A&P: lecture exam part 2

The Muscular System

Overview of the Muscular System

The muscular system consists exclusively of skeletal muscles, which are composed of muscle cells (fibers) organized into bundles called fascicles. The organization and function of these muscles are essential for movement, posture, and stability in the human body.

• Muscle fibers are grouped into fascicles.

• Muscles are classified by the arrangement of fascicles and their relationship to tendons.

• There are four main patterns of fascicle organization: parallel, convergent, pennate, and circular.

Fascicle Organization

Parallel Muscles

In parallel muscles, fibers run parallel to the long axis of the muscle. This arrangement allows for significant shortening and movement.

• Example: biceps brachii

• The muscle thickens in the center (belly) during contraction.

• Parallel muscles can contract up to about 30% of their resting length.

• Tension produced depends on the total number of myofibrils and is directly related to the cross-sectional area of the muscle.

• Approximately 1 in2 (6.45 cm2) of cross-section can develop about 50 lb (23 kg) of force.

Convergent Muscles

Convergent muscles have a broad origin and converge to a single attachment point (tendon, aponeurosis, or raphe). The direction of pull can vary depending on which fibers are activated.

• Example: pectoralis major

• Muscle fibers pull in different directions depending on stimulation.

Pennate Muscles

Pennate muscles have fibers that attach obliquely to a central tendon, resembling a feather. There are three types:

• Unipennate: Fibers on one side of the tendon (e.g., extensor digitorum).

• Bipennate: Fibers on both sides of the tendon (e.g., rectus femoris).

• Multipennate: Tendon branches within the muscle (e.g., deltoid).

• Pennate muscles do not shorten as much as parallel muscles but contain more myofibrils and generate more tension.

Circular Muscles

Circular muscles (sphincters) are arranged in concentric rings and function to open and close body openings.

• Example: orbicularis oris (mouth sphincter)

Levers in the Muscular System

Lever Mechanics

Bones act as levers (rigid structures), and joints serve as fulcrums (fixed points). Muscles provide the applied force (AF) to overcome resistance (R). Levers function to change the direction, distance, speed, or strength of movement.

• Three classes of levers are defined by the relative positions of the applied force, fulcrum, and resistance.

Lever Class	Arrangement	Example	Function
First-Class	Fulcrum between AF and R	Seesaw, neck extension	Balance force and resistance
Second-Class	Resistance between AF and Fulcrum	Wheelbarrow, standing on tiptoe	Small force moves large weight
Third-Class	AF between Resistance and Fulcrum	Biceps curl	Maximizes speed and distance

Muscle Attachments and Actions

Origins and Insertions

Each muscle has a fixed point of attachment (origin) and a moving point of attachment (insertion). Most muscles attach to the skeleton, with the origin typically proximal to the insertion.

Actions

Muscle contractions produce specific actions (movements), such as flexion, extension, and adduction. Actions are described in terms of the bone, joint, or region affected.

Muscle Interactions

• Muscles work in groups to maximize efficiency.

• Smaller muscles reach maximum tension before larger, primary muscles.

Functional Terminology

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

• Antagonist: Opposes the action of the agonist.

• Synergist: Assists the agonist or stabilizes the origin (fixator).

• Agonists and antagonists work in pairs (e.g., flexors–extensors, abductors–adductors).

Naming Skeletal Muscles

Descriptive Criteria

• Location: Indicates body region (e.g., temporalis muscle).

• Origin and Insertion: First part of the name is the origin, second is the insertion (e.g., genioglossus muscle).

• Fascicle Organization: Describes orientation (e.g., rectus = straight, transversus = across, oblique = angle).

• Relative Position: Externus (superficialis) = surface, internus (profundus) = deep, extrinsic = outside organ, intrinsic = inside organ.

• Structural Characteristics: Number of tendons (bi-, tri-), shape (trapezius, deltoid, rhomboid), size.

• Action: Movement produced (e.g., flexor, extensor, retractor), or occupation/habit (e.g., buccinator for trumpet players).

Muscle Size Terminology

• Longus = long

• Longissimus = longest

• Teres = long and round

• Brevis = short

• Magnus = large

• Major = larger

• Maximus = largest

• Minor = small

• Minimus = smallest

Divisions of the Muscular System

Axial and Appendicular Muscles

The muscular system is divided into axial and appendicular muscles based on location and function.

• Axial muscles (60%): Position the head and spinal column, move the rib cage.

• Appendicular muscles (40%): Support the pectoral and pelvic girdles and move the limbs.

Axial Muscles

• Muscles of the head and neck

• Muscles of the vertebral column

• Oblique and rectus muscles

• Muscles of the pelvic floor

Appendicular Muscles

• Position and stabilize the pectoral and pelvic girdles

• Move the upper and lower limbs

• Divided into muscles of the shoulders and upper limbs, and muscles of the pelvis and lower limbs

Effects of Aging on the Muscular System

• Skeletal muscle fibers decrease in diameter.

• Muscles become less elastic and develop more fibrous tissue (fibrosis).

• Decreased tolerance for exercise.

• Reduced ability to recover from muscular injuries.