Muscle
• Muscle—a Latin word for ―little mouse‖ • Muscle is the primary tissue in the:
• Heart (cardiac MT)
• Walls of hollow organs (smooth MT)
• Skeletal muscle
• Makes up nearly half the body’s mass Overview of Muscle Tissue
• Functions of muscle tissue
• Movement
• Skeletal muscle—attached to skeleton
• Moves body by moving the bones
• Smooth muscle—squeezes fluids and other substances through hollow organs
Overview of Muscle Tissue
• Functions of muscle tissue (continued)
• Maintenance of posture—enables the body to remain sitting or standing
• Joint stabilization • Heat generation
• Muscle contractions produce heat
• Helps maintain normal body temperature Functional Features of Muscles
• Functional features
• Contractility
• Long cells shorten and generate pulling force • Excitability
• Electrical nerve impulse stimulates the muscle cell to contract • Extensibility
an opposing muscle • Elasticity
• Can recoil after being stretched Types of Muscle Tissue
• Skeletal muscle tissue
• Packaged into skeletal muscles • Makes up 40% of body weight • Cells are striated
Types of Muscle Tissue
• Cardiac muscle tissue—occurs only in the walls of the heart • Smooth muscle tissue—occupies the walls of hollow organs
• Cells lack striations
Similarities of Muscle Tissue
• Cells of smooth and skeletal muscle
• Are known as fibers
• Muscle contraction
• Depends on two types of myofilaments (contractile proteins) • One type contains actin
• Another type contains myosin
• These two proteins generate contractile force
Similarities of Muscle Tissues
• Plasma membrane is called a
sarcolemma
• Cytoplasm is called sarcoplasmSkeletal Muscle
• Each muscle is an organ
• Consists mostly of muscle tissue • Skeletal muscle also contains
• Connective tissue • Blood vessels • Nerves
Basic Features of a Skeletal Muscle
• Connective tissue and fascicles
• Sheaths of connective tissue bind a skeletal muscle and its fibers together
• Epimysium—dense regular connective tissue surrounding entire muscle
• Perimysium—surrounds each fascicle (group of muscle fibers)
• Endomysium—a fine sheath of connective tissue wrapping each muscle cell
Basic Features of a Skeletal Muscle
• Connective tissue sheaths are continuous with tendons
• When muscle fibers contract, pull is exerted on all layers of connective tissue are tendon
• Sheaths provide elasticity and carry blood vessels and nerves Connective Tissue Sheaths in Skeletal Muscle
Basic Features of a Skeletal Muscle
• Nerves and blood vessels
• Each skeletal muscle supplied by branches of • One nerve
• One artery
• One or more veins
Basic Features of a Skeletal Muscle
• Nerves and blood vessels (continued)
• Nerves and vessels branch repeatedly • Smallest nerve branches serve:
• Individual muscle fibers
• Neuromuscular junction—signals the muscle to contract Basic Features of a Skeletal Muscle
• Muscle attachments
• Most skeletal muscles run from one bone to another • One bone will move, other bone remains fixed
• Origin—less movable attachment • Insertion—more movable attachment Basic Features of a Skeletal Muscle
• Muscle attachments (continued)
• Muscles attach to origins and insertions by CT • Fleshy attachments—CT fibers are short
• Indirect attachments—CT forms a tendon or aponeurosis • Bone markings present where tendons meet bones
• Tubercles, trochanters, and crests Microscopic and Functional Anatomy of Skeletal Muscle Tissue
• The skeletal muscle fiber
• Fibers are long and cylindrical
• Are huge cells—diameter is 10–100µm
• Length—several centimeters to dozens of centimeters • Each cell formed by fusion of embryonic cells
• Cells are multinucleate
• Nuclei are peripherally located Diagram of Part of a Muscle Fiber Myofibrils and Sarcomeres
• Striations result from internal structure of myofibrils • Myofibrils
• Are long rods within cytoplasm • Make up 80% of the cytoplasm
• Are a specialized contractile organelle found in muscle tissue • Are a long row of repeating segments called sarcomeres
(functional unit of Skeletal MT) Sarcomere
• Basic unit of contraction of skeletal muscle
• Z disc (Z line)—boundaries of each sarcomere
• Thin (actin) filaments—extend from Z disc toward the center of the sarcomere
• Thick (myosin) filaments—located in the center of the sarcomere
• Overlap inner ends of the thin filaments • Contain ATPase enzymes
Sarcomere Structure
• A bands—full length of the thick filament
• Includes inner end of thin filaments
• H zone—center part of A band where no thin filaments occur • A bands and I bands refract polarized light differently
• A bands—anisotropic • I bands—isotropic
Sarcomere Structure (continued)
• M line—in center of H zone
• Contains tiny rods that hold thick filaments together
• I band—region with only thin filaments
• Lies within two adjacent sarcomeres Sarcomere Structure (continued)
Sarcoplasmic Reticulum and T Tubules
