The Muscular System: Structure, Function, and Major Muscle Groups

Chapter 9: The Muscular System

Structure of Skeletal Muscles

Skeletal muscles are complex organs composed of muscle fibers, connective tissues, blood vessels, and nerves. Their organization allows for efficient force generation and movement.

• Skeletal muscle fibers: Long, thin cells called muscle fibers, each surrounded by a thin layer of extracellular matrix (ECM) known as the endomysium.

• Fascicle: A bundle of 10-100 muscle fibers, surrounded by the perimysium (connective tissue sheath).

• Epimysium: Encloses all fascicles in a muscle; continuous with the fascia, which separates individual muscles.

• Connective tissues merge to form tendons that attach muscle to bone or other structures.

• Skeletal muscle is vascular and has an extensive nerve supply; it is voluntary (under conscious control) and must be stimulated by the nervous system to contract.

• Muscle knot: Also known as a myofascial trigger point.

Fascicle Patterns and Muscle Shapes

Muscles are classified by the arrangement of their fascicles, which influences their function and appearance.

• Parallel: Evenly spaced fascicles; straplike (e.g., sartorius).

• Convergent: Broad at one end, tapers to a single tendon; usually triangular (e.g., pectoralis major).

• Pennate: Fascicles attach to tendon at an angle, resembling a feather.

  • Unipennate: Fascicles on one side of tendon.

  • Bipennate: Fascicles on both sides of tendon.

  • Multipennate: Several tendons with fascicles arranged like multiple feathers.

• Circular: Encircle an opening (e.g., orbicularis oculi); often called sphincters.

• Fusiform: Thick in the belly, tapered at ends.

Naming Muscles

Muscles are named based on various characteristics:

• Size: Major, minor, longus, brevis, vastus.

• Location: Superior, inferior, medial, lateral.

• Attachment: Sternocleidomastoid (attaches to sternum, clavicle, mastoid process).

• Function: Flexor, extensor, adductor, abductor, levator.

Functions of Skeletal Muscles

The primary function is to contract and generate muscle tension, producing movement and heat.

• Agonist (Prime Mover): Main muscle responsible for movement.

• Antagonist: Opposes and slows the action of the agonist.

• Synergist: Assists the agonist, stabilizes joints, and ensures smooth movement.

• Fixator: Holds a bone in place, anchoring movement and reducing injury risk.

Muscle Origin and Insertion

Muscle actions are described by their points of attachment:

• Origin: More fixed attachment, usually stationary.

• Insertion: Attachment that moves during contraction.

• In muscles crossing multiple joints, origin and insertion may switch depending on movement and fixator activity.

Lever Systems in Body Movements

Muscles, bones, and joints form lever systems to move body parts efficiently.

• Lever: Bone

• Load: Weight of the body part or object moved

• Force: Muscle tension

• Fulcrum: Joint

Types of levers:

Class	Arrangement	Example
1st Class	Fulcrum in the middle	Atlanto-occipital joint (seesaw)
2nd Class	Load in the middle	Rising on toes (wheelbarrow)
3rd Class	Force in the middle	Most common in body (tongs)

• Mechanical advantage: Small force moves large load (fulcrum close to load).

• Mechanical disadvantage: More force needed to move load (fulcrum farther from load).

Muscle strain: Occurs when muscle is overstretched or overexerted, leading to tears.

9.2 Muscles of the Head, Neck, and Vertebral Column

Muscles of Facial Expression

These muscles insert into skin or connective tissue, not bone, allowing for facial movements and expressions.

• Epicranius: Two muscle bellies (frontalis and occipitalis) connected by the epicranial aponeurosis; elevates eyebrows.

• Orbicularis oculi: Circular muscle around the eye; closes eyelids.

• Levator palpebrae superioris: Opens eyelids; antagonist to orbicularis oculi.

• Corrugator supercilii: Produces squinting.

• Levator labii superioris: Elevates upper lip (sneering).

• Zygomaticus major/minor: Elevate corners of mouth (smiling).

• Risorius: Draws corners of mouth laterally.

• Orbicularis oris: Encircles mouth; controls lip movements (kissing muscle).

• Depressor anguli oris, depressor labii inferioris, mentalis: Frowning muscles.

• Buccinator: Compresses cheek.

• Platysma: Tenses skin of neck, depresses mandible.

Extrinsic Eye Muscles

Six skeletal muscles move each eye, four rectus and two oblique muscles.

• Rectus muscles: Superior, inferior, medial, lateral; move eye in corresponding directions.

• Oblique muscles: Superior (rotates eye inferiorly and laterally), inferior (rotates eye superiorly and laterally).

Muscles of Mastication and Swallowing

• Masseter: Elevates mandible; originates from zygomatic arch.

• Temporalis: Elevates and retracts mandible; originates from temporal bone.

• Medial and lateral pterygoids: Protract and move mandible side-to-side.

• Swallowing muscles: Digastric, stylohyoid, mylohyoid, geniohyoid (elevate hyoid and tongue); pharyngeal constrictors (move food into esophagus).

• Tongue muscles: Genioglossus (protrudes), hyoglossus (depresses), styloglossus (retracts and elevates).

Muscles That Move the Head and Neck

• Sternocleidomastoid: Rotates head to opposite side; flexes neck when both sides contract.

• Scalene muscles: Anterior, middle, posterior; lateral flexion of neck, assist in deep breathing.

• Trapezius: Superior part extends head; also moves scapula.

• Splenius capitis and cervicis: Extend and rotate head and neck.

Muscles of the Vertebral Column

• Erector spinae group: Main extensors of vertebral column; divided into iliocostalis (lateral), longissimus (middle), spinalis (medial) columns.

