1.1 Muscular System Notes

Study Guide: The Muscular System (Unit 5.1)

Basic Features of Muscles

• Excitability: Muscles react to signals, like nerves or chemicals.

• Contractibility: Muscles can shorten to create movement.

• Extensibility: Muscles can stretch without breaking.

• Elasticity: Muscles return to their original shape after stretching or contracting.

Muscle Attachments

• Origin: The fixed end of the muscle, usually closer to the center of the body, that doesn’t move.

• Insertion: The end of the muscle that moves the body part during contraction.

Types of Muscle Tissue

1. Skeletal Muscle:

   • Look: Striped (striated) and tube-shaped with many nuclei.

   • Control: Voluntary (you control it).

   • Job: Moves bones for body motion.

2. Smooth Muscle:

   • Look: Not striped (smooth), spindle-shaped, one nucleus.

   • Control: Involuntary (works on its own).

   • Job: Found in organs like the stomach; helps move things inside the body.

3. Cardiac Muscle:

   • Look: Striped, branched, one nucleus.

   • Control: Involuntary (keeps working without thinking).

   • Job: Found only in the heart to pump blood.

Parts of a Muscle Cell

• Sarcolemma: The cell’s outer layer (membrane).

• Sarcoplasm: The liquid inside the cell (like cytoplasm).

• Myofibrils: Long strands inside the muscle cell that help it contract.

• Sarcoplasmic Reticulum: Stores and releases calcium, which is needed for muscle contraction.

• T-Tubules: Pathways that carry electrical signals into the muscle cell.

Proteins in Muscles

• Actin (Thin Filament):

  • A protein that works with other proteins (troponin and tropomyosin) to allow muscles to contract.

• Myosin (Thick Filament):

  • A motor protein that pulls actin to create movement using energy from ATP.

Sarcomere: The Muscle’s Working Unit

• What is it? The part of the muscle cell that contracts.

• Key Parts:

  • Z Line: Marks the end of each sarcomere; pulled closer during contraction.

  • M Line: Middle of the sarcomere; keeps myosin in place.

  • I Band: Area with only actin; appears light.

  • A Band: Area where actin and myosin overlap; appears dark.

  • H Zone: Center of the A Band with only myosin; shrinks during contraction.

Steps of Muscle Contraction

1. Signal from Nerve:

   • A nerve sends a message to the muscle.

   • The nerve releases acetylcholine (Ach), which starts an electrical signal in the muscle cell.

2. Calcium Release:

   • The signal triggers the sarcoplasmic reticulum to release calcium (Ca²⁺).

   • Calcium is essential for contraction.

3. Sliding Filament Theory:

   • Calcium Action: Calcium binds to troponin, moving tropomyosin and uncovering binding sites on actin.

   • Cross-Bridge Formation: Myosin attaches to actin.

   • Power Stroke: Myosin pulls actin toward the center of the sarcomere, shortening the muscle.

   • Resetting: ATP helps myosin detach and prepare for another pull.

What Happens to the Sarcomere During Contraction?

• H Zone and I Band: Both get smaller or disappear.

• Z Lines: Move closer together.

• A Band: Stays the same (myosin length doesn’t change).