The Muscular System: Structure, Function, and Organization

The Muscular System

Overview of the Muscular System

The muscular system is composed of specialized tissues that produce movement, maintain posture, stabilize joints, and generate heat. Muscles are essential for all voluntary and many involuntary movements in the body.

• Produce movement: Muscles are responsible for locomotion and manipulation of the environment.

• Maintain posture and body position: Continuous muscle contractions stabilize the body.

• Stabilize joints: Muscles reinforce and support joint structures.

• Generate heat: Muscle contractions produce heat, contributing to body temperature regulation.

Muscle Structure and Attachments

Nerve and Blood Supply

Each muscle receives a dedicated nerve, artery, and veins. Contracting muscle fibers require large amounts of oxygen and nutrients, which are delivered by the blood supply.

Muscle Attachments

Muscles span joints and attach to bones at two main points:

• Origin: Attachment to the immovable or less movable bone.

• Insertion: Attachment to the movable bone.

Attachments can be:

• Direct (fleshy): Epimysium fused to periosteum of bone or perichondrium of cartilage.

• Indirect: Connective tissue wrappings extend beyond muscle as a tendon or aponeurosis.

Microscopic Anatomy of Skeletal Muscle

Muscle Fibers

Skeletal muscle fibers are long, cylindrical cells containing multiple nuclei. Key structures include:

• Sarcolemma: Muscle fiber plasma membrane.

• Sarcoplasm: Muscle fiber cytoplasm.

• Myofibrils: Densely packed, rodlike elements responsible for muscle contraction.

• Sarcoplasmic reticulum: Specialized endoplasmic reticulum that stores calcium ions.

• T tubules: Invaginations of the sarcolemma that help transmit action potentials.

Myofibrils and Sarcomeres

Myofibrils are composed of repeating units called sarcomeres, which are the functional units of muscle contraction. Sarcomeres contain:

• Striations: Alternating dark (A bands) and light (I bands) regions.

• Myofilaments: Thick (myosin) and thin (actin) filaments.

Molecular Composition of Myofilaments

• Thick filaments: Composed of myosin molecules, each with a tail and two heads. Myosin heads bind to actin and ATP.

• Thin filaments: Composed of actin subunits, tropomyosin, and troponin. Actin provides binding sites for myosin heads.

Muscle Contraction

Mechanism of Contraction

Muscle contraction is the activation of cross bridges between actin and myosin to generate force. Shortening occurs when the tension generated exceeds the opposing forces. The process is described by the sliding filament model:

• In the relaxed state, thin and thick filaments overlap only slightly.

• During contraction, myosin heads bind to actin, forming cross bridges and pulling thin filaments toward the center of the sarcomere.

• Neither filament changes length; they slide past each other.

Changes During Contraction

• Z discs are pulled toward the M line.

• I bands shorten.

• H zones disappear.

• A bands move closer together but do not change length.

Cross Bridge Cycle

The cross bridge cycle describes the sequence of events during muscle contraction:

1. Myosin head attaches to actin, forming a cross bridge.

2. Power stroke: Myosin head pivots, pulling actin filament toward the M line.

3. ATP attaches to myosin, causing it to detach from actin.

4. ATP is hydrolyzed, re-cocking the myosin head.

Types of Muscle Contraction

• Isometric contraction: Muscle tension increases but does not exceed the load; muscle does not shorten.

• Isotonic contraction: Muscle tension exceeds the load and the muscle shortens.

Muscle Organization and Fascicle Arrangements

Fascicle Arrangements

Fascicle arrangement determines muscle shape, range of motion, and power. Types include:

• Circular: Fascicles arranged in rings (e.g., orbicularis oris).

• Convergent: Broad origin, fascicles converge toward a single tendon (e.g., pectoralis major).

• Parallel: Fascicles parallel to the long axis (e.g., sartorius).

• Fusiform: Spindle-shaped with parallel fibers (e.g., biceps brachii).

• Pennate: Short fascicles attach obliquely to a central tendon. Types: unipennate, bipennate, multipennate.

Power depends on the number of muscle fibers; bipennate and multipennate muscles are more powerful but have a shorter range of motion.

Lever Systems in the Body

Components of a Lever System

• Lever: Rigid bar (bone) that moves on a fixed point (fulcrum).

• Fulcrum: Joint around which the lever moves.

• Effort: Force applied by muscle contraction.

• Load: Resistance moved by the effort (bone, tissue, added weight).

Classes of Levers

• First-class lever: Fulcrum is between load and effort (e.g., raising the head off the chest).

• Second-class lever: Load is between fulcrum and effort (e.g., standing on tiptoe).

• Third-class lever: Effort is applied between fulcrum and load (e.g., flexing the forearm by the biceps brachii).

Major Skeletal Muscles and Their Functions

Muscles of the Head

• Facial expression: Insert into skin, allowing for nonverbal communication; innervated by cranial nerve VII (facial nerve).

• Muscles of mastication: Four pairs, innervated by cranial nerve V; temporalis and masseter are prime movers of jaw closure, pterygoids assist in grinding, and buccinator aids in chewing.

Muscles for Breathing

• Inspiratory muscles: Diaphragm and external intercostals enlarge the rib cage during inhalation.

• Diaphragm: Separates thoracic and abdominal cavities.

Muscles of the Abdominal Wall

Four paired muscles form the lateral and anterior abdominal wall:

• Rectus abdominis

• External obliques

• Internal obliques

• Transversus abdominis

These muscles are innervated by intercostal nerves and are involved in trunk flexion, rotation, and functions such as urination, defecation, childbirth, vomiting, coughing, and screaming.

Muscles of the Anterior and Posterior Thorax

Most are extrinsic shoulder muscles responsible for movements such as elevation, depression, rotation, protraction, and retraction of the scapula.

Muscles Crossing the Shoulder Joint

• Insert on and move the humerus.

• Prime movers: pectoralis major, latissimus dorsi, deltoid.

• Rotator cuff muscles (supraspinatus, infraspinatus, teres minor, subscapularis) act as synergists and fixators, reinforcing the shoulder capsule and preventing dislocation.

Muscles Crossing the Elbow Joint

Muscles are divided into anterior (flexors) and posterior (extensors) compartments by fascia.

Muscles of the Forearm

• Divided into anterior and posterior groups, each with superficial and deep muscles.

• Responsible for movements of the wrist, fingers, thumb, and pronation/supination of the forearm.

• Pronator teres and pronator quadratus pronate the forearm; supinator assists biceps brachii in supination.

Muscles Crossing the Hip and Knee Joints

• Include flexors (iliopsoas), extensors (hamstrings), adductors (adductor magnus, longus, brevis, pectineus, gracilis), and abductors/rotators (gluteus maximus, medius, minimus, piriformis, obturator externus/internus, gemellus).

• Quadriceps are powerful knee extensors, inserting into the patella and tibial tuberosity via the patellar ligament.

Muscles of the Leg

• Anterior compartment: Toe extensors and ankle dorsiflexors (tibialis anterior, extensor digitorum longus, fibularis tertius, extensor hallucis longus).

• Lateral compartment: Plantar flexion and eversion (fibularis longus, fibularis brevis).

• Posterior compartment: Plantar flexors, divided into superficial and deep groups, innervated by the tibial nerve.

Muscles of the Foot

• Responsible for flexion, extension, abduction, and adduction of the toes.

• Support the arches of the foot; some leg tendons assist in these actions.

• Extensor digitorum brevis is a dorsal foot muscle that helps extend the toes.