Chapter 10: The Muscular System – Structure, Function, and Organization

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Muscular System Overview

Functional Organization of the Muscular System

The muscular system comprises nearly half of the body's weight and includes approximately 700 muscles. These muscles vary in size, shape, and function, and their performance depends on fiber organization and attachment to the skeleton. Muscles are divided into two main divisions:

Axial muscles: Support and position the axial skeleton.
Appendicular muscles: Support, move, and brace the limbs.

Fascicle Organization & Leverage

Types of Muscle Based on Fascicle Organization

Muscle fascicle arrangement determines muscle shape and function. The main types are:

Parallel muscles: Fascicles run parallel to the long axis. Most skeletal muscles are parallel. The central body is called the belly. Flat muscles with broad attachments are called aponeuroses. Example: Biceps brachii.
Convergent muscles: Fan-shaped with broad origin and fascicles converging to a single point. Fibers pull in different directions. Example: Pectoralis major.
Pennate muscles: Fascicles form an angle with the tendon. Types include:
- Unipennate: Fibers on one side of the tendon. Example: Extensor digitorum.
- Bipennate: Fibers on both sides. Example: Rectus femoris.
- Multipennate: Tendon branches within muscle. Example: Deltoid.
Circular muscles (sphincters): Fascicles arranged concentrically around an opening. Contraction constricts the opening. Example: Orbicularis oris.

Levers and Leverage in Muscle Action

Muscles modify force, speed, or direction by acting on levers (bones) and fulcrums (joints). There are three classes of levers:

First-class lever: Fulcrum between applied force and load (like a seesaw). Example: Joint between occipital bone and atlas.
Second-class lever: Load between applied force and fulcrum (like a wheelbarrow).
Third-class lever: Applied force between load and fulcrum. Most common in the body.

Muscle Names and Functional Terms

Origin, Insertion, and Action

Origin: End of muscle that remains stationary during contraction.
Insertion: End attached to the moving part.
Action: Specific movement produced by muscle contraction.

Muscle Groups Based on Function

Agonist (prime mover): Chiefly responsible for movement. Example: Biceps brachii for elbow flexion.
Antagonist: Opposes the agonist. Example: Triceps brachii for elbow flexion.
Synergist: Assists the agonist. Example: Brachioradialis for elbow flexion.
Fixator: Synergist that prevents movement at another joint.

Muscle Naming Terminology

Region of the body
Position, direction, or fascicle organization
Structural characteristics (origin, shape, etc.)
Muscle action

Axial and Appendicular Muscles

Axial Muscles

Arise from the axial skeleton and position the head and spinal column. Move the rib cage and assist in breathing. Four major groups:

Muscles of head & neck (facial expression, eye movement, mastication, tongue, pharynx, neck)
Muscles of vertebral column (stabilize, flex, extend, rotate)
Oblique and rectus muscles of trunk (form walls of thoracic and abdominopelvic cavities)
Muscles of pelvic floor (support pelvic organs)

Appendicular Muscles

Stabilize, position, and support limbs. Divided into upper and lower limb groups:

Upper limb: Position pectoral girdle, move arm, move forearm/hand, extrinsic/intrinsic hand muscles.
Lower limb: Move thigh, move leg, extrinsic/intrinsic foot muscles.

Axial Muscle Groups

Muscles of Facial Expression

Epicranius (frontalis & occipitalis): Raises eyebrows, wrinkles forehead.
Orbicularis oris: Purses and compresses lips.
Zygomaticus: Retracts and elevates corner of mouth.
Orbicularis oculi: Closes the eye.

Muscles of Mastication

Temporalis: Elevates mandible.
Masseter: Elevates mandible and closes jaw.

Muscles of Head and Neck

Sternocleidomastoid: Flexes neck.
Trapezius: Elevates, retracts, depresses, or rotates scapula upward.

Muscles of the Vertebral Column

Arranged in layers, extend from sacrum to skull. Many extensors, few flexors due to trunk muscles and gravity.

Oblique and Rectus Muscles of the Trunk

Oblique muscles: Fibers run at an angle to the body axis.
Rectus muscles: Fibers run parallel to the body axis.

Muscles of the Thorax

Pectoralis minor: Depresses/protracts shoulder, rotates scapula, elevates ribs.
External intercostals: Elevate ribs.
Diaphragm: Expands thoracic cavity, compresses abdominopelvic cavity.

Muscles of the Abdominal Wall

Rectus abdominis: Depresses ribs, flexes vertebral column, compresses abdomen.
External obliques: Compress abdomen, depress ribs, flex/bend spine.
Internal obliques: Compress abdomen, depress ribs, flex/bend spine.
Transverse abdominis: Compresses abdomen.

Muscles of the Pelvic Floor

Form the perineum, support pelvic organs. Superficial muscles differ between sexes; deep muscles are similar.

