45.1 How Muscles Contract?

• A muscle fiber is a long, thin muscle cell.

• Within each muscle cell are many threadlike, contractile structures called myofibrils.

  • The myofibrils inside muscle fibers often look striped, or striated, due to the alternating light-dark units called sarcomeres, which repeat along the length of a myofibril.

• In 1952, biologist Hugh Huxley examined cross sections of sarcomeres using electron microscopy.

  • He observed that there were two types of filaments, thin filaments and thick filaments, and that these filaments overlapped in the dark bands but not in the light bands.

  • Huxley and his collaborator Jean Hanson also observed that sarcomeres stripped of their myosin had no dark bands.

  • They concluded that the thick filaments must be composed of myosin, and the thin filaments must be composed of actin.

• Each thin filament is composed of two coiled chains of actin, a common component of the cytoskeleton of eukarγotic cells.

  • One end of a thin filament is anchored to a structure called the Z disc, which forms the wall between neighboring sarcomeres.

  • The other end of a thin filament is free to interact with thick filaments.

• Thick filaments are composed of multiple strands of myosin.

  • They span the center of the sarcomere and are free at both ends to interact with thin filaments.

• Besides actin, thin filaments contain two key proteins called tropomyosin and troponin, which work together to block the myosin binding sites on actin.

  • When calcium ions bind to troponin，the troponin-tropomyosin complex moves in a way that exposes the myosin binding sites on actin.

  • Myosin then binds, and contraction can begin.

45.2 Classes of Muscle Tissue

• Smooth muscle cells are unbranched, tapered at each end, and often organized into thin sheets.

  • Contraction of smooth muscle cells occurs when actin and myosin bind and move past one another, but these proteins are not arranged into sarcomeres as they are in skeletal and cardiac muscle.

• Smooth muscle is essential to the function of the lungs, blood vessels, digestive system, urinary bladder, and reproductive system.

• Smooth muscle is autorhythmic, meaning that it can spontaneously contract, with no stimulation by the nervous system.

  • However, the autonomic nervous system can also stimulate and inhibit contraction in smooth muscle.

  • Because smooth muscle is innervated only by autonomic neurons, it is involuntary.

• Cardiac muscle makes up the walls of the heart and is responsible for pumping blood throughout the body.

• Like smooth muscle, cardiac muscle is autorhythmic and involuntary-it contracts following spontaneous depolarizations, and the rate and strength of contractions are influenced by autonomic neurons.

• Skeletal muscle consists of exceptionally long, unbranched muscle fibers.

45.3 Skeletal Systems

• Hydrostatic skeletons (“still-water skeletons”), or hydrostats, are constructed of an extensible body wall in tension surrounding a fluid or deformable tissue under compression.

• Longitudinal and circumferential muscles in earthworms make up an antagonistic muscle group, a group of two or more muscles that reextend one another via the skeleton.

• Endoskeletons (“inside skeletons”) are rigid structures that occur within the body.

• Vertebrate skeletons are composed of three main elements:

  • Bones are made up of cells in a hard extracellular matrix of calcium phosphate with small amounts of calcium carbonate and protein fibers.

    • The meeting places where adjacent bones interact are called articulations, or joints.

  • Cartilage is made up of cells scattered in a gelatinous matrix of polysaccharides and protein fibers.

  • Ligaments are bands of fibrous connective tissue, primarily collagen, that bind bones to other bones.

    • Ligaments stabilize the joints.

• Bones attach to skeletal muscle via bands of fibrous connective tissue called tendons.

• The hamstring muscles in the back of your thigh are flexors, muscles that pull bones closer together, decreasing the joint angle between them.

• The quadriceps muscles in the front of your thigh are extensors, muscles that increase the angle of a joint.

  • They straighten your leg at the knee joint.

• Osteoblasts are bone-building cells一they secrete the protein- and calcium-rich extracellular matrix at hardens to form bone.

• An exoskeleton (“outside skeleton”) is an exterior skeleton that encloses and protects an animal’s body.

• Chitinous ingrowths of the skeleton form apodemes, where muscles attach.