Lecture 11

Overview of Muscle Fiber Types

• Discusses three different types of muscle fibers present in human anatomy compared visually.

Muscle Structure

• Muscles are composed of fibers that are essentially the same cells.

• Individual fibers are multinucleated, meaning they contain multiple nuclei within each fiber.

• Muscles are arranged in a parallel fashion, emphasizing efficiency in contraction.

Fascicles

• A fascicle is defined as a bundle of fibers, akin to straws in a container.

• Each fascicle represents a structural level in the muscle organization.

• Muscle fibers within fascicles are parallel to one another, maintaining structural integrity and organization.

Myofibrils

• Myofibrils are the contractile units within each muscle fiber.

• Each fiber can contain hundreds of myofibrils organized parallelly, adding to the muscle's contraction ability.

Sarcomeres

• Sarcomeres are identified as the functional units of the muscle, responsible for contractions.

• The patterns of sarcomeres reflect in the muscle’s external appearance, contributing to striations.

• Bounded by Z lines, which demarcate individual sarcomeres, they play a significant role in muscle contraction.

Contractile Proteins

• Sarcomeres contain two primary types of myofilaments:

  • Actin (thin filaments)

  • Myosin (thick filaments)

• Actin and myosin interact to facilitate muscle shortening during contraction, functioning like a tug-of-war mechanism.

Zones of the Sarcomere

• A Band: Region containing myosin filaments, including overlapping actin filaments.

• I Band: Area with only actin filaments; the length of the I band decreases during contraction.

• H Zone: The region in the center of the sarcomere where myosin is present without actin overlapping; decreases to zero during maximum muscle contraction.

Functionality During Contraction

• When muscles contract, Z discs move closer together, reducing the length of the sarcomere.

• The M line remains stationary while Z discs draw nearer during contraction, maintaining the integrity of the structure.

• The spring-like properties of titin filaments help in returning the muscle to its relaxed state following contraction.

Muscle Connective Tissues

• Epimysium: Surrounds the entire muscle, serving as a protective layer.

• Perimysium: Surrounds fascicles within muscles.

• Endomysium: Envelops individual muscle fibers, providing support and insulation.

Pathology Related to Muscle Structure

• Discussion of Duchenne muscular dystrophy, associated with dystrophin degradation leading to muscle atrophy.

• Stresses the importance of connective tissues in maintaining muscle fiber integrity and function.