Chapter 9: Muscular System

Compare and contrast the functional characteristics of muscle tissue.

*Excitability* = the ability to receive and respond to stimuli *Contractility* = the ability to shorten forcibly when stimulated *Extensibility* = the ability to be stretched or extended *Elasticity* = the ability to bounce back to original length.

Describe the functions of muscle tissue.

*Movement* = locomotion and manipulation, vision, and facial expression (skeletal), blood pumping (cardiac), food digesting, and urination (smooth) *Posture Maintenance* = skeletal *Joint Stability* = skeletal *Heat Generation* = skeletal

Compare and contrast the types of muscle tissues in terms of structure, control, location, type of contraction, and function.

*Skeletal Muscle:* -long, thin and multi-nucleated fibers -striations -voluntary control -arranged into packages called muscles that attach to and cover the bony skeleton -contracts rapidly and vigorously, but tires easily; may exert great force *Cardiac Muscle:* -network of branched fibers connected by gap junctions (intercalated disks) -striations -involuntary control -only in heart -contracts at rhythmic, steady rate set by "pacemaker" *Smooth Muscle:* -lacks striations -involuntary control -walls of hollow visceral organs and blood vessels -contractions are slow and sustained.

Define the terms tendon, aponeurosis, raphe, and syncytium.

*Tendon* = *Aponeurosis* = *Raphe* = *Syncytium* =

Compare and contrast the ultrastructure of thick and thin filaments.

*Thick Filaments* are composed of the protein *myosin*. They have a rod-like tail (axis) that terminates into two globular heads or *cross bridges*. These cross bridges interact with active sites on thin filaments. *Thin Filaments* are composed of the protein *actin*. They are coiled helical structures that have tropomyosin and troponin inside. *Tropomyosin* is a rod-shaped protein that spirals around the actin backbone to stabilize it. *Troponin* is a group of three complex polypeptides. One binds to actin, another to tropomyosin, and another that binds to calcium ions. Both tropomyosin and troponin control actin's interaction with myosin during contraction.

Describe three similarities among the three muscle tissues.

1) All muscle cells are elongated = muscle fibers 2) Muscle contraction depends on two kinds of myofilaments (actin and myosin) 3) All muscle cells contain a cell membrane and cytoplasm.

Illustrate how skeletal muscle is wrapped in four layers of connective tissue.

Each muscle fiber (cell) is wrapped in a thin, delicate layer of CT called *endomysium*. Many muscle fibers are bundled together into groups called fascicles. Each fascicle is wrapped in a second layer of CT made of collagen called *perimysium*. Many fascicles are bundled together to form a skeletal muscle. Each skeletal muscle is covered by a third layer of dense, fibrous CT called *epimysium*. Each skeletal muscle is then covered by a fourth, very tough fibrous layer of CT called *deep fascia*.

Illustrate how the myofibrils that compose skeletal muscle fibers are composed of sarcomeres. Label the thick filaments, thin filaments, A-band, I-Band, and Z-line.

Each muscle fiber is composed of *myofibrils*. Each myofibril is composed of two types of protein filaments. *Thick filaments* are primarily composed of *myosin* and *thin filaments* are primarily composed of *actin*. Striations are caused by the arrangement of thick and thin filaments within the myofibrils. The *A-Band* is the dark area where the thin and thick filaments overlap. The *I-Band* is the light area where thin filaments are alone. The length of each myofibril is divided into sarcomeres and where they meet is an area called the *Z-Line*.

Identify the terms used for a muscle fiber's cell membrane and cytoplasm.

The cell membrane of a muscle cell is called *sarcolemma* and the cytoplasm of a muscle cell is called *sarcoplasm*.

Explain the significance of the special membranous organelles found in skeletal muscle tissue.

There are two special membranous organelles within the sarcoplasm of a muscle fiber. The *sarcoplasmic reticulum* is a network of membranous channels that surrounds each myofibril and runs parallel to it. It has high concentrations of calcium ions compared to the sarcoplasm, and when it's stimulated by a muscle impulse, the membrane becomes more permeable to calcium ions. This causes the calcium ions to diffuse out of the SR and into the sarcoplasm. The *transverse tubules* are a set of membranous channels that extend into the sarcoplasm as invaginations continuous with the sarcolemma. TT's are filled with extracellular fluid and extend deep into the cell. Each TT runs between two enlarged portions of SR called *cisternae*. One TT is associated with two SR and are termed the *triad*, which is where actin and myosin overlap.