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36. Musculoskeletal Systems. Chapter 36 Musculoskeletal Systems. Key Concepts 36.1 Cycles of Protein–Protein Interactions Cause Muscles to Contract 36.2 The Characteristics of Muscle Cells Determine Muscle Performance
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36 Musculoskeletal Systems
Chapter 36 Musculoskeletal Systems • Key Concepts • 36.1 Cycles of Protein–Protein Interactions Cause Muscles to Contract • 36.2 The Characteristics of Muscle Cells Determine Muscle Performance • 36.3 Muscles Pull on Skeletal Elements to Generate Force and Cause Movement
Chapter 36 Opening Question How have musculoskeletal systems evolved to maximize force generation and do so at minimal metabolic cost?
Concept 36.1 Cycles of Protein–Protein Interactions Cause Muscles to Contract • Three types of vertebrate muscle: • Skeletal—voluntary movement, also breathing • Cardiac—beating of heart • Smooth—involuntary, movement of internal organs • All use same sliding filament contractile mechanism.
Concept 36.1 Cycles of Protein–Protein Interactions Cause Muscles to Contract • Skeletal muscle (striated): • Cells are called muscle fibers—are large and multinucleate • Form from fusion of embryonic myoblasts • One muscle consists of many muscle fibers bundled together by connective tissue.
Concept 36.1 Cycles of Protein–Protein Interactions Cause Muscles to Contract • Muscle contraction due to interaction of contractile proteins: • Actin—thin filaments • Myosin—thick filaments • Each muscle fiber has several myofibrils—bundles of actin and myosin filament.
Concept 36.1 Cycles of Protein–Protein Interactions Cause Muscles to Contract • Each myofibril consists of sarcomeres—repeating units of overlapping actin and myosin filaments. • Each sarcomere is bounded by Z lines, which anchor actin.
Concept 36.1 Cycles of Protein–Protein Interactions Cause Muscles to Contract • Other parts of sarcomere: • A band in center—contains myosin • H zone and I band—no overlap of actin and myosin • M band within H zone—contains proteins
Concept 36.1 Cycles of Protein–Protein Interactions Cause Muscles to Contract • Titin—the largest protein in the body, runs the full length of the sarcomere. • Bundles of myosin filaments are held in the center of the sarcomeres by titin. • When muscle contracts, sarcomeres shorten and band pattern changes.
Concept 36.1 Cycles of Protein–Protein Interactions Cause Muscles to Contract • The sliding filament contractile model of muscle contraction depends on structure of actin and myosin: • Myosin molecule has two polypeptide chains coiled together, ending in a globular head • Myosin filament is many molecules in parallel • Actin filament is actin monomers in a long, twisted chain
Figure 36.3 Actin and Myosin Filaments Overlap in Myofibrils
Concept 36.1 Cycles of Protein–Protein Interactions Cause Muscles to Contract • Muscle cells are excitable—membranes can conduct action potentials. • Muscle contraction is initiated by action potentials from a motor neuron at the neuromuscular junction. • A motor unit—all the muscle fibers activated by one motor neuron.
Concept 36.1 Cycles of Protein–Protein Interactions Cause Muscles to Contract • One muscle may have many motor units. • To increase strength of muscle contraction—increase rate of firing of motor neuron or recruit more motor neurons to fire (more motor units activated).
Concept 36.1 Cycles of Protein–Protein Interactions Cause Muscles to Contract • Action potentials in muscle fiber also travel deep within the cell. • T tubules (transverse tubules) descend into the sarcoplasm (muscle fiber cytoplasm). • T tubules run close to the sarcoplasmic reticulum—a closed compartment that surrounds every myofibril.
Figure 36.5 T Tubules Spread Action Potentials into the Fiber
Concept 36.1 Cycles of Protein–Protein Interactions Cause Muscles to Contract • Two proteins span space between T tubules and sarcoplasmic reticulum and are physically connected. • The dihydropyridine (DHP) receptor on the T tubule membrane is voltage-sensitive. • The ryanodine receptor in the sarcoplasmic reticulum membrane is a Ca2+ channel.
Concept 36.1 Cycles of Protein–Protein Interactions Cause Muscles to Contract • When an action potential reaches the DHP receptor it changes conformation. • Ryanodine receptor then allows Ca2+ to leave the sarcoplasmic reticulum. • Ca2+ ions diffuse into the sarcoplasm and trigger interaction of actin and myosin and sliding of filaments.
Concept 36.1 Cycles of Protein–Protein Interactions Cause Muscles to Contract • Actin filament is actin monomers in a long, twisted molecule • Tropomyosin twists around actin; troponin attached at intervals • Myosin heads can bind specific sites on actin molecules to form cross bridges. Myosin changes conformation, causes actin filament to slide.
Figure 36.6 Release of Ca2+ from the Sarcoplasmic Reticulum Triggers Muscle Contraction
Concept 36.1 Cycles of Protein–Protein Interactions Cause Muscles to Contract • Cardiac muscle is also striated—cells are smaller than skeletal muscle and have one nucleus (uninucleate). • Cardiac muscle cells also branch and interdigitate—can withstand high pressures. • Intercalated discs provide mechanical adhesions between cells and contain gap junctions.
