Muscular System: Muscle Fiber Structure and Organization

Comprehensive practice flashcards reviewing skeletal muscle development, tissue organization, vasculature, electrical signaling, and sarcomere microanatomy.

How do skeletal muscle fibers develop during embryogenesis, and why are they multinucleated?

They develop through the fusion of myoblasts ("baby cells") to become very large cells; being multinucleate allows for increased production of enzymes and proteins.

What are myosatellite cells, and what is their function?

They are unfused myoblasts that do not fuse into the muscle fiber, functioning to aid in muscle repair.

What is the sarcolemma, and what electrical property change triggers contraction?

The sarcolemma is the cell membrane of a muscle fiber surrounding the sarcoplasm (cytoplasm); a change in transmembrane potential ($-90\,mV$) triggers muscle contraction.

What are Transverse tubules (T tubules) and what is their role in muscle contraction?

Narrow tubules continuous with the sarcolemma extending deep into the muscle fiber that transmit action potentials ("electricity") through the cell to allow the entire muscle fiber to contract simultaneously.

What are myofibrils and what are the two main types of myofilaments within them?

Lengthwise subdivisions (threads) within a muscle fiber composed of bundles of myofilaments: thin filaments (made mostly of the protein actin) and thick filaments (made of the protein myosin).

What is the Sarcoplasmic Reticulum (SR) and what does it store?

A membranous structure surrounding each myofibril that forms chambers (terminal cisternae) attached to T tubules and stores high concentrations of Calcium.

What structural components make up a triad in skeletal muscle?

A triad is formed by one T tubule and two terminal cisternae.

What is a sarcomere?

The repeating pattern of myofilaments, the structural units that form myofibrils, and the smallest contractile units of muscle.

What creates the striped or striated appearance within myofibrils?

The alternating dark, thick filaments (A bands) and light, thin filaments (I bands).

What is the M line within the A band of a sarcomere?

The center of the A band located at the midline of the sarcomere, consisting of proteins that connect the thick filaments.

What characterizes the H band of a sarcomere?

The area around the M line that contains thick filaments and no thin filaments.

What is the zone of overlap in a sarcomere?

The densest, darkest area on a light micrograph where thick and thin filaments overlap and interact so contraction occurs.

What structures compose the I band and where are the Z lines located?

The I band contains thin filaments only; Z lines are located at the centers of the I bands at the two ends of a sarcomere.

What is the structure of F-actin (filamentous actin) in thin filaments?

Two twisted rows of the globular protein G-actin, where the active sites on G-actin bind to myosin.

What is the function of nebulin in the thin filament?

Nebulin holds F-actin together.

What is tropomyosin and what is its role in resting muscle tissue?

A double-stranded protein that lies on top of the actin strand and prevents actin-myosin interaction.

What is the function of troponin in the thin filament?

Troponin binds tropomyosin to actin.

How do muscle connective tissue layers form attachments to bone?

Collagen fibers of the endomysium, perimysium, and epimysium come together at the ends of muscles to attach to bone as either a tendon (bundle) or an aponeurosis (flat sheet, e.g., palm).

What are the primary functions of the extensive vasculature supplying skeletal muscles?

To supply large amounts of oxygen, supply nutrients (glucose, fatty acids), and carry away waste (lactic acid, $CO_2$).

How are skeletal muscles controlled by the nervous system?

They are voluntary muscles controlled by nerves of the central nervous system.