Lewis/Life - Review of Key Concepts

Review of Key Concepts - Chapter 34

The musculoskeletal system helps to maintain homeostasis by enabling an animal to move in response to environmental stimuli.
An animal's skeleton supports its body, protects soft tissues, and enables the animal to move. A hydrostatic skeleton consists of tissue containing constrained fluid. A braced framework, which has solid components, can be on the organism's exterior as an exoskeleton or within the body as an endoskeleton. Exoskeletons include shells that thicken and grow throughout life and jointed suits of armor that are periodically shed and replaced. An endoskeleton grows along with the organism.
The vertebrate skeleton supports, protects, and 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. Bones in different vertebrate species can differ in width and density.
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 (calcium and phosphate). In compact bone, osteocytes form osteons. The central channel for blood supply is an osteonic canal. Canaliculi connect lacunaes 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.
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 striated, voluntary, and move bones.
Sliding protein filaments in muscles provide movement in diverse organisms. Tendons attach an intact whole skeletal muscle to bones. Within a muscle are bundles of muscle fibers. Each muscle fiber is a long cylindrical cell composed primarily of thick and thin myofilaments. The thick myofilaments are bundles of myosin molecules, 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.
A muscle fiber cell is a chain of many 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 are pulled past one another so that they overlap more.
When a motor neuron stimulates a muscle fiber, the neurotransmitter acetylcholine is released at a neuromuscular junction. Electrical charges enter the muscle cell causing the sarcoplasmic reticulum to release calcium ions, which bind to the troponin molecules on actin myofilaments. As a result, troponin moves and no longer prevents actin from binding 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 actin myofilament.
After the nerve impulse, calcium ions are actively pumped back into the sarcoplasmic reticulum and tropomyosin moves to prevent actin and myosin interactions. This leads to muscle relaxation. The energy for muscle contraction comes first from stored ATP, which is quickly exhausted. For a short time new ATP can form from creatine phosphate stored in muscle cells, and then aerobic respiration generates ATP. If ATP use exceeds the ability of muscle cells to produce ATP aerobically, the cells switch to anaerobic respiration. An important source of energy for muscle contraction is glycogen.
When stimulated, a muscle cell responds in an all-or-none fashion, although not all of the many muscle cells in a whole muscle contract. When a muscle cell is stimulated once, it contracts and relaxes, a response called a twitch. If the stimulation rate increases, the muscle cell does not completely relax between pulses, and the response strengthens. At a high rate of stimulation, muscle cells reach a sustained state of maximal contraction called tetanus.
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.
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. Muscles form antagonistic pairs, enabling bones to move in two directions.
Muscles are always partially contracted, generating muscle tone. Muscle spindles control muscle tone by sending impulses to the brain or spinal cord when a muscle is stretched.
A muscle exercised regularly increases in size (hypertrophy) 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.