• Sarcoplasmic reticulum
• A specialized smooth ER
• Interconnecting tubules surround each myofibril
• Some tubules form cross-channels called terminal cisternae • Cisternae occur in pairs on either side of a
t tubule
Sarcoplasmic Reticulum and T Tubules
• Sarcoplasmic reticulum
• Contains calcium ions—released when muscle is stimulated to contract
• Calcium ions diffuse through cytoplasm • Trigger the sliding filament mechanism
• T tubules—deep invaginations of sarcolemma
• Triad—T tubule flanked by two terminal cisterns Mechanism of Contraction
• Two major types of contraction
• Concentric contraction—muscle shortens to do work
• Eccentric contraction—muscle generates force as it lengthens • Muscle acts as a ―brake‖ to resist gravity
• ―Down‖ portion of a pushup is an example Mechanism of Contraction
• Sliding filament mechanism
• Explains concentric contraction
• Myosin head attach to thin filaments at both ends of a sarcomere
• Then pull thin filaments toward the center of the sarcomere
• Thin and thick filaments do not shorten
• Initiated by release of calcium ions from the SR • Powered by ATP
Sliding Filament Mechanism
• Contraction changes the striation pattern
• Fully relaxed—thin filaments partially overlap thin filaments • Contraction—Z discs move closer together
• Sarcomere shortens
• I bands shorten, H zone disappears • A band remains the same length Microscopic and Functional Anatomy of Skeletal Muscle Tissue
• Muscle extension
• Muscle is stretched by a movement opposite that which contracts it
• Muscle fiber length and force of contraction
• Greatest force produced when a fiber starts out slightly stretched • Myosin heads can pull along the entire length of the thin
filaments The Role of Titin
• Titin—a spring-like molecule in sarcomeres
• Resists overstretching
• Holds thick filaments in place • Unfolds when muscle is stretched Innervation of Skeletal Muscle
• Motor neurons innervate skeletal muscle tissue
• Neuromuscular junction is the point where nerve ending and muscle fiber meet
• Axon terminals—at ends of axons • Store neurotransmitters
• Synaptic cleft—space between axon terminal and sarcolemma The Neuromuscular Junction
Motor Units
10
Muscle Tissue
Types of Skeletal Muscle Fibers
• Skeletal muscle fibers are categorized according to two characteristics
• How they manufacture energy (ATP) • How quickly they contract
• Oxidative fibers—produce ATP aerobically
• Glycolytic fibers—produce ATP anaerobically by glycolysis
Types of Skeletal Muscle Fibers
• Skeletal muscle fibers
• Are divided into three classes • Slow oxidative fibers
• Red slow oxidative fibers
• Fast glycolytic fibers
• White fast glycolytic fibers
• Fast oxidative fibers
• Intermediate fibers
Types of Skeletal Muscle Fibers
• Slow oxidative fibers
• Red color due to abundant myoglobin
• Obtain energy from aerobic metabolic reactions • Contain a large number of mitochondria
• Richly supplied with capillaries
• Contract slowly and resistant to fatigue • Fibers are small in diameter
Types of Skeletal Muscle Fibers
• Fast glycolytic fibers
• Contain little myoglobin and few mitochondria • About twice the diameter of slow-oxidative fibers
• Contain more myofilaments and generate more power • Depend on anaerobic pathways
Types of Skeletal Muscle Fibers
• Fast oxidative fibers
• Have an intermediate diameter
• Contract quickly like fast glycolytic fibers • Are oxygen-dependent
• Have high myoglobin content and rich supply of capillaries • Somewhat fatigue-resistant
• More powerful than slow oxidative fibers Disorders of Muscle Tissue
• Muscle tissues experience few disorders
• Heart muscle is the exception • Skeletal muscle
• Remarkably resistant to infection • Smooth muscle
• Problems stem from external irritants Disorders of Muscle Tissue
• Muscular dystrophy
• A group of inherited muscle destroying disease
• Affected muscles enlarge with fat and connective tissue • Muscles degenerate
• Types of muscular dystrophy
• Duchenne muscular dystrophy • Myotonic dystrophy
Disorders of Muscle Tissue
• Myofascial pain syndrome
• Pain is caused by tightened bands of muscle fibers
• Fibromyalgia
• A mysterious chronic-pain syndrome • Affects mostly women
• Symptoms—fatigue, sleep abnormalities, severe musculoskeletal pain, and headache
Muscle Tissue Throughout Life
• Muscle tissue develops from myoblasts
• Myoblasts fuse to form skeletal muscle fibers
Muscle Tissue Throughout Life
• Cardiac muscle
• Pumps blood three weeks after fertilization
• Satellite cells
• Surround skeletal muscle fibers
• Resemble undifferentiated myoblasts
• Fuse into existing muscle fibers to help them grow Muscle Tissue Throughout Life
• With increased age
• Amount of connective tissue increases in muscles • Number of muscle fibers decreases
• Loss of muscle mass with aging
• Decrease in muscular strength is 50% by age 80 • Sarcopenia—muscle wasting