The Muscular System: Structure, Function, and Contraction

Excitability

The ability of muscle tissue to respond to a stimulus from a motor neuron or hormone.

Contractability

The ability of muscle tissue to shorten when stimulated.

Extensibility

The ability of muscle tissue to lengthen or stretch, even past its original shape.

Elasticity

The ability of muscle tissue to recoil or bounce back to the original shape and length after being stretched.

Skeletal Muscle

Voluntarily controlled, cylindrical, multinucleated, and striated muscle tissue connected to bones that contracts slowly or very quickly.

Cardiac Muscle

Involuntarily controlled, branched, uninucleated, and striated muscle tissue found in the heart that produces mostly slow and steady contractions except during short periods of activity.

Smooth Muscle

Involuntarily controlled, nonstriated, and uninucleated muscle tissue arranged in uniform layers within the walls of internal organs (e.g., stomach) that produces slow contractions sustained for long periods of time.

Tendon

A fibrous connective structure that attaches skeletal muscle to bone.

Fascicle

A bundle of muscle fibers situated within a skeletal muscle.

Muscle Fiber

A long, thin muscle cell covered by sarcoplasmic reticulum.

Sarcoplasmic Reticulum

A specialized membrane covering each muscle fiber that transmits an impulse to the muscle fiber.

Myofibril

Thread-like organelle of a muscle fiber structured into long, striated functional units called sarcomeres.

Sarcomere

The contractile unit of a muscle fiber located in the space between two Z-lines.

Myofilaments

Protein filaments—categorized as actin (thin) and myosin (thick)—responsible for the contracting activity of muscle fibers.

Actin

The thin myofilament that attaches at the Z-line and gets pulled toward the M-line during muscle contraction.

Myosin

The thick myofilament featuring rounded extensions called heads that bind to actin to pull the muscle fiber toward the M-line.

Epimysium

The protective membrane covering the whole skeletal muscle.

Perimysium

The protective connective tissue membrane covering a muscle fascicle.

Endomysium

The delicate protective membrane covering an individual muscle fiber.

Sliding Filament Theory

The process of muscle contraction where myosin heads bind to twisted actin filaments and pull them toward the M-line, drawing Z-lines closer together and shortening the sarcomere using ATP.

Z-line

The structural boundary where actin filaments are attached, defining the borders of an individual sarcomere.

M-line

The central line in the sarcomere toward which actin and myosin interact to pull the muscle fiber, shortening the fiber.

Calcium Ions ($\text{Ca}^{2+}$)

Ions required during contraction to expose the active sites on actin for myosin heads to attach.

Oxygen Debt

A condition occurring during intense physical exertion where a difference arises between the amount of oxygen available and the amount required, resulting in being out of breath.

Muscle Fatigue

A state reached when muscles are worked strenuously, the glucose supply is exhausted, and ATP is no longer efficiently used for cross-bridges.

Origin

The point of attachment of a muscle tendon to the more stationary bone.