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