BS161 Anatomy and Physiology 2024/25 - Lecture 15

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

• The lecture covers muscle tissue, specifically skeletal muscle, including its structure, function, and the mechanisms of contraction.
• Learning objectives include describing muscle structure, muscle contraction, sarcomere structure and organisation of myofilaments in relation to sliding filament theory, and the pathology of named diseases in various body systems.
• Muscle tissue is divided into three types: skeletal, cardiac, and smooth.
• Skeletal muscles are attached to the skeletal system, allowing movement.
• Six functions of skeletal muscles include producing skeletal movement, maintaining posture and body position, supporting soft tissues, guarding entrances and exits, maintaining body temperature, and storing nutrient reserves.
• Skeletal muscle is organized with muscle tissue (muscle cells/fibres), connective tissues, nerves, and blood vessels. Tendons attach muscle to bone.
• Muscles have three layers of connective tissue: epimysium, perimysium, and endomysium.
• Epimysium—an outer layer of collagen fibres connected to fascia.
• Perimysium—surrounds bundles of muscle fibres (fascicles), providing blood vessels and nerve supply to fascicles.
• Endomysium—surrounds individual muscle cells (muscle fibres), containing capillaries and nerve fibres. It also has satellite cells for repair.
• Skeletal muscle cells (fibres) are long, develop through fusion of mesodermal cells (myoblasts), and contain hundreds of nuclei.
• The contractile unit of a muscle fibre is a sarcomere.
• A band: thick filaments (myosin).
• I band: thin filaments (actin).
• Z line: defines the boundaries of a sarcomere, forming the dark bands.
• M line: centre of the A band.
• H band: gap around the M line, containing only thick filaments.
• Myofibrils—lengthwise subdivisions within muscle fibre, made up of bundles of thin/thick myofilaments. Myofilaments are responsible for muscle contraction—thin (actin) and thick (myosin).
• Thin filaments (actin): made of the globular protein actin, and they have troponin (globular protein) which is attached to tropomyosin (double strand) and the active sites on G-actin bind to myosin. These are controlled by Ca2+.
• Thick filaments (myosin): contain about 300 twisted myosin subunits, and contain titin which recoils after stretching. Myosin molecules consist of a tail (that binds to other myosin molecules) and a head (that consists of two globular protein subunits that reach the nearest thin filament).
• Muscle contraction is based on the sliding filament theory. Thin filaments (actin) slide along the thick filaments (myosin). The width of the A band stays the same, Z lines move closer, and the H band gets smaller.
• Cross-bridge formation occurs when myosin heads interact with active sites on actin filaments.
• The contraction cycle has 6 stages:
  • Contraction Cycle Begins (arrival of Ca2+)
  • Active-Site Exposure
  • Cross-Bridge Formation
  • Myosin Head Pivoting (power stroke)
  • Cross-Bridge Detachment
  • Myosin Reactivation (ATP used)
• The neuromuscular junction (NMJ) is a special intercellular connection between the nervous system and skeletal muscle fibres. It controls calcium ion release into the sarcoplasm. The area of the sarcolemma in contact with axon terminals is the motor end plate.
• NMJ consists of axon terminal of neuron, synaptic cleft (space between neuron and muscle), motor end plate (area of sarcolemma on muscle fibre).
• The events at the NMJ include acetylcholine release, binding to receptors, generating action potential, ACh removal (by AChE).
• Cardiac muscle cells (cardiocytes) are smaller, have a single nucleus, have short wide T tubules, have SR with no terminal cisternae.They are aerobic with high myoglobin/mitochondria, and they have intercalated discs.
• Intercalated discs connect cardiocytes mechanically, chemically, and electrically, making the heart work as a single unit.
• Smooth muscle tissue forms around other tissues, including in the skin (arrector pili muscles), blood vessels and airways, reproductive and glandular systems, and digestive and urinary systems. It forms sphincters and produces contractions.
• Smooth muscle is non-striated, has a different internal organization of actin and myosin, and has diverse functional characteristics, differing from skeletal and cardiac muscle.
• The structure of smooth muscle includes long spindle-shaped cells with a central nucleus, no T tubules or sarcomeres, scattered myosin fibers with several heads per fiber, thin filaments attached to dense bodies, and dense bodies transmitting contractions.