The Muscular System

• muscles are responsible for all types of body movement

• three basic muscle types are found in the body

  • skeletal muscle

  • cardiac muscle

  • smooth muscle

Characteristics of Muscles

• skeletal and smooth muscles are elongated (muscle cell = muscle fiber)

• contractions of muscles are due to the movement of microfilaments

• all muscles share some terminology

  • prefixes myo and myo refer to “muscle”

  • prefix sacro refers to “flesh”

Skeletal Muscle Characteristics

• most are attached by tendons to bones

• cells are multinucleate

• striated - have visible banding

• voluntary - subject to conscious control

Connective Tissue Wrappings of Skeletal Muscle

• cells are surrounded and bundled by connective tissue

  • endomysium - encloses a single muscle fiber

  • perimysium - wraps around a fascicle (bundle) of muscle fibers

  • epimysium - covers the entire skeletal muscle

  • fascia - on the outside of the epimysium

Skeletal Muscle Attachment

• epimysium blends into a connective tissue attachment

  • tendons - cord-like structures

    • mostly collagen fibers

    • often cross a joint due to toughness and small size

  • aponeuroses - sheet-like structures

    • attach muscles indirectly to bones, cartilages, or connective tissue coverings

• sites of muscle attachment

  • bones

  • cartilages

  • connective tissue coverings

Smooth Muscle Characteristics

• lacks striations

• spindle-shaped cells

• single nucleus

• involuntary - no conscious control

• found mainly in walls of hollow organs

Cardiac Muscle Characteristics

• striations

• usually has a single nucleus

• branching cells

• jointed to another muscle cell of an intercalated disc

• involuntary

• found only in the heart

Skeletal Muscle Functions

• produce movement

• maintain posture

• stabilize joints

• generate heat

Microscopic Anatomy of Skeletal Muscle

• sarcolemma - specialized plasma membrane

• myofibrils - long organelles inside muscle cell

• sarcoplasmic reticulum (SR) - specialized smooth ER

• myofibrils are aligned to give distinct bands

  • I band = light band

    • contains only thin filaments

  • A band = dark band

    • contains the entire length of the thick filaments

• sarcomere - the contractile unit of a muscle fiber

• organization of the sarcomere

  • myofilaments

    • thick filaments = myosin filaments

      • composed of the protein myosin

      • has ATPase enzymes

      • myosin filaments have heads (extensions or cross bridges)

      • myosin and actin overlap somewhat

    • thin filaments = actin filaments

      • composed of the protein actin

      • anchored to the Z disc

• at rest, there is a bare zone that lacks actin filaments called the H Zone

• Sarcoplasmic Reticulum (SR)

  • stores and releases calcium

  • surrounds myofibril

Stimulation and Contraction of SIngle Skeletal Muscle Cells

• Excitability (responsiveness or irritability) - ability to receive and respond to a stimulus

• Contractility - the ability to shorten when an adequate stimulus is received

• Extensibility - the ability of muscle cells to be stretched

• Elasticity - the ability to recoil and resume resting length after stretching

The Nerve Stimulus and Action Potential

• skeletal muscles must be stimulated by a motor neuron (nerve cell) to contract

• motor unit - one motor neuron and all the skeletal muscle cells stimulated by that neuron

• Neuromuscular Junction

  • association site of the axon terminal of the motor neuron and muscle

• Synaptic Cleft

  • the gap between nerve and muscle

  • nerve and muscle do not make contact

  • the area between nerve and muscle is filled with interstitial fluid

Transmission of Nerve Impulse to Muscle

• Neurotransmitter - chemical released by nerve upon arrival of nerve impulse

  • the neurotransmitter for skeletal muscle is acetylcholine (ACh)

• acetylcholine attaches to receptors on the sarcolemma

• sarcolemma becomes permeable to sodium (Na+)

• sodium rushes into the cell generating an action potential

• once started, muscle contraction cannot be stopped

The Sliding Filament Theory of Muscle Contraction

• activation by the nerve causes myosin heads (cross-bridges) to attach to binding sites on the thin filaments

• myosin heads then bind to the next site of the thin filament and pull them toward the center of the sarcomere

• this continued action causes the sliding of the myosin along the actin

• the result is that the muscle is shortened (contracted)