The Muscular System

Introduction to the Muscular System

The muscular system consists exclusively of skeletal muscles, which are responsible for movement, posture, and heat production. The organization of muscles affects their power, range, and speed of movement.

Fascicle Arrangement

Types of Fascicle Arrangements

Skeletal muscle fibers are grouped into bundles called fascicles. The arrangement of these fascicles determines the muscle's function and efficiency. Muscles are classified based on fascicle patterns:

• Parallel muscles: Fascicles run parallel to the long axis of the muscle. These muscles can be flat or cylindrical with a central belly. Example: biceps brachii. Tension depends on the number of myofibrils.

• Convergent muscles: Muscle fibers spread out like a fan and converge on a single attachment site (tendon, aponeurosis, or raphe). Example: pectoralis major. Fibers can pull in different directions.

• Pennate muscles: Fascicles are at an angle to the tendon. These muscles do not move their tendons as far as parallel muscles but contain more myofibrils and develop more tension. Types include unipennate (e.g., extensor digitorum), bipennate (e.g., rectus femoris), and multipennate (e.g., deltoid).

• Circular muscles (sphincters): Fascicles are arranged concentrically around an opening. Contraction decreases the diameter of the opening. Example: orbicularis oris.

Levers in the Muscular System

Classes of Levers

Muscles attach to bones, which act as levers. The site of attachment affects force, speed, and range of movement. Each joint acts as a fulcrum. Levers can change the direction, distance, speed, and strength of applied force. There are three classes of levers:

• First-class lever: The fulcrum is between the applied force and the load. Example: extension of the neck and lifting the head.

• Second-class lever: The load is between the applied force and the fulcrum. Example: ankle extension (plantar flexion) by calf muscles.

• Third-class lever: The applied force is between the load and the fulcrum. This is the most common lever in the body, maximizing speed and distance at the expense of force. Example: flexion at the elbow.

Origins, Insertions, and Muscle Actions

Origins and Insertions

The origin is the fixed point of attachment of a muscle, usually proximal, while the insertion is the movable point, usually distal. Muscle actions are described by their effect on bones or joints (e.g., flexion, extension, adduction).

Muscle Interactions

• Agonist (prime mover): Responsible for a specific movement.

• Antagonist: Opposes the action of the agonist.

• Synergist: Assists the agonist.

• Fixator: Stabilizes the origin of the agonist by preventing movement at another joint.

Agonists and antagonists work in pairs; when one contracts, the other stretches (e.g., flexors and extensors).

Naming Skeletal Muscles

Muscle Naming Conventions

Muscle names provide descriptive information about their region, position, direction, fascicle arrangement, structural characteristics, and action.

• Region: Examples include abdominal, brachial, femoris, etc.

• Position/Direction: Terms like externus, internus, extrinsic, intrinsic, transversus, oblique, rectus.

• Structural Characteristics: Origin and insertion (e.g., genioglossus), number of tendons (biceps, triceps, quadriceps), shape and size (deltoid, trapezius, rhomboid).

• Action: Terms such as flexor, extensor, pronator, supinator, abductor, adductor, etc.

Some names reflect occupations or habits (e.g., buccinator means "trumpeter").

Axial and Appendicular Muscles

Divisions of the Muscular System

The muscular system is divided into axial and appendicular muscles:

• Axial muscles: Constitute 60% of skeletal muscles. They position the head and vertebral column, move the rib cage, and form the pelvic floor.

• Appendicular muscles: Move and support the pectoral and pelvic girdles and limbs.

Principal Axial Muscles

Muscles of the Head and Neck

Axial muscles are grouped by location and function:

• Muscles of facial expression: Originate on the skull. Examples: orbicularis oris (mouth), buccinator (cheek).

• Muscles of mastication: Move the mandible. Examples: masseter, temporalis, pterygoid muscles.

• Muscles of the tongue: Names end in "glossus" (e.g., genioglossus, hyoglossus).

• Muscles of the pharynx: Initiate swallowing.

• Extrinsic eye muscles: Control eye position.

• Muscles of the anterior neck: Control larynx position, depress mandible, and support tongue and pharynx.

Muscles of the Vertebral Column

• Erector spinae muscles: Superficial (spinalis, longissimus, iliocostalis) and deep layers (semispinalis, multifidus, interspinales, intertransversarii, rotatores).

• Spinal flexors: Longus capitis, longus colli (neck); quadratus lumborum (lumbar region).

Oblique and Rectus Muscles

• Oblique muscles: Compress underlying structures and rotate the vertebral column. Examples: scalene (cervical), intercostal (thoracic), external/internal oblique, transversus abdominis (abdominopelvic).

• Rectus muscles: Flex the vertebral column. Example: rectus abdominis.

• Diaphragm: Major muscle for breathing, divides thoracic and abdominopelvic cavities.

Muscles of the Pelvic Floor

• Support pelvic organs, flex sacrum and coccyx, control movement through urethra and anus.

• Perineum divided into urogenital and anal triangles; pelvic diaphragm forms muscular foundation.

Principal Appendicular Muscles

Muscles of the Shoulders and Upper Limbs

• Muscles that position the pectoral girdle: Trapezius, serratus anterior, subclavius, pectoralis minor, rhomboid major/minor, levator scapulae.

• Muscles that move the arm: Deltoid, supraspinatus, subscapularis, teres major/minor, infraspinatus, coracobrachialis, pectoralis major, latissimus dorsi.

• Rotator cuff: Supraspinatus, infraspinatus, teres minor, subscapularis.

• Muscles that move the forearm and hand: Biceps brachii, triceps brachii, brachialis, brachioradialis, supinator, pronator teres, pronator quadratus, flexor/extensor carpi muscles.

• Muscles that move the fingers: Extrinsic (forearm origin) and intrinsic (hand origin) muscles.

Muscles of the Pelvis and Lower Limbs

• Muscles that move the thigh: Gluteal muscles (maximus, medius, minimus), lateral rotators, adductors, iliopsoas.

• Muscles that move the leg: Flexors (hamstrings: biceps femoris, semitendinosus, semimembranosus, sartorius, popliteus), extensors (quadriceps femoris: vastus muscles, rectus femoris).

• Muscles that move the foot and toes: Extrinsic (gastrocnemius, soleus, fibularis, tibialis posterior/anterior, extensor/flexor digitorum/hallucis longus), intrinsic (originate on tarsal/metatarsal bones).

Functional Relationships and Effects of Exercise

Integration with Other Body Systems

The muscular system is supported by:

• Cardiovascular system: Delivers oxygen and nutrients, removes carbon dioxide.

• Respiratory system: Responds to oxygen demand.

• Integumentary system: Disperses heat from muscle activity.

• Nervous and endocrine systems: Direct responses of all systems.

Exercise produces responses in multiple body systems, enhancing muscular efficiency and overall health.

*Additional info: The notes have been expanded with academic context, definitions, and examples to ensure completeness and clarity for college-level Anatomy & Physiology students.*