Topic 19- Musculoskeletal System

Outcomes:

Comprehend the Levels of Muscle Structure

Sequence the Process of Muscle Contraction

Hypothesize and Diagnose Variability in Contraction

Compare and contast muscle types

I. Vertebrate Skeletal System

A. Introduction to Muscle

• Function: Enables voluntary movement; stimulated by neurons and attached to bones via tendons.

• Abundance: Most abundant tissue in the body.

• Origin: Derived from mesoderm, which also forms:

  • Cardiac muscle

  • Smooth muscle (gut)

  • Kidney cells

  • Red blood cells

• Structure:

  • Muscle fiber = muscle cell

    • Multinucleated (high protein output)

    • Elongates rather than divides (no cytokinesis)

    • Bundled together to form whole muscles

  • Myofibrils

    • Long, contractile fibers inside each muscle fiber

    • Made of repeating units called sarcomeres

  • T-Tubules (Transverse Tubules)

    • Invaginations of the sarcolemma (plasma membrane)

    • Carry action potentials deep into the cell

    • Ensure simultaneous contraction throughout the fiber

  • Sarcoplasmic Reticulum (SR)

    • Network surrounding myofibrils

    • Stores and releases Ca²⁺ to initiate contraction

B. Muscle Fibers

• Myofibrils:

  • Run the length of the muscle fiber

  • Composed of thick and thin filaments:

    • Thin Filaments:

      • Two strands of actin

      • Covered by tropomyosin, anchored by troponin

      • Tropomyosin blocks myosin binding sites at rest

    • Thick Filaments:

      • Made of ~350 myosin molecules

      • Myosin heads have ATPase activity to drive contraction

C. Sarcomere

• Definition: Functional unit of contraction in muscle fibers

• Structure:

  • Z lines: Boundary of each sarcomere; anchor thin filaments

  • M line: Middle; anchors thick filaments

  • Overlap: Region where thick and thin filaments interact

• Contraction:

  • Sarcomeres shorten (filaments slide past each other)

  • Z lines move closer, filament lengths remain constant

  • Greater overlap = stronger contraction

D. Sliding Filament Model

1. At Rest

• Tropomyosin blocks actin binding sites

• No Ca²⁺ → no contraction

2. Neural Signal Initiation

• Motor neuron releases acetylcholine (ACh) at neuromuscular junction

• ACh binds to receptors → Na⁺ influx → muscle action potential

3. Calcium Release

• AP travels through T-tubules

• SR releases Ca²⁺ into cytosol

4. Exposure of Active Site

• Ca²⁺ binds troponin → conformational change

• Tropomyosin moves → exposes myosin-binding sites on actin

5. Cross-Bridge Cycle

1. Cross-Bridge Formation:

   • Energized myosin head binds actin (ADP + Pi attached)

2. Power Stroke:

   • ADP + Pi released → myosin pulls actin toward M line

3. Cross-Bridge Detachment:

   • New ATP binds → myosin releases actin

4. Reactivation of Myosin Head:

   • ATP hydrolyzed → myosin returns to high-energy (cocked) state

• Cycle repeats as long as ATP and Ca²⁺ are available

6. Relaxation

• Ca²⁺ is pumped back into SR

• Tropomyosin re-covers actin

• Muscle fiber returns to resting length

E. Energy Use in Muscle

• Muscle contraction is ATP-intensive

• ATP is not stored long-term, must be regenerated continuously:

  • Creatine Phosphate:

    • Transfers phosphate to ADP → rapid ATP regeneration

  • Glycogen:

    • Stored in muscle → glucose → ATP

  • Cellular Respiration:

    • Oxygen + glucose → long-term ATP production

F. Rigor Mortis

• Definition: Post-death muscle stiffening

• Cause:

  • No ATP → myosin heads cannot detach from actin

  • Ca²⁺ stays in cytosol → sustained contraction

  • Cross-bridges remain locked

II. Other Muscle Types

A. Cardiac Muscle

• Location: Heart walls only

• Function: Rhythmic, coordinated contraction → pumps blood

• Spontaneous Activity: Contains pacemaker cells

• Intercalated Discs:

  • Specialized junctions between cells

  • Allow ion flow (Na⁺, Ca²⁺) → synchronized contractions

• Autonomous Rhythm:

  • Heart beats independently of nervous input

  • Can keep beating outside the body (e.g., during transplants)

B. Smooth Muscle

• Location: Digestive tract, bladder, blood vessels, reproductive organs

• Structure:

  • Involuntary, non-striated, not attached to bones

  • No sarcomeres, no T-tubules, underdeveloped SR

• Contraction:

  • Slow and sustained

  • Regulated by hormones and autonomic nervous system

  • Found in peristalsis, blood flow regulation, and organ function