lecture 16 pt1

Striated Muscle Overview

• Striated muscle is specialized tissue found in skeletal muscles and is characterized by its banded appearance due to the arrangement of contractile proteins.

• Terminology:

  • Plasma lemma in general cells is called sarcolemma in muscle cells.

  • Endoplasmic reticulum is referred to as sarcoplasmic reticulum in muscle cells.

  • Cytoplasm is called sarcoplasm in muscle cells.

• The prefix "sarco" refers specifically to striated muscle tissue.

Myofibrils and Muscle Contraction

• Myofibrils:

  • Myofibrils are the contractile units of striated muscle and are composed of two key proteins: actin and myosin.

  • When you exercise, you do not increase the number of muscle cells, but rather increase the amount of myofibrils, making the muscle stronger.

• Muscle Bands:

  • Thick bands are comprised of myosin, while thin bands are comprised of actin.

  • A bands represent thick myosin bands, and I bands represent thin actin bands.

  • The area between Actin filaments is called the H zone.

  • Z bars are tiny bands that help hold everything in place within myofibrils.

Muscle Relaxation and Contraction

• During muscle contraction, the fibers pull together, causing the muscle to become shorter and thicker.

• When muscles are not used (e.g., due to a cast), they undergo atrophy, leading to a decrease in muscle size and strength.

Mechanism of Contraction

• Cross Bridges:

  • Myosin heads (cross bridges) bind to specific grooves on actin, pulling them together.

  • Binding is influenced by electrical charges: positive charges from actin and negative charges from myosin attract each other.

  • Energy Consumption:

    • Approximately 65% of energy (ATP) is used for muscle contraction, and 35% is used for body heat.

• Rigor Mortis:

  • Occurs post-mortem as myosin cross bridges remain bound to actin due to lack of ATP, leading to muscles being locked in a contracted state.

Role of Tropomyosin and Troponin

• Tropomyosin is a protein that covers actin sites to prevent muscle contraction when relaxation occurs.

• Troponin is essential for muscle contraction as it interacts with calcium to pull tropomyosin away from the actin binding sites, thus allowing contraction to occur.

Muscle Fiber Types

• Myoglobin:

  • Myoglobin is a protein similar to hemoglobin that carries oxygen within muscle cells.

  • Muscle fibers can be categorized into:

    • Red Fibers: Contain high myoglobin; used for posture and long-term contractions (e.g., neck muscles).

    • White Fibers: Used for quick actions and responses, such as sprinting.

Genetic Determination of Muscle Composition

• Muscle fiber composition (red vs. white) is genetically determined.

• Different people have varying proportions of these fibers uniquely influencing their performance in activities such as sprinting versus distance running.

Sliding Filament Theory

• The muscle contraction mechanism is referred to as the sliding filament theory, where actin and myosin slide past each other, facilitated by the interactions of troponin and tropomyosin in response to calcium ions.

Neuromuscular Junction

• At the myoneural junction, nerves stimulate muscle fibers to contract.

• Acetylcholine is the neurotransmitter released at the synapse that makes the muscle membrane permeable to sodium ions, generating an electrical impulse that reaches the muscle threshold for contraction.

• Threshold levels prompt the release of calcium from the sarcoplasmic reticulum, triggering contraction.

Conclusion

• Understanding the structure and mechanism of striated muscle is crucial for comprehending muscle contraction, muscle health, and functional fitness.

• The details about the sliding filament theory and muscle fiber compositions are essential topics to review for the exam.