Three basic muscle types are found in the body

Skeletal muscle
Cardiac muscle
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
    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