The Musculoskeletal System
34.1 A Musculoskeletal System Helps an Animal to Respond to Its Environment
1. The musculoskeletal system helps to maintain homeostasis by enabling an animal to move in response to environmental stimuli.
34.2 Types of Skeletons
2. An animal’s skeleton supports its body, protects soft tissues, and enables the animal to move.
3. A hydrostatic skeleton consists of tissue containing constrained fluid.
4. A braced framework, which has solid components, can be on the organism’s exterior as an exoskeleton or within the body as an endoskeleton.
5. The vertebrate skeleton supports, protects, attaches to muscles, and stores minerals. The axial skeleton consists of the skull, vertebral column, breastbone, and ribs. The appendicular skeleton includes the limbs and the limb girdles (pectoral and pelvic) that support them.
6. Cartilage and bone make up skeletons. Cartilage entraps a great deal of water, which makes it an excellent shock absorber. Bone has a rigid matrix and derives its strength from collagen and its hardness from minerals. Spongy bone has many spaces. In compact bone, osteocytes form osteons. The central channel for blood supply is an osteonic canal. Canaliculi connect lacunae that house bone cells and the osteonic canal. Bone cells extend through canaliculi and pass materials from cell to cell. Bone continually degenerates and builds up.
34.3 Muscle Diversity
7. Many cells and organisms have movement mechanisms based on actin and myosin.
8. Invertebrates have smooth and striated muscles.
34.4 Vertebrate Muscles
9. Smooth muscle cells are spindle-shaped and involuntary, and they line organs, push food through the digestive tract, and regulate blood flow and pressure. Cardiac muscle cells, in the heart, are striated and involuntary and are joined by intercalated disks into a branching pattern. Skeletal muscle cells are multinucleate, striated, voluntary, and move bones.
10. Sliding protein filaments in muscles provide movement. Tendons attach an intact whole skeletal muscle to bones. Each skeletal muscle fiber is a long cylindrical cell composed primarily of thick and thin myofilaments. The thick myofilaments are myosin, each with a head (cross bridge) attached to a shaft. The heads project outward from each end of a myosin bundle. The thin myofilaments are composed of actin plus troponin and tropomyosin.
11. A muscle fiber is a chain of contractile units, called sarcomeres. Myofilaments within a sarcomere give the tissue its striated appearance. According to the sliding filament model, muscle contraction occurs when the thick and thin myofilaments move past one another so that they overlap more.
12. When a motor neuron stimulates a muscle fiber, acetylcholine is released at a neuromuscular junction. Electrical waves spread along the muscle cell membrane, causing the sarcoplasmic reticulum to release calcium ions, which bind to the troponin molecules on myofilaments. As a result, troponin moves and no longer prevents actin from binding to myosin. Once myosin cross bridges touch actin, ATP attached to the myosin head splits and the head moves, causing the actin myofilament to slide past the myosin myofilament. ADP and inorganic phosphate are released. A new ATP binds to the myosin head and the cross bridge to actin breaks, the myosin head returns to its original position, and a new cross bridge forms further along the myofilament.
13. After the nerve impulse, calcium ions are actively pumped back into the sarcoplasmic reticulum and tropomyosin moves to prevent actin and myosin interactions. The muscle relaxes.
14. The energy for muscle contraction comes first from stored ATP, then from creatine phosphate stored in muscle cells, and then from aerobic respiration, and finally from anaerobic metabolism. An important source of energy for muscle contraction is glycogen.
15. A nerve cell and the muscle fibers it touches are a motor unit.
16. When stimulated, a muscle cell responds in an all-or-none fashion, although not all of the cells in a whole muscle contract. When a muscle cell is stimulated once, it contracts and relaxes. If the stimulation rate increases, the muscle cell does not completely relax and the response strengthens. At a high rate of stimulation, muscle cells reach a sustained state of maximal contraction, tetanus.
17. Muscles form antagonistic pairs, which enable bones to move in two directions.
34.5 Skeletal Muscle and Bone Function Together
18. Joints attach bones. Some joints, such as those holding the skull bones in place, are immovable. Freely moving synovial joints consist of cartilage and connective tissue ligaments that contain lubricating synovial fluid.
19. Most voluntary muscles attach to bones, forming lever systems. When a muscle contracts, a bone moves. The muscle end attached to the stationary bone is its origin. The end attached to the movable bone is called the insertion.
20. A muscle exercised regularly increases in size (hypertrophies) because each muscle cell thickens. An unused muscle shrinks (atrophies). Regular exercise causes microscopic changes in muscle cells that enable them to use energy more efficiently.