Skeletal Muscle Functional Anatomy

• Functional Anatomy of Skeletal Muscle

Objectives

• Students should be able to draw and label a skeletal muscle at all anatomical levels
• Students should be able to diagram the structure of the thick and thin myofilaments and label the constituent proteins
• Students should be able to describe the functional importance of the subunits

Muscular System Functions

• The muscular system functions in body movement (locomotion)
• The muscular system functions in maintenance of posture
• The muscular system functions in respiration via diaphragm and intercostal muscles contractions
• The muscular system functions in communication (verbal and facial)
• The muscular system aids digestion of food
• The muscular system aids maintenance of blood pressure
• The muscular system functions in heartbeat
• The muscular system functions in production of body heat (thermogenesis)

Categorization of Muscle

• Striated muscles include skeletal and cardiac muscle
• Unstriated muscle is smooth muscle
• Skeletal muscle is voluntary
• Cardiac and smooth muscle are involuntary

Skeletal muscle

• Skeletal muscle is striated
• Skeletal muscle is under voluntary control and innervated by somatic nerves
• Skeletal muscle lacks anatomical connection between fibers
• Skeletal muscle contraction requires external innervation
• Skeletal muscles form somatic musculature attached to bones
• Skeletal muscles are responsible for locomotion, facial expressions, posture, respiratory movements, other types of body movement
• Skeletal muscle has well-developed cross striations
• Skeletal muscles need external innervations to contract
• Skeletal muscles lack anatomical and functional connections between cells

Skeletal Muscles: Microscopy

• Skeletal muscles are multinucleated cylindrical cells with nucleus at periphery
• Skeletal muscles exhibit cross striations

Cardiac muscle

• Cardiac muscle is striated
• Cardiac muscle is under involuntary control and innervated by autonomic nerves
• Cardiac muscle has connection between fibers (gap junctions and intercalated discs)
• Cardiac muscle contracts without external innervation

Smooth muscle

• Smooth muscle is non-striated
• Smooth muscle is under involuntary control and innervated by autonomic nerves
• Connections between smooth muscle fibers can be present or absent
• Some types of smooth muscle can contract without external innervation, while others cannot

Criteria of muscles

• Excitability is the ability to generate action potentials
• Action potentials activate muscular contraction
• Contractility is the ability of a muscle to shorten and generate pulling force

Skeletal muscle fibers

• Skeletal muscle is made up of many muscle fibers
• Muscle fibers are building blocks of the muscular system
• Each muscle fiber is a single cell that are multinucleated, long, cylindrical, and surrounded by a cell membrane known as the sarcolemma

Contractile proteins

• Muscle fibers are made up of myofibrils
• Each myofibril consists of filaments
• Thick filaments are myosin filaments
• Thin filaments are actin, tropomyosins and troponins (I, T & C)
• Myofibrils are responsible for the striation and actual muscle contraction

Myosin

• Myosin is a component of thick filament
• Myosin has tail ends intertwined around each other
• Myosin has globular heads that project out at one end
• Tails are oriented toward the center of filament
• Heads protrude outward at regular intervals
• The heads have two important sites critical to the contractile process
• An actin-binding site: heads form cross bridges between thick and thin filaments through this site
• A myosin ATPase (ATP-splitting) site

Actin

• Actin is the primary structural component of thin filaments
• Actin is spherical in shape
• Thin filament also has two other proteins: tropomyosin and troponin
• Each actin molecule has special binding site for attachment with myosin cross bridge
• Binding results in contraction of muscle fiber

Actin and Myosin

• Actin and myosin are often called contractile proteins
• Actin and myosin are not unique to muscle cells but are more abundant and more highly organized in muscle cells

Tropomyosin and Troponin

• Tropomyosin and Troponin are regulatory proteins
• Tropomyosin covers actin sites for binding with myosin and blocks interaction that leads to muscle contraction
• Troponin is made of three polypeptide units
• Troponin T binds to tropomyosin, interlocking them to form a troponin-tropomyosin complex
• Troponin I binds to actin in thin myofilaments to hold the actin-tropomyosin complex in place
• Troponin C binds with Ca2+ to produce a conformational change in Troponin I and initiates skeletal muscle contraction

Sliding filament mechanism

• Cross-bridge interaction between actin and myosin brings about muscle contraction via the sliding filament mechanism

Arrangement of myofibrils

• The sarcomere is the functional unit of skeletal muscle
• Sarcomeres are found between 2 Z lines
• The A band is made up of thick filaments along with portions of thin filaments that overlap on both ends of thick filaments
• The H zone is a lighter area within middle of A band where thin filaments do not reach
• The M line extends vertically down middle of A band within center of H zone
• The I band consists of remaining portion of thin filaments that do not project into A band

Sarcoplasmic Reticulum

• The sarcoplasmic reticulum is a modified smooth endoplasmic reticulum
• The sarcoplasmic reticulum consists of a fine network of interconnected compartments that surround each myofibril
• The sarcoplasmic reticulum functions to store Ca+2

Transverse Tubules

• T tubules run perpendicularly from surface of muscle cell membrane into central portions of the muscle fiber
• Since membrane is continuous with surface membrane, the action potential on surface membrane also spreads down into T-tubule
• Spread of action potential down a T tubule triggers release of Ca2+ from sarcoplasmic reticulum into cytosol
• The function of T tubule is to trigger the release of Ca2+

Sarcoplasm

• The sarcoplasm contains large quantities of potassium, magnesium, phosphate and numbers of protein enzymes
• The sarcoplasm also has numbers of mitochondria to supply the myofibrils with energy (ATP)