Chapter 9 - The Muscular System

Muscle Fiber Organization

• The internal organization of a muscle fiber includes several key components:

  • Myofibril: Long, thread-like structures that contract to cause muscle movement.

  • Mitochondria: Organelles responsible for ATP production that provide energy for muscle contractions.

  • Sarcolemma: The cell membrane surrounding a muscle fiber.

  • Thin Filament: Composed primarily of actin; important for muscle contraction.

  • Thick Filament: Composed mainly of myosin; essential for the contraction process.

  • Triad: A structure consisting of a T tubule and two terminal cisternae that play a crucial role in calcium signaling for contraction.

  • Sarcoplasmic Reticulum: A specialized endoplasmic reticulum that stores calcium ions important for muscle contraction.

  • Sarcoplasm: The cytoplasm of muscle fibers where myofibrils are suspended.

Sarcomere Structure

• Key components within the sarcomere include:

  • A band: The dark band of the sarcomere where thick filaments are located.

  • I band: The light band where thin filaments are present, that do not overlap with thick filaments.

  • H band: The lighter area in the A band where there are no thin filaments.

  • Z line: The structure at either end of the sarcomere to which thin filaments are anchored.

  • M line: The middle line of the sarcomere, where thick filaments are anchored.

  • Titin: A large protein that helps anchor thick filaments to the Z line and maintains the structure of the sarcomere.

  • Zone of Overlap: The area where thin and thick filaments overlap, crucial for muscle contraction.

Relaxed and Contracted Sarcomeres

• Relaxed Sarcomere:

  • The I bands are wider; the space between thick filaments is larger, allowing for muscle relaxation.

• Contracted Sarcomere:

  • The I bands narrow; thick and thin filaments slide past each other, reducing the space between the Z lines and resulting in contraction.

The Neuromuscular Junction

• Key components involved in muscle contractions:

  • Arriving Action Potential: Signals the motor neuron to release neurotransmitters.

  • Synaptic Cleft: The gap between the motor neuron and the muscle fiber where neurotransmitters diffuse.

  • Acetylcholine (ACh): The neurotransmitter released, binding to receptors on the sarcolemma at the motor end plate.

  • AChE (Acetylcholinesterase): An enzyme that breaks down ACh after muscle contraction.

  • Junctional Folds: Increases the surface area of the motor end plate, enhancing the reception of ACh.

Steps in Initiating Muscle Contraction

1. ACh Released: ACh binds to receptors on the sarcolemma.

2. Action Potential: Travels down the T tubules, triggering events leading to contraction.

3. Calcium Release: Sarcoplasmic reticulum releases Ca²⁺ ions into the sarcoplasm.

4. Active-Site Exposure: Calcium facilitates the exposure of active sites on actin.

5. Cross-Bridge Formation: Myosin heads attach to actin, initiating contraction.

6. ACh Removal: ACh is hydrolyzed by AChE, halting further stimulation.

7. Recapture of Ca²⁺: Calcium is resequestered by the sarcoplasmic reticulum.

8. Active Sites Covered: Myosin can no longer bind to actin, ending contraction.

9. Contraction Ends: Muscle fiber returns to resting state as tension is released.

10. Relaxation: The muscle passively returns to its resting length.

Motor Units

• Motor units consist of motor neurons and the muscle fibers they innervate, which may vary in number:

  • Motor Unit 1: Responsible for fine control with fewer muscle fibers.

  • Motor Unit 2: Moderately sized for balance and coordination.

  • Motor Unit 3: Larger units for gross movements involving many muscle fibers.