Concept 36.1 Cycles of Protein–Protein Interactions Cause Muscles to Contract • Pacemaker and conducting cells initiate and coordinate heart contractions. • Heartbeat is myogenic—generated by the heart muscle itself. • Autonomic nervous system modifies the rate of pacemaker cells but is not necessary for their function.
Concept 36.1 Cycles of Protein–Protein Interactions Cause Muscles to Contract • Excitation and contraction of cardiac muscle differs from skeletal muscle: • T tubules are larger • DHP proteins in T tubules are Ca2+ channels and are not connected to the ryanodine receptors • Ryanodine receptors are ion-gated Ca2+ channels, sensitive to Ca2+.
Concept 36.1 Cycles of Protein–Protein Interactions Cause Muscles to Contract • In cardiac cells, action potential spreads through T tubules, opens voltage-gated Ca2+ channels and Ca2+ flows into sarcoplasm. • Increase in Ca2+ opens the Ca2+ channels in sarcoplasmic reticulum—large increase in Ca2+ in sarcoplasm initiates fiber contraction • Ca2+-induced Ca2+ release
Concept 36.1 Cycles of Protein–Protein Interactions Cause Muscles to Contract • Smooth muscle—in most internal organs, under autonomic nervous system control. • Smooth muscle cells are arranged in sheets—have electrical contact via gap junctions. • Action potential in one cell can spread to all others in the sheet.
Concept 36.1 Cycles of Protein–Protein Interactions Cause Muscles to Contract • Plasma membrane of smooth muscle cells is sensitive to stretch. • Stretched cells depolarize and fire action potentials, which start contraction. • Important for digestion.
Concept 36.1 Cycles of Protein–Protein Interactions Cause Muscles to Contract • Neural influences on smooth muscle come from autonomic nervous system. • Acetylcholine in digestive tract causes depolarization and action potentials, causing contraction. • Norepinephrine causes the same cells to hyperpolarize, leading to fewer contractions.
Figure 36.7 Neurotransmitters and Stretch Alter the Potential of Smooth Muscle Cells (Part 1)
Figure 36.7 Neurotransmitters and Stretch Alter the Potential of Smooth Muscle Cells (Part 2)
Concept 36.1 Cycles of Protein–Protein Interactions Cause Muscles to Contract • Smooth muscle contraction: • Ca2+ influx to sarcoplasm stimulated by stretching, action potentials, or hormones • Ca2+ binds with calmodulin—activates myosin kinase, which phosphorylates myosin heads; can then bind and release actin
Concept 36.2 The Characteristics of Muscle Cells Determine Muscle Performance • In skeletal muscle—minimum unit of contraction is a twitch. • Twitch measured in terms of tension, or force it generates. • A single action potential generates a single twitch. Force generated depends on how many fibers are in the motor unit.
Concept 36.2 The Characteristics of Muscle Cells Determine Muscle Performance • Tension generated by entire muscle depends on: • Number of motor units activated • Frequency at which motor units are firing
Concept 36.2 The Characteristics of Muscle Cells Determine Muscle Performance • Single twitch—if action potentials are close together in time, the twitches are summed, tension increases. • Twitches sum because Ca2+ pumps can not clear Ca2+ from sarcoplasm before next action potential arrives. • Tetanus—action potentials are so frequent there is always Ca2+ in the sarcoplasm.
Concept 36.2 The Characteristics of Muscle Cells Determine Muscle Performance • How long muscle fiber can sustain tetanic contraction depends on ATP supply. • ATP is needed to break the actin–myosin bonds, and “re-cock” the myosin heads. • To maintain contraction, actin–myosin bonds have to keep cycling.
Concept 36.2 The Characteristics of Muscle Cells Determine Muscle Performance • A low level of tension is maintained by some muscles. • Muscle tone—a small but changing number of motor units are contracting. • Muscle tone is constantly being adjusted by the nervous system.
Concept 36.2 The Characteristics of Muscle Cells Determine Muscle Performance • Type of muscle fiber determines endurance and strength. • Skeletal muscle fibers can express genes for different variants of myosin with different ATPase activity. • Faster or slower ATPase activity determines different muscle characteristics.
Concept 36.2 The Characteristics of Muscle Cells Determine Muscle Performance • Slow-twitchfibers (oxidative or red muscle). • Contain myoglobin, oxygen binding protein, and many mitochondria; well-supplied with blood vessels. • Maximum tension develops slowly but is highly resistant to fatigue.
Concept 36.2 The Characteristics of Muscle Cells Determine Muscle Performance • Slow-twitch fibers have reserves of glycogen and fat—can produce ATP as long as oxygen is available. • Muscles with high proportion of slow-twitch fibers are good for aerobic work (e.g., long distance running, cycling, swimming, etc.).
Concept 36.2 The Characteristics of Muscle Cells Determine Muscle Performance • Fast-twitch fibers (glycolytic or white muscle). • Fewer mitochondria, fewer blood vessels, little or no myoglobin. • Develop greater maximum tension faster, but fatigue more quickly. • Cannot replenish ATP for prolonged contraction.
