The Muscular System

• The muscular system consists of over 600 different muscles.
• It is composed of connective tissue and muscle tissue.

Functions of the Muscular System

• Creates movement by working with bones, ligaments, and tendons.
• Moves food through the digestive system.
• Moves blood through the body by pumping the heart.
• Produces heat.
• Provides protection to inner organs.

Tendons and Ligaments

• Tendons attach muscles to bones; they consist of flexible connective tissue strands.
• Muscle contraction shortens the tendon, which pulls on the bone.
• Ligaments are fibrous tissues that connect bone to bone.

Types of Muscle

• Skeletal muscles attach to bones for movement.
• They also help protect inner organs.
• Cardiac muscles are found only in the heart.
• They cause the heart to beat.
• Smooth muscles line the digestive tract and blood vessels.
• These muscles move food and blood through the body.

Voluntary vs. Involuntary Muscles

• Voluntary muscle action is under control.
• Most skeletal muscle movement is voluntary.
• Walking, talking, running, writing, jumping, and stretching are examples of voluntary actions.
• Involuntary muscle action is not under your control.
• Cardiac and smooth muscles are involuntary.
• Heart beating, food moving through the digestive tract, and reflexes are examples of involuntary actions.

How Muscles Work

• Muscles work in pairs to cause smooth, controlled movements.
• Many basic movements are a result of muscles bending and straightening.
• A flexor muscle contracts to bind a part of the body.
• An extensor muscle contracts to straighten a part of the body.
• Flexor and Extensor muscles typically work opposite of one another.

Skeletal Muscle Structure

• Skeletal muscle is encased by epimysium, a fascia of fibrous connective tissue.
• Endomysium surrounds muscle fibers.
• Sarcolemma lies beneath the endomysium and is a thin elastic membrane.
• Sarcoplasm, the cytosol, contains contractile proteins, nuclei, mitochondria, and sarcoplasmic reticulum.

Sarcomere

• Sarcomere is the basic functional unit of muscle.
• It is distinguished as the area between Z lines.
• Thicker filaments confine to A-band, including A lighter middle region called H-zone.
• Thinner filaments arise from the middle region of I band at the Z line.

Muscle Contraction

• During contraction, neither thicker nor thinner filaments change in length; they slide.
• They slide past each other.
• During contraction, the A band remains the same, while the I band and H zone decrease.

Actin-Myosin Orientation

• Myosin filaments (thick) have long, rod-shaped tails and two globular heads that form cross-bridges.
• Actin filaments (thin) are pear-shaped double helixes.
• Tropomyosin is a rigged rode shaped protein lies in groove on either side of actin.
• Troponin is a complex of three globular proteins.

The Sliding Filament Theory

• The theory proposes that muscle fibers can shorten or lengthen as thick and thin myofilaments slide past each other.
• The filaments themselves do not change length.
• Myosin cross-bridges attach, rotate, and detach from actin filaments using energy from ATP hydrolysis, which drives fibers shortening.

Mechanism of Muscle Contraction

• Muscle cells are packed with protein fibers called myofibrils, collections of sarcomeres separated by Z-discs.
• Sarcomeres are the smallest contractile units in muscle fibers.
• Sarcomeres contain two main types of protein fibers: myosin (thicker) and actin (thinner).
• Muscle contraction is produced by the sliding of myosin fibers over actin fibers in each sarcomere, resulting in muscle cell contraction.
• Striation in skeletal muscle tissue is due to the arrangement of thick and thin filaments.

Motor Units

• Motor unit synaptic bulbs of one motor neuron (nerve cell) innervate many muscle dispersed randomly in the overall muscle mass.
• The muscle cells and the motor neuron that innervates them are called a motor unit.
• When the neuron of a motor unit sends a nerve impulse, all muscle cells (fibers) of the motor unit contract together.