-2 um long in a resting muscle.
• Sarcomeres are collection of thread-like structures called filaments
• Myofilaments are 1,000 - 2,000 arranged parallel to the long axis of the sarcomere
• There are two types, thick and thin
• Thick myofilaments are found in the midzone of the sarcomere.
• They span the region of A-band, have a diameter of 15 nm, a length of 1.5 um, and contain myosin
• Thick myofilaments consists of two myosin heavy chains, and two myosin heads. -The myosin heads bind to active sites on actin molecules to form cross bridges. -They attach to the rod portion with a hinge region that bends and straightens during contrations
• The heads are ATPase enzymes
• Thin myofilaments run between thick filaments, span the region of A-band, have a diameter of 8 nm, a length of 1 um, and contain actin
• Thin myofilaments are associated with the regulatory, tropomyosin and troponin
• Globular (G) actin contains receptor sites for myosin head
• In relaxed muscle, tropomyosin covers the active sites on the G actin subunits.
• Troponin attaches to tropomyosin, binds to calcium, and regulates the interaction between actin and myosin.

Muscle Contraction

• Muscle contraction has an electrical and mechanical component.
• The electrical component involves sarcolemma, transverse tubules, and sarcoplasmic reticulum.
• The mechanical component involves myofibril and myofilament.

Skeletal Muscle fiber types

• Slow, Oxidative Fibers (Type I) are numerous, use oxidative phosphorylation, are fatigue resistant, and for the postural muscles of the back
• Fast, Oxidative-Glycolytic Fibers (Type lla) are numerous, have intermediate energy use and fatigue rate, and are for the major muscles of the legs
• Fast, Glycolytic Fibers (Type llb) are sparse, use anaerobic glycolysis, have high rates of fatigue, and are for the extraocular muscles