Appendicular Muscle Groups

Muscles That Position the Pectoral Girdle

Pectoralis minor and serratus anterior (chest)
Trapezius and levator scapulae (back)

Trapezius: Largest muscle of pectoral girdle, innervated by multiple nerves. Origin: Occipital bone; Insertion: Clavicle and scapula.

Muscles That Move the Arm

Latissimus dorsi and deltoid: Largest superficial muscles for arm movement.
Rotator cuff: Four muscles (Supraspinatus, Infraspinatus, Teres minor, Subscapularis) stabilize shoulder joint. Remembered by SITS acronym.

Muscles That Move the Forearm and Hand

Biceps brachii: Flexion at elbow/shoulder, supination.
Triceps brachii: Extension of elbow, extension/adduction at shoulder.
Brachialis: Flexion at elbow.
Brachioradialis: Flexion at elbow.

Special connective tissues include extensor retinaculum and flexor retinaculum, which hold tendons in place. Synovial tendon sheaths surround tendons crossing bony surfaces. Carpal tunnel syndrome results from inflammation, causing nerve compression and pain.

Muscles That Move the Hand and Fingers

Originate on humerus, radius, ulna, and interosseous membrane.
Located deep to wrist flexors/extensors.

Intrinsic Muscles of the Hand

Responsible for fine movements of thumb and fingers.
Flexion/extension, abduction/adduction at metacarpophalangeal joints.
Opposition/reposition of thumb.

Muscles That Move the Thigh

Gluteal group
Lateral rotator group
Adductor group
Iliopsoas group

Example: Adductor magnus produces flexion, extension, and adduction at the hip.

Muscles That Move the Leg

Flexors of the knee
Extensors of the knee: Quadriceps femoris group (Rectus femoris, Vastus medialis, Vastus intermedius, Vastus lateralis)

Muscles of the Pelvic Girdle and Thigh

Iliopsoas group: Flexion at hip.
Gluteus maximus: Extension/lateral rotation at hip.
Gluteus medius: Abduction/medial rotation at hip.
Sartorius: Flexion at knee, flexion/lateral rotation at hip.
Tensor fascia latae: Flexion/medial rotation at hip, supports knee.

Quadriceps Femoris Group

Rectus femoris: Extension at knee, flexion at hip.
Vastus medialis, intermedius, lateralis: Extension at knee.

All four insert on the patella via the quadriceps tendon.

Hamstring Muscles

Biceps femoris: Flexion at knee, extension/lateral rotation at hip.
Semimembranosus: Flexion at knee, extension/medial rotation at hip.
Semitendinosus: Flexion at knee, extension/medial rotation at hip.

Extrinsic Muscles That Move the Foot and Toes

Gastrocnemius and Soleus

Intrinsic Muscles of the Foot

Originate on tarsal/metatarsal bones, tendons, ligaments.
Provide padding and maintain arches.

Deep Fascia and Limb Compartments

Deep Fascia

Deep fascia separates limb muscles into compartments, containing infection or pressure. Clinical importance:

Compartment syndrome: Ischemia due to lack of blood flow from trauma can cause paralysis or death of muscle if not relieved within 2–4 hours.

Summary Table: Types of Muscle Fascicle Organization

Type	Fascicle Arrangement	Example	Key Features
Parallel	Parallel to long axis	Biceps brachii	Most skeletal muscles; contracts up to 30%
Convergent	Fan-shaped, broad origin	Pectoralis major	Fibers pull in different directions
Pennate	Angle with tendon	Deltoid, Rectus femoris	Unipennate, bipennate, multipennate types
Circular	Concentric around opening	Orbicularis oris	Constricts opening

Summary Table: Major Muscle Groups and Actions

Muscle Group	Example Muscles	Primary Actions
Facial Expression	Frontalis, Orbicularis oris, Zygomaticus	Move skin, facial expression
Mastication	Masseter, Temporalis	Elevate mandible, close jaw
Neck	Sternocleidomastoid, Trapezius	Flex neck, move scapula
Trunk	Rectus abdominis, External obliques	Flex vertebral column, compress abdomen
Upper Limb	Biceps brachii, Triceps brachii, Deltoid	Flexion/extension at elbow/shoulder
Lower Limb	Quadriceps, Hamstrings, Gluteus maximus	Extension/flexion at knee/hip

Key Equations and Concepts

Muscle Contraction and Tension

Tension depends on total number of myofibrils.
Muscle contraction: Shortens muscle up to 30% of its length.

Lever Mechanics

Levers amplify force, speed, or direction. The mechanical advantage (MA) is given by:

For levers:

Clinical Relevance

Carpal tunnel syndrome: Inflammation of flexor retinaculum and synovial tendon sheaths restricts movement and compresses the median nerve, causing pain and weakness.
Compartment syndrome: Ischemia from trauma can cause irreversible muscle damage if not treated promptly.

Additional info:

Muscle groups often work together to produce complex movements, with agonists, antagonists, and synergists coordinating actions.
Muscle names often reflect their location, shape, function, or attachment points.