• Transversospinal group: Semispinalis, multifidus, rotatores; extend and rotate vertebral column.

• Quadratus lumborum: Extends and laterally flexes vertebral column.

9.3 Muscles of the Trunk and Pelvic Floor

Muscles of Ventilation

Breathing is accomplished by skeletal muscles, primarily the diaphragm and intercostal muscles.

• Diaphragm: Main muscle of inspiration; contracts to flatten and increase thoracic cavity volume, drawing air into lungs.

• External intercostals: Elevate ribs during inspiration.

• Internal intercostals: Depress ribs during forced expiration.

• Accessory muscles: Sternocleidomastoid and scalenes assist in forced inspiration.

Abdominal Muscles

• Rectus abdominis: Flexes trunk; enclosed by rectus sheath, divided by tendinous insertions ("six-pack").

• External oblique: Lateral to rectus abdominis; fibers run downward and medially.

• Internal oblique: Deep to external oblique; fibers run upward and medially.

• Transversus abdominis: Deepest; fibers run horizontally, compresses abdominal cavity, increases intra-abdominal pressure.

• Linea alba: Midline connective tissue structure.

Muscles of the Pelvic Diaphragm and Perineum

• Pelvic diaphragm: Formed by levator ani muscles (pubococcygeus, iliococcygeus, ischiococcygeus); supports pelvic organs.

• Anococcygeal ligament: Connective tissue from rectum to coccyx.

• Perineum: Inferior to pelvic diaphragm; includes urogenital diaphragm (external urethral sphincter, deep transverse perineal muscles).

• External anal sphincter: Voluntary control of defecation.

• Perineal body: Central tendon of perineum.

• Bulbospongiosus and ischiocavernosus: Erect external genitalia during sexual excitement.

9.4 Muscles of the Pectoral Girdle and Upper Limb

Muscles That Move the Scapula

• Serratus anterior: Protracts scapula ("boxer's muscle").

• Pectoralis minor: Protracts and depresses scapula.

• Trapezius: Elevates scapula (shrugging).

• Levator scapulae: Elevates scapula.

• Rhomboid major and minor: Retract scapula.

Muscles That Move the Arm at the Shoulder Joint

• Pectoralis major: Flexes, adducts, and medially rotates arm.

• Coracobrachialis: Assists in flexion and adduction of humerus.

• Deltoid: Anterior fibers flex, central fibers abduct, posterior fibers extend arm.

• Latissimus dorsi: Extends, adducts, and medially rotates arm.

• Teres major: Assists in arm extension and medial rotation.

• Rotator cuff muscles: Supraspinatus (abduction), infraspinatus (lateral rotation), teres minor (lateral rotation), subscapularis (medial rotation); stabilize shoulder joint.

Muscles That Move the Forearm and Hand

• Biceps brachii: Flexes and supinates forearm.

• Brachialis: Main flexor of forearm.

• Brachioradialis: Assists in forearm flexion.

• Triceps brachii: Extends forearm.

Hand and finger muscles are divided into flexors (anterior/medial forearm) and extensors (posterior/lateral forearm):

• Flexors: Flexor carpi radialis, palmaris longus, flexor carpi ulnaris, flexor digitorum superficialis/profundus, flexor pollicis longus.

• Pronators: Pronator teres, pronator quadratus.

• Extensors: Extensor carpi radialis longus/brevis, extensor carpi ulnaris, extensor digitorum, extensor pollicis longus/brevis, extensor indicis.

• Abductor pollicis longus: Abducts thumb.

• Intrinsic hand muscles: Thenar muscles (abductor pollicis brevis, flexor pollicis brevis), lumbricals, dorsal/palmar interossei.

9.5 Muscles of the Hip and Lower Limb

Muscles of the Hip and Thigh

• Iliopsoas: Main flexor of thigh (iliacus + psoas major).

• Pectineus: Assists thigh flexion and adduction.

• Gracilis: Adducts thigh; straplike muscle of medial thigh.

• Adductor group: Adductor magnus, longus, brevis; adduct thigh.

• Sartorius: Flexes, abducts, laterally rotates thigh, and flexes leg.

• Quadriceps femoris: Four muscles (rectus femoris, vastus lateralis, intermedius, medialis); extend leg at knee, rectus femoris also flexes thigh.

• Gluteal group: Gluteus maximus (extends, abducts, laterally rotates thigh), gluteus medius/minimus (abduct, medially rotate thigh).

• Hamstrings: Semitendinosus, semimembranosus, biceps femoris; extend thigh, flex leg.

Muscles of the Leg, Ankle, and Foot

• Tibialis anterior: Dorsiflexes and inverts foot.

• Extensor digitorum longus: Dorsiflexes foot, extends toes.

• Extensor hallucis longus: Dorsiflexes foot, extends big toe (hallux).

• Fibularis longus/brevis: Evert foot.

• Triceps surae: Gastrocnemius and soleus; plantarflex foot via calcaneal tendon.

• Plantaris, tibialis posterior, fibularis longus/brevis: Assist in plantarflexion.

• Flexor hallucis longus: Flexes big toe.

• Flexor digitorum longus: Flexes toes 2-5.

• Intrinsic foot muscles: Control fine movements of toes.

Example: Lever System in the Body

When rising on your toes, the ball of the foot acts as the fulcrum (2nd class lever), the body weight is the load, and the calf muscles provide the force. This arrangement allows a small muscle force to lift the entire body weight.

Additional info: The notes above expand on the original content by providing definitions, examples, and organizing the material into a logical, textbook-style structure for comprehensive study.