Muscles

Muscles Do One Thing: Contract

• Actively, muscles can only contract.
  • They extend only passively.
• Therefore, the musculoskeletal system is based on muscles that come in antagonistic pairs.
• Bones provide a skeletal structure for our body; antagonistic muscle pairs move these bones.
  • See the example below.
  • Flexor: muscle that brings bones together
  • Extensor: muscle that separates bones

Figure 49.25(a), page 1014, Campbell's Biology, 5th Edition

Skeletal Muscle: Basic Anatomy

Muscle

Thousands of muscle cells (muscle fibers) held together by connective tissue

Muscle fiber (muscle cell)

• Elongated cell (the length of the muscle)
• Many nuclei per cell
• Surrounded by a cell membrane called the sarcolemma.
  • Sarco is Greek for flesh.
    • Sarcophagous is a stone coffin and literally means “flesh eater.”
  • Sarcolemma can conduct an action potential along its surface like a nerve axon does!
  • Invaginations of the sarcolemma, called T tubules, can run through the cell so that the action potential can go inside the muscle cell.

Myofibrils (subcellular structures)

• Many per cell
• Arranged lengthwise in cell
• The most important organelle in the muscle cell is the sarcoplasmic reticulum.
  • A membrane-bound structure, similar to the endoplasmic reticulum in normal cells
  • Completely surrounds the myofibrils.
  • Calcium ions are stored here.
  • Think of it as a bag of calcium ions surrounding the myofibrils.

Sarcomere

• Functional unit of skeletal muscle
• Made of two filaments:
  • Thick filaments: bundles of myosin
  • Thin filaments: actin
• The size of a sarcomere is from Z line to Z line, and thus can change as the muscle contracts.

Figure 38.17, page 646, Starr's Biology: The Unity and Diversity of Life, 7th Edition; Unknown source

Muscle Contraction

• Change in length of the sarcomere via changing the distance between Z lines.
• Actin and myosin don't change in length; they just slide past one another.

Transparency 177, Starr's Biology: The Unity and Diversity of Life, 7th Edition

Sliding-Filament Model

• A simplified view of muscle contraction:
  1. Globular head of myosin attaches to actin (cross-bridge forms between filaments).
  2. Myosin globular head pulls itself along the actin.
  3. Myosin globular head releases itself from the actin.
  4. Myosin globular head is reenergized by ATP and therefore returns to its original high-energy position.
• Think of this in terms of energy transfer during one complete cycle:
  • Before movement: high potential energy
  • After movement: low potential energy
  • ATP must transfer energy so the myosin has high potential energy to begin the cycle again.

Transparency 178, Starr's Biology: The Unity and Diversity of Life, 7th Edition

Details

• ATP hydrolysis: Setting the "Mouse Trap"
  • Hydrolysis: Word Derivation
    • Hydro: Water
    • Lysis: To split
    • ATP, in the presence of water (hydro), is split (lysis), into ADP and inorganic phosphate (P_i).
  • ATP hydrolysis is a process in which energy from ATP is either transferred to another chemical or the energy is used to do some kind of work. In this case it is used to do work.
    • Myosin globular head is moved into a higher-energy configuration (just like pulling the spring on a mouse trap).
• Cross-bridge formation occurs
  • The myosin globular head, in its high-energy state, binds to the actin.
• Release of ADP and P_i
  • As the myosin globular head releases the ADP and P_i, the myosin globular head relaxes to its low energy state causing the actin (the thin filament) to slide.
• ATP binding
  • The binding of ATP releases the myosin head.

Figure 49.28, page 1016, Campbell's Biology, 5th Edition

How Myosin Binds to Actin

• Actin filaments shown below (lines of orange spheres twined together) have myosin binding sites (dark orange spots).
• The myosin binding sites are covered by a long protein molecule, tropomyosin.
• Another protein, troponin, controls the position of the tropomyosin.
• When calcium ions bind to troponin, the tropomyosin molecule moves to expose the myosin binding sites, allowing the myosin to bind to the actin.

Figure 49.29, page 1017, Campbell's Biology, 5th Edition

Neuromuscular Junction: How Electrical Signals Cause Muscle Contraction

• An action potential arrives at the neuromuscular junction.
  • Acetylcholine is the neurotransmitter at every neuromuscular junction.
  • Acetylcholine diffuses across the neuromuscular junction, binding to receptors on the sarcolemma to cause an action potential to be created on the sarcolemma.
  • The action potential moves along the sarcolemma and spreads down into the T tubules.
  • The action potential causes the sarcoplasmic reticulum to release calcium ions into the cytoplasm of the cell.
• Calcium ions bind with the actin molecules.
• This causes the cross-bridge binding sites to open up so the myosin globular heads can attach and begin muscle contraction.

Figure 47.5, Purves's Life: The Science of Biology, 7th Edition