2.10 Anatomy Chapter 10

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2.10 Anatomy Chapter 10
2010-09-27 23:17:17
Skeletal Muscle Tissue

Overview of muscle tissue, skeletal muscle, disorders
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  1. Muscle Tissue Functions
    • 1. Movement: skeletal muscles attach to the skeleton and moves body via moving bones; muscle in visceral organs squeeze/move substances through the hollow organs
    • 2. Maintenance of posture: skeletal muscles contract continuously to maintain posture
    • 3. Joint stabilization: role of muscle tone in stabilizing and strengthening joints
    • 4. Heat generation: muscle contractions produce heat that plays a vital role in maintaining body temperature at 37*C
  2. Muscle Tissue
    Distinguishing Functional Characteristics
    • 1. Contractility: contracts forcefully; shorten and generate a pulling force
    • 2. Excitability: nerve signals and other factors excite muscle cells and stimulate them to contract
    • 3. Extensibility: muscle tissue can be stretched by the contraction of an opposing muscle
    • 4. Elasticity: muscle tissue can recoil passively and resume its resting length after being stretched
  3. Types of Muscle Tissue
    (1) Presence/ Absence of Striations
    • 1. Skeletal Muscle Tissue: 40% of body weight; striated, contraction under voluntary control
    • 2. Cardiac Muscle Tissue: only in the wall of the heart; striated; under involuntary control
    • 3. Smooth Muscle Tissue: found in walls of hollow internal organs other than heart; lack striations; under involuntary control

    • Muscle cells = fibers (except heart cells)
    • Plasma membrane = sarcolemma
    • Cytoplasm = sarcoplasm
    • Endoplasmic Reticulum = sarcoplasmic reticulum (specialized for storage of Calcium)
    • Contraction depends on myofilaments: specific types of microfilaments (actin/myosin)
  4. Basic Features of Skeletal Muscle
    Connective Tissue and Fascicles
    • 1. Epimysium: "overcoat" of dense, irregular connective tissue that surrounds the whole skeletal muscle
    • 2. Perimysium: fibrous connective tissue surrounding groups of skeletal muscle fibers called fascicles
    • 3. Endomysium: fine sheath of loose connective tissue (mainly reticular fibers) surrounding each muscle fiber

    *There is continuity between the sheaths, endomysium is connected to the perimysium, which is connected to the epimysium, which is further connected to the tendon. When the muscles contract, it pulls on these connective tissues and exerts its force to move the bone.
  5. Basic Features of Skeletal Muscle
    Nerves and Blood Vessels
    • Each skeletal muscle is supplied by:
    • - one nerve
    • - one artery
    • - one or more veins
    • all of which enter and exit the muscle near the middle of its length
    • - nerves and vessels branch repeatedly, with smallest branches going into individual fibers
    • *rich blood supply indicative of high metabolic demands
  6. Basic Features of Skeletal Muscle
    Muscle Attachments
    • The location on a bone where a muscle connects; each muscle extends from one bone to another, crossing at least one movable joint
    • - origin: attachment of the muscle on the less movable or fixed bone
    • - insertion: attachment on the more movable bone (bends bone towards origin)

    - aponeurosis: a cordlike tendon or flat sheet of connective tissue that attaches the muscle to bone in indirect attachments; usually attached to raised bone markings, ex. tubercles, trochanters, or crests
  7. Microscopic and Functional Anatomy of Skeletal Muscle Tissue
    Skeletal Muscle Fiber
    Myofibrils and Sarcomeres
    Long, cylindrical cells; formed by fusion of hundreds of embryonic cells, and are therefore multinucleated (nuclei are peripheral, lie just under the sarcolemma)

    • Hierarchy of muscle cells from biggest to smallest:
    • muscle fiber --> myofibril (rod-shaped "organelles") --> myofilament proteins (actin/myosin)
    • Clearly striated due to myofibrils: specialized contractile organelles
    • As we age, we get more myofibrils, not more fibers (fibers only get bigger)
    • Myofibril: long row of repeating segments called sarcomeres (basic unit of contraction)
    • Boundaries of a sarcomere are called Z discs (or Z lines)
    • - thin filaments (actin): extend from the Z disc toward the center of the sarcomere
    • - thick filaments (myosin): cylindrical bundle consisting largely of myosin molecules.
    • - A band: dark bands created by thick filaments, along with inner ends of thin filaments
    • - H zone: central part of an A band, where no thin filaments reach
    • - M line: center of the H zone that contains tiny rods that hold the thick filaments together
    • - I band: two regions on either side of the A band that contain only thin filaments (light bands); Z disc runs through middle of I band
    • *During contraction, H zone disappears; I band shortens
  8. Microscopic and Functional Anatomy of Skeletal Muscle Tissue
    Sarcoplasmic Reticulum and T Tubules
    • Sarcoplasmic Reticulum: an elaborate smooth endoplasmic reticulum whose tubules surround each myofibril, cisterns are SR tubules that form larger, perpendicular cross-channels.
    • - Store Ca2+ that are released to signal muscle contraction

    • T-tubules (Transverse Tubules): deep invaginations of the sarcolemma that run between each pair of cisternae; triad: 2 cisternae flanking a t-tubule
    • - Conduct electrical impulse to deepest region of muscle fiber
  9. Functional Anatomy of Skeletal Muscle
    Innervation of Skeletal Muscle
    motor neurons innervate muscle fibers; nerve endings serves each muscle fiber

    • Neuromuscular Junction (motor end plate): the point at which the nerve ending and fiber meet
    • - nerve ending is enlarged at the end of the axonal process, forming axon terminals, to store chemical messengers called neurotransmitters.
    • - synaptic cleft: the space between axon terminals and the sarcolemma of the muscle cell

    *axon terminals house vesicles that contain neurotransmitters (such as acetylcholine, ACh), that are released when signaled. The neurotransmitter diffuses across the synaptic cleft and binds to receptors on the sarcolemma, which in turn signal the muscle to contract.
  10. Disorders of Skeletal Muscle Tissue
    Muscular Dystrophy
    Muscular Dystrophy: a group of inherited muscle-destroying diseases; affected muscles enlarge with fat and connective tissue while muscle fibers degenerate.

    Fibromyalgia: a mysterious chronic-pain syndrome of unknown cause; symptoms include musculoskeletal pain, fatigue, sleep abnormalities, and headache.