The Muscular System

Introduction to the Muscular System

The muscular system is a complex network of nearly 700 muscles that enables movement, maintains posture, and supports bodily functions. The organization of muscle fibers into fascicles directly affects the power, range, and speed of muscle movement.

• Muscle fascicles are bundles of muscle fibers (cells) that determine muscle function.

• Muscle tension depends on the number of myofibrils and the cross-sectional area of the muscle.

• Each square inch of muscle cross-section can generate approximately 50 pounds of tension.

Muscle Fiber Structure

Muscle fibers contain myofibrils, which are the contractile units of the muscle.

• Myofibrils are composed of repeating units called sarcomeres, which are responsible for muscle contraction.

• Myofibrils are surrounded by the sarcoplasmic reticulum, which regulates calcium ion concentration during contraction.

Fascicle Arrangement and Classification of Skeletal Muscle Fibers

Patterns of Fascicle Organization

The arrangement of fascicles within a muscle determines its function and mechanical properties. There are four primary patterns: parallel, convergent, pennate, and circular.

• Parallel muscles: Fascicles run parallel to the long axis of the muscle. Example: Biceps brachii.

• Convergent muscles: Fascicles converge from a broad area to a single attachment point (tendon, aponeurosis, or raphe). Example: Pectoralis major.

• Pennate muscles: Fascicles are arranged at an angle to the tendon. These muscles generate more tension but less movement than parallel muscles. Types include:

  • Unipennate: Fascicles on one side of the tendon. Example: Extensor digitorum.

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  • Bipennate: Fascicles on both sides of the tendon. Example: Rectus femoris.

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  • Multipennate: Tendon branches within the muscle. Example: Deltoid.

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• Circular muscles (sphincters): Fascicles are arranged concentrically around an opening, controlling its diameter. Example: Orbicularis oris.

Levers and Muscle Mechanics

Levers in Skeletal Motion

Skeletal muscles produce movement by acting on bones, which function as levers. Joints serve as fulcrums, and muscles provide the applied force to overcome resistance (load).

• The function of a lever is to change the direction, distance, speed, or strength of the applied force.

• There are three classes of levers, based on the arrangement of the fulcrum, applied force, and load:

First-Class Lever

• Fulcrum is positioned between the load and applied force.

• Example: Seesaw, or the head moving on the neck.

Second-Class Lever

• Load is positioned between the fulcrum and applied force.

• Example: Wheelbarrow, or standing on tiptoe.

Third-Class Lever

• Applied force is positioned between the fulcrum and load.

• Most common lever type in the body; maximizes speed and distance.

• Example: Flexing the forearm at the elbow.

Muscle Attachment and Actions

Origins and Insertions

Muscles attach to bones at two points: the origin (fixed, usually proximal) and the insertion (moving, usually distal).

• Movement occurs when the insertion is pulled toward the origin.

Muscle Actions and Interactions

• Muscle actions are described by the movement they produce (e.g., flexion, extension, adduction).

• Muscles work in groups for efficiency; smaller muscles activate first, followed by larger ones.

• Muscle roles:

  • Agonist (prime mover): Produces a specific movement.

  • Antagonist: Opposes the action of the agonist.

  • Synergist: Assists the agonist or stabilizes its origin.

• Agonists and antagonists often work in pairs (e.g., flexors and extensors).

Naming Skeletal Muscles

Muscle Naming Conventions

The names of skeletal muscles are based on several descriptive criteria:

• Location: Indicates the region of the body (e.g., Genioglossus muscle from chin to tongue).

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• Origin and Insertion: The first part of the name is the origin, the second is the insertion.

• Fascicle Organization: Describes the orientation of fascicles (e.g., Rectus = straight, Transverse = across, Oblique = angled).

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• Relative Position: Indicates depth or location (e.g., Externus or superficialis = superficial; Internus or profundus = deep).

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• Structural Characteristics: Number of tendons (bi-, tri-), shape (trapezius, deltoid, rhomboid), and size (longus, brevis, magnus, maximus, minor, minimus, vastus).

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• Action: Describes the movement produced (e.g., flexor, extensor, adductor) or habits (e.g., risor = laughter).

Divisions of the Muscular System

Axial and Appendicular Muscles

The muscular system is divided into two main groups:

• Axial muscles: Position the head and spinal column, move the rib cage; comprise 60% of skeletal muscles.

• Appendicular muscles: Support the pectoral and pelvic girdles and limbs; comprise 40% of skeletal muscles.

Summary Table: Fascicle Arrangement Types

Summary Table: Muscle Roles

Additional info: Expanded explanations and tables were added for clarity and completeness, based on standard anatomy and physiology textbook content.