Chapter 6- The Muscular System

Three basic muscle types are found in the body

Skeletal muscle \n Cardiac muscle \n Smooth muscle

Muscle Types

  • Skeletal and smooth muscle cells are elongated (muscle cell = muscle fiber)
  • Contraction and shortening of muscles are due to the movement of microfilaments
  • All muscles share some terminology
    • Prefixes myo- and mys- refer to “muscle”
    • Prefix sarco- refers to “flesh”
  • Skeletal muscle
    • Most skeletal muscle fibers are attached by tendons to bones
    • Skeletal muscle cells are large, cigar-shaped, and multinucleate
    • Also known as striated muscle because of its obvious stripes
    • Also known as voluntary muscle because it is the only muscle tissue subject to conscious control
  • Skeletal muscle cells are surrounded and bundled by connective tissue
    • Endomysium—encloses a single muscle fiber (cell)
    • Perimysium—wraps around a fascicle (bundle) of muscle fibers
    • Epimysium—covers the entire skeletal muscle \n blends into a connective tissue attachment
    • Fascia—on the outside of the epimysium
  • Smooth muscle
    • No striations
    • Involuntary—no conscious control
    • Found mainly in the walls of hollow visceral organs (such as stomach, urinary bladder, respiratory passages)
    • Spindle-shaped fibers that are unicleate
    • Contractions are slow and sustained
  • Cardiac muscle
    • Striations
    • involuntary
    • Found only in the walls of the heart
    • Uninucleate
    • Branching cells joined by gap junctions called intercalated discs*-*
    • Contracts at a steady rate set by pacemaker

Muscle Function

  • Whereas all muscle types produce movement, skeletal muscle has three other important roles:
    • Maintain posture and body position
    • Stabilize joints
    • Generate heat

Microscopic Anatomy of Skeletal Muscle

  • Sarcolemma —specialized plasma membrane

  • Myofibrils—long organelles inside muscle cell

    • Light (I) bands and dark (A) bands give the muscle its striated (banded) appearance
  • Banding pattern of myofibrils

    • I band = light band
    • Contains only thin filaments
    • Z disc is a midline interruption
    • A band = dark band
    • Contains the entire length of the thick filaments
    • H zone is a lighter central area
    • M line is in center of H zone

Iband

Aband

Iband

Hzone

Zdisc

Zdisc

  • Sarcomere—contractile unit of a muscle fiber

    • Structural and functional unit of skeletal muscle
  • Organization of the sarcomere

    • Myofilaments produce banding(stripped pattern)
    • Thick filaments = myosin filaments
    • Thin filaments = actin filaments
  • Thick filaments = myosin filaments

    • Composed of the protein myosin
    • Contain ATPase enzymes to split ATP to release energy for muscle contractions
    • Possess projections known as myosin heads
    • Myosin heads are known as cross bridges when they link thick and thin filaments during contraction
  • Thin filaments = actin filaments

    • Composed of the contractile protein actin
    • Actin is anchored to the Z disc
  • At rest, within the A band there is a zone that lacks actin filaments called the H zone

  • During contraction, H zones disappear as actin and myosin filaments overlap

Sacromere

Zdisc

Mline

Zdisc

Thin Actin

Thick Myosin

  • Sarcoplasmic reticulum (SR)
    • Specialized smooth endoplasmic reticulum
    • Surrounds the myofibril
    • Stores and releases calcium

Stimulation and Contraction of Single Muscle Cell

  • Special functional properties of skeletal muscles
    • Irritability (responsiveness)—ability to receive and respond to a stimulus
    • Contractibility—ability to forcibly shorten when an adequate stimulus is received
    • Extensibility—ability of muscle cells to be stretched
    • Elasticity—ability to recoil and resume resting length after stretching

Nerve Stimulus and Action Potential

  • Skeletal muscles must be stimulated by a motor neuron (nerve cell) to contract
  • Motor unit—one motor neuron and the skeletal muscle cells stimulated by that neuron
  • Neuromuscular junction
    • Association site of axon terminal of the motor neuron and sarcolemma of a muscle
  • Neurotransmitter
    • Chemical released by nerve upon arrival of nerve impulse in the axon terminal
    • Acetylcholine (ACh) the neurotransmitter that stimulates skeletal muscle
  • Synaptic cleft
    • Gap between nerve and muscle, filled with interstitial fluid
    • Although very close, the nerve and muscle do not make contact

The Nerve Stimulus and Action Potential

Mechanism of Muscle Contraction: Sliding Filament

  • What causes filaments to slide?
    • Calcium ions (Ca2+) bind regulatory proteins on thin filaments and expose myosin-binding sites, allowing the myosin heads on the thick filaments to attach
    • Each cross bridge pivots, causing the thin filaments to slide toward the center of the sarcomere
    • Contraction occurs, and the cell shortens
    • During a contraction, a cross bridge attaches and detaches several times
    • ATP provides the energy for the sliding process, which continues as long as calcium ions are present