BS161 Anatomy and Physiology - Lecture 15

BS161 Anatomy and Physiology 2024/25 - Lecture 15: Muscle Tissue

• The lecture is about muscle tissue, specifically skeletal, cardiac, and smooth muscle.
• The learning objectives include describing muscle structure, muscle contraction, sarcomere structure, and the sliding filament theory, along with pathology of diseases in various body systems.
• Muscle tissue is a primary tissue type divided into skeletal, cardiac, and smooth muscle tissue.
• Skeletal muscles are attached to the skeletal system and enable movement.
• Six functions of skeletal muscle tissue: produce skeletal movement, maintain posture and body position, support soft tissues, guard entrances and exits, maintain body temperature, and store nutrient reserves.
• Skeletal muscle tissue is composed of muscle tissue (muscle cells or fibres), connective tissues, nerves, and blood vessels; tendons attach muscles to bones, and ligaments attach bones to bones.
• Muscles are composed of three layers of connective tissues: epimysium, perimysium, and endomysium.
• Epimysium is the exterior collagen layer that separates muscle from surrounding tissues.
• Perimysium surrounds muscle fiber bundles (fascicles) and includes blood vessels and nerve supply to fascicles.
• Endomysium surrounds individual muscle cells (muscle fibers), containing capillaries and nerve fibers, and myosatellite cells (stem cells).
• Skeletal muscle cells (fibers) are very long, developing through fusion of mesodermal cells (myoblasts), and containing hundreds of nuclei.
• The contractile unit of a muscle fiber is a sarcomere.
• Myofibrils, made up of thin and thick myofilaments, are responsible for muscle contraction.
• Thin filaments are composed of the protein actin, and thick filaments are composed of the protein myosin.
• The sarcolemma is the cell membrane of a muscle fiber (cell).
• Transverse tubules (T tubules) transmit action potentials through the cell allowing simultaneous contraction.
• The sarcoplasmic reticulum (SR) is a membranous structure surrounding myofibrils, transmitting action potentials to myofibrils, and is similar in structure to smooth endoplasmic reticulum.
• The functional organization in skeletal muscle features epimysium, perimysium, endomysium, muscle fascicles, muscle fibers, and myofibrils.
• Sarcomeres, the contractile units of muscle, show a striated pattern due to alternating thick (A bands) and thin (I bands) filaments. Within the A band, there's an H band (centered portion) containing only thick filaments, and a zone of overlap where both thick and thin filaments intersect.
• Thin filaments consist of F-actin (filamentous actin), nebulin (holds F-actin strands together), tropomyosin (a double strand preventing actin-myosin interaction), and troponin.
• Thick filaments contain about 300 twisted myosin subunits and titin strands for recoil after stretching. Myosin has a tail and two globular protein subunits for binding to actin, reaching thin filaments.
• Muscle contraction involves myosin heads interacting with actin filaments, forming cross-bridges, and actin filaments sliding over myosin (myosin doesn't move).
• Sliding filament theory describes thin filaments sliding towards the M-line; the A-band width stays the same, Z-lines move closer together, the H-band gets shorter, and the sarcomere gets shorter.
• The neuromuscular junction (NMJ) is the special intercellular connection between the nervous system and skeletal muscle fiber.
• The area of the sarcolemma in contact with axon terminals is called the motor end plate.
• Events at the NMJ include ACh release, ACh binding to receptors on the motor end plate, ACh breakdown by acetylcholinesterase, and action potential generation and subsequent calcium release.
• Cardiac muscle tissue is striated, found in the heart, with small cells (cardiocytes) containing a single nucleus, short T tubules, SR without terminal cisternae, high aerobic capacity (myoglobin & mitochondria), and intercalated discs.
• Intercalated discs are specialized contact points between cardiocytes, joining cell membranes via gap junctions and desmosomes.
• The discs maintain structure, increase molecular and electrical connections, and allow for action potential conduction in the heart.
• Smooth muscle tissue is non-striated, with a different internal organization of actin and myosin compared to skeletal and cardiac muscle. Cells are spindle-shaped, have a single central nucleus, lack T tubules, and possess scattered myosin fibres with more heads per thick filament connected to dense bodies.
• Smooth muscle, located around other tissues, regulates blood pressure (in blood vessels and airways), produces movements in digestive and reproductive systems (like sphincters), and controls actions like "goosebumps."
• Further study on cardiac and smooth muscle may be possible depending on available time.

This quiz covers Lecture 15 of BS161, focusing on muscle tissue, including skeletal, cardiac, and smooth muscle types. It explores the structure, contraction mechanisms, and the sliding filament theory, alongside the impact of various diseases on muscle function. Test your understanding of muscle tissue's role in the human body and its physiological importance.