2. Three types of vertebrate muscle:  Skeletal—voluntary movement, breathing  Cardiac—beating of heart  Smooth—involuntary, movement of internal organs All use sliding filament contractile mechanism.

3. Skeletal muscle: striated Cells are called muscle fibers—are large and multinucleate One muscle consists of many muscle fibers bundled together by connective tissue

4. Muscle fiber (cell) of skeletal muscle Made up of several myofibrils – bundles of contractile protein filaments Contractile proteins: Actin—thin filaments Myosin—thick filaments Each myofibril made up of sacromeres Repeating units of actin and myosin Bound by Z lines that anchor actin 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 When muscle contracts, sacromeres shorten and banding pattern changes

5. The sliding filament model of muscle contraction depends on structure of actin and myosin molecules. 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 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 5–10 nm

6. 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. 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.

7. Sarcoplasmic reticulum (in the muscle fiber) has Ca 2+ pumps. At rest Ca 2+ concentration is higher in sarcoplasmic reticulum, lower in sarcoplasm. Action potential reaches receptor proteins and opens the Ca 2+ channels, Ca 2+ flows out of sarcoplasmic reticulum and triggers interaction of actin and myosin. Actin filaments also include tropomyosin and troponin. Troponin has three subunits—one binds actin, one binds myosin, and one binds Ca 2+. At rest, tropomyosin blocks the binding sites on actin. When Ca 2+ is released, it binds to troponin, which changes conformation. Troponin is bound to tropomyosin—twisting of tropomyosin exposes binding sites on actin. Myosin head then binds to actin initiating sliding When Ca 2+ pumps remove Ca 2+ from sarcoplasm, contraction stops. ATP is needed to break the actin-myosin bonds, and “re-cock” the myosin heads

8. Cardiac muscle is also striated — cells are smaller than skeletal muscle and have one nucleus. Cardiac muscle cells also branch and interdigitate — can withstand high pressures. Intercalated discs provide mechanical adhesions between cells.

9. 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.

10. Skeletal systems are the rigid supports against which muscles can pull. Three types of skeletal systems in animals — hydrostatic, exoskeletons, and endoskeletons.

11. Hydrostatic skeleton consists of a volume of fluid enclosed in a body cavity surrounded by muscle. When muscles oriented in one direction contract, the fluid-filled body cavity bulges out in the opposite direction. Examples: Cnidarians, annelids, and other invertebrates.

12. Exoskeleton consists of a hardened outer surface to which muscles attach. Contractions of the muscles cause segments of the exoskeleton to move. Examples: Mollusks, arthropods.

13. Endoskeleton of vertebrates is an internal scaffold. An advantage is that growth can occur without shedding the skeleton. Human skeleton has 206 bones and can be divided: Axial skeleton includes skull, vertebral column, sternum, and ribs Appendicular skeleton includes pectoral and pelvic girdles, bones of the arms, legs, hands, and feet

14. Figure 48.15 The Human Endoskeleton

15. Muscles can exert force in only one direction — they create movement by working in antagonistic pairs. Flexor — the muscle that bends or flexes the joint Extensor — the muscle the straightens or extends the joint